integration of pdms membranes into thermoplastic ......integration of pdms membranes into...

Integration of PDMS membranes into thermoplastic microfluidic packages 1

Integration of PDMS Membranes into Thermoplastic Microfluidic

Packages

By Paul Maxwell Miller

A thesis submitted to the Faculty of Engineering and Industrial Sciences at

Swinburne University of Technology in fulfilment of the requirements for the degree of Doctor of Philosophy

Hawthorn, Victoria, Australia

June 2009


Declaration This thesis contains no material that has been accepted for the award of any other

degree or diploma in any university or college of advanced education and to the best

of my knowledge contains no material previously published or written by another

person except where due reference is made

_________________________ _________________ Signature Date


Acknowledgements Firstly I would like to thank my supervisors, Dr Igor Sbarski, Professor Tom Spurling

and Dr Thomas Gengenbach for all their support and effort during my PhD.

I would also like to thank various other people who have assisted me with my PhD,

including Professor Erol Harvey, Dr Matthew Solomon, Dr Matthias Schuenemann,

Martin Telgarski, Dr Jason Hayes, Sebastian Garst, David Thomson and Rowan

Cumming.

Thanks also to my DSTO colleagues, Dr. Dan Billing and Dr. John Coumbaros for

encouragement and/or badgering. I would also like to thank Dr. Mike Rogers for

editing my thesis when he had no obligation to.

Finally I would like to thank my family; my partner Louise Formby for editing and

support at the end; my former partner Amy Timoshanko, who supported me through

the unhappier days of my PhD. Very last thanks go to my dog, Homer, for always

lifting my spirits.


Abstract This project proposes a novel process to integrate a polydimethylsiloxane (PDMS)

membrane into a polycarbonate (PC) microfluidic device.

A novel composite material was manufactured by adhesively laminating UV/ozone

pretreated PDMS to polycarbonate using Loctite 3105 UV curable adhesive. Hot

embossing of this composite produced a microstructured substrate of PC with an

adhesively bonded PDMS layer at the surface.

This PDMS surface was then exposed to an oxidative media such as oxygen plasma

or UV radiation to produce a highly activated PDMS surface that can adhere to itself

upon contact.

The use of a novel system of PC bonded to PDMS using a UV curable adhesive

resulted in adhesion of sufficient strength and durability to withstand the hot

embossing process. Optimizing the UV curing parameters maximized joint strength

for PC to PDMS bonds. The surface chemistry of the delaminated PDMS was

analyzed and used to determine the dependence of adhesive failure mode on

UV/ozone treatment parameters.

The research conducted proves that an adhesively laminated composite of PDMS

and PC can be embossed in a reproducible way with both semi-circular and

rectangular tools. The dimensions of the embossed channels can also be modeled

in terms of both the embossing parameters and the UV exposure parameters.

In summary, this technique can be used to fabricate microfluidic structures in a

controlled fashion, and when combined with oxidative exposure of PDMS surfaces,

offers an effective way to integrate PDMS membranes into thermoplastic microfluidic

devices.


Table of Contents 1 Introduction .........................................................................................................17

1.1 Statement of the Problem............................................................................17 1.2 Project Outline .............................................................................................18 1.3 Microfluidic Devices and Technology ..........................................................19

1.3.1 Microfluidics..........................................................................................19 1.3.2 Advantages of Miniaturisation of Microfluidic Components .................19 1.3.3 Advantages of Polymers for Manufacturing Microfluidic Devices .............19 1.3.4 Integrated Microfluidic Components.....................................................21 1.3.5 Applications of Microfluidic Devices .....................................................21

1.4 Boundaries of the Research ........................................................................22 1.5 Structure of the Thesis.................................................................................23 1.6 Institutions....................................................................................................25

2 Literature Review ................................................................................................26 2.1 Overview ......................................................................................................26 2.2 Bonding and Adhesion of Polymers ............................................................26

2.2.1 Introduction...........................................................................................26 2.2.2 Mechanical Interlocking........................................................................26 2.2.3 Chemical Bonding ................................................................................27 2.2.4 Adsorption ............................................................................................27 2.2.5 Electrostatic ..........................................................................................27 2.2.6 Diffusion................................................................................................27 2.2.7 Weak Boundary Layers ........................................................................27

2.3 Surface Modification of Polymers ................................................................28 2.3.1 Introduction...........................................................................................28 2.3.2 Mechanical Processes .........................................................................28 2.3.3 Wet Chemical Processes .....................................................................28 2.3.4 Dry Chemical Processes ......................................................................29 2.3.5 Surface Treatment of PDMS in Microfluidics .......................................33

2.4 Polymer Microfabrication Methods ..............................................................33 2.4.1 Introduction...........................................................................................33 2.4.2 Laser Machining ...................................................................................34 2.4.3 Micromilling...........................................................................................34 2.4.4 Hot Embossing .....................................................................................35 2.4.5 Injection Molding...................................................................................38 2.4.6 Other Molding Techniques ...................................................................40 2.4.7 Casting..................................................................................................41 2.4.8 Plasma Etching.....................................................................................41

2.5 Sealing of Microfluidic Devices....................................................................42 2.5.1 Introduction...........................................................................................42 2.5.2 Solvent Bonding ...................................................................................42 2.5.3 Lamination ............................................................................................43 2.5.4 Adhesives .............................................................................................43 2.5.5 Thermal Bonding ..................................................................................44 2.5.6 Oxidative Sealing..................................................................................45 2.5.7 Problems With Sealing of Microfluidic Devices....................................46

2.6 PDMS in Microfluidic Devices......................................................................46 2.7 Polycarbonate in Microfluidic Devices.........................................................49 2.8 Integration of PDMS and Thermoplastics in Microfluidic Devices...............50


3 Materials & Methodology ....................................................................................52 3.1 Introduction ..................................................................................................52 3.2 Materials ......................................................................................................52

3.2.1 Polydimethylsiloxane (PDMS) ..............................................................52 3.2.2 Polycarbonate (PC) ..............................................................................53 3.2.3 Loctite 3105 UV Curable Adhesive ......................................................53

3.3 Thermal Testing...........................................................................................54 3.3.1 Introduction...........................................................................................54 3.3.2 Differential Scanning Calorimetry (DSC)..............................................55 3.3.3 Differential Photocalorimetry ................................................................56

3.4 Mechanical Testing......................................................................................57 3.4.1 Introduction...........................................................................................57 3.4.2 Tensile Mechanical Testing..................................................................57 3.4.3 Lap Shear Testing ................................................................................59 3.4.4 Peel Testing..........................................................................................60 3.4.5 Rheometry ............................................................................................61

3.5 Surface Modification of Polymers ................................................................62 3.5.1 UV Surface Treatment..........................................................................62 3.5.2 The Oxygen Plasma Reactor ...............................................................63

3.6 Spectroscopy Techniques ...........................................................................64 3.6.1 X-ray Photo Electron Spectroscopy .....................................................64

3.7 Microfabrication Technology........................................................................67 3.7.1 AVIA Laser Machining..........................................................................67 3.7.2 Micro Milling..........................................................................................68 3.7.3 Electroplating........................................................................................69 3.7.4 Hot Embossing .....................................................................................70

3.8 Microscopy Techniques ...............................................................................71 3.8.1 Overview of Microscopy Techniques ...................................................71 3.8.2 Laser Confocal Microscopy ..................................................................72 3.8.3 SEM Microscopy...................................................................................73 3.8.4 Preparation of Microcopy Samples ......................................................74

3.9 Software Packages......................................................................................75 3.9.1 Design Expert .......................................................................................75

4 Fabrication of Composite Materials ....................................................................81 4.1 Introduction ..................................................................................................81 4.2 XPS Analysis of PC, PDMS and Adhesive..................................................82

4.2.1 Adhesive Controls ................................................................................82 4.2.2 PDMS Controls.....................................................................................85 4.2.3 PC Controls ..........................................................................................86

4.3 XPS analysis of delaminated PC-PDMS surfaces ......................................86 4.3.1 Introduction...........................................................................................86 4.3.2 UV Pretreatment of PDMS: None.........................................................86 4.3.3 UV Pretreatment of PDMS: 30 seconds...............................................88 4.3.4 Overview of Surface Chemistry Dependence on UV Treatment .........96

4.4 Mechanical Testing of PC and PDMS .........................................................96 4.4.1 Tensile Mechanical Properties of Polycarbonate.................................96 4.4.2 Tensile Mechanical Properties of Sylgard 184 PDMS .........................98 4.4.3 Compressive Deformation of PC and PDMS .......................................99

4.5 Thermal Testing of Materials .....................................................................100 4.5.1 Glass Transition Temperature............................................................100


4.5.2 Curing Properties................................................................................102 4.6 Mechanical Testing of PDMS-PC Strength of Adhesion ...........................104

4.6.1 Testing in Lap Shear Mode ................................................................104 4.6.2 Testing in Peel Mode..........................................................................107

4.7 Conclusions ...............................................................................................110 5 Replication by Hot Embossing of Semicircular Microstructures .......................113

5.1 Introduction ................................................................................................113 5.2 Methodology for Hot Embossing ...............................................................114

5.2.1 Hot Embossing Parameters ...............................................................114 5.2.2 Hot Embossing Procedure and Analysis............................................115 5.2.3 Tool Dimensions.................................................................................117

5.3 Microstructuring of Polycarbonate .............................................................119 5.3.1 Embossing Results.............................................................................119 5.3.2 DOE Models and Comparison with Experimental Data .....................121 5.3.3 Empirical Derivations Based on DOE Models....................................125 5.3.4 Comparison of non-DOE Empirical Derivations with Experimental Data 126 5.3.5 Numerical Optimisation of Processing Conditions .............................129

Microstructuring of Non-bonded Laminates of PC and PDMS.............................134 5.3.6 Embossing Results.............................................................................134 5.3.7 DOE Models and Comparison with Experimental Data .....................140 5.3.8 Empirical Derivations Based on DOE Models....................................144 5.3.9 Comparison of non-DOE Empirical Derivations with Experimental Data 145 5.3.10 Numerical Optimisation of Processing Conditions .............................146

5.4 Microstructuring of Adhesively Bonded Laminates of PC and PDMS: 5 minutes UV Pretreatment Time and 25 minutes UV Curing Time .......................148

5.4.1 Results from Embossing ....................................................................148 5.4.2 DOE Analysis .....................................................................................154

5.5 Microstructuring of Adhesively Bonded Laminates of PC and PDMS: 20 minutes UV Pretreatment Time and 25 minutes UV Curing Time .......................154

5.5.1 Results from Embossing ....................................................................154 5.5.2 DOE Models and Comparison with Experimental Data .....................160 5.5.3 Empirical Derivations Based Upon DOE Models ...............................166 5.5.4 Comparison of Empirical Derivations with Experimental Data...........167 5.5.5 Optimisation of Processing Conditions ..............................................170

5.6 Conclusions ...............................................................................................172 6 Replication by Hot Embossing of Rectangular Microstructures .......................175

6.1 Introduction ................................................................................................175 6.2 Methodology for Fabrication of Rectangular Semicircular Microstructures 176

6.2.1 Tool Fabrication and Tool Dimensions...............................................176 6.2.2 Hot Embossing Parameters for PC – 1st Stage..................................177 6.2.3 Hot Embossing Parameters for PC – 2nd Stage.................................178 6.2.4 Hot Embossing Parameters for Composite Material..........................178 6.2.5 Analysis of Embossing Results ..........................................................179

6.3 Microstructuring of Polycarbonate .............................................................179 6.3.1 PC Embossing Results – 1st Stage ....................................................179 6.3.2 PC Embossing Results – 2nd Stage ...................................................184 6.3.3 Selection of Embossing Parameters for Composite Material ............185


6.4 Microstructuring of Composite Material .....................................................186 6.5 Microstructuring of Composite Material Using DOE..................................190

6.5.1 Embossing Result...............................................................................191 6.5.2 DOE Models and Comparison with Experimental Data .....................193 6.5.3 Empirical Derivations Based on DOE Models and Comparisons with Experimental Data.............................................................................................194 6.5.4 Total Error and Optimal Processing Conditions.................................204

6.6 Conclusions ...............................................................................................205 6.6.1 Conclusions from PC embossing .......................................................205 6.6.2 Conclusions from Composite Embossing – Conventional .................206 6.6.3 Conclusions from Composite Embossing – DOE analysis ................206

7 Sealing of PDMS microstructures.....................................................................207 7.1 Introduction ................................................................................................207 7.2 XPS Analysis of Heavily UV/Ozone Treated PDMS .................................208

7.2.1 Surface treatment of PDMS at 60.1 mm Working Distance...............208 7.2.2 Surface Treatment of PDMS at 30.1 mm Working Distance .............216

7.3 UV/Ozone Pretreatment Requirements for PDMS Autohesion.................220 Treatment Requirements for Oxygen Plasma Sealing of Microfluidic Composite Substrates with a PDMS Surface Layer ...............................................................221 7.4 Assembled Multilayer Structures ...............................................................222 7.5 Conclusions ...............................................................................................223

8 Conclusions.......................................................................................................225 8.1 Project Summary .......................................................................................225 8.2 Project Novelty...........................................................................................226 8.3 Project Conclusions ...................................................................................226

8.3.1 Fabrication of Novel Microfluidic Substrate Material..........................226 8.3.2 Hot Embossing with a Semi-circular Tool ..........................................227 8.3.3 Hot Embossing with Rectangular Tool ...............................................230 8.3.4 General Embossing Conclusions .......................................................232 8.3.5 Sealing of PDMS Microstructures ......................................................232

8.4 Analysis of Methodology and Conclusions................................................233 8.5 Analysis of Results and Recommendations for Future Research.............234

9 Bibliography ......................................................................................................236


List of Figures Figure 1-1: Layout of the process to produce the composite material.......................18 Figure 2-1: Channel blockage and distortion during adhesive (left) and thermal

bonding (right) .....................................................................................................46 Figure 3-1: TA Instruments TA2920 DSC..................................................................55 Figure 3-2: Zwick Z010 Tensile testing machine .......................................................58 Figure 3-3: Schematic of tensile test piece ................................................................58 Figure 3-4: Joint diagram for lap shear testing ..........................................................59 Figure 3-5: Schematic of the peel joint setup (block arrows show direction of

extension)............................................................................................................60 Figure 3-6: TA Instruments AR200 Rheometer .........................................................61 Figure 3-7: UV Exposure cabinet ...............................................................................63 Figure 3-8: Example of curve fitting for XPS data (carbon) .......................................66 Figure 3-9: The sample mount of the AVIA laser.......................................................68 Figure 3-10: Micro milling tool ....................................................................................68 Figure 3-11: Electroplating bath .................................................................................69 Figure 3-12: Hot Embossing sample chamber...........................................................71 Figure 3-13: Laser confocal microscope....................................................................72 Figure 3-14: Scanning electron microscope ..............................................................74 Figure 3-15: E5100 gold coating unit from Polaron Equipment Ltd...........................75 Figure 4-1: A novel method for the adhesion of PDMS to polycarbonate .................81 Figure 4-2: Surface chemistry of delaminated PDMS................................................87 Figure 4-3: Pictorial representation of PDMS delamination under no UV/ozone pre-

treatment .............................................................................................................87 Figure 4-4: Surface chemistry of delaminated PC .....................................................88 Figure 4-5: Surface chemistry of delaminated PDMS................................................89 Figure 4-6: Changes in delamination mode for 30 s UV/Ozone pre-treatment with

increasing curing time .........................................................................................90 Figure 4-7: Surface chemistry of delaminated PC .....................................................91 Figure 4-8: Surface chemistry of delaminated PDMS................................................92 Figure 4-9: Changes in delamination mode for 8 minutes UV/ozone pre-treatment

with increasing curing time..................................................................................92 Figure 4-10: Surface chemistry of delaminated PC ...................................................93 Figure 4-11: Surface chemistry of delaminated PDMS..............................................94 Figure 4-12: Delamination mode for PDMS...............................................................94 Figure 4-13: Surface chemistry of delaminated PC ...................................................95 Figure 4-14: XPS atomic ratios for delaminated PDMS.............................................96 Figure 4-15: Stress-strain behaviour of Lexan 8010 (curves are shifted to the right for

clarity)..................................................................................................................97 Figure 4-16: Stress-strain behaviour of Sylgard 184 PDMS (curves are shifted to the

right for clarity) ....................................................................................................98 Figure 4-17: Deformation of PC under compression .................................................99 Figure 4-18: Deformation of PDMS under compression..........................................100 Figure 4-19: mDSC scan of PC................................................................................101 Figure 4-20: mDSC scan of Loctite 3105.................................................................101 Figure 4-21: Photo-DSC scan of Loctite 3105 .........................................................103 Figure 4-22: Tensile failure stress for PC to PDMS lap joints as a function of UV

parameters ........................................................................................................105 Figure 4-23: Influence of UV/ozone pre-treatment times on failure stress of PC-

PDMS peel joints...............................................................................................108


Figure 4-24: Influence of UV/ozone curing time on failure stress of PC-PDMS peel joints ..................................................................................................................109

Figure 4-25: A novel method for the adhesion of PDMS to PC ...............................110 Figure 5-1 Project Flowchart with Chapter 5 components highlighted ....................113 Figure 5-2: Embossing sample stack.......................................................................115 Figure 5-3: Approximation of channel area (Red) against the actual channel shape

(Blue) using Equation 5-1 .................................................................................116 Figure 5-4: Laser confocal profile of semicircular shim tool (the black line shows the

outline of the embossing tool) ...........................................................................118 Figure 5-5: Comparison of experimental data and DOE model for channel width ..123 Figure 5-6: Comparison of experimental data and DOE model for ratio of width to

depth .................................................................................................................123 Figure 5-7: Channel width as a function of embossing temperature .......................124 Figure 5-8: Ratio of width to depth as a function of embossing temperature ..........125 Figure 5-9: Predicted and calculated channel area for embossed PC ....................127 Figure 5-10: Comparison of experimental and predicted data for channel depth in

embossed PC....................................................................................................128 Figure 5-11: Sum of % of reference value for calculated channel width, area and

ratio ...................................................................................................................132 Figure 5-12: Comparison of experimental and DOE predicted channel depth for PC

embossed through PDMS.................................................................................142 Figure 5-13: Comparison of experimental and DOE predicted channel depth at the

shoulders for PC embossed through PDMS.....................................................143 Figure 5-14: Comparison of experimental and DOE predicted channel area for PC

embossed through PDMS.................................................................................143 Figure 5-15: Comparison of experimental and DOE predicted width to depth ratios

for PC embossed through PDMS .....................................................................144 Figure 5-16: Comparison of experimental and derived channel width for PC

embossed through PDMS.................................................................................145 Figure 5-17: Comparison of experimental and DOE predicted channel depth........163 Figure 5-18: Comparison of experimental and DOE predicted depth at channel

shoulders...........................................................................................................163 Figure 5-19: Comparison of experimental and DOE predicted channel ratio of width

to depth .............................................................................................................164 Figure 5-20: Comparison of experimental and DOE predicted channel area..........164 Figure 5-21: Comparison of experimental and DOE predicted area of left section.165 Figure 5-22: Comparison of experimental and DOE predicted area of right section

..........................................................................................................................165 Figure 5-23: Comparison of experimental channel width with channel width derived

from DOE equations .........................................................................................168 Figure 5-24: Comparison of experimental channel width with DOE derived width of

channel halves ..................................................................................................169 Figure 5-25: Comparison of experimental channel width with DOE derived width of

channel half.......................................................................................................169 Figure 6-1: Flow Chart of Work Performed in Chapter 6 .........................................175 Figure 6-2: Embossed channel dimensions.............................................................176 Figure 6-3: Bottom width as % deviation from reference.........................................179 Figure 6-4: Top width as % of reference from reference .........................................180 Figure 6-5: Left wall angle as % of reference ..........................................................180 Figure 6-6: Right wall angle as % of reference........................................................181


Figure 6-7: Depth at middle as % of reference ........................................................182 Figure 6-8: Channel cross sectional area as % of reference...................................183 Figure 6-9: Ratio of channel width to depth as a % of reference.............................183 Figure 6-10: PC embossed with 600 Tool at 14.6 Bars ...........................................184 Figure 6-11: PC embossed with 600 Tool at 19.3 Bars ...........................................185 Figure 6-12: Total % deviation from reference for 600 tool .....................................185 Figure 6-13: % Bottom width as a % of reference ...................................................186 Figure 6-14: Top width as a % of reference.............................................................187 Figure 6-15: Left wall angle as a % of reference .....................................................188 Figure 6-16: Right wall angle as a % of reference...................................................188 Figure 6-17: Depth at middle as a % of reference ...................................................189 Figure 6-18: Total % error for embossed composite ...............................................190 Figure 6-19: Comparison of experimental and DOE total cross sectional area ......194 Figure 6-20: Comparison of experimental data and DOE mean model for middle

depth .................................................................................................................197 Figure 6-21: Comparison of experimental data and DOE mean model for depth from

left shoulder.......................................................................................................197 Figure 6-22: Comparison of experimental data and DOE mean model for depth from

left shoulder.......................................................................................................198 Figure 6-23: Comparison of experimental and DOE approximation of centre section

area ...................................................................................................................199 Figure 6-24: Comparison of experimental and DOE approximation for bottom width

..........................................................................................................................200 Figure 6-25: Comparison of DOE approximation and experimental data for average

of width of left and right section ........................................................................201 Figure 6-26: Comparison of DOE approximated and experimental right wall angle203 Figure 6-27: Comparison of DOE approximated and experimental left wall angle..203 Figure 6-28: Sum of total deviations from reference................................................205 Figure 7-1: Flowchart representing Chapter 7 work ................................................207 Figure 7-2: Atomic ratio O/C as a function of UV treatment time and ageing time..209 Figure 7-3: Atomic ratio (C1+C2)/C as a function of UV treatment time and ageing

time....................................................................................................................210 Figure 7-4: Atomic ratio C3/C as a function of UV treatment time and ageing time 211 Figure 7-5: Atomic ratios C4/C as a function of UV treatment time and ageing time

..........................................................................................................................212 Figure 7-6: Atomic ratio C5/C as a function of UV treatment time and ageing time 213 Figure 7-7: Atomic ratio Organo-Si/C as a function of UV treatment time and ageing

time....................................................................................................................214 Figure 7-8: Atomic ratios inorganic-Si/C as a function of UV treatment time and

ageing time........................................................................................................215 Figure 7-9: Atomic ratio O/C as a function of UV/ozone treatment time..................217 Figure 7-10: Atomic ratio (C1+C2)/C as a function of UV/ozone treatment time ....218 Figure 7-11: Atomic ratio C3/C, C4/C and C5/C as a function of UV/ozone treatment

time....................................................................................................................218 Figure 7-12: Atomic ratio for Si species on a UV/ozone treated PDMS surface .....219 Figure 7-13: Autohesion of PDMS after UV/ozone exposure at 30.1 mm working

distance.............................................................................................................220 Figure 7-14: Influence of UV/ozone pretreatment time and post-exposure ageing on

autohesion.........................................................................................................221


Figure 7-15: Cross section of PDMS layer between two embossed composite sections .............................................................................................................223

Figure 8-1: Layout of the Embossing Process.........................................................225 Figure 8-2: Evolution of delamination mechanism between PC and PDMS as amount

of UV/ozone surface treatment increases.........................................................227 Figure 8-3: Outline of potential alternative to the use of laminated structures ........235


List of Tables Table 2-1: Embossing properties for PC and PMMA (Becker and Heim 2000) ........37 Table 3-1: Components by weight of PDMS base resin ............................................52 Table 3-2: Components by weight of PDMS curing agent.........................................52 Table 3-3: Two variable matrix of the second order rotatable design........................77 Table 3-4: Full matrix of the second order rotatable design for two variables...........78 Table 3-5: Three variable matrix of second order rotatable design ...........................79 Table 3-6: Full matrix of the second order rotatable design for three variables ........80 Table 4-1: Control chemistries of uncured and cured Loctite 3105 ...........................83 Table 4-2: Control chemistry of delaminated Loctite 3105 ........................................85 Table 4-3: Control chemistry of PDMS ......................................................................86 Table 4-4: Control chemistry for Lexan PC................................................................86 Table 4-5: Tensile Mechanical Properties of Lexan 8010..........................................97 Table 4-6: Mechanical properties of PDMS ...............................................................98 Table 4-7: Design Expert Matrix for PC to PDMS adhesion....................................104 Table 5-1: Parameters for embossing with semicircular tool ...................................115 Table 5-2: Reference dimensions and values for semicircular shim .......................118 Table 5-3: DOE Data as real responses ..................................................................119 Table 5-4: DOE Data as % of reference value.........................................................120 Table 5-5: Numerical optimisation for ratio of channel width to depth.....................129 Table 5-6: Numerical optimisation for channel width ...............................................130 Table 5-7: The sum of channel width, depth, area and RWD (as a % of reference)

..........................................................................................................................131 Table 5-8: Optimal experimental responses as a function of parameters ...............133 Table 5-9: Results for embossing of PC through PDMS .........................................135 Table 5-10: Results for embossing of PC through PDMS .......................................136 Table 5-11: Embossing results for PC through PDMS (% of reference) .................137 Table 5-12: Embossing results for PC through PDMS (% of reference) .................138 Table 5-13: Variation in % of reference values........................................................139 Table 5-14: Average, Standard deviation and standard deviation as a percentage of

average .............................................................................................................140 Table 5-15: Numerical solutions for optimisation of channel depth .........................146 Table 5-16: Numerical solutions for simultaneous optimisation of channel depth,

cross sectional area and ratio of width to depth ...............................................146 Table 5-17: Results for embossing of 5 minutes pretreatment/25 minutes curing

composite material (real values).......................................................................148 Table 5-18: Results for embossing of 5 minutes pretreatment/ 25 minutes curing

composite material (real values).......................................................................149 Table 5-19: Results of embossing of 5 minutes pretreatment/25 minutes curing

composite material (% of reference).................................................................150 Table 5-20: Results of embossing of 5 minutes pretreatment / 25 minutes curing

composite material (% of reference).................................................................151 Table 5-21: Data range for each dimension as a % of reference ............................152 Table 5-22: Analysis of reproducibility experiments for embossing of 5 minutes

pretreatment/25 minutes curing composite materials.......................................153 Table 5-23: Comparison of Standard deviation as a % of average for PC embossed

through PDMS, and embossed composite 5 minutes pretreatment/25 minutes curing time.........................................................................................................153

Table 5-24: Results for embossing of 20 minutes pre-treatment/25 minutes curing composite material (real values).......................................................................155


Table 5-25: Results for embossing of 20 minutes pre-treatment/ 25 minutes curing composite material (real values).......................................................................156

Table 5-26: Results of embossing of 20 minutes pre-treatment/25 minutes curing composite material (% of reference).................................................................157

Table 5-27: Results of embossing of 20 minutes pre-treatment / 25 minutes curing composite material (% of reference).................................................................158

Table 5-28: Data ranges for embossed composite (20 minutes pre-treatment time/25 minutes curing time)..........................................................................................159

Table 5-29: Analysis of replicate experiments for embossing in composite material (20 minutes pre-treatment time/25 minutes curing time)..................................160

Table 5-30: Numerical solutions for channel depth at middle..................................170 Table 5-31: Numerical solutions for channel depth at left shoulder.........................171 Table 5-32: Numerical solutions for channel depth at right shoulder ......................171 Table 5-33: Numerical solutions for channel depth at left shoulder, middle, and right

shoulder ............................................................................................................172 Table 6-1: Dimensions of the rectangular embossing tool.......................................177 Table 6-2: Additional channel dimensions for DOE analysis of embossing ............177 Table 6-3: DOE Matrix for Embossing of Composite Material .................................191 Table 6-4: DOE data as real responses...................................................................191 Table 6-5: DOE data as real responses...................................................................192 Table 6-6: DOE data as % of reference value .........................................................192 Table 6-7: DOE data as % of reference values .......................................................193 Table 6-8: Mean, standard deviation and coefficient of variance for embossed data

..........................................................................................................................195 Table 6-9: Total Error for Embossed Composite .....................................................204 Table 7-1: Oxygen plasma treatment parameters and failure modes .....................222


Glossary of Acronyms ACS – Area Cross Section

ALS – Area Left Section

ARS – Area Right Section

BE – Binding Energy

BW – Bottom Width

CAD – Computer Aided Design

CD – Channel Depth

CDLS – Channel Depth Left Shoulder

CDRS – Channel Depth Right Shoulder

CNC – Computer Numerical Controlled

COC - Cyclo-olefin Copolymer

CRC – Cooperative Research Centre

CSA – Cross Sectional Area

CSIRO – Commonwealth Scientific and Industrial Research Organisation

CWLS – Channel Width Left Section

CWRS – Channel Width Right Section

DE – Design Expert

DNA – Deoxyribose Nucleic Acid

DOE – Design of Experiments

DPC –Differential Photo Calorimetry

DSC- Differential Scanning Calorimetry

EP – Embossing Pressure

ET – Embossing Time

GE – General Electric

FTIR – Fourier Transform Infra Red

HF – Hydrofluoric acid

IRIS – Industrial Research Institute Swinburne

LCM – Laser Confocal Microscope/Microscopy

LDPE – Low Density Polyethylene

LMW – Low Molecular Weight

mDSC – Modulated Differential Scanning Calorimetry

PC- Polycarbonate

PCR – Polymerase Chain Reaction


PDMS - Polydimethylsiloxane

POM – Polyoxy Methacrylate

PE – Polyethylene

PET – Polyethylene terepthalate

PETg – Polyethylene terephalate glycol

PMMA – Polymethyl methacrylate

PTFE – Polytetrafluoroethylene

PS – Polystyrene

RF – Radio Frequency

RWD – Ratio Width to Depth

SEM – Scanning Electron Microscope

SIMS – Secondary Ion Mass Spectroscopy

SR- Styrene Rubber

TA – Thermal Analysis

TW – Top Width

UV – Ultraviolet

UVCT – Ultraviolet curing time

UVPT – Ultraviolet pre-treatment time

XPS – X-Ray Photoelectron Spectroscopy

WBL – Weak Boundary Layer

WLS – Width Left Section

WRS – Width Right Section


1 Introduction 1.1 Statement of the Problem Elastomeric polydimethylsiloxane (PDMS) membranes are often integrated into

microfluidic devices because they can be pneumatically actuated to pump fluids and

gases through the channels, reservoirs and other components of microfluidic

devices.

This integration requires strong adhesion between PDMS and polycarbonate (PC),

which is technically difficult for the following reasons:

♦ Since PDMS is an elastomer, it does not soften and flow the way

thermoplastics do. This means that conventional fusion welding techniques,

such as frictional welding, ultrasonic welding, resistance welding and

microwave welding cannot be used.

♦ The very low surface energy of PDMS makes it difficult for adhesives to wet

the surface. In addition, although screen printing of adhesives can be

performed, this becomes increasingly difficult to perform as channels reduce

in size, making its application to truly micro scale channels technically

difficult.

What is required is an innovative method for the integration of PDMS into PC

thermoplastic microfluidic devices. This technology should enable strong adhesion

between the PDMS and the PC substrate, even at elevated temperatures above

100oC.

This project therefore deals with the development of fabrication technology for

microfluidic devices, particularly the development of integration technology that

overcomes the difficulties associated with incorporating PDMS membranes into

thermoplastic PC microfluidic devices.


1.2 Project Outline This project proposes to solve this problem using the process outlined in Figure 1-1;

Figure 1-1: Layout of the process to produce the composite material

A novel composite material was fabricated by adhesive lamination of UV/ozone

treated PDMS to PC. The adhesive used was Loctite 3105 UV curable. After

lamination, the composite material was re-exposed to UV/ozone to cure the adhesive

(step 3). Hot embossing of this novel microfluidic substrate material produced a

microstructured substrate of PC with an adhesively bonded PDMS surface layer

(step 4).

The PDMS surface of this embossed material was then likewise exposed to

UV/ozone or similar oxidative media such as oxygen plasma (step 5), producing an

activated surface that could irreversibly adhere to a separate, similarly treated PDMS

sheet or component.

In the current research, the primary components to be investigated are:

- an investigation into the adhesion of PDMS to PC;

- an experimental study on the microstructuring of the composite material; and

- an experimental investigation into the surface treatment and auto-adhesion of

PDMS.

1

2

3

4

5

6

Polycarbonate sheet

Expose sheet PDMS to UV/Ozone

Adhesively laminate treated PDMS onto PC, and re-expose to UV/ozone to cure adhesive

Microstructure the composite material via hot embossing

Expose both PDMS surfaces to UV/ozone

Press oxidised PDMS surfaces together to bond


1.3 Microfluidic Devices and Technology

1.3.1 Microfluidics Microfluidics involves the manipulation and sensing of micro/nanolitre volumes of

liquids and gases in the channels, reservoirs and valves of microfluidic devices.

These devices are constructed from multiple layers of materials such as

thermoplastics, elastomers, silicon and glass, and have miniaturised channel

networks replicated or machined into them through which the liquids and/or gases

can flow and be manipulated.

These networks function as components of these devices that, when integrated with

various other components such as detection systems, can create a functional

microfluidic device.

Fabrication and components of microfluidics have been reviewed extensively

(Becker and Gartner 2000; McDonald, Duffy et al. 2000; Auroux, Iossifidis et al.

2002; Becker and Locascio 2002; McDonald and Whitesides 2002; Ng, Gitlin et al.

2002; Reyes, Iossifidis et al. 2002; Ko, Yoon et al. 2003; Vilkner, Janasek et al.

2004), and will therefore not be discussed here.

1.3.2 Advantages of Miniaturisation of Microfluidic Components Miniaturisation produces a number of benefits in terms of fabrication and operation of

microfluidic devices. Such benefits include;

- reduced amounts of raw material needed to produce a device;

- reduced costs of any disposal, manufacturing or cleaning operations;

- reduced amounts of reagents needed to perform operations; and

- faster and more energy-efficient temperature control, since the device has

less thermal mass, and the smaller channels allow faster temperature

equilibration

1.3.3 Advantages of Polymers for Manufacturing Microfluidic Devices Most of the knowledge base for microfluidics has been developed from experience in

the microelectronics industry using similar methods and materials (Auroux, Iossifidis


et al. 2002; Reyes, Iossifidis et al. 2002). However, the use of such materials and

methods for microfluidics presents several problems for commercialisation, and a

great deal of recent research deals with the use of polymeric materials as

alternatives (Becker and Locascio 2002; Ng, Gitlin et al. 2002; Ko, Yoon et al. 2003).

Aside from the inherent advantages of miniaturisation, the use of polymeric materials

instead of glass and silicon has additional advantages (Auroux, Iossifidis et al. 2002;

Reyes, Iossifidis et al. 2002).

Due largely to the disposable nature of the devices, considerable cost benefit (both

to the manufacturer and consumer) can be gained from the use of relatively cheap

polymer materials (Becker and Gartner 2000).

Another benefit of utilising polymer materials is the reduction in fabrication time.

Although fabrication steps can be fully automated, when using glass and silica each

device is fabricated in a serial fashion, resulting in increased fabrication time and

cost. Polymers have great potential for simple, high volume manufacturing

(Martynova, Locascio et al. 1997; Roberts, Rossier et al. 1997; Schift, David et al.

2000; Xu, Locascio et al. 2000).

Certain chemicals used during fabrication of glass and silica devices (such as HF)

are extremely dangerous (Becker and Gartner 2000), and thus safety can be

dramatically increased by a switch to the use of polymeric materials.

