electrothermal hybrids for soft robotic applications · 2019. 11. 15. · table 2-5. thermal...
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Electrothermal Hybrids for Soft Robotic Applications
by
Yu-Chen Sun
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Mechanical and Industrial Engineering
University of Toronto
© Copyright by Yu-Chen Sun 2019
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Electrothermal Hybrids for Soft Robotic Applications
Yu-Chen Sun
Doctor of Philosophy
Mechanical and Industrial Engineering
University of Toronto
2019
Abstract
Throughout the evolution of human technologies, electronic-based actuators have been
unreplaceable. These actuators can provide superior power and speed in comparison to biological
tissue. However, complexity and bulkiness are two of the major weaknesses that prevent
electronic-based actuators from being implemented in biomimicking and biomedical applications.
Recent advances in material sciences have suggested the possibility of flexible material-based
actuators. These “smart material” actuators can be constructed by combining different composite
materials, and motion or deformation can be observed once the actuator is subjected to a proper
stimulus. This thesis presents a series of studies that focus on the development of novel active
hybrid composites. Starting with a well-known polymeric blend shape memory polymer (SMP)
and electrothermal actuator (ETA) system, the composites were first modified with nanoparticles
to achieve room temperature deformability. The plasticizing effect was then utilized to enhance
the actuation temperature, while conductive filler was implemented to enable electroactive ability.
Lastly, a novel 4D printing fabrication process was presented, which demonstrated the possibility
of using the material-based actuators in artificial muscles and soft robotic applications.
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Acknowledgments
I would like to use this opportunity to express my gratitude to everyone who supported me
throughout the course of my PhD program. I cannot thank enough to my parents, friends, and the
rest of my family for enormous amount of support they have provided.
I would also like to acknowledge my PhD supervisor, Professor Hani Naguib for the tremendous
amount of guidance, inspirations, and opportunities he has provided. As a truly dedicated
supervisor, his encouragement in research is one of the essential elements for the complementation
of this work.
Special thanks to my lifelong friends, Nazanin Khalili, Harvey Shi, Eunji In, and Marco Chu. I am
deeply grateful for all their supports and encouragement. This work would not have been possible
without you. Acknowledgement also goes to my past and present colleagues from the Smart
Polymers can Composites Lab (SAPL) for providing me with endless help in research.
Financially this work was supported by the Natural Sciences and Engineering Research Council
(NSERC) of Canada, Ontario Graduate Scholarship (OGS), Ontario Centres of Excellence (OCE),
Mitacs agency, Governments of Ontario and Canada.
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Table of Contents
Table of Contents
Acknowledgments ............................................................................................................... iii
Table of Contents ................................................................................................................. iv
List of Tables .....................................................................................................................viii
List of Figures ....................................................................................................................... x
List of Symbols, Abbreviations and Nomenclature .............................................................. xv
Chapter 1 Introduction .......................................................................................................... 1
1.1 Preamble..................................................................................................................... 1
1.2 Definition of soft robotics smart active materials ....................................................... 2
1.3 Design of electrically and thermally active materials ................................................. 5
1.3.1 Shape Memory Polymer .................................................................................... 5
1.3.2 Electrothermal actuator ..................................................................................... 7
1.3.3 4D printing and 4D active materials .................................................................. 9
1.4 Motivation and Objectives ........................................................................................ 10
1.4.1 Objectives ....................................................................................................... 12
1.5 Organization of the Thesis ........................................................................................ 13
1.6 Contribution ............................................................................................................. 15
1.7 References ................................................................................................................ 16
Chapter 2 Room Temperature Deformable Shape Memory Composite with Fine-tuned
Crystallization Induced via Nanoclay Particles .............................................................. 22
2.1 Introduction .............................................................................................................. 23
2.2 Experimental ............................................................................................................ 26
2.2.1 Materials ......................................................................................................... 26
2.2.2 Composite Fabrication .................................................................................... 26
2.2.3 Composite Characterization ............................................................................ 27
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2.3 Results and Discussion ............................................................................................. 29
2.3.1 SEM & Morphologies ..................................................................................... 29
2.3.2 Energy-dispersive X-ray Spectroscopy (EDS) ................................................. 31
2.3.3 FTIR ............................................................................................................... 34
2.3.4 DSC ................................................................................................................ 36
2.3.5 X-ray Diffraction ............................................................................................ 41
2.3.6 TGA ............................................................................................................... 43
2.3.7 Storage Modulus and Tan Delta ...................................................................... 45
2.3.8 Shape Memory Recovery ................................................................................ 47
2.4 Conclusion................................................................................................................ 51
2.5 References ................................................................................................................ 52
Chapter 3 Toward the Low Actuation Temperature of Flexible Shape Memory Polymer