Due to the isotropic nature of the etching process, only shallow, mainly semicircular

channels can be fabricated in glass and silica substrates. For many applications,

features such as high aspect ratio channels, channels with a defined but arbitrary

wall angle, or channels with different heights are desirable. These cannot be

achieved with standard microfabrication methods in glass or silica (Becker and

Gartner 2000), but are relatively easy to achieve with modern polymer processing

technology. The use of polymers as substrate materials increases the commercial

feasibility of mass fabricated microfluidic devices.


Some common polymers that have been used to date include polycarbonate (PC),

polymethylmethacrylate (PMMA), polystyrene (PS), polydimethylsiloxane (PDMS),

and polyethylene terephthalate (PET).

1.3.4 Integrated Microfluidic Components As microfluidic technology advances, more complicated devices emerge that require

the integration of more components to perform an increasing range of fluidic

functions. The components that may be required include complex combinations of

valves, pumps, filters, membranes and switches. Multiple components may also be

required on the same device. A large amount of information on microfluidic

components is available in the scientific literature (Ko 1995; Roberts, Rossier et al.

1997; Chou, Austin et al. 2000; Dolnik, Liu et al. 2000; McDonald, Duffy et al. 2000;

Kameoka, Craighead et al. 2001; Kim and Knapp 2001; Liu, Ganser et al. 2001;

Auroux, Iossifidis et al. 2002; Becker and Locascio 2002; Guadioso and Craighead

2002; Ng, Gitlin et al. 2002; Ko, Yoon et al. 2003), and will not be extensively

reviewed here.

1.3.5 Applications of Microfluidic Devices The applications of microfluidics are extensive, and there are similarly extensive

reviews available in the literature (Dolnik, Liu et al. 2000; Ehrnstrom 2002;

Khandurina and Guttman 2002; Kopf-Sill 2002; Rossier, Reymond et al. 2002). A

brief number of applications are listed below.

Microfluidics is in no way limited to one branch of science. For example, microfluidic

components comprise a major part of flow cytometry, used extensively in the study

of complex biological systems such as living cells. Much of our knowledge of the

immune system is derived from the study of immune cells using flow cytometers.

Rare cell pathways and even intracellular signalling pathways can be analysed using

this technology, thus significantly advancing the progress of biological sciences

(Andersson and Berg 2003).

Microfluidics are also important for genomics. Both DNA sequencing and

polymerase chain reaction (PCR) are highly dependent on microfluidic devices and


produce hugely significant amounts of information, such as the information

generated by the human genome project (Chou, Austin et al. 2000; Dolnik, Liu et al.

2000; Lee, Chen et al. 2001; Schneega and Kohler 2001; Auroux, Iossifidis et al.

2002; Chou, Changarni et al. 2002; Khandurina and Guttman 2002; Vinet, Chaton et

al. 2002; Liu 2003).

Advanced protein analysis techniques such as immunoassays and enzyme assays

also require microfluidic applications. More information can be found in the literature

(Laurell, Marko-Varga et al. 2001; Khandurina and Guttman 2002; Lion, Rohner et al.

2003; Marko-Varga, Nilsson et al. 2003).

In the field of chemistry, the dimensions of microfluidic channels present advantages

for industrial catalysis. For instance, reaction efficiency could be increased because

the high surface to volume ratio of microfluidic channels allows greater contact area

between the reactants and the catalysts. In addition, the laminar flow in the

channels could allow high efficiency separation of the reaction products. More

information can be found in the literature (Becker and Gartner 2001; Tokeshi,

Kikutani et al. 2003).

1.4 Boundaries of the Research This project is limited to:

- microstructuring of PC itself within the parameter ranges specified

- manufacturing and microstructuring of non-bonded composites of PC and

PDMS within the parameter ranges specified;

- manufacturing and microstructuring of adhesively bonded composites of PC

and PDMS within the parameter ranges specified; and

- sealing of PDMS against itself by exposure to oxidative media.

- The equations derived or calculated throughout this project cover only the

specific range of variables used during the experiments. The boundary

conditions for each equations should be considered to be the limit of the

parameter space that was tested.

The scope of this research could potentially be extended to selection of alternative

membrane materials such as other elastomers, selection of alternative thermoplastic


materials such as PET or PMMA, different thicknesses of PDMS, and the selection

of alternative adhesives such as silicones or epoxies.

1.5 Structure of the Thesis The thesis is divided into eight chapters;

Chapter 1 – Introduction

The introduction chapter describes the problem to be solved, the proposed solution

to the problem and what is novel about the proposed solution. It also specifies the

research outcomes for the project. Microfluidics is introduced, as are the

advantages of miniaturisation and the advantages of polymers over traditional

microtechnology materials. The applications of microfluidic devices are discussed to

give some context to the research that will be performed. The boundaries of the

research are also described.

Chapter 2 – Literature review

The literature that is relevant to the project is previewed to give the reader some

context to the research, why it is novel, and how it compares with previous research.

The bonding and adhesion of polymers is reviewed, with a brief description of the

various theories of adhesion. Surface modification technologies for polymers are

reviewed, as are polymer microfabrication methods, to give the reader some insight

into the variety of techniques that are commonly employed for machining of polymers

on the micro scale.

Methods for sealing of microfluidic devices, and the use and applications of PC and

PDMS are covered. Information from the previous sections is combined to review the

specific techniques employed to seal microfluidic devices.

Finally, the use and applications of PC and PDMS in microfluidic devices is

discussed, along with strategies for incorporating PDMS into thermoplastic

microfluidic devices.

Chapter 3 – Methodology


Chapter three includes a discussion of the materials, methods and equipment used

to gather data and results throughout the project.

First the materials and their properties are discussed. After this, the various

techniques employed, including thermal analysis, mechanical testing, microscopy

techniques and surface modification techniques, are discussed.

Finally the microfabrication tools, including hot embossing, are detailed.

A brief description of the specialist software packages used is included at the end of

the chapter.

Chapter 4 – Fabrication of Composite Microfluidic Materials

The properties of the base materials and manufacturing of the composite material

are researched, including thermal and mechanical properties of the base materials,

strength of adhesion between the materials, and the surface properties of the

materials when treated.

Chapter 5 – Embossing with a semicircular shim

The following materials were embossed using a semi-circular tool:

- polycarbonate

- two layer non-bonded structure of PDMS and PC

- a laminated composite of PC and PDMS.

Empirical relationships were calculated between the embossing parameters and the

dimensions of the embossed channels.

Chapter 6 – Embossing with a rectangular shim

Similar to Chapter 5, the same materials were embossed, but using a rectangular

tool.

In Chapter 6, empirical relationships were determined between the UV treatment

parameters used during the manufacture of the composite material and the

dimensions of the embossed channels.


Chapter 7 – Sealing of PDMS Microfluidic Surfaces

Chapter 7 demonstrates the ability of PDMS to auto-adhere when exposed to

oxidative media such as UV/ozone or oxygen plasma.

Chapter 8 – Conclusions

This chapter presents a brief summary of the work performed and what conclusions

can be made.

1.6 Institutions The project commenced in mid-2001. It was conducted at the Industrial Research

Institute Swinburne (IRIS) at Swinburne University of Technology, and was funded

by the Cooperative Research Centre (CRC) for Microtechnology. The project also

involved collaboration with CSIRO Division of Molecular Science (now CSIRO

Molecular and Health Technologies) at Clayton, Australia.


2 Literature Review

2.1 Overview This literature review deals with the current state of knowledge in several areas

relevant to the project. It contains the following sections:

- 2.2 reviews various theories of adhesion in polymers;

- 2.3 reviews surface modification techniques for polymers;

- 2.4 reviews microfabrication methods for polymers in microfluidics;

- 2.5 and 2.6 review the use of PDMS and PC, respectively, in microfluidics;

and

- 2.7 reviews integration of PDMS with thermoplastic in microfluidic devices.

2.2 Bonding and Adhesion of Polymers

2.2.1 Introduction It is generally considered that the properties of a particular adhesive bond are the

result of several different adhesion mechanisms. The major adhesion mechanisms

are briefly reviewed in the following sections (Brewis 1982; Allen 2003).

2.2.2 Mechanical Interlocking The basic principle of mechanical interlocking is that total bond strength is equal to

total surface area multiplied by bond strength per unit area. Therefore, an increase

in surface area will produce an increase in bond strength, and surface area may be

increased by, for example, mechanically roughening the surface.

An example of increased surface area enhancing adhesion is the case of wood

being roughened with glass paper prior to adhesive application. However, there is

also clear evidence that this is not always the case, and that in some instances there

is in fact a negative correlation between surface roughness and bond strength

(Leeden and Frens 2002; Allen 2003). If the surfaces have poor wettability, then an

increase in surface irregularities may in fact reduce the contact area between

adhesive and adherend, and therefore reduce the total bond strength (Leeden and

Frens 2002).


2.2.3 Chemical Bonding Primary chemical bonds across an interface will increase the adhesion strength to a

level that is not possible with secondary bonds such as Van der Waals interactions

or hydrogen bonding, and the extent of interfacial primary bonding is strongly

dependent on the reactivity of the opposing surfaces (Allen 2003).

2.2.4 Adsorption Interfacial adhesion will occur because of interatomic and intermolecular forces

established at the interface, provided that intimate contact is achieved (Leeden and

Frens 2002). Most commonly, interfacial forces will result from Van der Waals and

Lewis acid-base interactions (Leeden and Frens 2002).

2.2.5 Electrostatic Since two different materials have different electronic band structures, an electron

transfer mechanism may be present that would allow the transfer of electrons and

thus equalize the Fermi levels. This would produce an electrical double layer at the

interface, and the resulting electrostatic forces contribute to adhesion strength

(Brewis 1982; Allen 2003) .

2.2.6 Diffusion At elevated temperatures and pressures polymer molecules have sufficient mobility

to diffuse across an interface and into another polymer material, resulting in some

proportion of the chains being present in both parts. This interdiffusion results in a

rigid bond (Leeden and Frens 2002).

2.2.7 Weak Boundary Layers Beyond the actual interface itself there is an interfacial zone, which is the product of

alterations or modifications to the substrate materials. Within this interfacial zone

exist material property gradients, differing significantly to the properties of the bulk

substrate (Brewis 1982; Allen 2003).


This raises the possibility that within the interfacial zone exists a weak boundary

layer (WBL) that will always be the main factor in determining the strength of

adhesion, even if the failure appears to be interfacial.

The recorded energy of adhesion will therefore always be equal to the cohesive

energy of the WBL. However surface pre-treatments of polymers are often carried

out to remove weak boundary layers (Brewis 1982; Allen 2003).

2.3 Surface Modification of Polymers

2.3.1 Introduction Surface modifications of polymers are generally carried out for two reasons in

microfluidics:

- to improve the surface properties of the material in terms of performing

chemical, biological or analytical functions; and

- to improve the adhesive properties of the surface.

Reviews on surface modification technology are available (Ratner 1995). Polymer

surface modifications can be performed by a variety of processes reviewed in the

following sections.

2.3.2 Mechanical Processes These methods are largely related to the process of improving adhesion by

mechanical interlocking, mentioned in section 2.2.2. A greater strength of adhesion

can be achieved by increasing the effective surface area. Some examples of

mechanical processes are:

- sand/grit blasting (Leahy, Young et al. 2003);

- grinding or brushing; and

- abrading (Leeden and Frens 2002).

2.3.3 Wet Chemical Processes Depending on the specific treatment, these techniques perform a variety of functions

such as removal of surface layers, activation of the substrate, and


deposition/incorporation of new functional groups (Glasgow, Beebe et al. 1999;

Brewis and Dahm 2001; Leahy, Young et al. 2003). Wet chemical methods have

also found some application in microfluidics (Henry, Waddell et al. 2002; Breisch,

Heij et al. 2004).

Some examples of wet chemical processes are;

- chromic sulfuric acid treatment (CSA or pickling);

- organic solvent treatment;

- coating with chemically active substances (priming);

- deposition of active metals; and

- incorporation of surface active materials into the bulk (Leger 2000; Fallahi,

Mirzadeh et al. 2003).

2.3.4 Dry Chemical Processes Dry chemical treatments were used during this project. The following sections

contain a discussion of several of these methods, including:

- laser surface modification (Buchanan and Dodiuk-Kenig 1999; Liu 2003);

- UV/ozone treatment;

- corona discharge treatment (Uehara 1999); and

- plasma treatment (Wertheimer, Martinu et al. 1999).

Other types of dry chemical processes are;

- thermal treatment;

- flame treatment (Brewis and Mathieson 1999); and

- ion etching.

2.3.4.1 Laser Surface Modification This technique relies on the application of laser energy to functionalise surfaces or

alter the topography of the surface in such a way that the adhesive characteristics of

the surface are improved (Abbasi, Mirzadeh et al. 2002; Fallahi, Mirzadeh et al.

2003). This occurs because the application of laser energy in the presence of

oxygen or air oxidises the surface, increasing its wettability.


The use of lasers enables surface modifications over very small areas, and this

technology has been applied to surface modification in microfluidic devices (Roberts,

Rossier et al. 1997), and to create initiation sites for grafting reactions (Abbasi,

Mirzadeh et al. 2002).

2.3.4.2 UV/ozone This kind of surface modification is similar to laser surface modification, but is more

applicable to treating larger areas. Surface modification with UV/ozone increases

the hydrophilicity of the surface. As hydrophilicity increases, it becomes easier for

an adhesive to wet the surface, and good surface wetting is important for strong

adhesion (Kang, Neoh et al. 1999).

UV/ozone pretreatment is used as an initiation step for the grafting of polymers onto

surfaces (Ikada 1992; Kang, Neoh et al. 1998; Kang, Neoh et al. 1999), and this has

been applied to microfluidics (Hu, Ren et al. 2002). UV treatment has also been used

to improve microfluidic hydrophilicity (Liu, Ganser et al. 2001; Schnyder, Lippert et al.

2003).

2.3.4.3 Plasma Treatment A plasma is a gas that contains both charged and neutral particles, including some of

the following: molecules, atoms, electrons, cations, anions and radicals (Graham

2001).

Charged particles such as these are always present in trace amounts. The

application of an energy source such as RF or microwave radiation to the gas

mixture will accelerate these charged particles, producing a chain reaction of

charged particles colliding with neutral particles, ionizing them, and producing more

charged particles. This is the basic mechanism of plasma generation (Inagaki 1996;

Conrads and Schmidt 2000).

There are a number of different reactions that can occur between plasma and a

polymeric surface:


Surface cleaning

This is a major advantage of surface modification by the application of radiation.

Most other cleaning techniques, such as solvent cleaning, are likely to leave a layer

or organic contamination on the surface. Surface contamination can also be the

result of additives such as release agents, pigments or processing aids, which are

deliberately included and can self-segregate to the surface during processing. A

plasma is one way of efficiently removing this surface contamination (Wertheimer,

Martinu et al. 1999).

Ablation/Etching

Ablation or etching of the surface is distinguished from cleaning by the amount of

material that is removed. Ablation is important for removal of surface layers and for

treatment of semicrystalline polymers. Since amorphous regions ablate much faster

than crystalline regions, the ablation of a semicrystalline surface will create a series

of peaks and troughs, with the troughs indicating the amorphous regions. This

increase in surface topography can increase adhesion via mechanical interlocking

(Wertheimer, Martinu et al. 1999).

Cross linking or branching of near-surface molecules

Crosslinking will often occur when an inert plasma is used to treat a polymer surface.

When using gases such as helium or argon, the plasma will break C-C or C-H

bonds. The resultant free radicals on the surface can react with each other to

produce a stable crosslinked structure (Nickerson; Wertheimer, Martinu et al. 1999)

Modification of surface chemical structure

Modification can occur due to interaction with, or deposition of, chemical species

from the plasma (Wertheimer, Martinu et al. 1999). This can range from

functionalising the surface with simple chemical groups such as chlorine atoms or

hydroxyl groups, to depositing complicated fluorinated polymers for increasing

hydrophobicity.

Oxygen and oxygen-containing plasmas are commonly used for modification of

polymer surfaces such as PE (Sapieha, Cerny et al. 1993) and PET (Sapieha, Cerny


et al. 1993; Wells, Badyal et al. 1993; Kang, Neoh et al. 1999; Wu, Xanthopoulos et

al. 2002), PTFE (Kang, Neoh et al. 1999) and PS (Wells, Badyal et al. 1993).

Other articles describe the argon, oxygen and nitrogen treatment of PE, PC, PET,

PS and PMMA (Gerenser 1993), argon and carbon dioxide treatment of PMMA

(Amor, Baud et al. 2000), oxygen and water plasma treatment of LDPE, POM,

PMMA, PET and SR, or ammonia and nitrogen/oxygen plasma treatments of

polyamide films.

Some reasons for the plasma treatment include:

- modification of wettability (Tomcik, Popovic et al. 2001); and

- adhesion enhancement (Kruse, Kruger et al. 1995; Schulz, Munzert et al.

2001; Shenton, Lovell-Hoare et al. 2001)

Literature about adhesion enhancement is extensive, including review articles

(Liston, Martinu et al. 1993).

Plasmas are often applied in microfluidics to seal PDMS devices but they are also

used to alter surface properties of the materials, such as cyclo-olefin copolymers

(COC) (Guadioso and Craighead 2002).

2.3.4.4 Corona Discharges A corona discharge is a type of low temperature plasma. The primary difference

between a corona discharge and a plasma is that a corona discharge occurs in air,

at atmospheric pressure. Its primary advantage over plasma treatment is that it is

much cheaper, since a vacuum is not required.

Corona discharges produce an oxidised surface that is more susceptible to chemical

bonding across the interface in order to improve adhesion. They are also used for

the removal of low molecular weight species from a surface.

Corona discharges have been used to treat a wide variety of materials, such as PE

(Carley and Kitze 1980; Sapieha, Cerny et al. 1993; Park and Jin 2001), PET


(Sapieha, Cerny et al. 1993), and PDMS (Kim, Chaudhury et al. 2000; Ro, Lim et al.

2002).

2.3.5 Surface Treatment of PDMS in Microfluidics PDMS has been treated by oxygen plasma (Morra, Occhiello et al. 1990), where it

was reported that oxygen plasma species interact preferentially with silicon, resulting

in an increase in the average number of oxygen atoms bonded to silicon. The

surfaces were also reported to be highly hydrophilic, but that ageing in air induces

hydrophobic recovery. The mechanism of hydrophobic recovery was concluded to

be the diffusion of polar groups introduced into the bulk, and from surface

condensation of silanols and consequent cross linking of the surface (Morra,

Occhiello et al. 1990).

Treatment by corona discharge (Kim, Chaudhury et al. 2000) has also been

performed. It was reported that the formation of low molecular weight (LMW) species

because of chain scissioning and their subsequent diffusion to the surface was the

principal cause of hydrophobic recovery, but that migration of any unreacted silicone

fluid to the surface could also cause hydrophobic recovery.

Other plasmas have been used to treat PDMS, including argon and hydrogen

plasmas (Olander, Wirsen et al. 2003), and air plasmas (Fateh-Alavi, Nunez et al.

2002). PDMS has also been coated with Parylene to improve problems resulting

from its high porosity (Shin, Cho et al. 2003).

2.4 Polymer Microfabrication Methods

2.4.1 Introduction A variety of microfabrication methods have been employed to generate

microstructures in polymers, including casting, laser ablation, hot embossing,

thermoforming and plasma etching. These methods are referred to frequently in the

literature (Sadeghipour, Chen et al. 1994; Roberts, Rossier et al. 1997; Xiang, Lin et

al. 1999; Schift, David et al. 2000; Wang and Morris 2000; Xu, Locascio et al. 2000;

Soane, Soane et al. 2001; Spanos, Ebbens et al. 2001; Shen, Pan et al. 2002;


Soper, Henry et al. 2002; Sia and Whitesides 2003; Heckele and Schomburg 2004;

Tesar, Tippetts et al. 2004; Vilkner, Janasek et al. 2004; Wheeler, Trapp et al. 2004).

2.4.2 Laser Machining Laser ablation is a common technique for the fabrication of microfluidic channels.

During the ablation process, a polymer is exposed to a pulsed UV laser, and the

absorption of that light induces bond breakage in the backbone (Becker and

Locascio 2002). It is thought that a combination of photodegradation and thermal

degradation occurs during the process (Becker and Locascio 2002). At bond

breakage, a shock wave is produced and particles are ejected from the surface, thus

creating a hole in the material. By moving the laser across the surface, this hole can

be repeated, creating a channel.

A typical laser ablation set up consists of an excimer laser, which delivers light

pulses at 193 nm (ArF) or 248 nm (KrF) with typical pulse frequencies of 10-100 Hz

(ArF) to several kHz (KrF), a mask or aperture, and an xy-table on which the

substrate is mounted. The mask defines the ablated region, while moving the

substrate on the xy-stage underneath the mask makes the complete pattern.

The use of lasers in the fabrication of microfluidic devices is common in research

environments (Rossier, Schwarz et al. 2000; Bianchi, Wagner et al. 2001; Lippert,

Hauer et al. 2003). It has been used to machine PS (Roberts, Rossier et al. 1997),

PMMA (Pugmire, Waddell et al. 2002), PC (Roberts, Rossier et al. 1997; Xu, Lin et

al. 1998; Xiang, Lin et al. 1999; Wen, Lin et al. 2000; Pugmire, Waddell et al. 2002),

PDMS (Graubner, Jordan et al. 2002) and PET (Roberts, Rossier et al. 1997;

Watanabe and Yamamoto 1997; Henry, Waddell et al. 2002).

2.4.3 Micromilling Micromilling can be used to fabricate microstructures directly (Marko-Varga, Ekstrom

et al. 2001), but it is more commonly utilised to fabricate tools for mass replication.

The mass production of polymer substrates containing microfluidic channel networks

is a two stage process:


(1) fabrication of a master;

(2) transference of the channel network onto the polymer substrate.

Producing a master can be achieved by various techniques depending on the

materials to be used and the degree of precision that is required. For relatively large

structures, such as greater than 100 µm, traditional computer numerical controlled

(CNC) micromachining can be used to produce master tools in materials such as

stainless steel. Stainless steel offers excellent mold insert lifetimes. Also, the

development times for micromachined tools can be shorter as no mask fabrication

and lithography step is involved.

There are, however, limitations on the types of structures that can be produced by

micromachining methods. Simple channel structures with straight walls are well-

suited geometries for these techniques. On the other hand, channel crossings, high

aspect ratio structures, very deep holes or very small structures cannot be

fabricated, or only with major drawbacks.

2.4.4 Hot Embossing Hot embossing is one of the most widely used replication methods for microfluidic

devices, and has been performed in a variety of materials, including;

- polyethylene terephthalate (Henry, Waddell et al. 2002)

- polymethyl methacrylate (Martynova, Locascio et al. 1997; Becker and Heim

2000; Kenny 2000; Xu, Locascio et al. 2000; Bacon 2001; Heyderman, H.

Schift et al. 2001; Lee, Chen et al. 2001; Wabuyele, Ford et al. 2001; Ko,

Yoon et al. 2003; Zhao and Chui 2003; Muck, Wang et al. 2004)

- polycarbonate (Becker and Heim 2000; Kenny 2000; Liu, Ganser et al. 2001;

Shen, Pan et al. 2002)

- cyclo olefin copolymer (Kameoka, Craighead et al. 2001; Monkkonen, Hietala

et al. 2002)

- polyethylene terephthalate glycol (Bacon 2001)

- copolyester (Jiang, Wang et al. 2001)


A similar technique, micro-compression moulding, has also been employed (Moon,

Lee et al. 2003), as has hot imprinting (Henry, Waddell et al. 2002).

After fabrication, the master is mounted in the hot embossing system together with a

planar polymer substrate. Both are heated separately in a vacuum chamber to a

temperature just above the Tg (glass transition temperature).

The vacuum is necessary for several reasons:

- it prevents the formation of air bubbles due to entrapment of air in small

cavities;

- it allows the removal of water vapour which is driven out of the polymer

substrate during the process; and

- it increases the lifetime of the tools by reducing the likelihood of corrosion

occurring at these elevated temperatures.

The tool is brought into contact with the substrate and then a controlled force is

applied. After a short time interval, and still applying the embossing force, the tool-

substrate sandwich is then cooled to below Tg.

In order to reduce the level of thermal stress present in the material as well as

minimize replication errors that may result from the different thermal expansion

coefficients, this thermal cycle should be as small as possible.

After reaching the lower cycle temperature, the embossing tool is mechanically

driven apart from the substrate, which now contains the desired features. This is

usually the most critical step, as now the highest forces act on the polymer

microstructure, particularly if a structure with vertical walls and a high aspect ratio is

desired. Therefore, an automated mold release is crucial for highly accurate

reproduction.


Table 2-1 presents some data for hot embossing of PMMA and PC.

Material Density

(103

kg/m3 )

Tg

(oC)

Young’s

Modulus

(GPa)

Embossing

Temp

(oC)

Deemboss

Temp

(oC)

Embossing

Force

(kN)

Hold time

(s)

PMMA 1.17-

1.20

106 3.1-3.3 120-130 95 20-30 30-60

PC 1.20 150 2.0-2.4 160-175 135 20-30 30-60

Table 2-1: Embossing properties for PC and PMMA (Becker and Heim 2000)

Thermocycling of the substrate in embossing is necessary to achieve high aspect

ratios and a good mold fill. High aspect ratio structures are desirable for a number of

reasons, including:

- increased channel surface area;

- higher packing densities of microstructural elements; and

- higher throughput due to a higher cross sectional area per unit substrate

area.

However, in addition to thermocycling, other steps of the embossing process must

be executed properly in order to successfully emboss high aspect ratio structures.

One such feature is de-embossing.

To prevent damage to the embossed structure during de-embossing, the frictional

forces between the embossing tool and the substrate must be minimised. If the

frictional forces during de-embossing exceed the local tensile strength of the

polymer, the microstructure will be seriously damaged (Becker and Heim 2000).

A possible cause of increased friction is the roughness of the embossing master.

Therefore, during master fabrication, it is critical that sidewall roughness is

minimized.

The frictional forces mentioned above become particularly crucial when sidewall

angles approach 90o. A small deviation from vertical reduces the problem of

sidewall roughness dramatically, and if the design allows this deviation, it can be

very advantageous in the fabrication process (Becker and Heim 2000).


Simple friction may not be the only cause of damage during de-embossing, however.

Adhesion forces may be generated directly between the tool and the substrate, and

these too can produce substantial damage to the microstructure if they are strong

enough.

In order to reduce the prospect of direct adhesion, both surfaces should offer as few

chemical bonding sites as possible, and while the use of release agents may

eliminate the problem altogether, the prospect of contamination in life sciences

applications is prohibitive (Becker and Heim 2000). Surface coating of the tool to

reduce the likelihood of adhesion could also be considered.

The geometrical parameters of the tool and the substrate are dependent to differing

degrees on its temperature. As both the tool and the substrate are thermally cycled,

this microscopic expansion and contraction of the materials can produce unwanted

stresses that damage the microstructure (Becker and Heim 2000). Therefore, it is

important that the thermal cycle be as small as possible.

While embossing is mostly done at elevated temperature as described above, some

research has also been performed on embossing at room temperature, otherwise

known as cold embossing (Xu, Locascio et al. 2000; Marko-Varga, Ekstrom et al.

2001).

This process would have the advantage of decreased cycle times since thermal

cycling is not necessary and reduced cost since no energy is expended on heating

the material. However, embossing at room temperature would likely reduce tool life

substantially since the materials are much harder at room temperature, and

replication accuracy would likely be poor due to yielding of the material.

2.4.5 Injection Molding Due to its extensive use for fabrication of macro-sized polymer parts, it is not

surprising that injection moulding has been applied to the fabrication of micro-sized

parts as well. The process starts with the raw polymer material, which is used in


granular form. These granules are fed into the cylinder, a heated screw, where the

pellets start to melt. This melt is then transported forward towards the mold cavity.

The molten polymer is then injected under high pressure into the cavity, which

contains the mold insert as the master structure (Callister 2006).

For injection moulding of macroscopic parts the cavity can be held at a temperature

below the solidification temperature of the polymer. This allows rapid fabrication by

permitting cycle times of the order of several seconds for most applications.

For injection molding of microscopic parts, the mold temperature needs to be closer

to the melting point of the polymer material to allow the polymer to flow into all the

smaller structures of the mold insert. The cavity is then cooled to allow the ejection

of the microstructured part.

This vario-thermal processing allows the fabrication of smaller structures than the

cold-cavity process, but increases the cycle time due to the heating and cooling.

Approximate cycle times for macro-injection moulding are a minute or less, whilst

micro-injection moulding cycles times are several minutes or greater.

For accurate micro-scale moulding thermal shrinkage has to be taken into account

when designing the mould because the material will likely shrink slightly as the

polymer solidifies.

Some research on injection moulding of microfluidic substrates has been performed

(McCormick, Nelson et al. 1997; Heckele and Schomburg 2004; Su, Shah et al.

2004). These authors injection moulded PMMA into an electrophoretic separation

design which was then sealed by thermal lamination. Their results indicated that the

geometry of the injection moulded part did not correspond well with the geometry of

the nickel electroform or the silicon master, which was attributed to premature

ejection of the parts from the mould. The authors were of the opinion that

optimisation of the process would produce improvements in channel resolution.

Other work has been done on the injection moulding of polymer nanovial arrays

(Marko-Varga, Ekstrom et al. 2001), and CD platforms (Schift, David et al. 2000), as


well as PC injection moulding (Su, Shah et al. 2004), and PMMA injection moulding

(Hulme, Fielden et al. 2004).

2.4.6 Other Molding Techniques Photoreaction injection moulding is a technology that was developed to avoid the

long cycle times of variothermal injection moulding. It consists of injecting a

monomer solution into the mould, and then photo-polymerising the mixture by

exposure to UV light (Joh and Pishko 2003). This technique has been used to

fabricate PETG hydrogel microstructures (Joh and Pishko 2003), and PMMA

microstructures (Muck, Wang et al. 2004). This technique requires a UV transparent

mould.

Another variation on conventional injection moulding reported in the literature is

called gas assisted injection moulding (Lai, Cao et al. 2000). This technique consists

of injecting a gas into a tool cavity partially filled with resin, while the resin injection

continues to prevent solidification of the flow front. After the resin injection is

completed, the gas injection continues, forming a gas bubble that keeps the resin

flow front moving until the resin skin reaches the end of the mold.

Once the material has completely filled the article, and the resin skin is fully

established, the gas bubble continues to be pressurized to avoid resin shrinkage.

Finally, the article is cooled adequately to establish skin strength, and the gas is

finally vented. The advantages of this technique over conventional injection moulding

include increased output, reduced resin consumption, reduced design constraints,

and lower costs.

Thermoforming as a fabrication method for microfluidics has also been reported

(Truckenmuller, Rummler et al. 2002). This consists of a molten polymer sheet

being placed on top of a mould. The mould is evacuated, and the molten polymer

sheet is sucked down into the mould by the vacuum. The polymer sheet is left to

solidify against the mould.


2.4.7 Casting Casting, or replica molding, is a technique that is used mainly for the fabrication of

PDMS microfluidic systems but could be used with a variety of curable elastomers

and thermosetting polymers. Casting, along with rapid prototyping, are collectively

known as soft lithography, or a group of nonphotolithographic methods for replicating

a pattern.

PDMS is produced by the reaction of its two components, the resin and the curing

agent (both of which consist of a variety of siloxane chemicals, see 3.2.1 for more

details). These can be mixed together, poured into a master mould containing the

microfluidic features, and then left to cure. The replica will be a negative relief of the

master (ridges on the master will appear as trenches in the replica). When fully

cured, the PDMS can then be removed from the mould with the microfluidic design

incorporated in the material. An advantage of PDMS is its very low stick, which

makes it easy to remove from moulds and casts. PDMS casting is distinct from the

photoreaction injection moulding process in that PDMS is thermally cured, while

photoreaction injection moulding is radiation cured.

Replica moulding is a very popular technique, having been used by a number of

authors for microfluidic devices and technology, (Hozokawa, Fujii et al. 1999;

Anderson, Chiu et al. 2000; McDonald, Duffy et al. 2000; Chabyinc, Chiu et al. 2001;

Ismagilov, Ng et al. 2001; Jiang, Wang et al. 2001; Kim and Knapp 2001; McDonald,

Metallo et al. 2001; Chiou, Lee et al. 2002; Ro, Lim et al. 2002; Chen, Acharya et al.

2003; Shin, Cho et al. 2003; Wu, Odom et al. 2003).

2.4.8 Plasma Etching Plasma etching is a technique that uses highly reactive plasmas of various gases to

generate holes and channels (Rossier, Schwarz et al. 2000). A plasma will

preferentially etch a polymer over a metal, so the surface of the polymer substrate is

partially covered by a metal mask to restrict the etching process.

The following passage is from an article (Rossier, Reymond et al. 2002) on the

plasma etching process;


The starting material is a polyimide foil of approximately 50 micron thickness, which

is coated on both sides with 5 micron copper. A photoresist layer is deposited on the

foils and structured by photolithography with the help of a high-resolution printer.

The copper is then chemically etched in order to produce the pattern of the

microchannels. The structured layers are then exposed to an oxygen-plasma, which

allows etching holes and microchannels in the plastic substrate. As both sides of the

substrate are exposed to the plasma, through-holes can be fabricated, thereby

providing reservoirs and/or access to the microchannel. Electrode fabrication can be

integrated in the same manner by etching 100 micron discs in the copper foils. A

last copper etching is then performed to produce conductive pads on which gold is

electroplated in order to get a surface which is suitable for electrochemistry.

2.5 Sealing of Microfluidic Devices

2.5.1 Introduction The microchannels are normally open (3 walled) after the fabrication step, and must

be sealed in any working device. This closing operation must be performed without

clogging or distorting the channels, changing their physical parameters or changing

their dimensions. The following sections describe a variety of bonding methods,

and then describe some of the methods applied to PC and PDMS.

2.5.2 Solvent Bonding The concept of solvent bonding is similar to diffusion bonding. A solvent is applied to

the polymer surfaces, solvating the surface layers and dramatically increasing the

mobility of polymer chains at the surface. When the two layers are pressed together,

the polymer chains on each surface become intertwined; as the solvent evaporates

or is driven off, chain mobility is reduced back to its original state, the newly

intertwined chains form a strong bond. Solvent bonding was reported for the sealing

of an all polyimide microfluidic device (Glasgow, Beebe et al. 1999).


2.5.3 Lamination In this process a thin PE/PET foil (typical thickness about 5-10 µm) is rolled onto the

structure with a heated roller (Roberts et al, 1997; Soane et al, 1998). The PE layer

melts and seals the channel while the high melting temperature PET backing layer

prevents excessive flow in the molten PE layer. However, when very small channels

are used, the PE may block the channels (Liu, Ganser et al. 2001). This thermal

laminiation technique of PE/PET layers has been employed by other authors as well

(Bianchi, Wagner et al. 2001).

A similar sealing method has been reported (McCormick, Nelson et al. 1997) which

consisted of a thermal lamination of a 2 mm thick sheet of Mylar coated with a

thermally activated adhesive. The same authors (McCormick, Nelson et al. 1997)

reported sealing of the device to be a significant challenge, and that another

significant problem with polymers as sealing materials or substrates is their inherent

autofluorescence. Autofluorescence would interfere in cases where microfluidic

devices use fluorescence as part of their detection system.

2.5.4 Adhesives Similar to lamination is glueing, where adhesives are used to attach the channel

plate to the sealing plate (Ekstrom et al, 1990); (Xu, Lin et al. 1998; Xiang, Lin et al.

1999; Soane, Soane et al. 2001). Various types of adhesives have been

investigated:

- thermo-melting adhesives (Soane, Soane et al. 2001), where the adhesive

formulation includes medium molecular weight components that, upon

heating, melt and diffuse into the two opposed surfaces, interpenetrating the

two surfaces and creating a stable interface for the assembled microstructure

- liquid curable adhesives (Soane, Soane et al. 2001), where one of the

surfaces, usually the cover, is coated with a film of a liquid curable adhesive.

The fluid layer is then cured until tacky, and the coated surface is contacted

with the opposing planar surface (usually of the base plate in which the

materials have been formed). The curable adhesive material is then fully


cured to seal the surfaces together, forming the enclosed microchannel

structure.

Another method employs elastomeric bonding materials (Soane, Soane et al. 2001),

which take the place of the adhesive film. The elastomeric material can be applied

as a solution, as an emulsion, or in formulations of two reactive components (Kim

and Knapp 2001). Elastomeric bonding materials can be used in contact adhesive

formulations, in which one or more small molecule components are mixed to provide

tack; they do not require application of pressure to provide tack.

Alternatively, elastomeric bonding materials can be used as pure elastomers to

provide closure of the microchannel structures and to seal small irregularities in the

generally planar opposed surfaces by application of pressure to exploit the

compressibility of the elastomeric materials.

Some authors (Soane, Soane et al. 2001) prefer a bonding process that results in

interpenetration into the two opposed planar surfaces, providing a stable sealed

interface between the substrate and the sealing plate. Where a bonding material is

used, a thin film or layer of the bonding material at the interface results.

2.5.5 Thermal Bonding Also reported (Wang and Morris 2000; Kameoka, Craighead et al. 2001) has been

the technique of heating the channel and sealing plates and then applying pressure

to seal the channels. Caution must be used during this technique in order to ensure

that the structures are not damaged.

Thermal bonding has been reported for PC (Wen, Lin et al. 2000; Liu, Ganser et al.

2001; Wabuyele, Ford et al. 2001), COC (Kameoka, Craighead et al. 2001;

Guadioso and Craighead 2002) and PMMA (Martynova, Locascio et al. 1997; Chen,

Sung et al. 2001; Wabuyele, Ford et al. 2001).

A high success rate for thermal bonding has been reported (Martynova, Locascio et

al. 1997). A few devices were not well sealed due to bubble entrapment, but these


failed devices could generally be salvaged by reheating the device for a longer time

at the same temperature, indicating that temperature and time for sealing the

channel are critical (Martynova, Locascio et al. 1997).

A general process for thermal bonding has been described (Soane, Soane et al.

2001) in which the planar surfaces of the base plate and the cover are aligned and

confined to a mechanical fixture, in which they are progressively heated under

pressure to a temperature 2-5oC above the glass transition temperature of the

polymer. In this first step, small irregularities in the surface accommodate each

other, while maintaining the physical integrity of the channels.

Then, the temperature is maintained above the glass transition temperature of the

polymer for enough time for polymer molecules to interpenetrate and create a bond.

Above the glass transition temperature the molecules have sufficient mobility to

entangle and orient in the surfaces of the two plates. In a final step of the bonding

process the temperature is slowly reduced in order to maintain a stress free interface

that provides a stable assembled microchannel structure.

2.5.6 Oxidative Sealing When polydimethylsiloxane (PDMS) is cast onto a master mold containing 3D

microstructures, those microstructures replicated in the PDMS, creating 3-walled

structures. The fourth channel wall can be made out of PDMS or some other

material (McDonald, Duffy et al. 2000). Sealing can occur in two ways:

- reversible, conformal sealing with a flat surface (Gent and Vondracek 1982;

Chaudhury and Whitesides 1992; McDonald, Metallo et al. 2001; Fujii 2002;

Wu, Odom et al. 2003); and

- irreversible sealing to certain substrates upon exposure of both surfaces to an

air plasma (Chaudhury and Whitesides 1991; Hu, Ren et al. 2002; Shin, Cho

et al. 2003), (Chabyinc, Chiu et al. 2001), (Duffy, McDonald et al. 1998;

Rocklin, Ramsey et al. 2000; McDonald, Metallo et al. 2001).

This technique will be described in more detail in section 2.6.


2.5.7 Problems With Sealing of Microfluidic Devices The primary problem with most sealing techniques for polymeric microfluidic devices

is channel blockage or channel distortion or some other artifact of the sealing

process that inhibits or even destroys the functionality of the final device. There are

a number of ways that this can occur, but the primary problems are adhesive

overflow and the loss of channel integrity during thermal welding (Soane, Soane et

al. 2001). These problems are depicted in Figure 2-1.

Figure 2-1: Channel blockage and distortion during adhesive (left) and thermal bonding (right)

In Figure 2-1 (left) the device has been adhesively bonded, but the pressure applied

during bonding has caused the adhesive to flow over into the device channels. This

can cause severe problems with channel blockage.

Figure 2-1 (right) presents a different problem. When using thermal bonding

techniques such as laser welding or resistance welding, heat is applied and the

polymer flows such that the two surfaces fuse together. However, under pressure,

the substrate can flow too much, and cause the channel walls to lose their integrity.

It is often difficult to introduce enough thermal flow to cause sealing while preventing

thermal flow in the channel walls and other microfluidic features.

2.6 PDMS in Microfluidic Devices PDMS is a widely used material for microfluidic devices, with many papers published

describing specific devices and device components (Duffy, McDonald et al. 1998;

Graubner, Jordan et al. 2002), (Anderson, Chiu et al. 2000), (Wu, Odom et al. 2003),


(McDonald, Metallo et al. 2001), (Chabyinc, Chiu et al. 2001; Ko, Yoon et al. 2003),

and others providing reviews of PDMS microfluidic technology (McDonald and

Whitesides 2002), (McDonald, Duffy et al. 2000), (Fujii 2002; Ng, Gitlin et al. 2002).

PDMS is an elastomer, consisting of a three-dimensional polymer network. The

three dimensional structure of elastomers generally gives them good temperature

stability because they do not undergo melt flow as thermoplastics such as

polyethylene do. PDMS is also highly flexible at room temperature, has excellent

optical clarity and good resistance to chemical attack.

Its resistance to chemical attack has led to its use in applications requiring

biostability (Belanger and Marois 2001; Fallahi, Mirzadeh et al. 2003).

However, it is often necessary to perform surface modification in order to alleviate

several disadvantages, including:

- its extreme hydrophobicity, which makes it difficult to fill PDMS devices with

aqueous solutions (Hu, Ren et al. 2002);

- its tendency to adsorb other molecules onto the surface, with some molecules

actually migrating into the polymer matrix itself (Hu, Ren et al. 2002). For

instance, a hydrophilic surface has been reported to reduce non-specific

absorption of proteins (Hu, Ren et al. 2002; Ng, Gitlin et al. 2002); and

- its poor adhesion, due to its highly hydrophobic surface (which inhibits good

wetting of the surface), and its three dimensional network structure (which

prevents thermal welding).

The most commonly used surface modification is the technique of oxidising the

surface of PDMS in order to produce a strong seal. PDMS is composed of repeating

units of –O-Si(CH3)2-. At the surface, the oxidisation process removes the methyl

groups and replaces them with silanol groups. These silanol groups are unstable

and can react with other silanol groups on other surfaces via condensation. These

new bonds are formed irreversibly and cannot be broken without rupturing the bulk of

the PDMS.


This process is commonly used for the irreversible sealing of PDMS devices (Ng,

Gitlin et al. 2002), (McDonald, Duffy et al. 2000), (McDonald, Metallo et al. 2001),

(Chabyinc, Chiu et al. 2001), (Duffy, McDonald et al. 1998), and can be

accomplished using oxygen or air plasma, a corona discharge or UV/ozone

exposure in air.

Simple mechanical clamping without oxidative pretreatment has also been employed

to produce irreversible bonding (Jiang, Wang et al. 2001), however it is possible that

an excess of curing agent in one part reacted with an excess of resin in the other

part.

This oxidation technique can also bond PDMS irreversibly to glass, silicon, silicon

oxide, quartz, silicon nitride, polyethylene, polystyrene and glassy carbon, but not to

PC, PMMA or PI (polyimide) (Duffy, McDonald et al. 1998).

The effects of surface oxidation on PDMS has been studied by various groups.

Some authors investigated how the hydrophobicity of PDMS changes after exposure

to corona discharge (Kim, Chaudhury et al. 2000). These authors found that the rate

of hydrophobic recovery increased considerably as the applied voltage and humidity

during discharge increased (Kim, Chaudhury et al. 2000). Low molecular weight

species were generated during exposure to corona discharge, and the authors

concluded that these could cause hydrophobic recovery. Higher applied voltages

created larger amounts of LMW species, and thus higher voltages would cause

faster recovery rates, since more LMW species would be present.

Oxidised samples stored under high vacuum also exhibited lower recovery rates

than those aged in air, which may indicate atmospheric moisture enhances

hydrophobic recovery (Kim, Chaudhury et al. 2000).

The presence of free silicone fluid, whether present in the original material or, more

significantly, created during exposure, would increase the rate of hydrophobic

recovery by diffusing to the surface and restoring the hydrophobic nature of the

original surface (Kim, Chaudhury et al. 2000).


The conclusion that low molecular weight (LMW) silicone species were primarily

responsible for hydrophobic recovery somewhat contradicts other studies (Morra,

Occhiello et al. 1990) that had proposed a variety of mechanisms acting

simultaneously, including elimination of polar groups by chemical interaction (silanol

condensation), reorientation of polar groups into the bulk, as well as the diffusion of

untreated species to the surface (Morra, Occhiello et al. 1990).

More sophisticated surface treatments are required for specific tailoring of the

surface properties of PDMS. This was demonstrated by a one-step procedure for

covalent attachment of polymers to the surface of PDMS microchannels by UV graft

polymerisation (Hu, Ren et al. 2002). Acrylic acid, acrylamide, dimethylacrylamide,

2-hydroxyethylacrylate, and polyethyleneglycolmonomethoxyl acrylate were grafted

onto PDMS to yield hydrophilic surfaces. A possible drawback to this technique is

that it may not be possible to irreversibly bond the grafted surfaces. Reversible

bonding was not attempted (Hu, Ren et al. 2002).

2.7 Polycarbonate in Microfluidic Devices Polycarbonate is a common material for use in microfluidic devices for a number of

reasons, including:

- it is optically clear;

- it has excellent temperature stability;

- it can be easily and accurately machined using a variety of techniques

including laser ablation and hot embossing; and

- it can be diffusion bonded to itself.

The use of polycarbonate has been widely reported in the literature (Roberts,

Rossier et al. 1997), (Bianchi, Wagner et al. 2001) and (Liu, Ganser et al. 2001),

(Wen, Lin et al. 2000), (Xiang, Lin et al. 1999), (Xu, Lin et al. 1998), (Marko-Varga,

Ekstrom et al. 2001).

A simple device has been described (Roberts, Rossier et al. 1997) which involved

the fabrication of fluid channels and reservoir networks by firing 200 mJ pulses from

a UV excimer laser at substrates moving in a predefined computer controlled pattern.

Efficient sealing was accomplished by film lamination, and the ablated structures


were observed to be well defined, with high aspect ratios (Roberts, Rossier et al.

1997).

However, later publications (Bianchi, Wagner et al. 2001) showed that such film

lamination procedures, while providing for easy and efficient sealing, produced very

poor flow properties. Since the two materials (substrate and laminate) have different

electroosmotic flow potentials, the flow is non-uniform across the width of the

channel, leading to decreased separation efficiency (Bianchi, Wagner et al. 2001).

To solve this problem, either new sealing techniques will need to be developed, or

the film lamination procedure will require some sort of pre- or post-sealing treatment

of the surfaces to match the electroosmotic potentials to appropriate levels.

Different surface modifications of polycarbonate have been investigated, including;

- sulfonation of polycarbonate in fuming sulfuric acid to increase hydrophilicity

(Soper, Henry et al. 2002);

- exposure to microwave oxygen plasma to increase hydrophilicity (Tomcik,

Popovic et al. 2001); and

- ozonation followed by graft copolymerization to improve auto-adhesion (Kang,

Neoh et al. 1998).

2.8 Integration of PDMS and Thermoplastics in Microfluidic Devices

PDMS is used to fabricate various microfluidic components, such as valves, pumps

and membranes. In addition to being used on its own, it can be found integrated

with thermoplastic components in devices that are hybrids of several different

materials.

For example, an immunosensing biochip has been constructed from a PDMS top

substrate (molded by polymer casting) and a PMMA bottom substrate (fabricated by

hot embossing) (Ko, Yoon et al. 2003). The two surfaces were bonded with pressure

and hermetically sealed (Ko, Yoon et al. 2003). Two inlet ports and an air vent were

opened through the PDMS top substrate, and gold electrodes for electrochemical

detection were patterned onto the PMMA bottom substrate (Ko, Yoon et al. 2003).


Hermetic sealing has been used to seal PDMS to PMMA and PC (Hozokawa, Fujii et

al. 1999; Xu, Locascio et al. 2000), and PDMS to PC (Ismagilov, Ng et al. 2001).

Another PMMA/PDMS hybrid device (Hozokawa, Fujii et al. 1999) consisted of a

PDMS part bonded to a PMMA part, with the PMMA part having access holes for

sample loading and pneumatic control.

A capillary gel electrophoresis chip of PDMS and PMMA has also been described

(Fujii 2002), as has a ten layer device for high throughput residue analysis of food

contaminants, which integrates aluminium, copolyester, PDMS and polyvinylidene

fluoride (PVDF) (Jiang, Wang et al. 2001). Polyvinylidene fluoride was placed

between PDMS sheets which had capillary moulded microchannels in them.

Copolyester was used to provide structural support to the soft PDMS, and the device

was sealed by clamping the polymers between aluminium plates (Jiang, Wang et al.

2001).


3 Materials & Methodology

3.1 Introduction This chapter details the materials and experimental methods used in the project, for

the purpose of providing sufficient information for the reader to replicate the work

that was performed during the project.

3.2 Materials

3.2.1 Polydimethylsiloxane (PDMS) The PDMS that was used in this project was Dow Corning

(http://www.dowcorning.com) product Sylgard® 184, which is one of the most

common silicone elastomers. It has two components (resin and curing agent) that

are mixed and then cured to form an elastomer.

The composition of the two components is shown in Table 3-1 (base resin), and

Table 3-2 (curing agent) (DowCorning 2005). This information is reproduced from

Dow Corning’s datasheet for Sylgard 184® (DowCorning 2001).

Resin Weight% range

Dimethyl siloxane, dimethylvinyl-terminated >60

Dimethylvinylated and trimethylated silica 30-60

Tetra(trimethylsiloxy) silane 1-5

Table 3-1: Components by weight of PDMS base resin

Curing Agent Weight % range

Dimethyl, methylhydrogen siloxane 40-70

Dimethyl siloxane, dimethylvinyl-terminated 15-40

Dimethylvinylated and trimethylated silica 10-30

Tetramethyl tetravinyl cyclotetrasiloxane 1-5

Table 3-2: Components by weight of PDMS curing agent


To convert the two components into an elastomer, they are mixed in a 10:1 mass

ratio (resin:curing agent). In line with manufacturer’s recommendations

(DowCorning 2001) the material was cured for approximately 3 hours at 75oC.

3.2.2 Polycarbonate (PC) Polycarbonate (PC) was selected as the thermoplastic material because it is

commonly used as a microfluidic substrate, and because it is transparent, has good

thermal stability and can be laser machined. GE Polymer Shapes

(http://www.gepolymershapes.com, now SABIC polymer shapes) manufactured the

polycarbonate that was used in this project, with the grade number 8010.

3.2.3 Loctite 3105 UV Curable Adhesive For this project an adhesive that could strongly adhere PDMS to PC was required.

Preliminary experiments into adhesion between PC and PDMS indicated that

cyanoacrylate instant adhesives and epoxy adhesives were not effective in adhering

PC to PDMS. Combining these adhesives with UV/ozone surface treatments also

resulted in minimal to no adhesion either.

However, experiments with Loctite 3105 were quite successful in adhering PC to

UV/ozone surface treated PDMS. Loctite 3105 was therefore selected as the

adhesive for the project.

Loctite 3105 (http://www.loctite.com.au) is a no mix, low viscosity adhesive which

cures rapidly to form flexible, transparent bonds when exposed to UV radiation

and/or visible light of sufficient intensity (Loctite 1999). Table 3-3 shows the

adhesive’s composition, which reproduces the Loctite datasheet (Loctite 2005).


Component Component weight % range

Proprietary acrylate monomer 30-60

Proprietary urethane monomer 30-60

Photoinitiator 24650-42-8 1-5

Proprietary substituted silane 1-5

Proprietary photoinitiator 1-5

Modified acrylamide 10-30

Table 3-3: Components of Loctite 3105 UV curable adhesive

3.3 Thermal Testing

3.3.1 Introduction Testing for thermal properties was conducted because the project involves

processing of materials at elevated temperatures.

A variety of techniques were used to determine the thermal properties of materials.

The principal techniques were differential scanning calorimetry (DSC) using a TA

Instruments Model 2920 (http://www.tainstruments.com), and differential

photocalorimetry (DPC), using a TA Instruments model Q1000.

DSC measures the difference in heat flow between a sample and an inert reference

as both are subjected to a linear change in temperature. Sudden fluctuations in this

heat flow may be indicative of thermal transitions such as melting point and glass

transition temperature.

DSC can reveal a wide variety of thermal information such as melting points, glass

transition points and percentage crystallinity of semicrystalline materials.

DPC is a technique related to DSC. DPC measures the difference in heat flow

between a sample and an inert reference as both are subjected to radiation; in this

case, UV radiation.


3.3.2 Differential Scanning Calorimetry (DSC) DSC scans were performed on PC and Loctite 3105 from room temperature up to

about 200oC. The purpose of these DSC scans was to measure the glass transition

temperature of each material. The glass transition temperature is an important

property for hot embossing, which will be discussed later in the methodology.

A DSC scan was not performed on PDMS, since its glass transition temperature is

well below zero and of no practical interest for this project, since all the processing

takes place at room temperature or at highly elevated temperatures. PDMS would

also display no melting point due to its elastomeric nature.

The TA instruments 2920 is shown in Figure 3-1.

Figure 3-1: TA Instruments TA2920 DSC

The sample masses were approximately 5-20 mg. The mass of these samples was

precisely determined and recorded and then the samples were sealed inside

aluminium sample pans using a pressing tool.

During experiments the system was purged with inert nitrogen to ensure the sample

would not oxidise.


The DSC was controlled through software called Thermal Analysis. After the sample

was loaded and the gas purge commenced, the test program was loaded. The test

program varied depending on the specific experiment performed, but a typical test

program for a DSC experiment is as follows:

Equilibrate at 30 oC

Ramp 3 oC per minute to 250 oC

Ramp 3 oC per minute to 50 oC

Repeat once

The TA instruments package comes with an analysis program called Universal

Analysis, which can perform analysis on any data set collected by a TA Instrument.

Once the experiment is completed, the results can be viewed and analysed in the

Universal Analysis software, including calculation of glass transition temperature.

3.3.3 Differential Photocalorimetry Differential photocalorimetry (DPC) can be used to analyse UV curable materials.

The use of DPC was intended to determine how long the adhesive would take to

cure upon exposure to UV radiation. However, since the UV lamps used in the

Q1000 and the UV lamps used in the project’s UV curing box are different, there is

only rough correlations that can be drawn between the two.

The TA Q1000 measures the heat flow from a material as it is exposed to UV/Vis

radiation in a temperature-controlled environment. For UV curable adhesives,

exposure to UV radiation will commence the curing reaction, and this curing reaction

has an associated enthalpy of reaction, which the DPC can detect. The variation in

enthalpy of reaction over time gives the enthalpic curve, which can then be used to

determine the properties of the UV curable adhesive.

A Loctite 3105 sample, 11.3 mg, was exposed to the UV light for approximately 3.5

minutes. The curing reaction is exothermic, and the maximum heat flow, which is

the maximum rate of chemical reaction, is represented as a peak.


3.4 Mechanical Testing

3.4.1 Introduction Mechanical properties at room temperature were determined using the Zwick Z010

(http://www.zwick.co.uk) mechanical tester (Figure 3-2). Several different modes of

testing were performed. Bulk tensile testing of the raw PDMS and PC was

performed for the purpose of simple characterization.

Lap shear testing of adhesively bonded PDMS and PC was performed to

approximate the shearing stresses that the sample would encounter during

embossing, although it was not possible to perform this test at the elevated

temperatures encountered during hot embossing.

Peel testing of adhesively bonded PDMS and PC was performed to approximate

stresses that might act to delaminate the composite structure after embossing.

A thermal chamber would have been useful to accurately determine adhesive

strengths during embossing, but none was available.

Rheometry was used to measure deformation during compression at elevated

temperature for PDMS and PC, similar to what the material would undergo during

embossing.

3.4.2 Tensile Mechanical Testing Figure 3-3 is a schematic of the test pieces used during mechanical testing. The

gauge length of the pieces was 25 mm, which is measured from the top of the

straight section in the middle of the piece to the bottom. PDMS pieces were

produced by casting.

The tests were performed at a testing speed of 50 mm/min.


Figure 3-2: Zwick Z010 Tensile testing machine

Figure 3-3: Schematic of tensile test piece

45 mm length Gauge length 25 mm

20 mm5 mm width at middle

10 mm


3.4.3 Lap Shear Testing For testing of bond strength, a lap shear joint configuration was used, and the

dimensions are shown in Figure 3-4. To create the lap joint, the following steps were

used:

- the PDMS and PC dumbbells were cut in two;

- the PDMS dumbbells were UV treated, while Loctite 3105 was daubed onto

the narrow ends of the PC dumbbells halves;

- after UV treatment, the PDMS sections of the dumbbells were removed from

the UV cabinet, and placed on top of the PC dumbbells in the lap shear

configuration (Figure 3-4), with the UV treated side facing the PC; and

- the assembled part was then placed back in the UV cabinet for curing of the

adhesive.

Figure 3-4: Joint diagram for lap shear testing

During lap shear testing, the assembled sample is placed in the Zwick machine and

extended. Force and extension are recorded by the instrument. When the sample

fails, the test stops automatically. After the test, failure stress can be calculated by

dividing the breaking force by the bond area, according to the following equation:

)(

)()(2mmAreaBond

NFailureatForcePaStressFailure = Equation 3-1

Lap joint area is 5 mm x ≈ 1 mm

≈ 5 mm2


This failure stress can be plotted as a function of UV treatment parameters to

determine how UV treatment affects failure stress in shear. This information can be

used to maximise adhesion in shear, which will reduce the likelihood of delamination

during embossing.

3.4.4 Peel Testing In addition to lap shear testing, peel testing of the PC to PDMS joint was performed.

To create the peel joints, the following procedure was used:

- 250 µm thick PDMS and 178 µm thick PC were cut into rectangles 2.5 cm

long x 2 cm wide;

- reinforced PDMS was produced by laminating it onto a layer of 178 µm thick

PC using the Loctite 3105 adhesive under UV exposure conditions of 5 min

pretreatment and 5 min curing at an exposure distance of 30.1 mm;

- the reinforced PDMS was then bonded to PC using Loctite 3105 in a peel joint

configuration as in Figure 3-5; it was necessary to mask part of the PDMS

surface to prevent adhesive spreading along the surface; and

- the bonded joint area was 5 mm long x 2 cm wide.

The peel joint setup is shown in Figure 3-5.

Figure 3-5: Schematic of the peel joint setup (block arrows show direction of extension)

PC laminated onto PDMS sheet (red is

adhesive layer) This section of the PDMS

surface is masked to

limit adhesion to the joint

area

PC piece of peel joint is peeled back for testing


Peel testing was performed according to a similar procedure as lap shear testing.

The assembled sample is placed in the Zwick machine and extended. Force and

extension are recorded by the instrument. When the sample fails, the test stops

automatically. After the test, failure stress can be calculated by equation 3-1.

This failure stress can be plotted as a function of UV treatment parameters to

determine how UV treatment affects failure stress in peel. Maximising adhesion in

shear will reduce the likelihood of delamination after embossing.

3.4.5 Rheometry Rheometry was performed using the TA Instruments AR200 Rheometer (Figure 3-6).

Figure 3-6: TA Instruments AR200 Rheometer

It was used to determine the extent of dimensional changes during compression at

elevated temperature. This was performed to greater understand the embossing

processes that would be utilised later in the project.

The process involved the use of the plate and plate system. The material disc,

which was 25 mm in diameter and about 250 µm thick, was clamped between the

plates at a set temperature of 155 oC, and a small compressive force (up to 50 N, the

limit of the instrument) was applied to the material.


155 oC was used as an approximation of the embossing temperature of

polycarbonate. As the compressive force is applied the change in distance between

the plates is recorded.

Compressive force was converted into compressive pressure, and compressive

pressure was plotted against % change in distance. This gives an estimate of how

much the materials will be compressed at these conditions in the embossing

machine.

It would have been additionally useful to investigate compression at higher loads, but

the rheometer’s limit was 50 N. Higher loads would have enabled an investigation

elastic and plastic sample deformation at the actual conditions used during the

embossing phase of the project, and allowed embossing conditions to be optimised

to maximise embossing feature resolution while minimising bulk deformation of the

embossed material.

3.5 Surface Modification of Polymers

3.5.1 UV Surface Treatment UV/ozone surface treatment was selected as the primary surface treatment for the

project due to its availability and success achieved in preliminary experiments into

adhesion between PC and PDMS using Loctite 3105 UV curing adhesive (refer to

Section 3.2.3).

UV/ozone exposures were performed in a custom built UV cabinet (Figure 3-7). This

cabinet had four UV lamps mounted at the top of the cabinet, which could be turned

on independently or all at once. Ozone was generated by the interactions of the UV

radiation with the oxygen molecules inside the cabinet.


Figure 3-7: UV Exposure cabinet

Samples were mounted on a retractable exposure shelf. The distance between the

UV lamps and the shelf could also be controlled.

The specifications for each UV lamp were as follows;

Manufacturer: Heraeus Amba Australia (http://www.heraeus-amba.com.au)

Power: 17 W

Spectrum Wavelength: 254 nm and 185 nm (most power at 254 nm)

Typical UV Efficiency at 254 nm: 40%

Typical UV-efficiency at 185 nm: 5-10%

Model Number: NNQ

3.5.2 The Oxygen Plasma Reactor The oxygen plasma reactor was built at CSIRO Division of Molecular Science (now

Division of Molecular & Health Technologies). The plasma was used to oxidise the

surface of PDMS for autohesion.

Plasma treatments were carried out in a reactor of a conventional design; it

employed two capacitively coupled electrodes in a horizontal configuration within a

cylindrical glass vessel (7.4 L volume). Samples were placed on the base electrode


(155 mm diameter, copper, earthed) for the treatment. A second copper disc was

used as the upper (active) electrode.

The separation between the two electrodes was approximately 150 mm. The

oscillator used in this study was a commercial plasma generator (ENI HPG-2)

equipped with matching networks and operating from 125 to 375 kHz. Ancillary

fittings, such as the pressure sensor (MKS Baratron), valves, and the pumping line,

were standard items.

A basic procedure for the treatments is as follows:

Preparations to treatments:

- ensure the equipment is clean and free from debris;

- fill the cold trap with liquid nitrogen, and open the valve between the cold trap

and the vacuum pump;

- open the fine vacuum control valve, coarse vacuum control valve and the

transducer isolation valve; and

- pump the reactor down to low pressure.

Treatment:

- isolate the reactor and slowly introduce oxygen into the reactor

- switch the power on, begin timing and adjust power to compensate for drift, if

any occurs

- when treatment is complete, turn off the power at the generator

- switch the generator off and disconnect the power leads

3.6 Spectroscopy Techniques

3.6.1 X-ray Photo Electron Spectroscopy X-ray photoelectron spectroscopy was used to gather information on the surface

chemistry of project materials. It is often used in combination with other surface

analysis techniques such as secondary ion mass spectrometry (SIMS) and fourier

transform infrared spectroscopy (FTIR), but neither of these techniques were

available.


In x-ray photoelectron spectroscopy, an x-ray beam is directed at the surface of the

material, causing electrons to be ejected from the surface. The number and kinetic

energy of electrons that are emitted from the surface are recorded.

The energy of the x-rays being used and the kinetic energy of the emitted electrons

can be used to calculate the electron binding energy according to the following

equation:

φ−−= kineticphotonbinding EEE Equation 3-2

where Ebinding is the energy of the electron emitted

Ephoton is the energy of the X-ray photons

Ekinetic is the kinetic energy of the emitted electron

Φ is the work function of the spectrometer (not the material)

Electron binding energy (BE) is the energy required to release an electron from its

atomic or molecular orbital. Binding energy is therefore unique to the atom from

which it was emitted, and the number of electrons emitted allows the calculation of

the amount of different atoms.

For the XPS work in this project, small sections of PC and PDMS of approximately

1cm2 in area were used.

XPS analysis was performed using an AXIS-HSi spectrometer (Kratos Analytical,

http://www.kratos.com) with a monochromated Al Kα source at a power of 180 W (12

kV × 15 mA), a hemispherical analyser operating in the fixed analyser transmission

mode and the standard aperture (1 mm × 0.5 mm). The total pressure in the main

vacuum chamber during analysis was of the order of 10-8 mBar. Vacuum conditions

are important to ensure that atmospheric gas molecules do not interfere with either

the x-ray beam or the emitted electrons.


Each specimen was analysed at an emission angle of 0°, as measured from the

surface normal. A circular area with a diameter of approximately 1 mm was analysed

on each sample.

All elements present were identified from survey spectra. The atomic concentrations

of the detected elements were calculated using integral peak intensities and the

sensitivity factors supplied by the manufacturer.

To obtain more detailed information about chemical structure, oxidation states etc.,

high resolution spectra were recorded from individual peaks at 40 eV pass energy

(yielding a typical peak width for polymers of 1.0 eV).

These data were quantified using a minimisation algorithm in order to calculate

curvefits and thus to determine the contributions from specific functional groups. An example of a typical curvefit for carbon is shown in Figure 3-8, showing the main

peak at 285.0 eV (aliphatic hydrocarbons) and several peaks shifted to higher

binding energy indicating chemically different bonding environments (eg, different

functional groups).

0

1000

2000

3000

4000

5000

6000

282284286288290292

Inte

nsity

(cou

nts)

Binding Energy (eV)

Figure 3-8: Example of curve fitting for XPS data (carbon)


A curvefit rarely is mathematically unique in terms of the number, position and shape

of spectral components it requires. It must therefore be based not only on

mathematical criteria but also on the available knowledge about the analysed

sample, and a curvefit might need to be refined several times. Quantitative results

are often ambiguous and it is important to be aware of the limitations of the

technique when attempting an interpretation.

The accuracy associated with quantitative XPS is in the range of 10% - 15%.

Precision (ie. reproducibility) is usually better than 5%. The latter is relevant when

comparing similar samples.

3.7 Microfabrication Technology

3.7.1 AVIA Laser Machining Laser machining was used to machine polycarbonate for PDMS molds and

mechanical testing pieces.

The AVIA laser 355/3000 (Coherent Scientific, http://www.coherent.com.au) is a

compact diode-pumped solid-state Q-switched laser that provides ultra violet (355

nm) output with pulse repetition rates from a single shot up to >100 kHz.

The AVIA laser (Figure 3-9) consists of the laser head and power supply connected

by an umbilical. The umbilical cord contains fiber optic cables to transmit light from

the diode bars in the power supply to the laser head and also houses electrical and

RF cables that provide control and monitoring signals between the laser head and

the power supply.


Figure 3-9: The sample mount of the AVIA laser

For more information on how laser machining works see Section 2.4.2.

3.7.2 Micro Milling Micro milling was used to machine a master relief in polymethyl methacrylate

(PMMA). The master relief was then used in electroplating to generate an

embossing tool.

The Isel CNC Machine CPM 3020 micro milling tool (http://www.techno-isel.com/) is

a 3D machining tool (Figure 3-10). It can perform a variety of tasks, but for this

project it was used to micromachine a master relief. An electroplated tool was

produced from this master relief, and the electroplated tool was then used for hot

embossing.

Figure 3-10: Micro milling tool


The instrument is computer controlled. A CAD design is loaded into the software,

from which the instrument machines. The machine itself has a maximum machining

range of 20 cm x 30 cm x 15 cm (x, y and z directions).

Before machining can begin, the operator has to set the machining speed

(approximately 5 mm/sec), and the machining depth (approximately 0.2 mm for

machining in the x and y directions), and the zero point for machining.

3.7.3 Electroplating Electroplating is a process for generating a master mold from the negative relief

produced by the micromilling machine. It involves plating the micro milled polymer

with nickel, and then removing the electroplated nickel so that it may be used as a

hot embossing tool.

The electroplating bath used in this project (Figure 3-11) was built at the Industrial

Research Institute Swinburne.

Figure 3-11: Electroplating bath


A brief description of the electroplating process is as follows;

- sample cleaning, which is important to produce a smooth, unblemished

electroplated tool;

- the acrylic master is surface modified with a chemical solution to improve the

adhesion of the electroplating solution;

- a silver solution is sprayed onto the surface, which provides the seed layer

onto which the nickel is electroplated;

- the master is then inserted into a holding frame for insertion into the

electroplating bath;

- the electroplating bath contains a nickel solution (pH 3.8-4.0) of:

- nickel sulfamate (250 - 650 g/L)

- nickel chloride 0 - 30 g/L

- boric acid 30 - 40 g/L

- electroplating duration depends the required thickness of the plate. Generally,

these tools were electroplated overnight, and the base plate, not including

feature dimensions, was 700 µm thick; and

- tool separation after electroplating is finished.

3.7.4 Hot Embossing Hot embossing is a technique for replicating microfluidic structures in thermoplastic

substrates. A brief description of the process follows:

- a polymer sheet is heated to a temperature slightly above its glass transition

temperature;

- a tool containing three dimensional features is pressed against the polymer

under conditions (temperature, pressure time) which depend on the properties

of the polymer;

- while keeping the polymer under pressure, the temperature is reduced to

below the glass transition temperature;

- the pressure is removed and the embossing chamber is opened; and

- the part is removed from the embosser and the tool (shim) is pulled off the

substrate.


Figure 3-12 shows the sample chamber for the hot embossing process.

Figure 3-12: Hot Embossing sample chamber

The pressure displayed on the control panel is the hydraulic pressure in the system,

not the pressure on the substrate. The relationship between hydraulic pressure in

the system and embossing pressure on the substrate can be expressed as;

5.1133SPP samplesample

hydraulic

×= Equation 3-3

Where Phydraulic is pressure in the hydraulic system in bars

Psample is pressure on the substrate in bars

Ssample is the area of the substrate in mm2

1133.5 mm2 is the area of the hydraulic cylinder (38 mm in diameter)

3.8 Microscopy Techniques

3.8.1 Overview of Microscopy Techniques Microscopy techniques were used to measure the dimensions of the embossed

channels and image the channel cross sections. Two techniques were used, Laser

Confocal Microscopy (LCM), and Scanning Electron Microscopy (SEM).


3.8.2 Laser Confocal Microscopy An Olympus OLS 1200 (http://www.olympusmicroimaging.com/) laser confocal

microscope (LCM) was the primary tool for evaluation of embossed samples (Figure

3-13).

Figure 3-13: Laser confocal microscope

The primary advantage of a confocal microscope in the context of this project is its

ability to produce in-focus images of specimens with varying topography. It

accomplishes this using a process known as optical sectioning. A laser scanning

microscope converges a laser beam into a very small spot using an objective and

then scans the specimen in the x and y directions while staying constant in the z-

axis. Only those surfaces that are in focus in that particular z-axis plane are

reflected from the surface back up into the optics of the microscope.