Composites with Room Temperature Deformability via Induced Plasticizing Effect ...... 57
3.1 Introduction .............................................................................................................. 58
3.2 Experimental ............................................................................................................ 62
3.2.1 Materials ......................................................................................................... 62
3.2.2 Composite Fabrication .................................................................................... 62
3.2.3 Composite Characterization ............................................................................ 63
3.3 Results and Discussion ............................................................................................. 65
3.3.1 SEM & Morphology ....................................................................................... 65
3.3.2 FTIR ............................................................................................................... 66
3.3.3 DSC ................................................................................................................ 67
3.3.4 TGA ............................................................................................................... 70
3.3.5 Storage Modulus and Tan Delta ...................................................................... 71
3.3.6 Rheology ........................................................................................................ 73
3.3.7 Shape Memory Properties ............................................................................... 74
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3.4 Conclusion................................................................................................................ 79
3.5 References ................................................................................................................ 80
Chapter 4 Hybrid Electroactive Shape Memory Polymer Composites with Room
Temperature Deformability ........................................................................................... 84
4.1 Introduction .............................................................................................................. 85
4.2 Materials & Experimental ......................................................................................... 90
4.3 Results and Discussion ............................................................................................. 91
4.3.1 Polymer Blend Morphologies and SWCNT Dispersions.................................. 91
4.3.2 Chemical Bonding Verification ....................................................................... 94
4.3.3 Thermal Properties and Degradation Behaviours of SMP Composites ............. 95
4.3.4 Rheological Behaviour .................................................................................... 99
4.3.5 Mechanical Property Characterization ........................................................... 101
4.3.6 Shape Memory Characterization – High Temperature Deformation............... 104
4.3.7 Room Temperature Deformability................................................................. 107
4.3.8 Electrical Conductivity, Joule heating, and Electroactive Demonstration ....... 109
4.4 Conclusion.............................................................................................................. 111
4.5 References .............................................................................................................. 112
Chapter 5 Shape Programming of Polymeric-based Electrothermal Actuator (ETA) via
Artificially Induced Stress Relaxation .......................................................................... 117
5.1 Introduction ............................................................................................................ 118
5.2 Materials & Experimental ....................................................................................... 121
5.2.1 Fabrication of Dual Layer ETA ..................................................................... 121
5.2.2 Characterization ............................................................................................ 122
5.3 Results and Discussion ........................................................................................... 123
5.3.1 ETA Material Properties ............................................................................... 123
5.3.2 ETA Morphology .......................................................................................... 126
5.3.3 Actuation of ETA.......................................................................................... 127
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5.3.4 Shape Programming of ETA ......................................................................... 131
5.3.5 PDMS post-curing and thermo-mechanical verification................................. 135
5.3.6 Nuclear magnetic resonance analysis ............................................................ 139
5.3.7 ETA Applications with Different Programmed Shape ................................... 140
5.4 Conclusion.............................................................................................................. 143
5.5 References .............................................................................................................. 144
Chapter 6 4D Printed Hybrids with Localized Shape Memory Behaviour: Implementation in
Functionally Graded Structure ..................................................................................... 148
6. Introduction .............................................................................................................. 149
6.2 Materials & Experimental ....................................................................................... 153
6.3 Results and Discussion ........................................................................................... 155
6.3.1 Plasticizer Induced Microscopic Morphological Changes .............................. 155
6.3.2 Chemical Composition Verification .............................................................. 157
6.3.3 Thermal Behaviour of Plasticized PLA Composites ...................................... 158
6.3.4 Viscoelastic and Rheological Characterizations ............................................. 161
6.3.5 Temperature Dependent Mechanical Properties............................................. 162
6.3.6 Characterization for Tensile Shape Memory Properties ................................. 164
6.3.7 Plasticized 4D Materials and Their Recovery Behaviours.............................. 166
6.3.8 Functionally Graded 4D Material with Localized Recovery Ability .............. 168
6.4 Conclusion.............................................................................................................. 170
6.5 References .............................................................................................................. 171
Chapter 7 Conclusions and Recommendations .................................................................. 177
7.1 Conclusions ............................................................................................................ 177
7.2 Recommendations for Future Works ....................................................................... 180
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List of Tables
Table 2-1. Isothermal crystallization half time under different isothermal temperature conditions.