The laser beam is then refocused onto a different z plane and the surface is re-

scanned. After the surface has been fully scanned (which may involve hundreds of

z-axis sections), the data is reconstructed with a computer, allowing three-

dimensional reconstructions of an object’s topography.

In order to properly reflect light back into the microscope, the surface must be

reflective. For polymers, this usually requires the surface be metal coated. Metal

coating was performed using a gold sputter coater (Section 3.8.4).


One of the drawbacks of LCM is that is not measure curved or rough surfaces very

well because these types of surfaces do not tend to reflect the light back up into the

optics. This phenomenon will be addressed later in the results sections.

3.8.3 SEM Microscopy Scanning electron microscopy (SEM) is a technique used for the characterisation of

materials, including polymers. It can produce high clarity images of complex surface

textures and morphologies, at very high magnification.

The electron source is on top of the column and generates the electron beam. Gas

molecules in the air deflect electrons so the column is evacuated before scanning

(typical pressure 1 x 10-3 Pa or lower). Within the electron gun is the filament, which

is the source of the electron beam. The filament is made of tungsten and is heated to

generate a fine beam of electrons.

When the electron beam hits the sample, the interaction of the electrons and the

atoms in the sample results in the emission of a variety of signals. Depending on the

sample, these signals can include secondary electrons (electrons from the sample

itself), backscattered electrons (beam electrons from the filament that bounce off

nuclei of atoms in the sample), X-rays, light, heat, and even transmitted electrons

(electrons that pass through the sample).

The SEM has several detectors to view the electron signals from the sample,

including a secondary electron detector and a backscattered electron detector.

Unlike the light microscope in which light forms an instant "real image" of the

specimen, the electrons in an SEM do not form a real image. Instead, the SEM

scans its electron beam line by line over the sample, and the detected electrons are

used to compose the image.

To get a good image using SEM, the specimens to be viewed must be electrically

conductive, and devoid of water and solvents that could vaporize in the vacuum


causing problems in the column. This is achieved for polymers by gold coating the

samples.

The SEM used during this project (Figure 3-14) is a Jeol JSM-T330 Scanning

Electron Microscope (http://www.jeol.com).

Figure 3-14: Scanning electron microscope

The advantage of SEM over optical instruments is that it completely avoids the

problem with depth of field, enabling it to generate images of complex topography

that are far superior than those that optical instruments could provide.

3.8.4 Preparation of Microcopy Samples LCM and SEM samples were gold coated using a Polaron Equipment Ltd SEM

coating unit E5100 (Figure 3-15). Quorum Technologies Limited now owns Polaron

Equipment Ltd as their website indicates (http://www.quorumtech.com/history.htm).


Figure 3-15: E5100 gold coating unit from Polaron Equipment Ltd

In conventional sputter coating a gold target is bombarded with heavy gas atoms (air

in this case). Gold atoms ejected from the target by the ionised gas cross the

plasma to deposit onto any surface within the coating unit including the specimen.

The procedure for using the vacuum gold coater was as follows:

- the samples were placed inside the chamber and the pressure was stabilised

at about 13 Pa; and

- gold coat samples for about 4 minutes, while maintaining pressure and

current at approximately 13 Pa and 25 mA, respectively.

When the coating procedure was complete, the pressure was equalised by slowly

opening the top valve. The samples can then be removed and analysed.

3.9 Software Packages

3.9.1 Design Expert The Design Expert Version 6 application (Stat-Ease Inc; http://www.statease.com)

was used to design experiments, process results of experiments, identify the

coefficients of regression equations and verify their adequacy.

A response surface method (RSM) was used for modelling in this project. An RSM

is a collection of mathematical and statistical techniques for modelling and analysis


of problems in which a response of interest is influenced by several variables and the

objective is to optimise this response (Montgomery 1997). This method is

particularly suited to this project because the project involves the optimisation of

embossing parameters and UV treatment parameters.

In most response surface methods, the relationship between the response and the

variables is unknown; therefore the first step in RSM is to find a suitable

approximation for the actual relationship between the response and the variables.

The central composite design was chosen as the method of identifying an

approximation for the behaviour of the processes undertaken during this project.

This is because the central composite allows a second order model to be created

without needing to perform a full three level factorial experiment. This is helpful for

minimising the number of experiments necessary for accurately modelling the data.

The design consists of three distinct sets of experimental runs:

- A factorial design in the factors studied, each having two levels;

- A set of centre points, experimental runs whose values of each factor are the

medians of the values used in the factorial portion. This point is often

replicated in order to improve the precision of the experiment;

- A set of axial points, experimental runs identical to the centre points except for

one factor, which will take on values both below and above the median of the

two factorial levels, and typically both outside their range. All factors are

varied in this way.

This class of designs calculates the individual coefficients and also the variance in all

points equidistant from the centre of the design.

The program evaluates data to determine whether a first or second order equation

can be used to describe it.

The two variable matrix of the second order rotatable design is shown in Table 3-3,

while the full matrix is shown in Table 3-4. The results are represented with a first or

second order linear equation where Y is a response function as in equation 3-4.


Y = P0 + P1 X1 + P2 X2 + P11 X12 + P22 X2

2 + P12 X1 X2 Equation 3-4

X1 X2

-1 -1

+1 -1

-1 +1

+1 +1

0 -1.41

0 +1.41

-1.41 0

+1.41 0

0 0

0 0

0 0

0 0

0 0

Table 3-3: Two variable matrix of the second order rotatable design


X0 X1 X2 X12 X2

2 X1X2

+1 -1 -1 +1 +1 +1

+1 +1 -1 +1 +1 -1

+1 -1 +1 +1 +1 -1

+1 +1 +1 +1 +1 +1

+1 -1 -1 +1 +1 +1

+1 +1 -1 +1 +1 -1

+1 -1 +1 +1 +1 -1

+1 +1 +1 +1 +1 +1

+1 -1.68 0 2.28 0 0

+1 +1.68 0 2.28 0 0

+1 0 -1.68 0 2.28 0

+1 0 +1.68 0 2.28 0

+1 0 0 0 0 0

+1 0 0 0 0 0

+1 0 0 0 0 0

+1 0 0 0 0 0

+1 0 0 0 0 0

+1 0 0 0 0 0

+1 0 0 0 0 0

+1 0 0 0 0 0

Table 3-4: Full matrix of the second order rotatable design for two variables

The three variable matrix of the second order rotatable design is shown in Table 3-5,

while the full matrix is shown in Table 3-6.

The results are represented with a first or second order linear equation, where Y is a

response function as in equation 3-5.

Y = P0 + P1X1 + P2X2 + P3X3 + P11X1

2 + P22X22 + P33X3

2 +P12X1 X2 +P23X2 X3+P13X1X3

Equation 3-5


X1 X2 X3

-1 -1 -1

1 -1 -1

-1 1 -1

1 1 -1

-1 -1 1

1 -1 1

-1 1 1

1 1 1

-1.68 0 0

1.68 0 0

0 -1.68 0

0 1.68 0

0 0 -1.68

0 0 1.68

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

0 0 0

Table 3-5: Three variable matrix of second order rotatable design


X0 X1 X2 X3 X12 X2

2 X32 X1X2 X2X3 X1X3

+1 -1 -1 -1 +1 +1 +1 +1 +1 +1

+1 +1 -1 -1 +1 +1 +1 -1 -1 +1

+1 -1 +1 -1 +1 +1 +1 -1 +1 -1

+1 +1 +1 -1 +1 +1 +1 +1 -1 -1

+1 -1 -1 +1 +1 +1 +1 +1 -1 -1

+1 +1 -1 +1 +1 +1 +1 -1 +1 -1

+1 -1 +1 +1 +1 +1 +1 -1 -1 +1

+1 +1 +1 +1 +1 +1 +1 +1 +1 +1

+1 -1.68 0 0 2.28 0 0 0 0 0

+1 +1.68 0 0 2.28 0 0 0 0 0

+1 0 -1.68 0 0 2.28 0 0 0 0

+1 0 +1.68 0 0 2.28 0 0 0 0

+1 0 0 -1.68 0 0 2.28 0 0 0

+1 0 0 +1.68 0 0 2.28 0 0 0

+1 0 0 0 0 0 0 0 0 0

+1 0 0 0 0 0 0 0 0 0

+1 0 0 0 0 0 0 0 0 0

+1 0 0 0 0 0 0 0 0 0

+1 0 0 0 0 0 0 0 0 0

+1 0 0 0 0 0 0 0 0 0

Table 3-6: Full matrix of the second order rotatable design for three variables


4 Fabrication of Composite Materials

4.1 Introduction

Figure 4-1: A novel method for the adhesion of PDMS to polycarbonate

The adhesive bonds between PC and PDMS need to be able to survive the hot

embossing process and Chapter 4 deals with determining the fabrication processes

and parameters that are required to fulfil this objective. Chapter 3 therefore deals

with the steps outlined in the flowchart in Figure 4-1.

Section 4.2 presents the XPS for the materials controls (data for PC, PDMS, and

adhesive. These are important as a point of reference for the delaminated surfaces

discussed later in Section 4.3.

An investigation into the surface chemistry of the adhesion of PDMS to PC is

presented in Section 4.3. PDMS was adhesively laminated to PC using Loctite 3105

with a variety of UV pre-treatment and UV curing conditions. The PDMS was then

delaminated and the surface chemistry analysed using XPS. This chemistry was

compared with unmodified PDMS surfaces and cured and uncured samples of pure

1

2

3

4

5

6

Polycarbonate sheet



Microstructure the composite materialvia hot embossing




adhesive. From this information, it could be determined how the adhesive joint

delaminated.

XPS was used because it was the only major surface analysis technique available.

An FTIR was available in another department at Swinburne University, and some

experiments were performed using solid films as the background media and then

scanning modified surfaces. However since no surface capable accessory such as

ATR (attenuated total reflectance) was available, these results were not considered

reliable, and were not included.

Mechanical testing results for PC and PDMS are described in Section 4.4, while

thermal and UV testing of materials appears in Section 4.5. Section 4.6 investigates

the strength of the PDMS to PC joint. This joint strength was determined in two

modes; lap shear and peel.

The conclusions from the work performed in Chapter 4 are presented in Section 4.7.

4.2 XPS Analysis of PC, PDMS and Adhesive

4.2.1 Adhesive Controls XPS data were collected for the uncured adhesive, the aerobically cured adhesive

and for the that was adhesive delaminated from PC surfaces. Aerobic curing was

performed by placing a droplet of adhesive onto a slide and exposing it to UV/ozone.

The collection of these controls provided a reference point against which other XPS

data could be compared. With control data for PC and PDMS, it allowed general

conclusions to be drawn about the composition of the delaminated surfaces. Table

4-1 displays binding energies and atomic ratios X/C (atomic concentration of

element/species X relative to the total concentration of carbon C) for a variety of

functionalities for both the uncured adhesive and for aerobically cured adhesive.


Table 4-1: Control chemistries of uncured and cured Loctite 3105

The limitation of XPS is that these functional environments can be the same across a

number of functional groups, and without other evidence it is difficult to directly

assign specific functional groups to particular peaks. As mentioned previously, FTIR

was attempted but no appropriate surface scanning accessory was available.

To describe the CHx species, C1+C2 was adopted as an appropriate notation

because many CHx groups are present at 285 eV (such as -C-CH3), but depending

on the carbon environment, some CHx species could be shifted to slightly higher

binding energies. Given the similar binding energies for these species, the

differences are difficult to quantify, and C1+C2 was used for all CHx species.

The chemistry of the adhesive changes significantly during cure, with a large

decrease in the O/C ratio (from 0.3438 to 0.2537) and an increase in the (C1+C2)/C

ratio (from 0.6726 to 0.7862). This increase in the (C1+C2)/C ratio indicated that the

numbers of saturated hydrocarbon groups, (such as -CH3, or -CH2-) increased during

curing.

The curve fitted C3 components have chemical shifts of +1.7 eV for the uncured

adhesive (from 285 eV to 286.7 eV), and +1.4 eV for cured adhesive (from 285 eV to

286.4 eV). These are potentially due to the presence of C with a single bonded O or

N substituent. Candidate groups could be -C-O-C-, -C-(OH) and C-O-(C=O), or

perhaps C-C≡N (chemical shift 1.4 eV) but in different proportions and thus

producing a different chemical shift. For instance, the mean chemical shift reported

Atomic BE (eV) X/C BE (eV) X/CSpecies uncured uncured cured cured

O 532.3 0.3438 532.1 0.2537C1+C2 285-285.6 0.6726 285-285.6 0.7862

C3 286.7 0.2386 286.4 0.1354C4 288.0 0.0094 287.7 0.0244C5 289.5 0.0792 289.1 0.0538

Organo-Si 102.4 0.1341 102.3 0.1028SiOx 103.4 0.0072 103.6 0.0132N1 398.1 0.0005 398.1 0.0008N2 400.1 0.0384 400.1 0.0173


for -C-O-C- is 1.5 eV, while that for C-O-(C=O) is 1.6 eV, so a C3 chemical shift of

1.4 eV would likely be the product of increased –C-O-C- over –C-O-(C=O).

However, as stated it is not possible to specifically identify all functional groups on

the basis of XPS analysis alone.

C4 groups were recorded in significantly higher quantities in the cured adhesive,

(C4/C ratios 0.0244 cured to 0.0094 uncured), and at a slightly lower binding energy

(287.7 eV cured to 288.0 eV uncured). C4 groups are generally the result of C with

two substituents bonded to it, such as in the case of C=O (2.8-3.0 eV chemical shift),

O-C-O (2.8-3.1 eV), N-C-O (2.8 eV mean chemical shift), N-C=O (3.0-3.6 eV).

The C5 peak assignments change from 289.5 eV (uncured) to 289.1 eV (cured). A

chemical shift of 4.5 eV (uncured) is possibly due to (C=O)C(C=O), while the

chemical shift of 4.1 eV (cured) is probably due to C-O-C=O. Again, it must be

repeated that the limitation of XPS is that it cannot definitively assign these peaks to

specific functional groups. As previously stated, in these instances other surface

chemistry techniques would have been very useful, but none were available.

For silica and nitrogen functionalities, the SiOx/C ratio is approximately twice as high

in the cured adhesive compared to uncured. The N1/C amounts are almost

negligibly small in both uncured and cured adhesive, while the N2/C peak is twice as

high in the uncured adhesive.

The cured sample in Table 4-1 was cured aerobically, which means it was exposed

to air during curing. Loctite’s product information states that when the adhesive is

cured in contact with air, the free radicals created by the decomposition of the

photoinitiator can be scavenged by oxygen prior to initiating polymerization. This can

lead to incomplete cure of the adhesive at the adhesive/oxygen interface, yielding a

tacky surface (Loctite 2007 ).

Although some area of the bond will be exposed to oxygen during cure (aerobic cure),

other areas (such as at the center) will not (anaerobic cure). To gather information

about curing in the middle of the bond, the was cured between two surfaces, to prevent

oxygen from scavenging the free radicals.


Bonding two sheets of polycarbonate together with Loctite 3105 and then

delaminating the sheets after curing produced the final adhesive control sample.

Due to the excellent adhesion between the adhesive and the polycarbonate,

delamination was cohesive through the adhesive layer. This meant the delaminated

surfaces offered an excellent adhesive surface for XPS analysis. For practical

purposes the delaminated sample was considered to represent the optimal adhesive

chemistry for adhesion. The binding energies and X/C ratios for the delaminated

surfaces are shown in Table 4-2.

Table 4-2: Control chemistry of delaminated Loctite 3105

The (C1+C2)/C ratio is significantly lower for the delaminated sample, while the

C2/C, C3/C, C4/C and C5/C ratios have all increased, indicating that saturated

hydrocarbons (C1 groups) have been more extensively oxidized or otherwise

converted into other functional groups. The elevated ratios of C3 and C4

environments as well as the slightly lowered O binding energy, suggest carboxylic

acid groups.

4.2.2 PDMS Controls The binding energies and X/C ratios of the PDMS are shown in Table 4-3. The

silicone backbone has the repeating unit of SiO(CH3)2. This should produce an O/C

and Si/C ratio of 0.5, as there is one Si and O atom and 2 carbon atoms in the

PDMS repeating unit. The variations in the X/C ratios in Table 4-3 are the result of

the PDMS having a variety of components, including silicate clays, as the

Atomic BE (eV) X/CSpecies delaminated delaminated

O 530.5 0.2508C1+C2 285-285.6 0.6904

C3 286.5 0.2275C4 287.1 0.0402C5 289.1 0.0813

Organo-Si 102.4 0.0067N2 399.1 0.0480


compositional table for PDMS showed (Tables 3-1 and 3-2, Materials & Methodology

Chapter). Additionally, surface compositions are always different from the bulk.

Table 4-3: Control chemistry of PDMS

4.2.3 PC Controls The surface chemistry of Lexan PC was also investigated. Table 4-4 shows the

main components of the PC surface. Substantial amounts of aliphatic hydrocarbons

[(C1+C2)/C = 0.77] due to the benzene rings, C3 and C4 groups from the –C-O-C-

and –O-(C=O)-C- groups in the polycarbonate backbone.

Table 4-4: Control chemistry for Lexan PC

4.3 XPS analysis of delaminated PC-PDMS surfaces

4.3.1 Introduction As part of understanding the development of adhesion during cure, XPS analysis

was performed on delaminated PC and PDMS surfaces at various combinations of

UV pre-treatment times and curing times. The XPS data that were gathered were

compared to XPS data from the controls to look for both similarities and variations.

From these patterns the delamination mode of the composite can be inferred.

4.3.2 UV Pretreatment of PDMS: None At zero surface treatment, the surface chemistry of the PDMS side is largely

independent of the curing time that is used (Figure 4-2).

Atomic BE (eV) X/CSpecies PDMS PDMS

O 532.8 0.6700C1+C2 285-285.6 0.9900

C3 286.3 0.0100C4 288.0 0.0000

Organo-Si 102.0 0.5800SiOx 103.3 0.1600

Atomic BE (eV) X/CSpecies lexan lexan

O 532.9 0.16C1+C2 284.8 0.77

C3 286.2 0.1276C4 290.4 0.0494

Organo-Si 101.9 0.0034SiOx 103.3 0.0013


When the XPS results from the PDMS side of the delaminated bond are compared

with the XPS results from unmodified PDMS (Table 4-3), the X/C ratios are similar.

This indicates that the adhesive has not adhered to the PDMS, and has instead

cured alongside it; the adhesive has then peeled away during delamination, leaving

a relatively adhesive free PDMS surface. This delamination behaviour is displayed

pictorially in Figure 4-3.

Figure 4-2: Surface chemistry of delaminated PDMS

Figure 4-3: Pictorial representation of PDMS delamination under no UV/ozone pre-treatment

Delamination

PDMSPC

Adhesive

Atomic Ratios for the PDMS side of a delaminated PC to PDMS lap joint using no UV pre-treatment with various UV

curing times

00.10.20.30.40.50.60.70.80.9

1

0 10 20 30 40 50 60

UV curing time (minutes)

Ato

mic

ratio

X/C

O C1+C2 C3 Organo-Si


The surface chemistry of the delaminated PC (Figure 4-4) is intermediate between

the surface chemistry of the delaminated adhesive (Table 4-2) and the control values

for PDMS (Table 4-3). This suggests that during the delamination process in Figure

4-3, significant amounts of PDMS contamination was left on the adhesive surface.

Figure 4-4: Surface chemistry of delaminated PC

4.3.3 UV Pretreatment of PDMS: 30 seconds When the PDMS has been pre-treated with UV/ozone for 30 seconds prior to

bonding (Figure 4-5), there are two clear stages in the development of the surface

chemistry. When the curing times are increased to 1, 2, 20 and 60 minutes, the

surface chemistry is very similar to that in Figure 4-2, indicating that the surface is

largely composed of adhesive-free PDMS for those parameters.

However, when the curing time is 10 minutes, the XPS analysis shows markedly

different surface chemistry. In fact the results are similar to those obtained for the

Atomic Ratios for PC side of a delaminated PC to PDMS lap joint using no UV pre-treatment with various UV curing times

0.00.1

0.2

0.3

0.4

0.5

0.60.7

0.8

0.9

1.0

0 10 20 30 40 50 60 70UV curing time (minutes)

Ato

mic

ratio

(X/C

)



adhesive itself. This indicates that the delamination mode at 1, 2, 20 and 60 minutes

curing time is different to the delamination mode at 10 minutes curing time.


These results suggest that at 10 minutes curing time, delamination occurs cohesively

through the adhesive layer rather than between the adhesive and the PDMS. The

adhesive is, therefore, strongly present in the surface layer of the delaminated

PDMS.

However, at all other curing times, these results suggest delamination occurs

between the adhesive and the PDMS; hence, the XPS data are similar to that of the

unmodified PDMS. This delamination behaviour is shown pictorially in Figure 4-6.

Atomic Ratios for the PDMS side of a delaminated PC to PDMS lap joint using 30 seconds of UV pre-treatment with

various UV curing times

00.10.20.30.40.50.60.70.80.9

1

0 10 20 30 40 50 60


Ato

mic

ratio

(X/C

)



Figure 4-6: Changes in delamination mode for 30 s UV/Ozone pre-treatment with increasing curing time

With increasing cure time, the delamination changes from adhesive, to cohesive, and

then back to adhesive. Initially, the adhesive delamination would probably be due to

the adhesive not having sufficient time to cure. Full curing of the adhesive produces

a change to cohesive delamination, and then as curing time is increased, the

delamination mode reverts back to adhesive.

The most likely explanation is that after, 60 minutes of curing, the adhesive is over

cured and the bond to the PDMS has lost mechanical strength. If the delamination

mode had moved into the PDMS this would be reflected in the surface chemistry of

the PC surface, but as Figure 4-7 shows, that is not the case.

The surface chemistry of the delaminated PC (Figure 4-7) is closer to the surface

chemistry of the delaminated adhesive (Table 4-2) than it is to the control values for

PC. Again, this is additional evidence that the process occurring under these

conditions is that represented in Figure 4-6.

PDMSPC

Adhesive

Delamination



4.3.3.1 UV Pre-treatment of PDMS: 8 minutes The XPS results for delaminated PDMS after UV/ozone pre-treatment for 8 minutes

(Figure 4-8) indicate that when an 8 minute UV/pre-treatment is applied, the surface

is similar to the adhesive controls, except for when a UV pre-treatment of 2 minutes

is applied.

Atomic Ratios for PC side of a delaminated PC to PDMS lap joint using 30 seconds UV pre-treatment with various UV curing times

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0


Ato

mic

ratio

(X/C

)




This indicates that the delamination mode exhibits the opposite pattern to that for 30

seconds UV/ozone pre-treatment. For 30 seconds pre-treatment, the delamination

mode went from adhesive to cohesive to adhesive, whereas for 8 minutes pre-

treatment the delamination mode went from cohesive to adhesive to cohesive. This

is indicated pictorially in Figure 4-9, although how this might be the case is uncertain.

Figure 4-9: Changes in delamination mode for 8 minutes UV/ozone pre-treatment with increasing curing time

PDMS

PC Adhesive

Atomic Ratios for the PDMS side of a delaminated PC to PDMS lap joint using 8 minutes of UV pre-treatment with


00.10.20.30.40.50.60.70.80.9

1

0 10 20 30 40 50 60UV curing time (minutes)

Ato

mic

rat

io (X

/C)



The surface chemistry of the delaminated PC (Figure 4-10) is more closely aligned to

the surface chemistry of the delaminated adhesive (Table 4-2) except for at 2

minutes curing time, where some of the atomic ratios spike to other levels, and then

revert back to ratios indicative of adhesive when the curing time is increased further.


4.3.3.2 UV Pretreatment of PDMS: 18 minutes When UV/ozone pre-treatment of the PDMS is 18 minutes, the XPS data show that

atomic ratios are largely independent of curing time, for both the PDMS side (Figure

4-11) and the PC side (Figure 4-13). The atomic ratios for these two sides are also

effectively identical, indicating that the same material is on the surface of both PC

and the PDMS.

Atomic Ratios for PC side of a delaminated PC to PDMS lap joint using 8 minutes UV pre-treatment with various UV curing times

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0


Ato

mic

ratio

(X/C

)




The atomic ratios that are evident in the data are similar to those shown for that of

the adhesive. This indicates that the PDMS surface has strongly adhered to the

adhesive, and delamination has occurred in the adhesive layer itself, as shown in

Figure 4-12.

Figure 4-12: Delamination mode for PDMS

PDMSPC

Adhesive

Atomic Ratios for the PDMS side of a delaminated PC to PDMS lap joint using 18 minutes of UV pre-treatment and


00.10.20.30.40.50.60.70.80.9

1

0 10 20 30 40 50 60UV curing time (minutes)

Ato

mic

ratio

(X/C

)



The surface chemistry of the delaminated PC (Figure 4-7) is more similar to the

surface chemistry of the delaminated adhesive (Table 4-2) than it is to the control

values for PC. Again, this is additional evidence for the pictorial explanation of

delamination under these conditions (Figure 4-12).


Atomic Ratios for PC side of a delaminated PC to PDMS lap joint using 18 minutes UV pre-treatment with various UV curing times

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0


Ato

mic

ratio

(X/C

)



4.3.4 Overview of Surface Chemistry Dependence on UV Treatment Figure 4-14 provides an overview of how the surface chemistry of PDMS is

dependent on UV/ozone exposure parameters. From left to right it shows selected

XPS data for PDMS, three combinations of UV pre-treatment and curing parameters,

and the adhesive. The three combinations of UV pre-treatment and curing

parameters were selected to cover broadly the UV pre-treatment/UV curing

parameter space, and demonstrate the evolution of the surface chemistry of the

delaminated PDMS as UV/ozone exposure times are increased.

Figure 4-14: XPS atomic ratios for delaminated PDMS

4.4 Mechanical Testing of PC and PDMS

4.4.1 Tensile Mechanical Properties of Polycarbonate Figure 4-15 and Table 4-5 show the results of a set of mechanical tests on

dumbbells of PC. They indicate that Lexan 8010 has a modulus of 0.6 GPa, and

undergoes significant yielding before failure.

Atomic ratios for control PDMS and control adhesive compared with atomic ratios for several delaminated PDMS

samples

0.00

0.20

0.40

0.60

0.80

1.00

1.20

PDMS 0,10 8,10 18,20 adhesivePDMS control, delaminated PDMS surfaces expressed as

pretreatment time and UV curing time in minutes, and adhesive control

Atom

ic R

atio

X/C

O C3 C4 C5 C1+C2


0 20 40 60 80 100 120

0

20

40

60

Strain in %

Stre

ss in

MPa

Figure 4-15: Stress-strain behaviour of Lexan 8010 (curves are shifted to the right for clarity)

Table 4-5: Tensile Mechanical Properties of Lexan 8010

6 samples Modulus

(MPa)

Max Stress

(MPa)

Strain at max

force (%)

Strain at

break (%)

Average 597.10 65.68 13.48 83.48

Standard deviation 20.65 0.75 0.20 34.82


4.4.2 Tensile Mechanical Properties of Sylgard 184 PDMS Figure 4-16 and Table 4-6 show the data from tensile tests on PDMS. They indicate

that, as would be expected, the soft elastomer PDMS has a much lower modulus

than PC, 1.54 MPa compared to 597 MPa.

0 50 100 150-2

0

2

4

6

8

10

Strain in %

Stre

ss in

N/m

m²

Figure 4-16: Stress-strain behaviour of Sylgard 184 PDMS (curves are shifted to the right for clarity)

6 Samples E Modulus

(MPa)

Max Force

(MPa)

Strain at Max

Force (%)

Strain at

Break (%)

Average 1.54 2.92 110.02 110.02

Standard deviation 0.41 0.96 22.79 22.79

Table 4-6: Mechanical properties of PDMS


4.4.3 Compressive Deformation of PC and PDMS The compressive deformation of PC and PDMS at elevated temperature (155 oC)

was determined using a plate and plate rheometer, and measuring the change in the

gap as a compressive load was applied to the materials. The results are displayed

in Figures 4-17 and 4-18. The force limit was approximately 50 N, so extensive

deformation was not possible. However, it can be seen that over the ranges tested,

compressive deformation of the sample was linear with applied pressure.

Figure 4-17: Deformation of PC under compression

Compressive Pressure vs % Strain for PC at 155 oC

P = 0.1224∆G + 0.3089R2 = 0.9995

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14

Change in gap (%)

Com

pres

sive

Pre

ssur

e (B

ar)


Figure 4-18: Deformation of PDMS under compression

The differences between the extent of compression can be attributed to the different

nature of each material. PC, being a rigid thermoplastic, compresses much less

than does PDMS, a soft elastomer. This behaviour will likely be similar during

embossing of the composite materials; PC will compress on slightly, while PDMS will

compress significantly.

4.5 Thermal Testing of Materials

4.5.1 Glass Transition Temperature Figure 4-19 shows the enthalpy flows for PC as it is heated at 3 oC/minute. The

glass transition temperature can be identified on an enthalpic curve as a small ridge

such as that shown in Figure 4-19. The Universal Analysis software program

calculated the glass transition temperature for PC as 147.59 oC

The glass transition temperature is important for hot embossing, since it is generally

performed at a temperature slightly above the glass transition. This will be

discussed in later chapters.

Compressive Pressure vs % Strain for PDMS at 155 oC

P = 0.053∆G + 0.3804R2 = 0.9991

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14

Change in Gap (%)

Com

pres

sive

Pre

ssur

e (B

ar)


Figure 4-19: mDSC scan of PC

Figure 4-20 shows the enthalpic curve for Loctite 3105 as it is thermally cycled.

Universal Analysis calculated the glass transition temperature of Loctite 3105 as

approximately 151.44 oC.

Figure 4-20: mDSC scan of Loctite 3105

151.44°C(I)151.26°C

152.40°C

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

Heat

Flow

(W/g)

0 50 100 150 200

Temperature (°C)

Sample: loctite 3105 1hr UV exposure 2ndSize: 4.8000 mgMethod: cyclic DSCComment: loctite 3105 1hr UV exposure, heaviest laminator setting x2, s

DSCFile: D:...\loctite3105UV1hrcure 2nd scan.001Operator: Paul MillerRun Date: 24-Jul-04 14:57Instrument: 2920 MDSC V2.6A

Exo Up Universal V3.7A TA Instruments

147.59°C(I)

143.21°C147.98°C

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14He

at F

low (W

/g)

0 50 100 150 200 250

Temperature (°C)

Sample: PC 200303cSize: 14.5000 mg

Comment: PC granule modulated DSC scan

DSCFile: D:...\mDSC analysis\polycarbonate1Operator: PaulRun Date: 20-Mar-03 10:57Instrument: 2920 MDSC V2.6A

Exo Down Universal V3.7A TA Instruments


As was mentioned in Chapter 3 - Materials and Methodology, mDSC was not

performed on PDMS because its TGA was not detectable by the mDSC apparatus

used.

These results indicate that the glass transitions of PC and Loctite 3105 are

reasonably close (147.59 oC vs 151.44 oC), indicating that it should be possible to

emboss both at the same temperature, with minimal compromise for selecting the

embossing temperature. If the two temperatures had been substantially different,

then, during embossing one or the other would have undergone either too much

plastic yielding, or would have had too little melt strength to retain any embossed

features.

4.5.2 Curing Properties Figure 4-21 shows the photo-DSC scan for curing of Loctite 3105. Curing

commenced 0.5 minutes after exposure, and the maximum rate of cure was at 0.57

minutes after exposure.


Figure 4-21: Photo-DSC scan of Loctite 3105

This result will likely not be directly applicable to the embossing phase of the project,

since different UV lamps with different intensities will be used, furthermore since the

PDMS is between the UV lamp and the adhesive during the embossing phase, it will

likely absorb a portion of the UV/ozone exposure itself. The experiment indicates

that under certain UV/ozone exposure conditions, curing occurs quite rapidly.

However, due to the limitations of the instrument, it was not possible to run an

experiment that would be directly comparable, so all that can be concluded from the

photo DSC scan is that under certain UV/ozone exposure conditions, curing occurs

quite rapidly.

0.57min

0.50min308.9J/g

-10

0

10

20

30

Heat

Flow

(W/g

)

0 1 2 3 4 5 6 7

Time (min)

Sample: Loctite 3105 no1Size: 11.3000 mgMethod: PCAComment: PCA

DSCFile: D:...\Paul Miller no3.001Operator: LHRun Date: 28-Apr-04 11:05Instrument: DSC Q1000 V7.3 Build 249

Exo Up Universal V3.7A TA Instruments


4.6 Mechanical Testing of PDMS-PC Strength of Adhesion

4.6.1 Testing in Lap Shear Mode Testing the strength of adhesion of PC-PDMS lap joints was performed using UV

pre-treatment time and UV curing time as the variables. Design Expert was used to

help plan the experiments.

Two values for area were used: the design area for the joint, which was 5 mm2; and

the estimated area for the joint, which was determined by examining each sample

after testing and estimating the amount of bonding area. Separate analyses were

performed to determine whether the calculation of joint area would seriously affect

the results.

It should be noted that the exposure times for UV pre-treatment and UV curing

varied between 0.5 and 20 minutes. Therefore, it is not possible to draw conclusions

regarding the behaviour of the system outside those parameters. The full set of

Design Expert parameters is shown in Table 4-7. The last five experiments are

performed at identical parameters to test reproducibility.

Factor 1 Factor 2

UV pretreatment

time (minutes)

UV curing

time (minutes)

3.36 3.36

17.14 3.36

3.36 17.14

17.14 17.14

0.50 10.25

20.00 10.25

10.25 0.50

10.25 20.00

10.25 10.25

10.25 10.25

10.25 10.25

10.25 10.25

10.25 10.25

Table 4-7: Design Expert Matrix for PC to PDMS adhesion


Results for Tensile Failure Stress Figure 4-22 shows how failure stress in PC to PDMS lap joints varies according to

UV treatment parameters. Failure stress generally increases as UV treatment

parameters are increased. The following sections perform a statistical analysis of

the collected data.

Figure 4-22: Tensile failure stress for PC to PDMS lap joints as a function of UV parameters

Tensile Failure Stress for PC to PDMS lap joints as a function of UV parameters

0

5

10

15

20

25

6.7 10.8 10.8 20.5 20.5 20.5 30.3 30.3 34.3

Sum of UV pretreatment time and UV curing time (minutes)

UV E

xpos

ure

Tim

e (m

inut

es)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Failu

re S

tress

(MPa

)UV pretreatment time (minutes) UV curing time (minutes)

Failure Stress in MPa (Area: Individually estimated) Failure Stress in MPa (Area: Single value for all)


Equations for Individually Estimated Area Value A statistical analysis of the data presented above revealed that failure stress could

be represented by the following function:

Failure Stress (MPa)

= 5.78 x 10-3 + 0.0216A + 0.0203B - 1.53 x 10-3 AB Equation 4-1

where A= UV pre-treatment time (minutes)

B= UV curing time (minutes)

When pre-treatment time and curing time are between 0.5 and 20 minutes.

Derivation of Equation 4-1 and solving for dFS/dA=0 and dFS/dB=0 results in optimal

failure strength parameters of;

A or UV PDMS pre-treatment = 13.3 minutes

B or UV adhesive curing = 14.1 minutes

When these values are substituted into Equation 4-1, the maximum failure stress is

293 kPa.

Results for Single Area Value A statistical analysis of the data presented above revealed that failure stress could

be represented by the following function:

Failure Stress (MPa)

= -0.035 + 0.029A + 0.028B - 2.13 x 10-3 AB Equation 4-2

where A = UV pre-treatment time (minutes)

B = UV curing time (minutes)

When pre-treatment time and curing time are between 0.5 and 20 minutes.


Derivation of Equation 4-2 and solving for dFS/dA = 0 and dFS/dB = 0 results in

optimal failure strength parameters of:

A or UV PDMS pre-treatment = 13.2 minutes

B or UV adhesive curing = 13.6 minutes

When these values are substituted into Equation 4-2, the maximum failure stress is

345 kPa.

The maximum failure stresses for both single area values and individually estimated

area values are obtained at very similar pretreatment times and curing times, while

the single area value maximum has a significantly largely maximum failure stress.

This is probably due to underestimation of the bond area.





This result is useful for designing experiments for determining optimal UV

parameters for the composite. However, since the failure mode in the lap shear test

is much simpler than the possible failure modes for the composite during or after

embossing, testing in peel mode was carried out also.

4.6.2 Testing in Peel Mode For testing in peel mode, peel joints were formed as described in Section 3.4.4.

Design Expert was not used and UV/ozone exposure conditions were as follows:

- UV pre-treatments: 0, 1, 2, 3, 8, 18, 25, 35 minutes

- UV curing times: 2, 3, 5, 10, 15, 20 minutes

Figure 4-23 indicates that pre-treatment time is of much greater significance for

failure stress.


Figure 4-23: Influence of UV/ozone pre-treatment times on failure stress of PC-PDMS peel joints

The data show several stages; firstly, for pre-treatment times of less than 2 minutes

the bond strength is so low that the joint fails before testing; secondly, there is a

rapid rise in failure stress for pre-treatments higher than 2 minutes; and thirdly, the

failure stress appears to reach a maximum when the pre-treatment time increases to

approximately 18 minutes.

Figure 4-24 demonstrates the effect of UV curing time on failure stress. At very low

pre-treatment times it does not matter how much time is spent curing the adhesive,

peel strength is still negligible. At lower pre-treatment times, such as 2 minutes, the

maximum failure stress in peel is achieved at 10 minutes curing time. For 3 minutes

pre-treatment time, the maximum failure stress is achieved at a slightly higher curing

time of 15 minutes. The maximum failure stress for 8 minutes pre-treatment time is

also achieved after 15 minutes.

Influence of UV Pre-treatment and Curing times on Failure Stress in PC-PDMS peel joints

0

100

200

300

400

500

600

700

800

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 27.5 30 32.5 35 37.5

UV pre-treatment time (minutes)

failu

re s

tress

(kPa

)

2 minutes UV Cure 3 minutes UV Cure 5 minutes UV cure

10 minutes UV cure 15 minutes UV cure 20 minutes UV cure


At higher pre-treatment times, it does not appear to matter how long the adhesive is

cured for, as long as it is cured for at least 3 minutes.

The maximum failure stress achieved in peel mode is 758 kPa at 18 minutes UV pre-

treatment time and 15 minutes UV curing time; however, 746 kPa was achieved at

18 minutes pre-treatment time and 3 minutes curing time.

Figure 4-24: Influence of UV/ozone curing time on failure stress of PC-PDMS peel joints

Influence of UV Pre-treatment and Curing times on Failure Stress in PC-PDMS peel joints

0

100

200

300

400

500

600

700

800

2 3 5 10 15 20


failu

re s

tress

(kPa

)

0 minutes UV Pretreatment 1 minutes UV Pretreatment 2 minutes UV Pretreatment

3 minutes UV pretreatment 8 minutes UV pretreatment 18 minutes UV pretreatment

25 minute UV pretreatment 35 minutes UV pretreatment


4.7 Conclusions

Figure 4-25: A novel method for the adhesion of PDMS to PC

The adhesive bonds between PC and PDMS need to be able to survive the hot

embossing process and Chapter 4 determined the fabrication processes and

parameters that are required to fulfil this objective. Glass transition temperatures

(important for selecting the embossing temperature), surface chemistry and

mechanical strength of adhesion were all investigated.

Glass Transition Temperatures The glass transition temperatures for PC (147.59 oC) and Loctite 3105 (151.44 oC)

are within several degrees of each other so hot embossing through both should be

feasible.

Surface Chemistry of Delaminated Surfaces It was found that, depending on the UV treatments, surface chemistry of the

delaminated PDMS surface varied between unmodified PDMS surfaces and Loctite

3105 adhesive. This was because the delamination mode was strongly dependant

on the UV treatments. Depending on the UV treatments, the delamination mode

could be adhesive (at the interface between PDMS and Loctite 3105), or cohesive

Start with sheet polycarbonate

UV/ozone exposure of PDMS sheet

Manufacture the composite material by adhesively laminating PDMS to PC

Cure adhesive by exposing to UV/ozone


(failure through the adhesive layer or substrate layer, not the interface). At higher

pre-treatment and curing times (10 minutes or greater for both), the failure was

cohesive through the adhesive.

Mechanical Strength of Adhesive Joints Joint strength was determined in two modes: lap shear and peel. During and after

embossing it is likely that multiple stresses will be acting on the composite, and thus

bond strength in several failure modes was determined.





Peel testing of adhesively bonded PDMS and PC was performed to approximate

stresses that might act to delaminate the composite structure after embossing.

A thermal chamber would have been useful to accurately determine adhesive

strengths at elevated temperatures such as those that would be encountered during

embossing, but none was available.

It was found that when testing in lap shear mode, the maximum failure stress was

between 400 and 500 kPa, while when testing in peel mode the maximum failure

stress was between 700 and 800 kPa.

There appears to be a wide parameter space in which the maximum or close to the

maximum adhesion can be obtained. This parameter space is approximately from 5

to 35 minutes of UV pre-treatment time, and 3 minutes or greater of curing time.

The Next Step This chapter has demonstrated that PC can be bonded to PDMS such that the bond

is stronger than the cohesive strength of the PDMS, and that the glass transition

temperatures of PC and PDMS are similar.


These two facts suggest that the proposed embossing process for composite

materials is feasible, however whether the adhesion will withstand the thermo-

mechanical stresses of the embossing process in unknown. This question is the

subject of Chapters 5 & 6.


5 Replication by Hot Embossing of Semicircular Microstructures

5.1 Introduction The project flowchart that was originally introduced in Chapter 1 is reintroduced here

as Figure 5-1. Chapter 4 focused on the first three stages of the flowchart, on the

manufacturing of the composite materials and the properties of the adhesive joint.

Chapter 5 focuses on the highlighted component of Figure 5-1, the embossing of the

substrate material.

Figure 5-1 Project Flowchart with Chapter 5 components highlighted

The purpose of Chapter 5 is to demonstrate that the proposed embossing process is

feasible for several systems of PC and PDMS across a range of processing

conditions.

These systems include:

- Polycarbonate. The use of polycarbonate is to provide a point of comparison

for embossing into composite structures. Using the polycarbonate data it

should be possible to isolate the affect of the PDMS on the embossed shape.

1

2

3

4

5

6

Polycarbonate sheet







- Dual layer, non bonded laminated composites of PC and PDMS. PC was

embossed through a PDMS layer, after which the PDMS layer was removed

and the embossing in the underlying layer was analysed.

- Dual layer, adhesively bonded laminated composites of PC and PDMS. 5/25

and 20/25 were selected as combinations of UV pre-treatment time

(minutes)/UV curing time (minutes), respectively. These were selected to

represent the adhesion when it was low and high, respectively, but not so low

as to increase the chances of delamination, which would make analysis

extremely difficult.

5.2 Methodology for Hot Embossing

5.2.1 Hot Embossing Parameters Experimental design software, Design Expert, was used for the design of the

experiments and also for the analysis of the results.

Table 5-1 displays the parameter sets that were used to perform the experiments.

The ranges for these parameters were as follows:

- embossing temperature was varied from 140 to 170oC;

- embossing time was varied from 0.3 minutes to 6.7 minutes; and

- embossing substrate pressure was varied between 2.3 and 47 Bars

These parameter ranges were selected based upon preliminary experiments which

indicated embossing feasibility within these general ranges.

The Design-Of-Experiment (DOE) set consisted of 20 experiments, including six at

identical parameters. Identical experiments were performed to assess

reproducibility.


Experiment

Embossing Temperature

(oC)

Embossing Time

(minutes)

Embossing Pressure

(Bar)

1 146.08 1.62 11.40 2 163.92 1.62 11.40 3 146.08 5.42 11.40 4 163.92 5.42 11.40 5 146.08 1.62 37.95 6 163.92 1.62 37.95 7 146.08 5.42 37.95 8 163.92 5.42 37.95 9 140.00 3.52 24.68 10 170.00 3.52 24.68 11 155.00 0.33 24.68 12 155.00 6.71 24.68 13 155.00 3.52 2.35 14 155.00 3.52 47.00 15 155.00 3.52 24.68 16 155.00 3.52 24.68 17 155.00 3.52 24.68 18 155.00 3.52 24.68 19 155.00 3.52 24.68 20 155.00 3.52 24.68

Table 5-1: Parameters for embossing with semicircular tool

5.2.2 Hot Embossing Procedure and Analysis The experimental procedure used for the embossing is listed in the experimental

methodology section (Chapter 3). The sample stack is shown in Figure 5-2.

Figure 5-2: Embossing sample stack

It consists of the metal base plates on top and bottom, embossing padding, PC

substrate and electroplated embossing tool.

Base plates (red) with padding (green)

Electroplated embossing tool

(black)

Substrate to be embossed

(blue)


The sample was left in the sample chamber for five minutes under vacuum to allow

thermal equilibration time to occur; the de-embossing temperature was arbitrarily

selected as 100oC, which is a temperature well below the glass transition

temperature.

Laser confocal microscopy (LCM) was used to measure the dimensions of the

embossed channel. From the LCM data, distances were measured or calculated.

After de-embossing, the samples were gold coated, and then scanned with laser

confocal microscopy to produce a three dimensional surface map. Data points could

be extracted from this surface map to provide the locations of various channel

features.

Figure 5-3 shows the dimensions that could be calculated directly from the LCM

data:

- the width at the top of the channel

- the width of the two halves

- the depth at the middle of the channel

- the depth from each channel shoulder

Figure 5-3: Approximation of channel area (Red) against the actual channel shape (Blue) using Equation 5-1

Depth at MiddleDepth at Left

Shoulder Depth at Right

Shoulder

Width at Top

Width of Left

Section

Width of Right

Section


Channel area was approximated according to Equation 5-1, which based on Figure

5-3. The red section in Figure 5-3 is the approximated area and the blue is the

channel outline.

DepthWidthtioncrosschannelofArea **5.0sec = Equation 5-1

This approach was also used to approximate the area of the left and right halves.

5.2.3 Tool Dimensions Figure 5-4 is a cross sectional height map taken from the laser confocal microscope

image of the hot embossing tool. It can be used to calculate the dimensions of the

hot embossing tool. It shows several distinct features.

One is the tool plate, away from the feature, which is flat and can be measured with

LCM. Another is the tool tip, which is a small region at the very tip of the tool, which

is reasonably flat as well. The laser can be reflected from this section quite well, so

the tool tip can be detected quite easily.

However, the curved surfaces of the tool do not reflect the laser well, so the confocal

microscope detects height at those points as almost nothing. The curved surfaces

can only be inferred from the tool tip and the tool plate.


Figure 5-4: Laser confocal profile of semicircular shim tool (the black line shows the outline of the embossing tool)

Reference dimensions calculated from this chart are shown in Table 5-2. During the

discussion of results in this chapter, there will be numerous mentions of the “control”

or “reference” values. These terms refer to the values in Table 5-2.

Reference Dimension Reference Value

Depth 362.3 µm

Width 700.7 µm

Area 126931.8 µm2

Ratio Width to Depth 1.93

Table 5-2: Reference dimensions and values for semicircular shim

Profile of Semicircular Shim using Laser Confocal Microscopy

0

100

200

300

400

500

600

700

800

900

1000

0 250 500 750 1000 1250 1500 1750 2000 2250 2500

X dimension (µm)

Z d

imen

sio

n (

µm

)

Z dimension

Light is scattered off curved surfaces, such as

the sides of the embossing tool, which is why LCM

gets a negligible reading at these points.

LCM can acquire measurements from flat surfaces such as the tip of the tool and the tool

plate


5.3 Microstructuring of Polycarbonate

5.3.1 Embossing Results Tables 5-3 and 5-4 display the channel dimensions of the embossed polycarbonate.

Table 5-3 shows the real values, whilst Table 5-4 shows the data as a % of the

reference values.

Experiment Number

Channel

width (µm)

Channel depth

(µm)

Cross sectional

Area (µm2)

Ratio width to Depth

1 886.67 260.00 115266.67 3.41

2 806.67 300.00 121000.00 2.69

3 1100.00 296.67 163166.67 3.71

4 616.67 353.33 108944.44 1.75

5 840.00 340.00 142800.00 2.47

6 650.00 340.00 110500.00 1.91

7 943.33 336.67 158794.44 2.80

8 630.00 356.67 112350.00 1.77

9 930.00 203.33 94550.00 4.57

10 616.67 346.67 106888.89 1.78

11 803.33 340.00 136566.67 2.36

12 753.33 333.33 125555.56 2.26

13 1030.00 340.00 175100.00 3.03

14 820.00 276.67 113433.33 2.96

15 770.00 340.00 130900.00 2.26

16 863.33 300.00 129500.00 2.88

17 733.33 333.33 122222.22 2.20

18 706.67 353.33 124844.44 2.00

19 926.67 326.67 151355.56 2.84

20 706.67 330.00 116600.00 2.14

Table 5-3: DOE Data as real responses


Experiment

Number Channel width

(% of reference) Channel depth

(% of reference) Cross sectional

Area (% of reference)

Ratio width to Depth (% of reference)

1 126.54 71.76 90.81 176.70

2 115.13 82.80 95.33 139.32

3 156.99 81.88 128.55 192.12

4 88.01 97.53 85.83 90.43

5 119.88 93.84 112.50 128.01

6 92.77 93.84 87.06 99.06

7 134.63 92.92 125.11 145.18

8 89.91 98.45 88.51 91.52

9 132.73 56.12 74.49 236.98

10 88.01 95.68 84.21 92.17

11 114.65 93.84 107.59 122.42

12 107.51 92.00 98.92 117.10

13 147.00 93.84 137.95 156.96

14 117.03 76.36 89.37 153.57

15 109.89 93.84 103.13 117.34

16 123.21 82.80 102.03 149.11

17 104.66 92.00 96.29 113.99

18 100.85 97.53 98.36 103.63

19 132.25 90.16 119.25 146.98

20 100.85 91.08 91.86 110.95

Table 5-4: DOE Data as % of reference value

Referring to Table 5-4, the range for each channel dimension as a function of the

reference dimensions is as follows:

- channel width: varies from 88.01% to 156.99%, and interval of 68.98%

- channel depth: varies from 56.12% to 98.45%, an interval of 42.33%

- cross sectional area: varies from 74.49% to 137.95%, an interval of 63.46%

- ratio of width to depth: varies from 90.43% to 236.98%, an interval of 146.55%

Of the four dimensions, channel depth has the smallest interval of variation, but is

the only dimension whose interval does not overlap 100%. 100% is the optimal

value, since a value of 100% would indicate that the experimental value is the same

as the reference value.


Channel depth also has the highest reproducibility of the four dimensions. The last

six experiments in the set are performed at identical parameters to assess

reproducibility. For channel depth, one standard deviation, expressed as a

percentage of the average of these results is equal to 5.35%, whilst one standard

deviation expressed in a similar fashion is equal to 11.6% for channel width, 9.29%

for channel area, and 15.71% for channel ratio of width to depth.

Looking more closely at the results, the average channel depth value for all twenty

experiments is 88.42, with a standard deviation of 10.5. All the values except one

fall within 3 standard deviations of the average (99% confidence interval). This

outlier is experiment 9, which returned a value for channel depth of 56.12%. Looking

at the parameters table, Table 5-1, experiment 9 was performed at an embossing

temperature of 140 oC, which is significantly below the glass transition temperature

of 147.59 oC that was recorded in chapter 4.

At a lower temperature, the force required to yield the specimen and the area that

will undergo yielding is greater. For experiment 9, this is why the embossed channel

is both significantly shallower and wider than the reference channel.

The next section uses to DOE to model the results across the embossing parameter

space.

5.3.2 DOE Models and Comparison with Experimental Data Modelling of real values According to statistical evaluation of experimental results using DOE software, the

real values for channel width (CW) and ratio of width to depth (RWD) may be

modelled by the following equations:

EPETTCW *86.3*87.0*08.1376.2926 −+−= Equation 5-2

EPETTRWD *01.0*02.0*07.047.14 −−−= Equation 5-3


where T = Temperature (oC)

ET = Embossing Time (minutes)

EP = Embossing Pressure (Bars)

Modelling of % of reference values According to DOE software, channel width and ratio of width to depth as a % of

reference value may be predicted by the following equations;

EPETTCW *55.0*12.0*87.170.417 −+−= Equation 5-4

EPETTRWD *77.0*26.1*82.376.749 −−−= Equation 5-5

Where T = Temperature (oC)



According to statistical evaluation using DOE software, a first or second order model

cannot describe channel depth or area. Section 5.3.3 (the following section)

attempts to empirically derive models for these dimensions.

Figure 5-5 plots the calculations of Equation 5-4 versus the experimental data for

channel width in column 1 of Table 5-4, while Figure 5-6 shows the calculations of

Equation 5-5 versus the experimental data for ratio of channel width to depth in

column 4 of Table 5-4.

Comparing the two charts, the fit between experimental data and empirical model

model is much better for channel width than for ratio of width to depth. However,

both tend to follow a similar pattern, at least when plotted as a function of experiment

number. From experiment 1, the values decrease to experiment 2, then increase to

increase 3, and then decrease to experiment 4, and so on. This pattern stops after

experiment 14 for both channel width and ratio of width to depth, but this pattern is

most distinctive up to experiment 10.


Figure 5-5: Comparison of experimental data and DOE model for channel width

Figure 5-6: Comparison of experimental data and DOE model for ratio of width to depth

By comparing this pattern with the pattern in the parameters (Table 5-1), it is clear

that through the experiments the results increase and decrease when embossing

Comparison of experimental data with DOE model for channel width

0

20

40

60

80

100

120

140

160

180

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Experiment Number (1-20)

% o

f ref

eren

ce va

lue

Channel w idth (experimental data) as a % of reference value

Channel w idth (DOE model) as a % of reference value

Comparison of experimental data with DOE model for ratio of width to depth

0

25

50

75

100

125

150

175

200

225

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Experiment Number (1-20)

% o

f ref

eren

ce v

alue

Ratio of w idth to depth (experimental data) as a % of reference value

Ratio of w idth to depth (DOE model) as a % of reference value


temperature increases and decreases respectively. Comparing this trend with

Equations 5-4 and 5-5, it is clear that embossing temperature affects the results to a

significantly greater degree than either of the other two parameters, simply because

the value of temperature multiplied by the prefix for the temperature value is much

larger than any possible combination of the other parameters multiplied by their

respective prefixes.

For example, the smallest value for the second term (temperature) in each equation

is 261.8 for channel width and 534.8 for ratio of width to depth, whilst the largest

value for the third term (embossing time) is 0.8 for channel width and 8.5 for ratio of

width to depth.

Plotting channel width and ratio of width to depth against embossing temperature

(Figures 5-7 and 5-8) and comparing the trend line equations in Figures 5-7 and 5-8

against Equations 5-4 and 5-5 confirms that channel width and ratio of width to depth

are mostly dependent on embossing temperature, and if it were necessary,

Equations 5-2 to 5-5 could be simplified to contain only the embossing temperature

terms.

Figure 5-7: Channel width as a function of embossing temperature

Channel width as a function of embossing temperature

y = -1.8672x + 404.55

0

20

40

60

80

100

120

140

160

180

135 140 145 150 155 160 165 170 175 180

Embossing Temperature (oC)

Ch

ann

el W

idth

(%

of

refe

ren

ce) Channel Width


Figure 5-8: Ratio of width to depth as a function of embossing temperature

Referring back to the data in Figures 5-7 and 5-8, it is evident that the DOE predicted

values for channel width fit the data better than does the DOE predicted values for

ratio of width to depth. The total discrepancy between predicted and experimental

values, across all 20 experiments, is 197.91% for channel width, for an average

discrepancy of 9.90% per experiment, versus a total discrepancy of 762.00% for

ratio of width to depth, for a average discrepancy of 38.10% per experiment. The

large size of the errors in ratio of width to depth is perhaps due to the fact that ratio

of width to depth inevitably combines errors from both width and depth.

5.3.3 Empirical Derivations Based on DOE Models The equations for channel width and ratio of width to depth can be used to derive

equations for predicting the response of area and channel depth.

Equations 5-2 and 5-3 can be used to calculate channel cross sectional area and

channel depth, with the use of two other equations:

Ratio of width to depth as a function of embossing temperature

y = -3.8194x + 726.18

0

50

100

150

200

250

135 140 145 150 155 160 165 170 175 180


Cha

nnel

Wid

th (%

of r

efer

ence

)

Ratio of Width to Depth


DepthWidthAArea **5.0)( = Equation 5-6

And

DepthWidthDepthtoWidthofRatio = Equation 5-7

Rearrangeing 5-7 gives;

DepthtoWidthofRatioWidthDepth = Equation 5-8

Inserting equations 5-2 and 5-3 into 5-8 gives:

EPETTEPETTDepth

*01.0*02.0*07.047.14*86.3*87.0*08.1376.2926

−−−−+−

= Equation 5-9

Area can thus be found by substituting Equations 5-9 and 5-2 into Equation 5-6 and 5-7 to give

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

−−−

−+−

⎟⎟⎠

⎞⎜⎜⎝

⎛−+

−=

EPETT

EPETT

EPETT

Area

*01.0*02.0*07.047.14

*86.3*87.0*08.1376.2926

**86.3*87.0

*08.1376.2926*5.0 Equation 5-10

These two equations (5-9 and 5-10), can be used to calculate predictions for channel

depth and channel area, which can then be compared to the LCM data from the

samples.

5.3.4 Comparison of non-DOE Empirical Derivations with Experimental Data

Figure 5-9 plots the predicted channel area from Equation 5-10 with the experimental

channel area values in Table 5-4, while Figure 5-10 plots the predicted channel

depth from Equation 5-9 with the experimental channel depth values in Table 5-4.


Figure 5-9: Predicted and calculated channel area for embossed PC

By comparison with channel width and ratio of width to depth (Figures 5-7 and 5-8),

the predicted values for channel area and channel depth do not rise and fall with the

experimental values as closely as they did for channel width and ratio of width to

depth.

Comparison of experimental data with non-DOE model for channel area

0

20

40

60

80

100

120

140

160


% o

f ref

eren

ce v

alue

Channel area (experimental data) as a % of reference value

Channel area (non-DOE model) as a % of reference value


Figure 5-10: Comparison of experimental and predicted data for channel depth in embossed PC

However, compared to ratio of width to depth, the total discrepancy between

predicted and experimental values is significantly less for channel area and channel

depth than for ratio of width to depth. Total discrepancy across all twenty

experiments is 426.71% for channel depth, for an average discrepancy per

experiment of 21.34%, while for channel area the total discrepancy is 493.82%, for

an average discrepancy of 24.69%.

So while the predicted values may not track the experimental values as closely as

they did for channel width and ratio of width to depth (RWD), the discrepancy

between the predicted values and the experimental values is substantially smaller

than it was for the DOE predicted model for RWD. Discrepancy per experiment for

channel area and channel depth is slightly more than half the discrepancy per

experiment for RWD.

It can be concluded that the derived equations provide a reasonable level of fit to the

data. There are discrepancies in places, but importantly the trends in the DOE

Comparison of experimental data with non-DOE model for channel depth

0

20

40

60

80

100

120


% o

f ref

eren

ce v

alue

Channel depth (experimental data) as a % of reference value

Channel depth (non-DOE model) as a % of reference value


predicted and derived data sets generally match the trends in the experimental data

sets.

5.3.5 Numerical Optimisation of Processing Conditions

DOE statistical analysis allows the prediction of optimum parameters for embossing.

When numerical optimisation is applied to ratio of width to depth (Equation 5-5), the

results are shown in Table 5-5.

Embossing

Temperature (oC)

Embossing

Time (minutes)

Embossing

Pressure (Bars)

Ratio of Width to

Depth as a % of

reference

162.55 2.02 34.06 99.99

163.54 3.12 27.31 100.01

163.75 5.11 23.07 99.98

161.09 4.86 36.60 99.98

162.87 4.50 28.36 100.01

161.98 1.66 37.40 99.99

163.76 2.11 27.92 99.99

163.91 5.23 22.06 100.01

163.85 2.51 26.77 100.03

163.92 5.42 17.43 103.33

Table 5-5: Numerical optimisation for ratio of channel width to depth

The results in Table 5-5 indicate that the required embossing temperature for

achieving an optimal ratio of width to depth is relatively constant compared to the

required embossing pressure and especially the required embossing time.

The standard deviation as a percentage of the average for the ten experiments is

just 0.58% for embossing temperature, against 31.55% for embossing time and

22.56% for pressure. This suggests that for the optimum results, temperature must

be within a very small window, while embossing pressure and especially embossing

time can be quite variable, as long as temperature is within that small interval.


Table 5-6 shows the results when numerical optimisation is applied to the equation

for channel width, Equation 5-4.

Embossing

Temperature (oC)

Embossing

Time (minutes)

Embossing

Pressure (Bars)

Channel Width as a

% of reference

163.52 1.77 22.89 99.99

159.49 2.27 36.65 99.99

159.27 2.73 37.49 99.00

163.84 5.41 22.59 100.00

162.68 3.71 26.14 100.00

159.87 4.9 35.92 100.00

160.23 2.29 34.12 100.01

161.11 2.3 31.14 99.99

159.53 3.54 36.75 100.01

161.61 2.69 29.52 100.01

Table 5-6: Numerical optimisation for channel width

The importance of temperature for channel width is similar to ratio of channel width

to depth. The standard deviation for embossing temperature, as a percentage of the

mean is 1.07%, compared to 38.25% for embossing time and 18.51% for embossing

pressure. So, as for ratio of width to depth, is it most important to have the correct

embossing temperature, since good results can be obtained from a range of

embossing times and pressures, as long as the correct embossing temperature is

maintained.

To analyse total error, the data from the four columns of Table 5-4 were added

together to give Table 5-7.


Table 5-7: The sum of channel width, depth, area and RWD (as a % of reference)

DOE analysis of the data in Table 5-7 indicated that it could be modelled similarly to

channel width and ratio of width to depth, resulting in Equation 5-11.

EPETTerrorTotal *62.1*91.0*56.590.1337 −+−= Equation 5-11

Where T = Temperature (oC)



Figure 5-11 compares the experimental values for total error as a % of reference

(Table 5-7) with the predicted values for total error as a % of reference from

Equation 5-11. Equation 5-11 and Figure 5-11 display the same dependence on

temperature as did channel width and ratio of width to depth.

Experiment Total Error as a Number % of the Reference

1 465.822 432.583 559.544 361.805 454.246 372.727 497.848 368.399 500.32

10 360.0811 438.5112 415.5413 535.7614 436.3315 424.2116 457.1517 406.9518 400.3619 488.6420 394.76


Figure 5-11: Sum of % of reference value for calculated channel width, area and ratio

Numerical optimisation can also be applied to several results simultaneously. When

channel width as a % of reference (optimum value 100%), ratio of channel width to

depth as a % of reference (optimum value 100%), and total error as a % of reference

(optimum value 400%) are optimised simultaneously, the solutions are shown in

Table 5-8.

Comparison of experimental and predicted values for Total % of Reference

0

100

200

300

400

500

600


% o

f ref

eren

ce

Total % of Reference (experimental) Total % of Reference (predicted)


Temperature (oC)

Embossing Time

(minutes)

Embossing Pressure

(Bar)

Total Error (% of

reference)

Channel Width (%

of reference)

Ratio Width to Depth (%

of reference)

163.92 5.42 18.97 399.92 101.85 102.14

163.92 5.39 18.86 400.07 101.91 102.25

163.92 5.28 18.89 399.92 101.88 102.36

163.92 4.93 18.69 399.92 101.94 102.96

163.92 4.69 18.56 399.92 101.99 103.38

161.65 5.42 26.76 399.92 101.80 104.76

163.92 3.76 18.04 399.92 102.16 104.96

160.36 5.42 31.20 399.92 101.77 106.26

163.92 3.58 17.94 399.92 102.19 105.26

162.28 1.62 22.46 399.93 102.52 110.49

Table 5-8: Optimal experimental responses as a function of parameters

Of the ten solutions, the average processing parameters are 163.17 oC for

embossing temperature, 4.55 minutes for embossing time, and 21.04 Bars for

embossing pressure. The standard deviations for these values as a percentage of

the averages are, respectively, 0.79%, 27.23% and 21.37%, again clearly indicating

that the range for embossing temperature is much more restricted than either

embossing temperature or pressure.


Microstructuring of Non-bonded Laminates of PC and PDMS

5.3.6 Embossing Results The experiments in this section replicate the experiments performed in Section 5.3

with one alteration. Embossing was performed through a sheet of PDMS that was

placed between the PC and the embossing tool.

Tables 5-9 and 5-10 show the real values, in micrometres, for the dimensions of the

embossed channels. Due to the more complex nature of the system, with

embossing taking place through a PDMS layer into the PC, additional dimensions

were calculated for this system than for embossing into PC. Additional dimensions

are the channel depth from the left and right shoulder of the embossed channel

(calculated as the difference between the height at the shoulder and the height at the

bottom of the embossed channel).

Also calculated were the width from the left and right shoulder to the middle of the

channel, and the area of the left and right halves of the channel. These calculated

by dividing the channel in half from the point of maximum depth and then calculating

the area using Equation 5-1.


Experiment Number

Channel depth

(µm)

Depth from left

shoulder (µm)

Depth from right

shoulder (µm)

Width at

top (µm)

Cross sectional area

(µm2)

1 287.50 277.00 298.00 1336.00 191997.50

2 339.00 322.00 356.00 1248.00 211094.00

3 279.50 269.00 290.00 1358.00 189581.00

4 324.50 316.00 333.00 1286.00 208398.50

5 259.00 247.00 271.00 1456.00 188576.00

6 342.00 332.00 352.00 1261.00 215426.00

7 225.50 216.00 235.00 1316.00 148065.50

8 325.50 323.00 328.00 1266.00 206004.00

9 251.00 242.00 260.00 1351.00 169303.00

10 340.50 320.00 361.00 1231.00 210244.00

11 322.00 304.00 340.00 1251.00 201906.00

12 308.00 292.00 324.00 1228.00 189336.00

13 296.00 272.00 320.00 1419.00 210456.00

14 303.50 286.00 321.00 1319.00 199134.50

15 286.00 267.00 305.00 1206.00 172648.00

16 286.50 266.00 307.00 1276.00 182479.50

17 297.00 277.00 317.00 1231.00 182493.50

18 300.50 283.00 318.00 1309.00 196091.00

19 280.00 264.00 296.00 1351.00 188732.00

20 313.00 298.00 328.00 1306.00 203684.00

Table 5-9: Results for embossing of PC through PDMS


Experiment Number

Ratio of Width to

Depth

Width from left

shoulder (µm)

Width from right

shoulder (µm)

Area left section

(µm2)

Area right section

(µm2)

1 4.65 673.00 663.00 93210.50 98787.00

2 3.68 650.00 598.00 104650.00 106444.00

3 4.86 698.00 660.00 93881.00 95700.00

4 3.96 673.00 613.00 106334.00 102064.50

5 5.62 726.00 730.00 89661.00 98915.00

6 3.69 651.00 610.00 108066.00 107360.00

7 5.84 691.00 625.00 74628.00 73437.50

8 3.89 648.00 618.00 104652.00 101352.00

9 5.38 703.00 648.00 85063.00 84240.00

10 3.62 583.00 648.00 93280.00 116964.00

11 3.89 631.00 600.00 90896.00 111010.00

12 3.99 600.00 628.00 87600.00 101736.00

13 4.79 691.00 728.00 93976.00 116480.00

14 4.35 718.00 601.00 102674.00 96460.50

15 4.22 593.00 613.00 79165.50 93482.50

16 4.45 653.00 623.00 86849.00 95630.50

17 4.14 631.00 600.00 87393.50 95100.00

18 4.36 688.00 621.00 97352.00 98739.00

19 4.83 701.00 650.00 92532.00 96200.00

20 4.17 700.00 606.00 104300.00 99384.00

Table 5-10: Results for embossing of PC through PDMS


Tables 5-11 and 5-12 show the embossing results as a % of the reference values. Experiment

Number Channel depth (%

of reference)

Depth from left shoulder (% of

reference)

Depth from right shoulder (% of

reference)

Width at top (% of

reference)


(% of reference)

1 79.35 74.18 84.85 190.67 151.03

2 93.57 86.23 101.37 178.11 166.06

3 77.15 72.04 82.57 193.81 149.13

4 89.57 84.63 94.82 183.54 163.93

5 71.49 66.15 77.16 207.80 148.34

6 94.40 88.91 100.23 179.97 169.46

7 62.24 57.85 66.91 187.82 116.47

8 89.84 86.50 93.39 180.68 162.05

9 69.28 64.81 74.03 192.81 133.18

10 93.98 85.70 102.79 175.69 165.39

11 88.88 81.41 96.81 178.54 158.83

12 85.01 78.20 92.26 175.26 148.94

13 81.70 72.84 91.12 202.52 165.55

14 83.77 76.59 91.40 188.25 156.65

15 78.94 71.51 86.85 172.12 135.81

16 79.08 71.24 87.41 182.11 143.55

17 81.98 74.18 90.26 175.69 143.56

18 82.94 75.79 90.55 186.82 154.25

19 77.28 70.70 84.28 192.81 148.46

20 86.39 79.81 93.39 186.39 160.23

Table 5-11: Embossing results for PC through PDMS (% of reference)


Experiment Number

Ratio of Width to Depth (% of

reference)

Width from left shoulder (% of

reference)

Width from right shoulder (% of

reference)

Area left section (%

of reference)

Area right section (%

of reference)

1 240.77 199.81 172.97 135.72 169.03

2 190.75 196.38 177.26 152.37 182.13

3 251.74 200.09 185.53 136.69 163.75

4 205.34 199.24 188.39 154.83 174.64

5 291.28 180.11 171.26 130.55 169.25

6 191.04 171.26 179.25 157.35 183.70

7 302.38 204.94 171.55 108.66 125.66

8 201.52 186.39 177.83 152.38 173.42

9 278.88 166.41 184.96 123.86 144.14

10 187.32 192.10 189.24 135.82 200.13

11 201.30 169.26 174.97 132.35 189.95

12 206.58 184.96 176.40 127.55 174.08

13 248.39 197.24 207.80 136.83 199.30

14 225.18 185.53 170.69 149.50 165.05

15 218.49 185.82 174.12 115.27 159.95

16 230.76 197.24 178.40 126.46 163.63

17 214.76 200.66 184.96 127.25 162.72

18 225.70 192.10 174.97 141.75 168.95

19 250.00 207.23 208.37 134.73 164.60

20 216.19 180.11 171.26 151.86 170.05

Table 5-12: Embossing results for PC through PDMS (% of reference)


Table 5-13 shows the range for each channel dimension from Tables 5-11 and 5-12

as a percentage of the reference dimensions.

Channel Dimension

Maximum (%)

Minimum (%)

Difference Range (%)

channel depth 94.40 62.24 32.16

depth from left shoulder 88.91 57.85 31.07

depth from right shoulder 102.79 66.91 35.88

channel width 207.80 172.12 35.68

channel area 169.46 116.47 52.99

ratio width to depth 302.38 187.32 115.06

width from left shoulder 207.23 166.41 40.82

width from right shoulder 208.37 170.69 37.68

area left section 157.35 108.66 48.69

area right section 200.13 125.66 74.48

Table 5-13: Variation in % of reference values

The variation in the results for embossing through PDMS is less than the variation for

embossing through PC. It is probably the case that the PDMS layer distributes the

force laterally across a larger surface area. This means that a change in embossing

parameter produces a smaller change in channel dimension compared to embossing

in PC, because the embossing energy is transferred to a larger area, and can thus

yield the larger area by a smaller amount.

Also unlike PC embossing, most data ranges do not overlap the reference value.

Only the depth from the right shoulder overlaps the reference. Several others come

close to overlapping the reference (channel depth, channel depth from right

shoulder, channel area and area of left section), while some are not close to the

reference at all (ratio of width to depth, channel width, and width from each

shoulder).

Although the data ranges do not overlap the reference as well, reproducibility is

much better for embossing through PDMS. Table 5-25 shows the standard


deviation of the six reproducibility experiments as a percentage of the average.

These percentages are significantly smaller than they were for embossing in PC,

indicating greater reproducibility for embossing through PDMS into PC.

Channel Dimension Average Standard Deviation

Standard Deviation as % of Average

channel depth 81.10 3.33 4.11 depth from left shoulder 73.87 3.51 4.75

depth from right shoulder 88.79 3.24 3.65 channel width 182.66 7.67 4.20 channel area 147.64 8.67 5.87

ratio width to depth 225.98 13.25 5.86 width from left shoulder 193.86 9.93 5.12

width from right shoulder 182.01 13.74 7.55 area left section 132.89 12.85 9.67

area right section 164.99 3.84 2.33 Table 5-14: Average, Standard deviation and standard deviation as a percentage of average

Greater reproducibility is potentially another result of the distribution of force across a

larger surface area. Transferring the embossing force to a larger surface area meant

deforming the larger area to a smaller extent.

5.3.7 DOE Models and Comparison with Experimental Data DOE statistical analysis of the data in Tables 5-9 to 5-12 indicated that a linear

model could describe the following parameters:

- channel depth (CD)

- channel depth from left shoulder (CDLS)

- channel depth from right shoulder (CDRS)

- channel cross sectional area (CSA)

- ratio of width to depth (RWD)


Modelling of real values

EPETTCD *36.0*71.3*53.3226 −−+−= Equation 5-12

EPETTCDLS *23.0*86.2*41.381.228 −−+−= Equation 5-13

EPETTCDRS *49.0*55.4*65.391.224 −−+−= Equation 5-14

EPETTCSA *2.342*02.2941*63.157270.31679 −−+−= Equation 5-15

EPETTRWD *006.0*04.0*07.021.15 ++−= Equation 5-16

Modelling of % of reference values

EPETTCD *10.0*02.1*97.062.62 −−+−= Equation 5-17

EPETTCDLS *062.0*77.0*91.028.61 −−+−= Equation 5-18

EPETTCDRS *14.0*30.1*04.104.64 −−+−= Equation 5-19

EPETTCSA *27.0*31.2*24.192.24 −−+= Equation 5-20

EPETTRWD *32.0*16.2*71.389.787 ++−= Equation 5-21

where CD = channel depth

CSA= channel cross sectional area

RWD = ratio of width to depth

T = temperature (oC)

ET = embossing time (minutes)

EP = embossing pressure (Bars)


Figure 5-12: Comparison of experimental and DOE predicted channel depth for PC embossed through PDMS

The predicted DOE models in Figures 5-12, 5-13, 5-14 and 5-15 all demonstrate a

good degree of fit with the experimental data, better than for embossing in PC.

Average discrepancies per experiment between data and model are significantly less

for PC through PDMS, with just 3.07 % for channel depth, 6.4% for channel area,

and 13.7% for ratio of width to depth. Given the higher reproducibility for the data

from embossing PC through PDMS, it is perhaps not surprising that the DOE models

show a higher degree of fit with the experimental data as well.

Comparison of experimental data with DOE model for channel depth

40

50

60

70

80

90

100

110

120


% o

f ref

eren

ce v

alue

Channel Depth (experimental data) as a % of referenceChannel Depth (DOE model) as a % of reference


Figure 5-13: Comparison of experimental and DOE predicted channel depth at the shoulders for PC embossed through PDMS

Figure 5-14: Comparison of experimental and DOE predicted channel area for PC embossed through PDMS

Comparison of experimental data with DOE models for channel depth at left shoulder and right shoulder

40

50

60

70

80

90

100

110

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Experiment number (1-20)

% o

f ref

eren

ce v

alue

Channel depth (experimental data) as a % of reference valueChannel depth at left shoulder (DOE model) as a % of referenceChannel depth at right shoulder (DOE model) as a % of reference value

Comparison of experimental data with DOE model for cross sectional area

100

120

140

160

180

200

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20Experiment number (1-20)

% o

f ref

eren

ce v

alue

Cross sectional area (experimental data) as a % of reference value Cross sectional area (DOE model) as a % of reference


Figure 5-15: Comparison of experimental and DOE predicted width to depth ratios for PC embossed through PDMS

Similar to embossing in PC, the data patterns in Figures 5-12 to 5-15 suggest that

temperature is again the dominant parameter, and this will be discussed in more

detail later in Section 5.4.

5.3.8 Empirical Derivations Based on DOE Models

DOE could not describe channel width with a first or second order equation.

However, using equations 5-22 and 5-23 and rearranging, expressions for channel

width could be derived:

( )DepthDepthtoWidthofRatioWidth *⎟⎠

⎞⎜⎝

⎛= Equation 5-22

DepthWidthAArea **5.0)( = Equation 5-23

Substituting equations 5-12 and 5-16 into equation 5-22 gives

Comparison of experimental data with DOE model for channel width-to-depth ratio

150170190210230250270290310330350

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


% o

f ref

eren

ce v

alue

Channel w idth to depth ratio (experimental data) as a % of referenceChannel w idth to depth ratio (DOE model) as a % of reference


⎟⎟⎠

⎞⎜⎜⎝

⎛−−

+−⎟⎟⎠

⎞⎜⎜⎝

⎛++

−=

EPETT

EPETT

CW*36.0*71.3

*53.386.226*

*006.0*04.0*07.021.15

Equation 5-24

5.3.9 Comparison of non-DOE Empirical Derivations with Experimental Data

Figure 5-16 plots equation 5-24 against the experimental results recorded in Table 5-

10. A reasonable degree of fit is obtained, particularly for the first ten experiments.

Figure 5-16: Comparison of experimental and derived channel width for PC embossed through PDMS

The average discrepancy per experiment between the experimental data and the

values predicted by the derived model is 11.84 %, which compares favourably with

the model predicted for ratio of width to depth (13.7 %).

Comparison of experimental data with non-DOE model for channel width

150

160

170

180

190

200

210

220

230

240

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


% o

f ref

eren

ce v

alue

Channel w idth (experimental data) as a % of reference

Channel w idth (non-DOE model) as a % of reference


5.3.10 Numerical Optimisation of Processing Conditions

Using DOE to optimise the embossing parameters for channel depth produced eight

solutions (Table 5-15).


Embossing Time (minutes)

Embossing Pressure (Bars)

Channel Depth (% of reference)

163.92 1.62 17.25 93.72

163.92 1.72 16.4 93.70

163.92 2.13 12.86 93.64

163.92 2.21 12.23 93.62

163.92 2.25 11.83 93.62

163.92 2.56 11.4 93.34

163.92 1.62 22.65 93.18

163.92 1.62 30.48 92.39

Table 5-15: Numerical solutions for optimisation of channel depth

However, when channel depth, cross sectional area and ratio of width to depth are

optimised simultaneously (channel width cannot be optimised because DOE could

not model it), the five solutions are shown in Table 5-16.


(oC)

Embossing Time

(minutes)

EmbossingPressure

(Bars)

Channel Depth (%

of reference)

Cross sectional area (% of reference)

Ratio of width to

depth (% of reference)

162.59 5.42 37.95 86.46 153.47 209.16

162.71 5.42 37.95 86.58 153.61 208.73

162.26 5.42 37.95 86.14 153.06 210.40

163.00 5.42 37.95 86.86 153.98 207.65

161.73 5.42 37.95 85.62 152.40 212.36

Table 5-16: Numerical solutions for simultaneous optimisation of channel depth, cross sectional area and ratio of width to depth


Three observations can be made when comparing the numerical solutions for

channel depth by itself and the numerical solutions for when depth, area and RWD

are optimised simultaneously.

First, the embossing temperatures are very similar compared to the embossing

temperatures in the numerical simulations for PC. Second, embossing time/pressure

solutions for channel depth by itself are substantially different to the time/pressure

solutions for when depth, area and RWD are optimised simultaneously. Third, the

numerical solutions for channel depth are significantly closer to 100% when

optimised by itself.


5.4 Microstructuring of Adhesively Bonded Laminates of PC and PDMS: 5 minutes UV Pretreatment Time and 25 minutes UV Curing Time

5.4.1 Results from Embossing Tables 5-17 and 5-18 present the real values of the results from embossing of

adhesively laminated composites fabricated with 5 minutes UV pre-treatment and 25

minutes UV curing.

Experiment

Number Channel

depth

(µm)

Depth from left

shoulder (µm)

Depth from right

shoulder (µm)

Width at

top (µm)


(µm2)

1 252.32 254.91 249.74 1702.83 214804.96

2 357.22 341.61 372.84 1367.24 244112.86

3 293.39 282.12 304.66 1555.50 228298.95

4 94.38 83.16 105.61 1364.60 64379.54

5 295.44 292.92 297.96 1532.78 226305.69

6 387.17 383.29 391.06 1226.70 237452.30

7 303.79 296.33 311.26 1406.53 213475.60

8 387.52 373.50 401.54 1329.66 258003.62

9 158.65 164.70 152.60 1036.90 82541.82

10 365.22 356.65 373.80 1321.96 242610.38

11 340.36 337.68 343.03 1467.70 249712.83

12 353.90 343.02 364.78 1340.45 237085.23

13 275.64 271.00 280.28 1698.77 234094.70

14 307.85 298.30 317.40 1460.52 224935.70

15 330.49 316.97 344.02 1390.01 229728.87

16 315.45 306.13 324.78 1445.18 228162.42

17 360.31 358.32 362.30 1351.60 243519.44

18 337.61 322.93 352.29 1503.97 255237.08

19 332.03 323.10 340.96 1397.66 231980.15

20 324.31 316.54 332.08 1490.60 241577.28

Table 5-17: Results for embossing of 5 minutes pretreatment/25 minutes curing composite material (real values)


Experiment Number

Ratio of Width to

Depth

Width from left

shoulder (µm)

Width from right

shoulder (µm)

Area left section

(µm2)

Area right section

(µm2)

1 6.75 840.75 862.08 107156.32 107648.63

2 3.83 689.56 677.68 117779.65 126333.21

3 5.30 767.60 787.91 108277.67 120021.27

4 14.46 683.82 680.78 28432.22 35947.32

5 5.19 812.12 720.65 118944.10 107361.60

6 3.17 618.61 608.10 118551.71 118900.59

7 4.63 726.30 680.23 107611.75 105863.85

8 3.43 638.59 691.07 119258.73 138744.89

9 6.54 566.37 470.53 46640.57 35901.25

10 3.62 520.37 801.59 92794.72 149815.66

11 4.31 755.37 712.34 127537.71 122175.12

12 3.79 680.05 660.41 116633.95 120451.27

13 6.16 856.35 842.42 116037.73 118056.96

14 4.74 717.31 743.21 106986.70 117949.00

15 4.21 692.47 697.54 109746.38 119982.49

16 4.58 698.99 746.19 106989.25 121173.17

17 3.75 664.43 687.17 119038.14 124481.29

18 4.45 659.51 844.46 106489.60 148747.48

19 4.21 704.99 692.67 113892.61 118087.54

20 4.60 762.08 728.53 120612.36 120964.93

Table 5-18: Results for embossing of 5 minutes pretreatment/ 25 minutes curing composite material (real values)


Tables 5-19 and 5-20 present the results from embossing of adhesively laminated

composites fabricated with 5 minutes UV pre-treatment and 25 minutes UV curing

as a % of reference values

Experiment


of reference)


reference)


reference)

Width at top (% of

reference)


(% of reference)

1 69.65 68.27 71.11 243.02 168.97

2 98.60 91.49 106.16 195.13 192.03

3 80.98 75.55 86.75 222.00 179.59

4 26.05 22.27 30.07 194.75 50.64

5 81.55 78.45 84.84 218.76 178.02

6 106.87 102.65 111.35 175.07 186.79

7 83.85 79.36 88.63 200.74 167.93

8 106.96 100.03 114.33 189.77 202.96

9 43.79 44.11 43.45 147.98 64.93

10 100.81 95.51 106.43 188.67 190.85

11 93.94 90.43 97.67 209.47 196.43

12 97.68 91.86 103.87 191.31 186.50

13 76.08 72.58 79.81 242.45 184.15

14 84.97 79.89 90.38 208.44 176.94

15 91.22 84.89 97.95 198.38 180.71

16 87.07 81.98 92.48 206.25 179.48

17 99.45 95.96 103.16 192.90 191.56

18 93.19 86.48 100.31 214.64 200.78

19 91.65 86.53 97.08 199.47 182.49

20 89.51 84.77 94.56 212.74 190.03

Table 5-19: Results of embossing of 5 minutes pretreatment/25 minutes curing composite material (% of reference)


Experiment Number

Ratio of width to depth (%

of reference)


reference)


reference)


of reference)

Area right section (%

of reference)

1 348.95 228.55 259.02 156.02 184.19

2 197.90 187.45 203.62 171.49 216.16

3 274.14 208.67 236.74 157.66 205.36

4 747.60 185.89 204.55 41.40 61.51

5 268.26 220.77 216.53 173.19 183.70

6 163.83 168.16 182.71 172.62 203.45

7 239.40 197.44 204.38 156.69 181.14

8 177.42 173.60 207.64 173.65 237.40

9 337.95 153.96 141.38 67.91 61.43

10 187.16 141.46 240.85 135.11 256.34

11 222.97 205.34 214.03 185.70 209.05

12 195.85 184.87 198.43 169.82 206.10

13 318.67 232.79 253.12 168.96 202.00

14 245.31 194.99 223.31 155.78 201.82

15 217.47 188.24 209.59 159.80 205.30

16 236.89 190.01 224.20 155.78 207.33

17 193.96 180.62 206.47 173.32 213.00

18 230.34 179.28 253.73 155.05 254.52

19 217.66 191.65 208.12 165.83 202.06

20 237.66 207.16 218.90 175.62 206.98

Table 5-20: Results of embossing of 5 minutes pretreatment / 25 minutes curing composite material (% of reference)

The data intervals for each dimension are in Table 5-21. Of all the dimensions, the

following have intervals that overlap the reference (100%):

- channel depth

- channel depth at left shoulder

- channel depth at right shoulder

- channel area

- channel area of left section

- channel area of right section


Channel Dimension

Maximum (%)

Minimum (%)






area right section 256.34 61.43 194.91 Table 5-21: Data range for each dimension as a % of reference

A summary of the reproducibility experiments for the embossing is presented in

Table 5-22. Taking standard deviation as a percentage of the average as a measure

of reproducibility, then the reproducibility of this data set is approximately as good as

the reproducibility of the embossing through PDMS into PC data set (Table 5-23).


Channel Dimension Average Standard deviation

Standard deviation as % of average





area right section 214.86 19.75 9.19 Table 5-22: Analysis of reproducibility experiments for embossing of 5 minutes pretreatment/25 minutes curing composite materials

Channel Dimension


PC embossed

through PDMS


5 minutes pre-treatment/25

minutes curing time

channel depth 4.11 4.56 depth from left shoulder 4.75 5.53

depth from right shoulder 3.65 3.95 channel width 4.20 4.22 channel area 5.87 4.36

ratio width to depth 5.86 7.42 width from left shoulder 5.12 5.29

width from right shoulder 7.55 8.10 area left section 9.67 5.38

area right section 2.33 9.19 Table 5-23: Comparison of Standard deviation as a % of average for PC embossed through PDMS, and embossed composite 5 minutes pretreatment/25 minutes curing time


5.4.2 DOE Analysis DOE did not identify any first or second order trends in the embossing data for the

5/25 composite (5 minutes pre-treatment time, 25 minutes curing time). This could

be attributed to insufficient UV pre-treatment of the PDMS, resulting in poor

adhesion. As was demonstrated in Figure 4-22: Influence of UV/ozone pretreatment

time on failure stress of PC-PDMS peel joints, the joint strength after 5 minutes UV

pre-treatment is approximately half the maximum that was obtained. This poor

adhesion may have produced delamination of the structures during embossing.

However, due to the strong reproducibility of the replicate experiments, it seems

unlikely that delamination was taking place at the replicate embossing conditions. If

it were, a greater degree of scatter would be expected in the results.

As will be seen in the next section, when the UV pre-treatment of the PDMS was

increased to 20 minutes, much better results were obtained from DOE.

5.5 Microstructuring of Adhesively Bonded Laminates of PC and PDMS: 20 minutes UV Pretreatment Time and 25 minutes UV Curing Time

5.5.1 Results from Embossing Tables 5-24 and 5-25 show the real values for results of embossing of adhesively

laminated materials manufactured using 20 minutes UV pre-treatment and 25

minutes UV curing.


Experiment Number

Channel

depth (µm)

Depth from left shoulder

(µm)

Depth from right

shoulder (µm)

Width at

top (µm)

Cross sectional

area (µm2)

1 324.13 312.81 335.45 1399.95 227281.60

2 364.85 350.24 379.45 1312.34 239772.40

3 311.59 304.61 318.58 1436.86 224199.32

4 339.83 328.79 350.88 1330.29 226132.75

5 277.84 273.17 282.52 1531.01 212708.33

6 358.42 353.73 363.11 1345.53 241257.05

7 283.00 279.20 286.79 1547.15 219055.58

8 385.62 388.53 382.71 1341.99 258727.18

9 264.71 255.09 274.34 1435.65 190107.43

10 368.87 371.57 366.17 1350.15 249083.91

11 339.22 326.38 352.06 1357.17 230583.27

12 337.02 339.60 334.45 1363.99 229794.74

13 327.02 315.52 338.52 1545.61 253278.25

14 344.47 347.00 341.94 1383.26 238186.57

15 322.64 329.73 315.54 1407.96 227061.38

16 323.59 309.04 338.13 1400.66 226731.85

17 333.82 338.64 329.00 1356.44 226315.02

18 324.30 309.43 339.17 1379.82 223453.02

19 323.18 325.03 321.32 1419.04 229293.72

20 347.38 335.12 359.65 1390.29 241545.09

Table 5-24: Results for embossing of 20 minutes pre-treatment/25 minutes curing composite material (real values)


Experiment Number

Ratio of Width to

Depth

Width from left shoulder

(µm)

Width from right shoulder

(µm)

Area left

section (µm2)

Area right

section (µm2)

1 4.32 664.79 735.16 103977.45 123304.15

2 3.60 630.86 681.48 110478.02 129294.39

3 4.61 669.55 767.30 101977.50 122221.83

4 3.91 656.72 673.57 107962.65 118170.10

5 5.51 761.64 769.37 104028.22 108680.11

6 3.75 646.38 699.15 114321.32 126935.73

7 5.47 737.99 809.17 103023.24 116032.35

8 3.48 664.42 677.57 129073.27 129653.91

9 5.42 708.60 727.05 90379.72 99727.71

10 3.66 700.59 649.56 130160.65 118923.26

11 4.00 648.04 709.13 105754.59 124828.68

12 4.05 660.88 703.11 112218.31 117576.43

13 4.73 724.42 821.19 114282.83 138995.42

14 4.02 669.07 714.19 116082.49 122104.08

15 4.36 694.45 713.51 114491.24 112570.14

16 4.33 692.40 708.26 106989.21 119742.64

17 4.06 659.75 696.69 111711.00 114604.02

18 4.25 708.97 670.85 109688.36 113764.65

19 4.39 705.32 713.72 114625.33 114668.39

20 4.00 689.95 700.34 115607.50 125937.59

Table 5-25: Results for embossing of 20 minutes pre-treatment/ 25 minutes curing composite material (real values)


Tables 5-26 and 5-27 show the % of reference values for results of embossing of

adhesively laminated materials manufactured using 20 minutes UV pre-treatment

and 25 minutes UV curing.

Experiment


of reference)


reference)


reference)

Width at top (% of

reference)


(% of reference)

1 89.46 83.77 95.52 199.80 178.79

2 100.70 93.80 108.04 187.30 188.62

3 86.00 81.58 90.71 205.07 176.36

4 93.80 88.05 99.91 189.86 177.89

5 76.69 73.16 80.44 218.50 167.33

6 98.93 94.73 103.39 192.03 189.78

7 78.11 74.77 81.66 220.81 172.32

8 106.44 104.05 108.97 191.53 203.53

9 73.07 68.32 78.11 204.89 149.55

10 101.81 99.51 104.26 192.69 195.94

11 93.63 87.41 100.24 193.69 181.39

12 93.02 90.95 95.23 194.67 180.77

13 90.26 84.50 96.39 220.59 199.24

14 95.08 92.93 97.36 197.42 187.37

15 89.05 88.31 89.85 200.94 178.62

16 89.31 82.76 96.28 199.90 178.36

17 92.14 90.69 93.68 193.59 178.03

18 89.51 82.87 96.57 196.93 175.78

19 89.20 87.05 91.49 202.52 180.37

20 95.88 89.75 102.41 198.42 190.01

Table 5-26: Results of embossing of 20 minutes pre-treatment/25 minutes curing composite material (% of reference)


Experiment

Number Ratio of width to

depth (% of reference)


reference)


reference)


of reference)

Area right section (% of

reference)

1 223.33 180.72 220.89 151.40 210.98

2 185.99 171.50 204.76 160.86 221.23

3 238.44 182.01 230.55 148.48 209.13

4 202.41 178.52 202.38 157.20 202.20

5 284.92 207.05 231.17 151.47 185.96

6 194.11 175.71 210.07 166.46 217.20

7 282.68 200.62 243.12 150.01 198.54

8 179.95 180.62 203.58 187.94 221.85

9 280.43 192.63 218.45 131.60 170.64

10 189.26 190.45 195.17 189.52 203.49

11 206.87 176.17 213.07 153.98 213.59

12 209.27 179.66 211.26 163.39 201.18

13 244.38 196.93 246.74 166.40 237.83

14 207.64 181.88 214.59 169.02 208.93

15 225.64 188.78 214.38 166.70 192.61

16 223.82 188.22 212.81 155.78 204.89

17 210.11 179.35 209.33 162.66 196.09

18 220.00 192.73 201.56 159.71 194.66

19 227.04 191.74 214.45 166.90 196.20

20 206.94 187.56 210.43 168.33 215.49

Table 5-27: Results of embossing of 20 minutes pre-treatment / 25 minutes curing composite material (% of reference)


Table 5-28 shows the interval ranges for the channel dimensions. The only

dimensions that overlap the reference (100%) are:

- channel depth

- depth from left shoulder

- depth from right shoulder

If these three dimensions can be modelled, then they can potentially be numerically

optimised to provide solutions for achieving values that are 100% of reference.

Channel Dimension

Maximum (%)

Minimum (%)






area right section 237.83 170.64 67.19 Table 5-28: Data ranges for embossed composite (20 minutes pre-treatment time/25 minutes curing time)

Analysis of the embossing replicates indicates that the reproducibility of the

embossing results in the 20/25 material (20 minutes pre-treatment time and 25

minutes curing time) are better than the reproducibility for either PC embossed

through PDMS or for the 5/25 composite material.


Channel Dimension Average Standard Deviation

Standard Deviation as % of Average





area right section 199.99 8.68 4.34 Table 5-29: Analysis of replicate experiments for embossing in composite material (20 minutes pre-treatment time/25 minutes curing time)

5.5.2 DOE Models and Comparison with Experimental Data

DOE statistical analysis of the data in Tables 5-24 to 5-27 indicated that a first or

second order model could describe the following parameters;

- channel depth (CD)

- channel depth from left shoulder (CDLS)

- channel depth from right shoulder (CDRS)

- ratio of width to depth (RWD)

- channel cross sectional area (CSA)

- area of left section (ALS)

- area of right section (ARS)

Equations 5-25 to 5-31 model the real values from Tables 5-24 and 5-25, while

equations 5-32 to 5-38 model the % of reference values from Tables 5-26 and 5-27.


Real Values

EPETEPTETTEPETTCD

**35.0**12.0**07.0*95.19*86.19*28.033.318

+++−−+=

Equation 5-25

EPETEPTETTEPETTCDLS

**35.0**13.0**11.0*94.21*11.25*07.074.354

+++−−−=

Equation 5-26

EPETEPTETTEPETTCDRS

**34.0**11.0**03.0*96.17*61.14*64.094.281

+++−−+=

Equation 5-27

EPETEPTETTEPETTRWD**004.0**002.0**001.0

*40.0*36.0*0008.090.3−−−

++−= Equation 5-28

EPETEPTETTEPETT

EPETTCSA

**25.201**79.56**18.4*40.32*53.59*39.44

*25.11171*84.5809*30.1383781057222

+++++−

−−+−=

Equation 5-29

EPETTALS *160*03.776*13.95015.43118 +++−= Equation 5-30

EPETEPTETTEPETT

EPETTARS

**59.110**61.31**44.108*88.28*0.496*36.30

*77.6934*97.10033*34.9554594075222

++−++−

−++−=

Equation 5-31

% of Reference Values

EPETEPTETTEPETTCD**1.0**03.0**02.0

*51.5*48.5*08.086.87+++

−−+= Equation 5-32

EPETEPTETTEPETTCDLS**09.0**04.0**03.0

*87.5*73.6*02.095+++

−−−= Equation 5-33

EPETEPTETTEPETTCDRS

**1.0**03.0**007.0*11.5*16.4*18.027.80

+++−−+=

Equation 5-34

EPETEPTETTEPETTRWD

**24.0**13.0**08.0*67.20*77.18*04.054.201

−−−++−=

Equation 5-35


EPETEPTETTEPETT

EPETTCSA

**16.0**04.0**003.0*03.0*05.003.0

*78.8*57.4*88.1063.637222

+++++−

−−+−=

Equation 5-36

EPETTALS *23.0*13.1*38.178.62 +++−= Equation 5-37

EPETEPTETTEPETT

EPETTARS

**19.0**05.0**19.0*05.0*85.0*052.0

*87.11*17.17*35.161016222

++−++−

−++−=

Equation 5-38

where CW = channel width

CD = channel depth

CSA = channel cross-sectional area


CWLS = channel width left section

CWRS = channel width right section

CDLS = channel depth left section

CDRS = channel depth right section

ALS = cross-sectional area (left section)

ARS = cross-sectional area (right section)

The DOE predicted values (Equations 5-32 to 5-38) as a % of reference are plotted

against the experimental values as a % of reference in Figures 5-17 to 5-22.

The comparisons in Figures 5-17 and 5-22 suggest that embossing temperature is

again the key parameter for the results. To demonstrate this point, embossing

temperature was plotted on Figure 5-17 against channel depth. The results rise and

fall as embossing temperature rises and falls during experiments 1-10, and then

becomes approximately steady when embossing temperature is steady during

experiments 11-20.

This trend is repeated for channel depth at the shoulders (Figure 5-18), ratio of width

to depth (Figure 5-19), channel area (Figure 5-20), area of left section (Figure 5-21),

and to a lesser extent, area of right section (Figure 5-22).


Figure 5-17: Comparison of experimental and DOE predicted channel depth

Figure 5-18: Comparison of experimental and DOE predicted depth at channel shoulders

Comparison of experimental data with DOE model for channel depth

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


% o

f ref

eren

ce v

alue

Channel depth (experimental data) as a % of reference

Channel depth (DOE model) as a % of reference

Comparison of experimental data with DOE models for channel depth at shoulders

0

20

40

60

80

100

120

140

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


% o

f ref

eren

ce v

alue

Channel depth (experimental data) as a % of referenceChannel depth at left shoulder (DOE model) as a % of referenceChannel depth at right shoulder (DOE model) as a % of reference


Figure 5-19: Comparison of experimental and DOE predicted channel ratio of width to depth

Figure 5-20: Comparison of experimental and DOE predicted channel area

Comparison of experimental data and DOE model for ratio channel width to depth

0

50

100

150

200

250

300

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


% o

f ref

eren

ce v

alue

Ratio of channel width to depth (experimental data) as a % of referenceRatio of channel width to depth (DOE model) as a % of reference

Comparison of experimental data and DOE model for cross sectional area

100

120

140

160

180

200

220

240

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


% o

f ref

eren

ce v

alue

Cross sectional area (experimental data) as a % of reference

Cross sectional area (DOE model) as a % of reference


Figure 5-21: Comparison of experimental and DOE predicted area of left section

Figure 5-22: Comparison of experimental and DOE predicted area of right section

Comparison of experimental data and DOE model for cross sectional area of left section

100

120

140

160

180

200

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


% o

f ref

eren

ce v

alue

Area of left section (experimental data) as % of reference

Area of left section (DOE model) as a % of reference

Comparison of experimental data and DOE model for cross sectional area of right section

150

170

190

210

230

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


% o

f ref

eren

ce v

alue

Area of right section (experimental data) as a % of reference

Area of right section (DOE model) as a % of reference


5.5.3 Empirical Derivations Based Upon DOE Models

DOE could not find first or second order models to fit the data for channel width, or

the width of the left half of the channel, or the width of the right half of the channel.

However, relationships for these dimensions may be derived using the following

equations, which are based upon Equation 5-6.

CDCSACW

*5.0= Equation 5-39

CDLSALS

CWLS*5.0

= Equation 5-40

CDRSARS

CWRS*5.0

= Equation 5-41

where CW = channel width

CD = channel depth

CSA = channel cross-sectional area


CWLS = channel width left section

CWRS = channel width right section

ALS = cross-sectional area (left section)

ARS = cross-sectional area (right section)

CDLS = channel depth left section

CDRS = channel depth right section

Substituting equations 5-25 to 5-31 into equations 5-39 to 5-41 gives the following

equations:


⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜

⎝

⎛

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

+++−

−+

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

++++

+

+

=

EPETEPTETTEP

ETTEPETEPT

ETTEPET

CW

**35.0**12.0**07.0*95.19

*86.19*28.033.318*5.0

**25.201**79.56**18.4*40.32

*53.59T*44.39-EP*11171.25-ET*5809.84-T*13837.3081057-

2

22

Equation 5-42

( )

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

+++−

−−+++−

=

EPETEPTETTEP

ETTEPETTCWLS

**35.0**13.0**11.0*94.21

*11.25*07.074.354*5.0

*160*03.776*13.95015.43118 Equation 5-43

⎟⎟⎠

⎞⎜⎜⎝

⎛+++−−−

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

++−+

+−−

++−

=

EPETEPTETTEPETT

EPETEPTETTEP

ETTEPETT

CWRS

**34.0**11.0**03.0*96.17*61.14*64.094.281

*5.0

**59.110**61.31**44.108*88.28

*0.496*36.30*77.6934*97.10033*34.9554594075

2

22

Equation 5-44

5.5.4 Comparison of Empirical Derivations with Experimental Data

Figure 5-23 compares the data for the two derivations for channel width (Equation 5-

42 and 5-43) with the experimental data collected from the embossed structures. It

can be seen that the predictions of Equation 5-42 are a close match for the

experimental data, while the predictions of Equation 5-43 are consistently higher

than the experimental values. This is likely due to asymmetry between the right and

left hand sections of the embossed channel.


Figure 5-23: Comparison of experimental channel width with channel width derived from DOE equations

Figure 5-24 compares the derived width of the left half of the channel with half the

measured channel width. It can be seen that the predicted channel half width is

consistently greater than half the measured channel width, whilst in Figure 5-25 the

derived width of the right half of the channel is less than the experimental channel

half-width for a significant number of the experiments.

However for both Figure 5-24 and 5-25, the derived predictions obtain a reasonable

match for the experimental data. The two data sets generally track each other quite

well across the set of experiments.

Comparison of experimental data and non-DOE model for channel width

150

160

170

180

190

200

210

220

230

240

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


% o

f ref

eren

ce v

alue

Channel w idth at top (experimental data) as a % of reference w idth

Channel w idth at top (non-DOE model) as a % of reference w idth


Figure 5-24: Comparison of experimental channel width with DOE derived width of channel halves

Figure 5-25: Comparison of experimental channel width with DOE derived width of channel half

Comparison of experimental data with non-DOE model for width of channel left section

150

170

190

210

230

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20


% o

f ref

eren

ce v

alue

Half channel w idth (experimental data) as a % of referenceWidth of left section (non-DOE model) as a % of reference

Comparison of experimental data with non-DOE model for width of channel right section

150

170

190

210

230

250


% o

f ref

eren

ce v

alue

Half channel w idth (experimental data) as a % of referenceWidth of right section (non-DOE model) as a % of reference


5.5.5 Optimisation of Processing Conditions

Of the dimensions analysed, the only data intervals that overlapped the reference

points was channel depth at the left shoulder, middle and right shoulder. These

dimensions could also be modelled using a first or second order equation, so it is

possible to use numerical optimisation to solve for parameter solutions that give

100% of reference results.

DOE solutions for channel depth are shown in Table 5-30. Similar to numerical

solutions from previous systems, the embossing temperature for all the solutions is

in the range 160-164oC, while the variation in embossing time and pressure is

considerably higher.




Channel Depth (% of reference)

163.79 5.05 25.30 100.00

163.62 3.65 26.53 100.00

163.84 1.83 28.46 100.00

163.76 1.75 29.57 100.01

163.91 1.79 28.04 100.01

163.60 2.57 28.06 99.99

162.27 5.18 29.23 100.00

162.05 3.18 35.41 100.00

161.00 4.82 34.39 100.00

163.69 3.87 26.11 100.00

Table 5-30: Numerical solutions for channel depth at middle

The DOE solutions for optimisation of channel depth at left shoulder (Table 5-31) are

very similar to those for optimisation of channel depth at middle, with the small

change that the solutions for embossing pressure are slightly higher. Embossing

temperature solutions are still in the 160-164 oC range, while embossing time still

has a large degree of variation.





Channel Depth at left shoulder (% of reference)

163.87 3.33 36.37 100.00

163.57 4.76 32.37 100.00

163.82 3.06 37.92 100.00

162.37 4.46 36.93 100.01

163.56 3.38 37.32 100.00

163.65 3.30 37.35 99.99

163.66 5.12 31.28 100.01

163.92 2.54 37.95 99.18

163.92 2.20 36.82 98.24

163.90 1.62 12.49 91.84

Table 5-31: Numerical solutions for channel depth at left shoulder

The numerical solutions for channel depth at right shoulder are slightly different to

the solutions for channel depth at middle and left shoulder. Embossing temperature

is slightly lower, and there is greater variation in embossing pressure compared to

the other two dimensions.




Channel Depth at right shoulder (% of reference)

159.00 1.91 26.01 100.01

159.08 2.21 25.62 100.01

157.73 2.59 12.13 100.00

161.74 4.60 18.00 100.00

159.10 4.98 32.81 100.00

160.06 4.38 25.45 100.00

158.12 1.88 20.97 100.00

157.88 2.55 13.52 100.00

162.07 4.28 14.70 100.00

158.78 2.80 15.89 100.00

Table 5-32: Numerical solutions for channel depth at right shoulder


When channel depth at left shoulder, middle and right shoulder are optimised

simultaneously (Table 5-33), the potential parameter solutions are more limited.

Embossing temperature ranges from 158-160.5 oC, embossing time ranges from 3.5-

5.5, and with one exception embossing pressure ranges from 36-38 Bars.


(oC)

Embossing Time

(minutes)

Embossing Pressure

(Bars)

Channel Depth (%

of reference)

Channel Depth at

left shoulder

(% of reference)

Channel Depth at

right shoulder

(% of reference)

158.22 5.42 37.95 97.88 95.87 100.01

158.33 5.42 37.95 98.03 96.03 100.15

158.29 5.42 37.46 97.84 95.79 100.01

158.60 5.42 37.43 98.27 96.25 100.42

158.66 5.42 37.27 98.31 96.28 100.47

158.48 5.42 36.11 97.72 95.57 100.01

158.71 4.99 37.95 98.05 95.89 100.35

159.35 3.58 37.95 97.19 94.43 100.13

160.54 5.28 25.18 96.67 93.53 100.01

Table 5-33: Numerical solutions for channel depth at left shoulder, middle, and right shoulder

5.6 Conclusions This chapter used profilometry to assess the replication of semicircular

microstructures in the following systems;

- (1) polycarbonate

- (2) polycarbonate that has been embossed through a non-bonded layer of

PDMS

- (3) adhesively bonded laminated composite of PC and PDMS fabricated using

5 minutes pre-treatment time and 25 minutes curing time

- (4) adhesively bonded laminated composite of PC and PDMS fabricated using

20 minutes pre-treatment time and 25 minutes curing time

The majority of dimensions for systems 1, 2 and 4 could be modelled using DOE.

Dimensions that could not be modelled using a first or second order equation using


DOE could be derived using the equations for the modelled dimensions and

geometric approximations for the tool dimensions.

Although the tool dimensions could be modelled using DOE, this does mean that the

tool dimensions were always replicated accurately across the range of parameters.

In the polycarbonate system, numerical simulation yielded a number of solutions

where the tool dimensions were reproduced accurately.

When PC was embossed through a layer of PDMS, channel depth at middle, left and

right shoulders, and to a lesser extent, the channel cross sectional area and the

areas of the channel halves could be optimised to within approximately 20% of the

reference value. However, channel width was invariably at least 50% larger than the

reference.

Embossing in the 5/25 composite material produced a similar pattern to embossing

in PC through PDMS, with the data ranges for channel depths and cross sectional

areas overlapping the reference (albeit without being able to be modelled in DOE),

whilst channel widths were significantly larger than the reference. The inability of

DOE analysis to model any of the dimensions of the 5/25 material was attributed to

instability of the adhesive joint under load at elevated temperature. According to the

work performed in Chapter 4, when tested in shear at room temperature, joint

strength when pre-treatment time is 5 minutes is approximately half of what it would

be when pre-treatment time is 20 minutes. This reduced joint strength may have

resulted in the joint behaving unpredictably during embossing.

Embossing in the 20/25 composite material resulted in data ranges for channel depth

that overlapped the reference (and could be modelled in DOE), whilst channel width,

area, and RWD resulted in data ranges that were significantly above the reference

value.

Numerical optimisation of channel dimensions indicated that the most important

parameter was embossing time. Embossing temperature invariably fell in a narrow


range from 158 oC to 164 oC, but embossing time and pressure could be quite

variable by comparison.

The 20/25 material proved the feasibility of reproducibly embossing a laminated

composite of adhesively bonded PDMS and PC, and that the results of this

composite embossing could also be modelled using DOE software, provided that the

manufacturing of the composite was such that strong adhesion was obtained

between the two layers. If strong adhesion is not obtained then the results may not

be able to be modelled.


6 Replication by Hot Embossing of Rectangular Microstructures

6.1 Introduction Referring to the project flowchart (Figure 6-1), this chapter investigates the

replication of rectangular microstructures after semicircular microstructures were

investigated in Chapter 5. Rectangular microstructures are more common than

semicircular microstructures in polymeric microfluidics, so testing the feasibility of the

embossing process with rectangular microstructures is an important step in

developing the process and testing its feasibility.

Figure 6-1: Flow Chart of Work Performed in Chapter 6

The purpose of Chapter 6 is to;

- Determine how embossing parameters affect replication in PC

- Determine suitable embossing parameters for the composite material

- Demonstrate the feasibility of embossing a laminated composite of PDMS and

polycarbonate using a rectangular tool

- Model the dimensions of the embossed channels in terms of the UV

fabrication parameters

1

2

3

4

5

6

Polycarbonate sheet







6.2 Methodology for Fabrication of Rectangular Semicircular Microstructures

6.2.1 Tool Fabrication and Tool Dimensions The polycarbonate used for this embossing investigation was a GE Plastics product,

Lexan 8010, 750 µm thick.

The embossing tool was fabricated in several stages. First, an acrylic block was

CNC milled to produce a negative master. The design dimensions were 600 deep

and 600 wide, for an aspect ratio of 1:1. The tool is therefore in some places

throughout this chapter referred to as the 600 Tool. The tool used was then made by

nickel electroplating off the milled acrylic.

The dimensions of the electroplated tool were measured by laser confocal

microscopy, and these dimensions were used as reference values for comparison

with the dimensions of the embossed structures. The reference values, and what

they mean in terms of the dimensions of the channel, are shown in Table 6-1 and

Figure 6-2.

Figure 6-2: Embossed channel dimensions

Width at top

Width at Bottom

Depth at Middle Left Wall Angle

Right Wall Angle


Dimension Reference Value

Depth at Middle (µm) 499

Width at Bottom (µm) 658

Width at Top (µm) 743

Right Wall Angle ( o) 85.2

Left Wall Angle ( o) 85.1

Table 6-1: Dimensions of the rectangular embossing tool

In addition to these basic dimensions, some parts of this chapter utilise additional

dimensions, shown in Table 6-3.

Dimension Reference Value

Depth from left shoulder (µm) 490.40

Depth from right shoulder (µm) 508.00

Total cross-sectional area (µm2 ) 349781.91

Area left section (µm2) 10430.81

Area centre section (µm2) 371020.42

Area right section (µm2) 10599.21

Ratio width to depth 1.49

Width left top (µm) 42.54

Width right top (µm) 42.55

Table 6-2: Additional channel dimensions for DOE analysis of embossing

6.2.2 Hot Embossing Parameters for PC – 1st Stage The embossing parameters for the 1st stage of embossing were as follows:

- embossing temperature: 155oC;

- embossing time: 0.5, 1, 3 or 5 minutes;

- embossing substrate pressure: 2.6, 5.2, 9.9, 14.6 and 19.3 Bars

- embossing was performed in an evacuated chamber, to remove any moisture

in the press

- de-embossing temperature: 100o C


- de-embossing was not controlled, nor was the orientation of the embossing

stack in the chamber.

6.2.3 Hot Embossing Parameters for PC – 2nd Stage A 2nd stage of embossing was conducted in an attempt to gain greater control and

precision over the embossing process. Some additional parameters were controlled,

and some were limited.

The embossing parameters for the 2nd stage were as follows:


- embossing time: 0.5, 1, 3 or 5 minutes;

- embossing substrate pressure: 14.6 or 19.3 Bars


in the press


- de-embossing was controlled, as was the orientation of the embossing stack

in the chamber.

6.2.4 Hot Embossing Parameters for Composite Material From embossing in PC, a single set of conditions was selected for embossing in

composite material. The parameters were:


- embossing time: 5 minutes;

- embossing substrate pressure: 14.6 Bars


in the press


- de-embossing was controlled, as was the orientation of the embossing stack

in the chamber.


6.2.5 Analysis of Embossing Results The embossed samples were then sectioned and gold coated on the Polaron

Equipment Ltd E5100 Vacuum Gold Coater, and the channel dimensions were

measured by laser confocal microscopy. These measurements were then

expressed as a % of the reference values (Tables 6-1 and 6-2).

6.3 Microstructuring of Polycarbonate

6.3.1 PC Embossing Results – 1st Stage Figure 6-3 shows embossing results for bottom width for the 600 Tool. The results

indicate that embossing time or pressure does not strongly influence the results.

Figure 6-4 shows embossing results for top width. A very significant improvement in

embossing resolution can be observed when pressure is increased above 2.6Bar.

Figure 6-3: Bottom width as % deviation from reference

Bottom Width of Embossed Channels in Polycarbonate as a % of reference

60

70

80

90

100

110

120

2.6 5.2 9.9 14.6 19.27


Bot

tom

Wid

th (%

of r

efer

ence

)

0.5 minutes hot embossing 1 minute hot embossing3 minutes hot embossing 5 minutes hot embossing


Figure 6-4: Top width as % of reference from reference

Figure 6-5: Left wall angle as % of reference

Top width of Embossed Channels in Polycarbonate as a % of reference

0

50

100

150

200

250

300

2.6 5.2 9.9 14.6 19.27


Top

Wid

th (%

of r

efer

ence

)


Left wall angle of Embossed Channels in Polycarbonate as a % of reference

0

20

40

60

80

100

120

2.60 5.20 9.90 14.60 19.27


Left

Wal

l Ang

le (%

of

refe

renc

e)

0.5 minutes hot embossing 1 minute hot embossing

3 minutes hot embossing 5 minutes hot embossing


Figure 6-5 and Figure 6-6 show embossing results for left and right wall angles for

the 600 Tool. Good embossing resolution is obtained at higher embossing times and

pressures.

Figure 6-6: Right wall angle as % of reference

Figure 6-7 shows embossing results for middle depth for the 600 Tool. As with width

at bottom (Figure 6-3), the embossing resolution does not appear to be strongly

dependent on embossing time or pressure.

Right wall angle of Embossed Channels in Polycarbonate as a % of reference

0

20

40

60

80

100

120

2.60 5.20 9.90 14.60 19.27


Righ

t Wal

l Ang

le (%

of r

efer

ence

)



Figure 6-7: Depth at middle as % of reference

Figure 6-8 shows embossing results for cross sectional area, while Figure 6-9 shows

embossing results for channel ratio of width to depth. Both figures demonstrate that

the accuracy with which the embossing tool is replicated improves at higher

pressures and temperatures.

Middle Depth of Embossed Channels in Polycarbonate as a % of reference

70

75

80

85

90

95

100

105

110

115

120

2.60 5.20 9.90 14.60 19.27


Mid

dle

Dept

h (%

of r

efer

ence

)



Figure 6-8: Channel cross sectional area as % of reference

Figure 6-9: Ratio of channel width to depth as a % of reference

Channel Cross Sectional Area for Embossed Channels in Polycarbonate as a % of reference

80

90

100

110

120

130

140

150

160

170

180

2.60 5.20 9.90 14.60 19.27

Embossing pressure (Bars)

Chan

nel C

ross

Sec

tiona

l Ar

ea (%

of r

efer

ence

)


Ratio of Channel Width to Depth for Embossed Channels in Polycarbonate as a % of reference

5075

100125150175200225250275300325350

2.60 5.20 9.90 14.60 19.27


Ratio

of C

hann

el W

idth

to

Dept

h (%

of r

efer

ence

)



Some observations can be made from the preliminary results:

- Channel depth at middle and channel width at bottom are less dependent on

embossing temperature and time than are the other dimensions; and

- Embossing accuracy improves at higher pressure and longer embossing

times.

Since embossing accuracy improved at higher pressures, the higher-pressure

experiments were repeated, but extra controls were included in the experiments in

an effort to obtain greater stability and continuity across temperature and pressure.

6.3.2 PC Embossing Results – 2nd Stage Embossing at 14.6 Bars (Figure 6-10) indicates that embossing accuracy improves

as embossing time increases..

Figure 6-10: PC embossed with 600 Tool at 14.6 Bars

Similar influences can be seen in Figure 6-11 for embossing at 19.3 Bars, although

embossing accuracy seems markedly worse at short embossing times.

Dimensions of Embossed Channels in Polycarbonate with 600 Tool at 14.6 Bars as a % of reference

70

80

90

100

110

120

130

140

150

0.5 1 3 5

Embossing time (minutes)

Cha

nnel

dim

ensi

ons

(% o

f re

fere

nce)

Bottom Width Top Width Left Wall Angle Right Wall Angle Middle Depth


Figure 6-11: PC embossed with 600 Tool at 19.3 Bars

6.3.3 Selection of Embossing Parameters for Composite Material Selection of optimum embossing parameters was done by comparing total %

deviation from reference for embossing at both 14.6 and 19.3 Bars (Figure 6-12).

Figure 6-12: Total % deviation from reference for 600 tool

Dimensions of Embossed Channels in Polycarbonate with 600 Tool at 19.3 Bars as a % of reference

25

50

75

100

125

150

175

200

225

0.5 1 3 5


Cha

nnel

dim

ensi

ons

(%

of re

fere

nce)

Bottom Width Top Width Left Wall Angle Right Wall Angle Middle Depth

Total % Deviation from Reference for PC embossed using 600 tool

0

40

80

120

160

200

240

280

0.5 1 3 5


Tota

l % D

evia

tion

from

R

efer

ence

14.6 Bars 19.6 Bars


Analysis of the data in Figure 6-12 suggests that total % error is least when

embossed at 14.3 Bars pressure and 5 minutes embossing time. These parameters

will be used for the embossing of the composite materials.

6.4 Microstructuring of Composite Material Embossing of the composite material was performed at 14.6 Bars substrate pressure

for 5 minutes.

The composites were laminated together at a variety of UV treatments, from 5

minutes each of UV pre-treatment and UV curing, to 30 minutes of UV pre-treatment

and UV curing.

The embossing results from each experiment were then analysed as a % of the

reference values. Figure 6-13 shows the bottom width as a % of the reference.

Figure 6-13: % Bottom width as a % of reference

At low UV treatment levels, for both pre-treatment and adhesive curing, the bottom

width is high as a % of the reference, between 80 and 90%. As the treatment times

increase, the bottom width generally drops to between 50 and 60% of the reference.

Bottom Width as a % of Reference for Embossed Polycarbonate/PDMS Composite

0

10

20

30

40

50

60

70

80

90

100

5.0/5.0 5.0/17.5 5.0/30.0 13.3/14.1 17.5/5 17.5/17.5 17.5/30 30.0/5 30.0/17.5 30.0/30

PDMS UV pretreatment time (minutes)/UV exposure time to cure adhesive (minutes)

% o

f ref

eren

ce

Bottom Width


The result at 17.5 minutes pre-treatment and curing is slightly different to the results

from other high treatment levels, being just over 70% of the reference.

Figure 6-14 shows how the UV treatment parameters affect the top width as a % of

the reference. It is clear that the deviations from reference are largest when the

adhesion is lowest. The optimal treatment parameters are approximately 17.5

minutes UV pre-treatment of PDMS, with curing time being can be anywhere

between 5 and 30 minutes. and 30 minutes curing of the adhesive.

Figure 6-14: Top width as a % of reference

Figures 6-15 and 6-16 show how UV treatment parameters affect the left wall angle

and right wall angle. Both show clear evidence that greater bond strength between

the PDMS and PC (in the form of increased pre-treatment and curing times)

improves the accuracy of embossing in the composite material. At higher UV

treatment times, the accuracy of the embossing process is much improved over

lower UV treatment times, improving from 10% of the reference (for 5/5 UV pre-

treatment/UV curing) to 55% of the reference (for 17.5/17.5 UV pre-treatment/UV

curing).

Top Width as a % of Reference for Embossed Polycarbonate/PDMS Composite

100

120

140

160

180

200

220

240

260

280

5.0/5.0 5.0/17.5 5.0/30.0 13.3/14.1 17.5/5 17.5/17.5 17.5/30 30.0/5 30.0/17.5 30.0/30


% o

f ref

eren

ce

Top Width


Figure 6-15: Left wall angle as a % of reference

Figure 6-16: Right wall angle as a % of reference

Figure 6-17 shows how the UV treatment parameters affect the channel depth.

There are some correlations with the previous results in this figure. Like in the

Right Wall Angle as a % of Reference for Embossed Polycarbonate/PDMS Composite

0

10

20

30

40

50

60

70

80

90

100

5.0/5.0 5.0/17.5 5.0/30.0 13.3/14.1 17.5/5 17.5/17.5 17.5/30 30.0/5 30.0/17.5 30.0/30


% o

f re

fere

nce

Right Wall Angle

Left Wall Angle as a % of Reference for Embossed Polycarbonate/PDMS Composite

0

10

20

30

40

50

60

70

80

90

100

5.0/5.0 5.0/17.5 5.0/30.0 13.3/14.1 17.5/5 17.5/17.5 17.5/30 30.0/5 30.0/17.5 30.0/30


% o

f re

fere

nce

Left Wall Angle


embossing results for PC, channel depth is resolved very accurately (95% of

reference or higher).

Figure 6-17: Depth at middle as a % of reference

Also, similar to the previous results for embossing of the composite material,

embossing accuracy is significantly increased when bond strength is high (higher UV

treatment times).

Upon comparison of the data from Figures 6-13 to 6-17, it can be concluded that for

the system studied, the optimal results are depth at middle replicated to 100% of the

reference value, width at top replicated to approximately 160% of the reference

value, while width at bottom and the wall angles are replicated to approximately 50%

of the reference values. Optimal UV treatment parameters for the embossing are

approximately 17.5 minutes each of UV pre-treatment and curing.

Considering the data as total error clarifies the picture. The chart below (Figure 6-

18) makes clear that the replication accuracy is poor at low UV treatment

parameters, but that it increases until the UV pre-treatment is increased up to the

level of 17.5 minutes, and then fluctuates slightly but does not rise above the high

errors of the low UV parameter embossing.

Channel Depth as a % of Reference for Embossed Polycarbonate/PDMS Composite

0

10

20

30

40

50

60

70

80

90

100

110

5.0/5.0 5.0/17.5 5.0/30.0 13.3/14.1 17.5/5 17.5/17.5 17.5/30 30.0/5 30.0/17.5 30.0/30


% o

f R

efer

ence

Channel Depth at Middle


Looking again at the individual breakdown of the total error, the fluctuations in total

error at the high end of the UV parameter scale can be largely attributed to the

behaviour of the width at the top of the channel, while the other parameters remain

largely constant.

Figure 6-18: Total % error for embossed composite

6.5 Microstructuring of Composite Material Using DOE After a non-DOE study of the embossed composite demonstrated feasibility using

the rectangular tool, a DOE study was conducted to determine any empirical

relationships between UV exposure parameters and channel dimensions.

However, it was considered that any delamination between the adhesive substrates

would likely render the DOE model useless. To mitigate the risks of this happening,

the UV parameters employed for the DOE investigation of the composite embossing

were shifted up relative to the UV parameters employed in the conventional analysis.

The maximum UV treatment employed was 40 minutes, while the minimum

employed was 10 minutes. This stands in contrast to the 5 minutes minimum UV

exposure, 30 minutes maximum UV exposure that was used in the conventional

Total % Deviation for All Channel Dimensions in Embossed Composite Material Vs UV parameters

0

50

100

150

200

250

300

350

400

450

5.0/5.0 5.0/17.5 5.0/30.0 13.3/14.1 17.5/5 17.5/17.5 17.5/30 30.0/5 30.0/17.5 30.0/30


% D

evia

tion

from

Ref

eren

ce

Total % Deviation from Reference


analysis. The specific combinations of the UV parameters used in the DOE analysis

are shown in Table 6-3.

UV pre-treatment time (minutes)


14.39 14.39 35.61 14.39 14.39 35.61 35.61 35.61 10.00 25.00 40.00 25.00 25.00 10.00 25.00 40.00 25.00 25.00 25.00 25.00 25.00 25.00 25.00 25.00

Table 6-3: DOE Matrix for Embossing of Composite Material

6.5.1 Embossing Result Tables 6-4 and 6-5 present the results of the embossing of adhesively laminated

composites using the 600 Tool for 5 minutes at 14.6 Bars of substrate pressure. The

data is presented as real units (such as micrometers).

Table 6-4: DOE data as real responses

depth from depth fromFactor 1 Factor 2 bottom top width left wall right wall middle left rightA:UV Pre B:UV cure width angle angle depth shoulder shoulderMinutes Minutes µm µm degrees degrees µm µm µm

14.39 14.39 215.21 1351.32 47.38 44.24 582.09 560.00 604.1735.61 14.39 95.09 1141.12 52.74 35.01 480.67 490.00 471.3414.39 35.61 165.16 1236.21 42.16 44.23 503.09 482.50 523.6735.61 35.61 243.30 1301.27 44.31 44.55 518.63 483.08 554.1710.00 25.00 40.04 1476.44 44.89 26.38 474.34 471.00 477.6740.00 25.00 62.56 1268.74 40.32 41.45 522.09 511.67 532.5025.00 10.00 140.14 1513.98 36.20 39.47 533.00 520.00 546.0025.00 40.00 240.24 1261.23 42.93 45.22 495.07 460.79 529.3425.00 25.00 135.13 1401.37 40.42 44.23 576.58 554.00 599.1625.00 25.00 145.15 1336.31 41.21 41.59 524.92 528.00 521.8325.00 25.00 215.21 1536.50 38.50 39.38 533.75 523.33 544.1725.00 25.00 115.11 1196.17 47.57 51.63 634.42 621.17 647.6725.00 25.00 190.18 1388.85 41.19 39.97 512.67 492.66 532.67


Table 6-5: DOE data as real responses

Tables 6-6 and 6-7 present the results of the embossing of adhesively laminated

composites using the 600 Tool for 5 minutes at 14.6 Bars of substrate pressure. The

data is presented as a % of the reference values.

Table 6-6: DOE data as % of reference value

Factor 1 Factor 2 total CS area left area centre area right ratio width width widthA:UV Pre B:UV cure area section section section to depth left top right topMinutes Minutes µm2 µm2 µm2 µm2 µm µm

14.39 14.39 457087.48 144340.00 125270.51 165600.47 2.32 515.50 620.6135.61 14.39 295703.68 91353.15 45706.91 168741.02 2.37 372.87 673.1614.39 35.61 352555.68 128591.08 83089.52 126663.02 2.46 533.02 538.0335.61 35.61 401737.15 119542.98 126181.46 142240.51 2.51 494.92 563.0510.00 25.00 360477.65 111382.08 18992.37 221273.26 3.11 472.96 963.4440.00 25.00 347525.88 154291.53 32661.64 154442.30 2.43 603.09 603.0925.00 10.00 440513.84 184782.00 74694.62 167993.26 2.84 710.70 663.1425.00 40.00 372177.26 114156.11 118934.42 117835.11 2.55 495.48 525.5125.00 25.00 442561.98 180227.28 77913.26 172007.87 2.43 650.64 615.6025.00 25.00 388843.46 159215.76 76191.41 154906.46 2.55 603.09 588.0725.00 25.00 467513.60 172214.82 114868.34 166723.34 2.88 658.15 663.1425.00 25.00 415586.49 176427.81 73028.09 158648.34 1.89 568.05 513.0125.00 25.00 405482.59 138696.11 97498.63 152653.68 2.71 563.05 635.62

depth from depth fromFactor 1 Factor 2 bottom top width left wall right wall middle left rightA:UV Pre B:UV cure width angle angle depth shoulder shoulderMinutes Minutes

14.39 14.39 28.96 205.32 55.70 51.91 116.60 114.19 118.9335.61 14.39 12.79 173.39 62.00 41.07 96.29 99.92 92.7814.39 35.61 22.22 187.83 49.57 51.90 100.78 98.39 103.0835.61 35.61 32.74 197.72 52.10 52.27 103.89 98.51 109.0910.00 25.00 5.39 224.34 52.77 30.95 95.02 96.04 94.0340.00 25.00 8.42 192.78 47.40 48.64 104.58 104.34 104.8225.00 10.00 18.86 230.04 42.56 46.32 106.77 106.04 107.4825.00 40.00 32.32 191.64 50.47 53.05 99.17 93.96 104.2025.00 25.00 18.18 212.93 47.52 51.90 115.50 112.97 117.9425.00 25.00 19.53 203.04 48.45 48.80 105.15 107.67 102.7225.00 25.00 28.96 233.46 45.26 46.20 106.92 106.71 107.1225.00 25.00 15.49 181.75 55.92 60.58 127.09 126.67 127.4925.00 25.00 25.59 211.03 48.43 46.90 102.70 100.46 104.86


Table 6-7: DOE data as % of reference values

6.5.2 DOE Models and Comparison with Experimental Data DOE analysis revealed that only one parameter, total cross sectional area, could be

described by a first or second order equation. These equations are described in

terms of real and % values in Equations 6-1 and 6-2.

Real Values

UVCTUVPTUVCTUVPTUVCT

UVPTareaCStotal

**92.467*02.86*65.318*56.8518

*45.269610*53

2

2

6

+−

−−

+= −

Equation 6-1

% of Reference Values

UVCTUVPTUVCTUVPTUVCT

UVPTareaCStotal

**13.0*02.0*09.0*44.2

*77.053.151

2

2

+−

−−

+=

Equation 6-2

Where CS = cross sectional

UVPT = UV pre-treatment time (minutes)

UVCT = UV curing time (minutes)

Factor 1 Factor 2 total CS area left area centre area right ratio width width widthA:UV Pre B:UV cure area section section section to depth left top right topMinutes Minutes

14.39 14.39 130.68 1383.79 33.76 1562.39 155.93 1211.80 1458.5435.61 14.39 84.54 875.80 12.32 1592.02 159.45 876.52 1582.0414.39 35.61 100.79 1232.80 22.39 1195.02 165.04 1252.99 1264.4735.61 35.61 114.85 1146.06 34.01 1341.99 168.53 1163.42 1323.2710.00 25.00 103.06 1067.82 5.12 2087.64 209.07 1111.80 2264.2540.00 25.00 99.36 1479.19 8.80 1457.11 163.22 1417.70 1417.3725.00 10.00 125.94 1771.50 20.13 1584.96 190.79 1670.66 1558.5025.00 40.00 106.40 1094.41 32.06 1111.74 171.11 1164.74 1235.0425.00 25.00 126.53 1727.84 21.00 1622.84 163.25 1529.48 1446.7725.00 25.00 111.17 1526.40 20.54 1461.49 170.99 1417.70 1382.0725.00 25.00 133.66 1651.02 30.96 1572.98 193.35 1547.13 1558.5025.00 25.00 118.81 1691.41 19.68 1496.79 126.64 1335.33 1205.6625.00 25.00 115.92 1329.68 26.28 1440.24 181.96 1323.58 1493.82


Figure 6-19 compares the experimental values of total cross sectional area with

those predicted from DOE analysis. As expected, there is a good match between

the two.

Figure 6-19: Comparison of experimental and DOE total cross sectional area

6.5.3 Empirical Derivations Based on DOE Models and Comparisons with Experimental Data

Since only total cross sectional area could be predicted using DOE analysis, a

number of approximations are needed in order to develop empirical predictions for

other parameters.

Comparison of experimental and DOE predicted total cross sectional area

0

20

40

60

80

100

120

140

160

1 2 3 4 5 6 7 8 9 10 11 12 13


% o

f ref

eren

ce v

alue

Experimental total CSA DOE predicted total CSA


Dimension Mean Standard Deviation

Coefficient of Variance

Bottom Width (µm) 154.04 65.38 0.42

Top Width (µm) 1339.19 121.65 0.09

Left Wall Angle (o) 43.06 4.36 0.1

Right Wall Angle (o) 41.33 5.98 0.15

Middle Depth (µm) 530.10 44.54 0.08

Depth at Left Shoulder (µm) 515.25 43.94 0.09

Depth at Right Shoulder (µm) 544.95 48.79 0.09

Total CS area (µm2) 395982.06 49731.71 0.13

Area left section (µm2) 144247.75 29930.43 0.21

Area centre section (µm2) 81925.47 34660.10 0.42

Area right Section (µm2) 159209.90 24874.04 0.16

Ratio width to depth 2.54 0.30 0.12

Width left top (µm) 557.04 90.34 0.16

Width right top (µm) 628.11 113.85 0.18

Table 6-8: Mean, standard deviation and coefficient of variance for embossed data

Table 6-8 shows that top width, middle depth, depth at left shoulder and depth at

right shoulder all have a coefficient of variance of less than 0.1. For the purpose of

approximating the data patterns, a coefficient of variance of less than 0.1 was

considered sufficient for a mean model to be employed.

On the basis of this assumption, the following mean models were used:

19.1339=widthtop Equation 6-3

1.530=depthmiddle Equation 6-4

04.515=shoulderleftatdepth Equation 6-5


95.544=shoulderrightatdepth Equation 6-6

Figures 6-20 to 6-23 show how the mean models compare to the experimental data.

Generally the mean model fits the data quite well.

Figure 6-20: Comparison of experimental data and DOE mean model for top width

Comparison of experimental data and DOE mean model for top width

0

25

50

75

100

125

150

175

200

225

1 2 3 4 5 6 7 8 9 10 11 12 13


To

p w

idth

(% o

f re

fere

nce

)

Experimental data - top w idth DOE mean model - top w idth


Figure 6-20: Comparison of experimental data and DOE mean model for middle depth

Figure 6-21: Comparison of experimental data and DOE mean model for depth from left shoulder

Comparison of experimental data and DOE mean model for middle depth

0

20

40

60

80

100

120

140

1 2 3 4 5 6 7 8 9 10 11 12 13Experiment Number (1-13)

Mid

dle

Dep

th (%

of r

efer

ence

)

Experimental data - middle depth DOE mean model - middle depth

Comparison of experimental data and DOE mean model for depth from left shoulder

0

20

40

60

80

100

120

140

1 2 3 4 5 6 7 8 9 10 11 12 13


Dep

th fr

om le

ft s

houl

der

(% o

fre

fere

nce)

Experimental data - depth from left shoulder DOE mean model - depth from left shoulder


Figure 6-22: Comparison of experimental data and DOE mean model for depth from left shoulder

Another approximation used was to describe the area of the centre section of the

embossed channel using the following equation.

UVCTUVPT

UVCTUVPT

UVCTUVPTACS

**57.272

*60.114*89.200

*62.11355*75.3028179653

22

+

+−

−+=

Equation 6-7

Figure 6-23 shows how experimental data and the predicted model of Equation 6-7

compare. The match is quite good.

Comparison of experimental data and DOE mean model for depth from right shoulder

0

20

40

60

80

100

120

140

1 2 3 4 5 6 7 8 9 10 11 12 13


Dep

th fr

om r

ight

sho

ulde

r (%

of

refe

renc

e)

Experimental data - depth from right shoulder DOE mean model - depth from right shoulder


Figure 6-23: Comparison of experimental and DOE approximation of centre section area

The final approximation used was to describe the width from the left or right shoulder

as:

⎟⎠⎞

⎜⎝⎛ −

=2

& BWTWWRSWLS Equation 6-8

Where WLS = width from left shoulder

WRS = width from right shoulder

TW = top width

BW = bottom width

In order to calculate equation 6-8, a model to describe BW is required.

BWMDACS *= Equation 6-9

where ACS = area of centre section MD = middle depth BW = bottom width

Comparison of experimental and DOE approximation of centre section area

0

5

10

15

20

25

30

35

40

1 2 3 4 5 6 7 8 9 10 11 12 13


Cent

re S

ectio

n Ar

ea (%

of

refe

renc

e)

DOE approximation of centre section area Experimental centre section area


Substituting equations 6-4 and 6-7 into 6-9 and re-arranging gives an equation for

BW (Equation 6-10). This equation is compared with experimental data in Figure 6-

24.

1.530**57.272*60.114

*89.200*62.11355*75.3028179653

2

2

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

++

−−

+

=UVCTUVPTUVCT

UVPTUVCTUVPT

BW Equation 6-10

Figure 6-24: Comparison of experimental and DOE approximation for bottom width

Substituting 6-10 into 6-8 gives;

Comparison of experimental and DOE approximation of bottom width

0

5

10

15

20

25

30

35

40

1 2 3 4 5 6 7 8 9 10 11 12 13


Botto

m w

idth

(% o

f ref

eren

ce)

DOE approximation of bottom width Experimental bottom width


⎟⎠⎞

⎜⎝⎛

⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜

⎝

⎛

⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜

⎝

⎛

++

−

−+

−=21*

1.530**57.272

*60.114*89.200

*62.11355*75.3028179653

19.1339&

2

2

UVCTUVPTUVCTUVPT

UVCTUVPT

WRSWLS Equation 6-11

Figure 6-25 compares the predictions of equation 6-11 with the average of the

experimental widths. Again, the two data sets track each other fairly closely.

Figure 6-25: Comparison of DOE approximation and experimental data for average of width of left and right section

Now, equations for predicting the wall angles can be empirically derived.

Using Equations 6-5, 6-6, 6-11 and

⎟⎠⎞

⎜⎝⎛=⎟

⎠⎞

⎜⎝⎛=

DLSWLSand

DRSWRS

θθ tantan Equation 6-12

Comparison of DOE approximation and experimental data for average of width of left and right section

0

200

400

600

800

1000

1200

1400

1600

1800

1 2 3 4 5 6 7 8 9 10 11 12 13


Wid

th o

f lef

t/rig

ht s

ectio

n (%

of

refe

renc

e)

DOE approximated width of left/right sectionAverage of experimental width of left and right section


Equation 6-13 can then approximate the left wall angle, while the right wall angle can

be approximated by Equation 6-14.

⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜

⎝

⎛

⎟⎠⎞

⎜⎝⎛

⎪⎪⎪⎪⎪⎪⎪

⎭

⎪⎪⎪⎪⎪⎪⎪

⎬

⎫

⎪⎪⎪⎪⎪⎪⎪

⎩

⎪⎪⎪⎪⎪⎪⎪

⎨

⎧

⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜

⎝

⎛

⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜

⎝

⎛

++

−

−+

−⎟⎠⎞

⎜⎝⎛= −

04.5151*

1.530**57.272

*60.114*89.200

*62.11355*75.3028179653

19.133921tan

2

2

1 UVCTUVPTUVCTUVPT

UVCTUVPT

lwθ Equation 6-13

⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜

⎝

⎛

⎟⎠⎞

⎜⎝⎛

⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜

⎝

⎛

⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜

⎝

⎛

⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜⎜

⎝

⎛

++

−

−+

−⎟⎠⎞

⎜⎝⎛= −

95.5441*

1.530**57.272

*60.114*89.200

*62.11355*75.3028179653

19.133921tan

2

2

1 UVCTUVPTUVCTUVPT

UVCTUVPT

rwθ Equation 6-14

Figures 6-26 and 6-27 compare the experimental data for wall angles with the

predictions of equations 6-13 and 6-14. The predicted responses are similar to the

experimental data, particularly for the left wall angle, Figure 6-28.


Figure 6-26: Comparison of DOE approximated and experimental right wall angle

Figure 6-27: Comparison of DOE approximated and experimental left wall angle

Comparison of DOE approximated and experimental right wall angle

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7 8 9 10 11 12 13


Righ

t wal

l ang

le (%

of r

efer

ence

)

DOE approximated right wall angle Experimental right wall angle

Comparison of DOE approximated and experimental left wall angle

0

10

20

30

40

50

60

70

1 2 3 4 5 6 7 8 9 10 11 12 13Experiment Number (1-13)

Left

wall

angl

e (%

of r

efer

ence

)

DOE approximated left wall angle Experimental left wall angle


6.5.4 Total Error and Optimal Processing Conditions Table 6-9 shows % error channel width at top, channel depth and channel width at

bottom. These three dimensions are chosen because they are the directly measured

dimensions. All the other dimensions are calculated from these three. Total % error

is the sum of the errors from the three dimensions.

UV Pretreatment

(minutes)

UV Curing

(minutes)

Top Width

% error

Bottom Width

% error

Middle Depth

% error Total %

error 14.39 14.39 71.04 105.32 16.60 192.97 35.61 14.39 87.21 73.39 3.71 164.30 14.39 35.61 77.78 87.83 0.78 166.39 35.61 35.61 67.26 97.72 3.89 168.88

10 25 94.61 124.34 4.98 223.93 40 25 91.58 92.78 4.58 188.94 25 10 81.14 130.04 6.77 217.95 25 40 67.68 91.64 0.83 160.14 25 25 81.82 112.93 15.50 210.25 25 25 80.47 103.04 5.15 188.67 25 25 71.04 133.46 6.92 211.43 25 25 84.51 81.75 27.09 193.35 25 25 74.41 111.03 2.70 188.14

Table 6-9: Total Error for Embossed Composite

Figure 6-28 shows the data from the last column from Table 6-9. The lowest total %

error that could be achieved with this system is between 160 and 170 %, from

experiments 2, 3, 4 and 8 (shown in bold in Table 6-9). Experiment 8 was the best

overall, with 25 minutes UV pre-treatment time, and 40 minutes UV curing time.


Figure 6-28: Sum of total deviations from reference

6.6 Conclusions This chapter measured by profilometry the replication of rectangular microstructures

in (1) polycarbonate, and (2), an adhesively bonded composite of PC and PDMS. All

the microstructures were replicated using an electroplated hot embossing shim.

The purpose of this investigation was to demonstrate;

- Determine how embossing parameters affect replication in PC

- Determine suitable embossing parameters for the composite material

- Demonstrate the feasibility of embossing a laminated composite of PDMS and

polycarbonate using a rectangular tool

- Model the dimensions of the embossed channels in terms of the UV

fabrication parameters

6.6.1 Conclusions from PC embossing During PC embossing, it was discovered that some channel dimensions were more

likely to emboss accurately than others. Channel depth at middle, and channel width

at bottom could be embossed to within 10% of the reference or better, and these

results were often independent of embossing time and pressure.

Total % Error for Embossed Composite

0

25

50

75

100

125

150

175

200

225

250

1 2 3 4 5 6 7 8 9 10 11 12 13


Tota

l % E

rror f

or E

mbo

ssed

Com

posi

teTotal % Error


However, both wall angles and the width at the top of the channel were quite difficult

to emboss accurately under any conditions, though the results were better at higher

pressures and longer embossing times.

A significant problem throughout the embossing process was poor consistency

between results in the same series, in that an incremental increase in an embossing

parameter invariably did not produce a discernible pattern in the data. This could be

due to the complexity of the flow of molten polymer during embossing, where very

small changes in orientation of the substrate during embossing produce substantial

changes in the results.

Additional controls measures were implemented to attempt to reduce this problem,

and were significant to a small degree in improving the erratic nature of the results.

6.6.2 Conclusions from Composite Embossing – Conventional Embossing of the composite material was clearly demonstrated to be feasible, and

that the UV exposure parameters used for fabrication of the composite material had

a clear impact on the accuracy of the embossed channel dimensions, especially for

the channel depth at middle and the wall angles.

6.6.3 Conclusions from Composite Embossing – DOE analysis Using DOE, it was possible to determine empirical models for how the dimensions of

the embossed channels are affected by UV pre-treatment times and UV curing

times. These empirical relationships provide a method for controlling the dimensions

of the embossed channels based upon the UV manufacturing parameters and the

embossing parameters. It was determined that the optimal UV treatment parameters

were 25 minutes UV pre-treatment time, and 40 minutes UV curing time.


7 Sealing of PDMS microstructures 7.1 Introduction

Referring to the project flowchart, previous chapters have described the adhesion of

PC to PDMS (Chapter 4), the ability of the PC to PDMS composite to withstand the

embossing process (Chapter 5) and the optimisation of the UV parameters for

embossing (Chapter 6). Chapter 7 deals with the final step of sealing the

microstructured composite to another PDMS part or composite, highlighted in the

project flowchart (Figure 7-1).

Figure 7-1: Flowchart representing Chapter 7 work

The specific purpose of Chapter 7 was to identify the PDMS surface treatment

requirements for autohesion, both in terms of the exposure times and the surface

chemistry. Autohesion is the spontaneous self-adhesion of two PDMS surfaces that

are manually pressed together after UV/Ozone exposure.

Section 7.2 describes how the surface chemistry of the PDMS changes with

UV/ozone pretreatments of various intensities and lengths of time. It also examines

the influence of the length of time between treatment and analysis, which is known

as the ageing time, to determine how the surface chemistry of the PDMS changes as

1

2

3

4

5

6

Polycarbonate sheet







it is exposed to the atmosphere post UV/ozone exposure. This ageing study was

combined with the results of Section 7.3 to attempt to determine the surface

chemistry requirements for adhesion, including how long a PDMS sample may be

exposed to the atmosphere (aged) after treatment and still spontaneously self stick

to a similarly treated sample.

Experimentally, how long a PDMS sample may be exposed to the atmosphere

(aged) after UV/Ozone exposure and still spontaneously self adhere to a similarly

treated PDMS was determined in Section 7.3, as were oxygen plasma requirements

in Section 7.4.

7.2 XPS Analysis of Heavily UV/Ozone Treated PDMS

7.2.1 Surface treatment of PDMS at 60.1 mm Working Distance 7.2.1.1 Oxygen The O/C ratio for the treated PDMS increases steadily with the length of UV/ozone

treatment (Figure 7-2). For untreated PDMS the O/C ratio is 0.64, and this increases

to as much as 4.07 (PDMS treated for 4.5 hours and aged for 7 days).

The increase in the O/C ratio is relatively linear across the treatment time space

regardless of ageing time. Also, ageing time does not appear to influence the O/C

ratio until the PDMS has been exposed for at least 2.5 hours. At treatment times

greater than this, the PDMS that has been aged for 7 days has significantly higher

O/C ratio, particularly with respect to the freshly treated PDMS.


Figure 7-2: Atomic ratio O/C as a function of UV treatment time and ageing time

7.2.1.2 Carbon The (C1+C2)/C ratio steadily decreases with UV exposure (Figure 7-3), due to

extensive oxidation of the surface layers. The C1+C2 species are likely mainly

attributable to the –CH3 groups that are attached to the siloxane main chain. They

are therefore characteristic for the unmodified PDMS.

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

UV treatment time (hours)

Atom

ic R

atio

O/C

O fresh O 2 days O 7 days


Figure 7-3: Atomic ratio (C1+C2)/C as a function of UV treatment time and ageing time

According to the data, the (C1+C2)/C ratio on the surface recovers slightly during

ageing in air after UV/ozone treatment (Figure 7-3). This is probably due to surface

restructuring, i.e. the re-emergence of unmodified siloxane segments at the surface.

This phenomenon, which is commonly observed after surface modification of

hydrophobic polymers, may be expected to be particularly pronounced in the case of

PDMS whose polymeric segments have a comparatively high mobility.

The C3 component (average binding energy of 286.7 eV) represents C-O based

groups such as ether (C-O-C), or hydroxyl (-OH) groups (Figure 7-4). Hydroxyl

groups have been suggested to be the critical functional group in the facilitation of

oxidative PDMS autohesion that is common in the microfluidic field (McDonald, Duffy

et al. 2000). The data from this set of experiments suggests that the C3 groups

increase in a fairly linear manner with time of exposure, and that the influence of

ageing does not produce any readily identifiable trends in the data.

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5


Atom

ic ra

tio (C

1+C2

)/C

C1+C2 fresh C1+C2 2 days C1+C2 7 days


Figure 7-4: Atomic ratio C3/C as a function of UV treatment time and ageing time

It should also be noted that C-O species exist on the surface prior to any UV/ozone

exposure. This may be attributable to contamination on the surface, the chemical

functionalities contained within the filler particles, or even just the normal interaction

of the surface with the atmosphere may results in a small quantity of hydroxyl or

ester groups on the surface at any given moment.

Unlike C-O based groups, functionalities represented by C4 (Figure 7-5) and C5

(Figure 7-6) components were not detected on the unmodified surface. With a

chemical shift relative to the hydrocarbon peak of 3eV and 4.5eV respectively, these

groups are most likely carbonyls (C=O) and O-C-O based species (C4), and O-C=O

based groups such as carboxylic acids (C5). Both of these species increase with

UV/ozone exposure, but are only evident in small quantities.

0.00

0.03

0.05

0.08

0.10

0.13

0.15

0.18

0.20

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5


Atom

ic R

atio

C3/

C

C3 fresh C3 2 days C3 7 days


Figure 7-5: Atomic ratios C4/C as a function of UV treatment time and ageing time

Ageing of the treated surface produces no clearly discernible trends in the C4/C data

(Figure 7-5), but the C5/C ratio drops significantly during ageing, particularly

between the fresh and aged 2 days samples (Figure 7-6). Further ageing beyond 2

days produces minimal changes in the C5/C ratio. This change is probably similar to

the changes described in Figure 7-3, where the surface exhibits a slight reversion

towards its unmodified state.

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5


Atom

ic ra

tio C

4/C



Figure 7-6: Atomic ratio C5/C as a function of UV treatment time and ageing time

7.2.1.3 Organo-Silicon The Si spectrum consists of two clearly resolved peaks; the measured binding

energies of 102.2 eV and 103.3 eV are characteristic of organo-silicon groups such

as those present in the PDMS main chain (which will be discussed here) and of

inorganic silicon species respectively (which will be discussed in the next section).

The organo-Si/C ratio increases for approximately 1 hour of exposure, and then

decreases (Figure 7-7). This may be attributable to some kind of surface cleaning

reaction that occurs initially, where other species are removed from the surface,

exposing the organo-Si PDMS main chain.

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5


Atom

ic R

atio

C5/

C



Figure 7-7: Atomic ratio Organo-Si/C as a function of UV treatment time and ageing time

After 1 hour of UV/ozone exposure, the amount of organo-Si on the surface starts to

decrease, which is almost certainly due to completion of the cleaning mechanism

mentioned in the previous paragraph and the commencement of the oxidation of the

organo-Si PDMS main chain and the formation of inorganic-Si containing surface

species. The effect of ageing on the organo-Si/C ratio is ambiguous with respect to

the fresh and aged 2 days samples, but is slightly more pronounced when the

sample has been aged for 7 days. After ageing for 7 days, it seems as though the

amount of PDMS on the surface has recovered slightly from its highly oxidized state.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5


Atom

ic R

atio

Org

ano-

Si/C

Organo-Si fresh Organo-Si 2 days Organo-Si 7 days


7.2.1.4 Inorganic-Silicon The inorganic-Si on the surface comes from two potential sources; (1) from the

silica/silicate containing filler particles that comprise approximately 40 wt% of the

material, and (2) from the oxidation of the siloxane main chain into inorganic-Si

compounds.

Figure 7-8: Atomic ratios inorganic-Si/C as a function of UV treatment time and ageing time

The inorganic-Si/C ratio increases right through the extent of UV treatment,

increasing from 0.3 up to 1.7 (Figure 7-8). The influence of ageing on these samples

is evident for treatment times 1.5 hours and greater. The inorganic-Si/C ratio also

increases with ageing. When these results are compared with the results for organo-

Si/C (Figure 7-7), it appears that both organo- and inorganic Si increase relative to C

as ageing continues.

7.2.1.5 Overview The UV/ozone treatment of PDMS results in the oxidation of the PDMS material: the

relative concentrations of unmodified PDMS were observed to decrease substantially

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5


Atom

ic R

atio

Inor

gani

c-Si

/C

Inorganic-Si fresh Inorganic-Si 2 days Inorganic-Si 7 days


whereas the levels of oxygen containing functionalities increased dramatically. The

chain scissioning of the PDMS backbone produces unstable silicon containing

species, which may react with surrounding oxygen to produce low molecular weight

silica particles, while the removal of the top PDMS surface layer may expose

additional silica filler particles lying underneath, resulting in large increases in the

inorganic-Si/C ratio.

The carbon spectra becomes much more complex upon UV/ozone exposure, with

the saturated hydrocarbon groups being replaced by a range of carbon-oxygen

functional groups.

However, over time the treated surface was observed to recover partially, and both

saturated hydrocarbons and siloxane units will increase on the surface. This is

possibly due to surface restructuring, i.e. the rearrangement of polymer segments at

the surface.

7.2.2 Surface Treatment of PDMS at 30.1 mm Working Distance 7.2.2.1 Oxygen The surface treatment of PDMS by UV/ozone produces a rapid increase in the O/C

ratio, which increases from less than 1 to over 3.5. This increase in O/C ratio is due

to the rapid and extensive oxidation of the surface during UV/ozone treatment.


Figure 7-9: Atomic ratio O/C as a function of UV/ozone treatment time

7.2.2.2 Carbon Prior to UV treatment, C1 and C2 groups constituted virtually all the C groups on the

surface of the PDMS (as indicated by the ratio of C1+C2 to C being very close to 1),

however as UV treatment progressed, the atomic ratios of these groups (C1+C2)/C

declined to slightly over 0.8 (Figure 7-10). This suggests that the C1 and C2 groups

are being oxidised into other carbon containing functionalities. This is

demonstrated by Figure 7-11, which shows dramatic increases in C4, C5 and

particularly C3 groups during UV/ozone exposure.

Figures 7-10 and 7-11, when considered together, indicate that the methyl groups in

the PDMS chain are rapidly oxidized to other states, perhaps first to silanols, and

then as the UV treatment time increases, the silicone main chain is further

scissioned. This creates separate oxidised inorganic silicon particles on the surface,

which would account for the dramatic increases in both surface oxygen and surface

silicon shown in Figure 7-4, where O/C increases from less than 1 to over 3, and

where inorganic-Si/C increases from less than 0.5 to over 1.

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

UV/Ozone treatment time (hours)

Atom

ic R

atio

(O/C

)O


Figure 7-10: Atomic ratio (C1+C2)/C as a function of UV/ozone treatment time

Figure 7-11: Atomic ratio C3/C, C4/C and C5/C as a function of UV/ozone treatment time

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5UV/Ozone treatment time (hours)

Ato

mic

Rat

io (C

1+C

2)/C

C1+C2

0

0.02

0.04

0.06

0.08

0.1

0.12

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

UV/Ozone treatment time (hours)

Ato

mic

Rat

io X

/C (w

here

X =

C3,

C4

or C

5) C3 C4 C5


7.2.2.3 Silicon

Figure 7-12 demonstrates how the atomic ratios for organo- and inorganic-Si species

vary with UV/ozone surface treatment. The organo-Si/C ratio briefly increases,

probably due to a cleaning mechanism removing molecular debris from the surface

and exposing unmodified PDMS beneath (this behaviour, which was also attributed

to a cleaning mechanism, was seen in the behaviour of Si species discussed in

section 7.2.1.3). However, the organo-Si/C ratio eventually declined, which was

attributed to the completion of the cleaning mechanism, and the oxidation of the

PDMS main chain.

Figure 7-12: Atomic ratio for Si species on a UV/ozone treated PDMS surface

The oxidation of the PDMS main chain is also primarily responsible for the continual

increase in the inorganic-Si/C ratio, as the PDMS main chain is oxidised into other Si

containing species. This behaviour was also discussed in section 7.2.1.4.

0

0.25

0.5

0.75

1

1.25

1.5

1.75

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5UV/Ozone treatment time (hours)

Atom

ic R

atio

(Si/C

)

Organo-Si Inorganic-Si


7.3 UV/Ozone Pretreatment Requirements for PDMS Autohesion Pretreatments were performed and then the oxidized surfaces were manually

pressed together to test for autohesion. Figure 7-13 indicates that at a working

distance of 30.1mm, PDMS requires approximately 1.25 hours of exposure in order

to sufficiently oxidize the surface for autohesion to occur. At exposure levels above

this overexposure does not appear to be a significant problem, with autohesion

remaining viable up to at least 4.5 hours.

Figure 7-13: Autohesion of PDMS after UV/ozone exposure at 30.1 mm working distance

Figure 7-14 shows the influence of ageing time on autohesion tests. After 1.5 hours

of UV/ozone exposure, PDMS autohesion is still feasible until at least 3 minutes after

exposure has been stopped. By contrast, after 2 hours of UV/ozone exposure, the

PDMS can be aged for less than 1 minute before autohesion will no longer be

feasible. After 2.5 hours exposure, autohesion was not feasible after just 0.5

minutes of ageing in air.

Autohesion Of PDMS Surfaces as a Function of UV/Ozone Exposure Time

0

0.2

0.4

0.6

0.8

1

1.2

0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25 3.5 3.75 4 4.25 4.5

UV/Ozone Exposure Time (hours)

Aut

ohes

ion

(0=N

o; 1

=Yes

)

Autohesion


In summary, Figure 7-14 demonstrates that the longer the PDMS is treated, the less

the PDMS can be aged before autohesion becomes unfeasible.

Figure 7-14: Influence of UV/ozone pretreatment time and post-exposure ageing on autohesion

Figure 7-14 suggests that overexposure will eventually prevent autohesion from

occurring, but this was not recorded in Figure 7-13. Determining precisely where

overexposure and the limits of autohesion intersect would require additional time.

Treatment Requirements for Oxygen Plasma Sealing of Microfluidic Composite Substrates with a PDMS Surface Layer

A number of samples were also treated with oxygen plasma in order to determine

what range of parameters could ensure that the PDMS will bond irreversibly to itself.

The treatment parameters and results are presented in Table 7-1. The number of

experiments was limited by time constraints towards the end of the project.

Influence of UV/Ozone Treatment Time and Post-Exposure Ageing Time on Autohesion of PDMS Surfaces

0

0.5

1

1.5

2

2.5

3

0 0.5 1 1.5 2 2.5 3 5

Post UV/Ozone exposure ageing time (minutes)

UV/

ozon

e ex

posu

re ti

me

(hou

rs)

UV/Ozone Exposure 1.5 Hours

UV/Ozone Exposure 2 Hours

UV/Ozone Exposure 2.5 Hours


The first number in column 2 of Table 7-1 is the oxygen pressure when treatment

was commenced, and the second number is the oxygen pressure when the

treatment was terminated.

Sample No.

Oxygen Pressure (mBar)

Power (W)

Frequency (kHz)

Treatment time

(seconds)

Bond Failure Mode

1 0.073-0.263 20 125 30 Cohesive

2 0.080-0.647 20 125 30 Cohesive

3 0.054-0.365 20 125 30 Cohesive

4 0.045-0.65 20 125 flash Adhesive

5 0.114-0.329 10 125 30 Cohesive

6 0.058-0.117 20 125 10 Cohesive

Table 7-1: Oxygen plasma treatment parameters and failure modes

Table 7-1 indicates that oxygen plasma is effective at producing autohesion between

PDMS surfaces. The bond failure mode was manually tested as either cohesive

(failure through the bulk of the specimen, not at the interface) or adhesive (failure at

the interface of the two specimens). The only instance where oxygen plasma

treatment resulted in an adhesive failure was when a “flash” of plasma was used,

which means that the plasma was switched on and then switched off, so that the

there was only a “flash” of treatment.

Other than this, it appears that oxygen plasma will be able to produce autohesion in

PDMS samples over a range of parameters.

7.4 Assembled Multilayer Structures

The purpose of the PDMS oxidation and sealing step is to produce sealed

microchannels such as that show in Figure 7-11. Figure 7-11 is an SEM image of a

cross-sectioned channel. It shows a pair of microstructured composite layers on top

and bottom with a PDMS layer in between.


Figure 7-15: Cross section of PDMS layer between two embossed composite sections

A comprehensive investigation could not be achieved due to time constraints,

however the use of SEM cross-sections would have been very valuable for

evaluating stresses in the PDMS layer. For instance, measuring the thickness of the

PDMS in the section would have given an indication of whether it had been

compressed during processing.

7.5 Conclusions Chapter 7 investigated the UV/ozone and oxygen plasma treatment and ageing of

PDMS surfaces. This was intended to determine the requirements for producing

autohesion of PDMS surfaces. At a working distance of 30.1mm, the minimum

required treatment for autohesion is between 1 and 1.5 hours.

Overexposure is not a significant problem, since autohesion is still feasible up to at

least 4.5 hours exposure. However, with increasing levels of exposure, the time

interval after exposure during which autohesion is still viable decreases, until after

approximately 2.5 hours exposure, at which point the surfaces must be brought into

contact immediately after exposure for autohesion to still be viable.


There are advantages to each procedure, but shorter treatment times would increase

manufacturing efficiency and reduce energy requirements, whereas a larger post-

treatment window would increase manufacturing flexibility (ie, the opposing PDMS

surfaces would not need to brought into contact immediately to produce autohesion).


8 Conclusions 8.1 Project Summary This project proposed to solve the problem of integration of a PDMS membrane into

a thermoplastic PC microfluidic device using the following process;

Figure 8-1: Layout of the Embossing Process

Commencing from step 2, a sheet of PDMS is exposed to UV/ozone and then

adhesively laminated onto the polycarbonate sheet. The PC/PDMS laminate is then

re-exposed to UV/ozone to cure the adhesive (step 3). The purpose of step 2

(PDMS surface oxidisation) is to enable good adhesion to the adhesive, and thus to

the PC. This process of PDMS surface oxidisation for adhesion enhancement was

studied in Chapter 4. Re-exposing the assembled structure to UV/ozone was

required because the adhesive used was a UV curing adhesive. The process of

curing the adhesive in the assembled microstructure was studied in Chapter 4 as

well.

Hot embossing could be performed once the assembled structure was cured. This

was studied in Chapter 5 and Chapter 6. Chapter 5 achieved proof of concept using

a semi-circular tool of low aspect ratio, as well as modelling the embossed channel

1

2

Polycarbonate sheet

Expose sheet PDMS to UV/ozone

5

6

3

4

Adhesively laminate treated PDMS onto PC, and

re-expose to UV/ozone to cure adhesive

Microstructure the composite material Via hot embossing


Press oxidised PDMS surfaces

together to bond


dimensions in terms of embossing parameters. Chapter 6 achieved proof of concept

for a rectangular tool, and modelled the dimensions of the embossed channel in

terms of. Chapter 6 also optimised the UV/ozone exposure conditions for tool

replication.

Once a three-walled micro channel had been fabricated, a common technique for

PDMS to PDMS bonding (oxidative autohesion) was utilised to seal the channel

(steps 5 and 6). The extent of surface oxidation required for PDMS to PDMS

oxidative adhesion was researched in Chapter 7.

8.2 Project Novelty A novel microfluidic substrate material was fabricated by using Loctite 3105 UV

curable adhesive to laminate UV/ozone pretreated PDMS to polycarbonate.

Hot embossing of this novel microfluidic substrate material to produce a

microstructured substrate of PC with an adhesively bonded PDMS layer at the

surface was also novel to the extent that hot embossing has not been attempted on

such a material before. This meant that the hot embossing investigation revealed

information such as the optimal UV/ozone exposure for hot embossing (Chapter 6).

A sealing technique already well established in the literature for PDMS is the use of

oxidative media (such as oxygen plasmas or corona discharges) to produce a highly

activated PDMS surface that can chemically adhere to itself upon contact.

8.3 Project Conclusions

8.3.1 Fabrication of Novel Microfluidic Substrate Material The proposed approach of integrating PDMS elastomeric membranes into

microfluidic packages was demonstrated to be feasible.

Laminating UV/ozone pretreated PDMS to PC with a UV curable adhesive was

particularly successful, resulting in the development of strong adhesion. As the

strength of adhesion increased, the mode of delamination changed from adhesive

delamination between the adhesive and the PDMS, to cohesive failure of the PDMS

itself (Figure 8-2).


Figure 8-2: Evolution of delamination mechanism between PC and PDMS as amount of UV/ozone surface treatment increases.

The UV/ozone treatment requirements for producing strong PC to PDMS adhesion

were low. Only a few minutes pretreatment could results in a substantial increase in

bond strength. For lap shear testing, failure stress increased from zero at no

UV/ozone pretreatment or curing, to approximately 0.4 MPa (individually estimated)

or 0.5 MPa (single area value), whilst for peel testing, failure stress increased from

zero to between 0.7 and 0.8 MPa.

In particular, the amount of UV/ozone pretreatment of PDMS is the key parameter for

increasing the bond strength, as indicated by mechanical peel testing in Chapter 4.

The peel strengths mentioned in the previous paragraph (0.7 – 0.8 MPa) were

achieved using UV/ozone pretreatment times of 18 minutes or greater and UV/ozone

curing times of 5 minutes or greater.

8.3.2 Hot Embossing with a Semi-circular Tool A variety of substrate materials were embossed using a semi-circular tool at a variety

of temperatures, pressures and times. DOE software was used to minimise the

required number of experiments, calculate relationships between parameters and

optimise processing conditions. The conclusions from each set of substrate

materials are presented below.

PDMS PC Adhesive

PDMS PC

Adhesive

Delamination Delamination


(1) Polycarbonate In embossed PC, channel width and ratio of width to depth could be modelled using

a first order equation, and (using several approximations for the shape of the

channel) these could subsequently be used to derive equations for channel depth,

and the area of the channel cross section.

Simultaneous optimisation of channel width and ratio of width to depth produced a

number of processing solutions, in particular temperature approximately 164 oC,

embossing time 5 minutes 30 seconds, and embossing pressure 18.9 Bar. These

solutions are a very close match for the dimensions of the tool, with error margins of

less than 3%.

(2) Polycarbonate that has been embossed through a non-bonded layer of PDMS

For this system, sheet PDMS was laid over PC, and then embossing was performed

through the PDMS and into the PC. The PDMS was then removed, and the

dimensions of the embossed structures in the PC were analysed.

The following dimensions could be modelled by DOE software:

- channel depth

- channel depth at left shoulder

- channel depth at right shoulder

- cross sectional area

- ratio of width to depth

Based on the equations for the above dimensions, an equation for channel width

could be derived as well.

Optimisation of the processing conditions for this system indicated that, while the

system could be accurately modelled, it could not be optimised to match the

dimensions of the embossing tool (except for channel depth).

Based on the modelling that was done, the best result that could be achieved with

the materials and parameters used would be ratio of ratio to depth twice as high as


that for the tool; cross sectional area 50% higher than that for the tool; and depth

15% lower than that for the tool. This is likely due to the thick layer of PDMS

present.

To further develop this technique, thinner sections of PDMS would have to be used,

but that is outside the scope of this project.

(3) Adhesively bonded laminated composite of PC and PDMS fabricated using 5 minutes pre-treatment time and 25 minutes curing time

DOE did not identify any patterns in the data for this system that could be modelled

by first or second order equations. This was attributed to the adhesion between the

PDMS and PC being likely not sufficient to withstand the stresses imposed on it by

the embossing process.

Referring to Chapter 4, at 5 minutes pre-treatment time and 25 minutes curing time,

the failure stress of the joint (0.2 – 0.4 MPa in peel testing) is about half that that can

be achieved using higher pre-treatment times.

(4) Adhesively bonded laminated composite of PC and PDMS fabricated using 20 minutes pre-treatment time and 25 minutes curing time

The following dimensions could be modelled in terms of embossing parameters

using DOE software:

- channel depth

- channel depth from left shoulder

- channel depth from right shoulder

- ratio of width to depth

- channel cross sectional area

- area of left section

- area of right section

Using the models for these dimensions, equations for channel width, channel width

at left shoulder and channel width at right shoulder were derived as well.


For this system (as for PC and PC through PDMS), channel depth could be modelled

to very closely match the reference. One the best numerical solutions (100.00% of

the reference) being 164 oC embossing temperature, 3 minutes 20 seconds

embossing time, and 36.3 Bar embossing pressure. According to the DOE model,

this should give a channel depth that is equal to the reference depth.

Although they could be modelled, most of the other dimensions for this system were

between 200% and 300% of the reference. It is likely that the relatively thick PDMS

layer (250 µm thickness) prevented accurate resolution of the other features, but this

is beyond the scope this project and would require further study to confirm whether

thinner PDMS layers, such as 50 or 100 µm thick, would enable accurate embossing

resolution.

In summary, embossing with the semicircular shim in a composite substrate was

proved feasible and the relationship between embossed channel dimensions and

embossing parameters could be modelled using DOE software. However, the

adhesion between the PDMS and PC needs to be sufficient to withstand the

embossing process. When this adhesion is maximised, such as for the 20/25

system, all the dimensions could be modelled using equations obtained directly from

DOE or derived from DOE models.

8.3.3 Hot Embossing with Rectangular Tool In Chapter 5, embossing into a composite microfluidic substrate with a simple

semicircular tool was demonstrated to be reproducible and could be modelled using

DOE.

The focus of Chapter 5 was on proving that the composite substrate could be

successfully embossed, and modelling how embossing time, pressure and

temperature affected the dimensions of the embossed substrates. In Chapter 6, the

embossing was investigated with a rectangular tool, and the focus was shifted to

understanding how the UV pre-treatment parameters of the substrate manufacturing

process affected the embossing process in the composite material.


The following sections detail the conclusions for each substrate materials:

(1) Polycarbonate Channel depth at middle, and channel width at bottom were embossed to a good

degree of accuracy, within 10% deviation from reference, and these results were

often independent of embossing time and pressure.

However, both wall angles and the width at the top of the channel were quite difficult

to emboss accurately under any conditions, though the accuracy was better at higher

pressures and longer embossing times.

A significant problem throughout the embossing process was poor consistency

between results in the same series, in that incremental increases in an embossing

parameter did not produce a discernible pattern in the data. This could be due to the

complexity of the flow of molten polymer during embossing, where very small

changes in orientation of the substrate during embossing produce substantial

changes in the results.

Additional control measures were implemented in the 2nd stage embossing to

attempt to reduce this problem, and were significant to a small degree in reducing

the variability and improving the precision of the results.

The optimal parameters for embossing in polycarbonate were determined to be 14.6

Bars of substrate pressure and 5 minutes embossing time. These were thus used as

the standard embossing conditions, while the affect of the UV treatment parameters

of the substrate manufacturing process was investigated.

(2) Non-DOE Composite Embossing

Embossing of the composite material clearly demonstrated that embossing into the

composite material was feasible, and that the UV exposure parameters used for

fabrication of the composite material had a clear impact on the accuracy of the

embossed channel dimensions, especially for the channel depth at middle and the

wall angles.


(3) DOE Composite Embossing Using DOE, it was possible to model the following dimensions in terms of UV

exposure parameters:

- total cross sectional area

- top width

- middle depth

- depth at left shoulder

- depth at right shoulder

Using these DOE models, it was possible to derive non-DOE models for the other

channel dimensions.

These empirical relationships provide a method for controlling the dimensions of the

embossed channels based upon the UV manufacturing parameters and the

embossing parameters. It was determined that the optimal UV treatment parameters

were 25 minutes UV pre-treatment time, and 40 minutes UV curing time.

8.3.4 General Embossing Conclusions The research conducted in Chapters 5 and 6 prove that a laminated composite of

PDMS and PC can be embossed in a reproducible way with both semi-circular and

rectangular tools. The dimensions of the embossed channels can also be modeled

in terms of both the embossing parameters and the UV exposure parameters. In

summary, this technique can be used to fabricate microfluidic structures in a

controlled fashion.

8.3.5 Sealing of PDMS Microstructures Chapter 7 investigated the UV/ozone and oxygen plasma treatment and ageing of

PDMS surfaces. This was intended to determine the requirements for producing

autohesion of PDMS surfaces. At a working distance of 30.1mm, the minimum

required treatment for autohesion is between 1 and 1.5 hours.

Overexposure is not a significant problem, since autohesion is still feasible up to at

least 4.5 hours exposure. However, with increasing levels of exposure, the time


interval after exposure during which autohesion is still viable decreases, until after

approximately 2.5 hours exposure, at which point the surfaces must be brought into

contact immediately after exposure for autohesion to still be viable.

There are advantages to each procedure, but shorter treatment times would increase

manufacturing efficiency and reduce energy requirements, whereas a larger post-

treatment window would increase manufacturing flexibility (ie, the opposing PDMS

surfaces would not need to brought into contact immediately to produce autohesion).

8.4 Analysis of Methodology and Conclusions Although the use of DOE to predict embossing results was quite successful, a

problem throughout the project, was the difficulty experienced in obtaining

reproducible embossing results that demonstrated clearly the influence of embossing

parameters on embossing results.

A number of factors regarding the embossing apparatus were not optimal for the

generation of reproducible and reliable results, including;

(1) The large heavy duty pressure pump, which was probably not suited for

precise control of the embossing pressure over a period of several minutes.

This was the only hot embossing pump available for the majority of the

project, so it was not possible to use another system. An electrically

controlled hot embosser, which has excellent control over pressure and

position would have been par preferable.

(2) The absence of any automated system for de-embossing of the tools from the

embossing substrate. Due to the non-stick properties of PDMS, mold stick

was not significant for the composite materials, but mold stick was likely a

problem for embossing in PC. An attempt was made to mitigate this problem

by always de-embossing the same way, but this would only have provided a

partial solution, and funding was not available for a proper de-embossing

system.

(3) A lack of any system for precisely controlling the position of the embossing

stack inside the embossing chamber. Even if this had been possible, the

constant use of the apparatus (and not only by this project) resulted in

significant wear and tear on the surfaces of the hot embossing plates, which


would have produced slight variations over time in pressure distribution even

if accurate positioning had been possible.

These factors probably combined to produce embossing results that were difficult to

interpret.

8.5 Analysis of Results and Recommendations for Future Research

The results obtained for embossing of the composites were optimized using the

parameter set available and for the tools that were used. This resulted in reasonably

accurate values for channel depth at middle however for all the other dimensions the

results, while optimized, were still poorly resolved on an absolute basis.

Therefore any substantial improvement in the results on an absolute level cannot be

obtained simply by altering the parameters. For instance the tools could be made

narrower in order to make the channel width at top more accurate, but this would

only make the results for channel width at bottom worse, since they are already too

small.

If more accurate results are to be obtained on an absolute value, it is likely that the

thickness of the PDMS surface layer would have to be reduced. This however would

introduce significant complications to the process. For instance a thinner section of

PDMS will not be able to deform as much as a thicker section, and so may rupture

during embossing.

An alternative solution is to still use a PDMS surface layer, not in the form of a

laminate, but in the form of a surface segregated PDMS copolymer additive, by

synthesizing copolymers of PDMS and thermoplastic and compounding these as an

additive into another thermoplastic.


Figure 8-3: Outline of potential alternative to the use of laminated structures

A variety of thermoplastics have been copolymerised with PDMS including such

common microfluidic substrates as polycarbonate and polymethyl methacylate. The

silicone segments of the copolymer additive will segregate to the surface creating a

silicone rich surface, while the thermoplastic components serve to integrate the

copolymer into the bulk polymer, either directly or in concert with a compatibilizer.

This concept was not pursued for a variety of reasons. Primarily, a significant period

of time had elapsed in the PhD before this concept was developed, reducing the time

available. This was a particularly significant consideration given the ambitious scope

of the concept. Also, although the technology for copolymerising PDMS with

thermoplastics, while having substantial mention in the literature, had not resulted in

significant numbers of commercial products, meaning the copolymerisation

technology would have to be mastered as a part of the project, and the institute

where the project was conducted had few facilities for synthetic chemistry.

In summary this concept was considered not practical given the time and resources

available.

Starting with bulk material Blending in a PDMS copolymer Low-surface energy PDMS segregates to the surface


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