................................................................................................................................................ 37
Table 2-2. Thermal properties of PLA/TPU/MMT composites ................................................. 40
Table 2-3. Effect of annealing on thermal properties of PLA/TPU/MMT composites ............... 40
Table 2-4. Summary of 2θ and d-spacing of different PLA/TPU/MMT composites .................. 42
Table 2-5. Thermal degradation of different PLA/TPU composites .......................................... 44
Table 3-1. Summary of thermal properties of PLA/TPU/PEG composites ................................ 69
Table 3-2. Summary of initial degradation temperatures and maximum weight loss (PLA/TPU)
................................................................................................................................................ 70
Table 3-3. Summary of initial degradation temperatures and maximum weight loss (PEG) ...... 70
Table 3-4. Summary of tan Delta peak temperature of different PEG content ........................... 72
Table 4-1. Electroactive SMP performance from literature ....................................................... 86
Table 4-2. Summary of thermal properties of PLA/TPU/PEG/CNT composites ....................... 97
Table 4-3. Degradation temperature of PLA/TPU/PEG and PLA/TPU/CNT composites .......... 98
Table 4-4. Degradation temperature of PLA/TPU/PEG/CNT composites ................................. 98
Table 5-1. Thermal properties of PDMS ..................................................................................125
Table 5-2. ETA performance comparison chart ...................................................................130
Table 5-3. Programming and actuation behaviour comparison of ETA and SMP .....................133
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Table 6-1. Summary of thermal properties of PLA/PEG composites........................................159
Table 6-2. Summary of initial degradation temperatures and maximum weight loss ................160
Table 6-3. Mechanical properties under room temperature testing condition ............................163
Table 6-4. Mechanical properties under 70oC testing condition ...............................................163
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List of Figures
Figure 1-1. Smart active materials and their corresponding mechanics. ...................................... 4
Figure 2-1. SEM images of (a, b) PLA/TPU, (c, d) PLA/TPU/1 wt% MMT, and (e, f)
PLA/TPU/3 wt% MMT ............................................................................................................ 30
Figure 2-2. EDS spectrum of PLA/TPU/MMT composites ...................................................... 31
Figure 2-3. EDS mapping of PLA/TPU (a) SEM image, (b) carbon/nitrogen overlay, (c) carbon
mapping, and (d) nitrogen mapping .......................................................................................... 32
Figure 2-4. EDS mapping of PLA/TPU/3MMT (a) SEM image, (b) carbon/nitrogen overlay, (c)
carbon mapping, (d) nitrogen mapping, (e) oxygen mapping, and (f) silicon mapping .............. 33
Figure 2-5. FTIR Absorption spectra of PLA, TPU, PLA/TPU, and PLA/TPU/MMT .............. 35
Figure 2-6. (a) Complete FTIR data of different PLA/TPU/MMT sample and (b) close up view
of TPU characterization peaks .................................................................................................. 35
Figure 2-7. Isothermal DSC curve for (a) 100°C, (b) 110°C, (c) 120°C, and (d) 130°C ............ 37
Figure 2-8. Crystallization half time (a) effect of MMT and (b) effect of isothermal temperature
................................................................................................................................................ 38
Figure 2-9. DSC heating curve of (a) PLA/TPU/MMT and (b) annealed PLA/TPU/MMT ....... 39
Figure 2-10. XRD measurement of different PLA/TPU/MMT composites ............................... 41
Figure 2-11. Thermal degradation of PLA/TPU/MMT composites .......................................... 43
Figure 2-12. (a, b) Storage modulus and tan delta of PLA/TPU/MMT composites, and (c, d)
annealed composites................................................................................................................. 46
Figure 2-13. Shape memory cycle for (a) PLA/TPU, (b) PLA/TPU/1MMT, (c)
PLA/TPU/2MMT, and (d) PLA/TPU/3MMT composites ......................................................... 48
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Figure 2-14. Shape recovery ratio (Rr) and shape fixing ratio (Rf) of different wt% MMT in
PLA/TPU composites ............................................................................................................... 49
Figure 3-1. SEM images of (a, b) PLA/TPU; (c, d) PLA/TPU/5%PEG; (e, f)
PLA/TPU/10%PEG; (g, h) PLA/TPU/15%PEG; and (i, j) PLA/TPU/20%PEG ........................ 65
Figure 3-2. FTIR absorption spectra of PLA/TPU and PLA/TPU/PEG .................................... 66
Figure 3-3. (a) DSC curves of pristine materials, (b) DSC curves of TPU/PEG & PLA/PEG
composites, (c)DSC curves of PLA/TPU/PEG composites and (d) thermal degradation of
PLA/TPU/PEG composites ...................................................................................................... 68
Figure 3-4. (a) Storage modulus and (b) tan delta of different PLA/TPU/PEG composites ....... 72
Figure 3-5. Viscosity of different PLA/TPU/PEG composites.................................................. 73
Figure 3-6. Shape memory cycles of (a) 0PEG, (b) 5PEG, (c) 10PEG, (d) 15PEG, (e) 20PEG,
and (f) Rf under two different shape fixing conditions ............................................................... 75
Figure 3-7. (a) Recovery ratio (Rr) and (b) recovery time of PLA/TPU/PEG composites under
two different recovery temperatures environment ..................................................................... 77
Figure 3-8. Shape recovery of PLA/TPU/PEG composites in 50°C water bath: top to bottom: 0,
10, 20 wt% PEG; left to right: 0, 5, 10, 20 seconds ................................................................... 78
Figure 3-9. Shape recovery of PLA/TPU/PEG composites in 70°C water bath: top to bottom: 0,
10, 20 wt% PEG; left to right: 0, 5, 10, 20 seconds ................................................................... 78
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Figure 4-1. Plasticizing effect enhanced CNT dispersion and stretchability enhancement ........ 88
Figure 4-2. Design of PEG/CNT hybrid SMP composites ........................................................ 89
Figure 4-3. SEM images of PLA4TPU6 with (a, d) PEG0; (b, e) PEG5; (c, f) PEG10 ............. 92
Figure 4-4. SEM images of PLA4TPU6 with (a, g) CNT2; (b, h) CNT2PEG5; (c, i)
CNT2PEG10; (d, f) CNT4; (e, k) CNT4PEG5; (f, l) CNT4PEG10 ........................................... 92
Figure 4-5. TEM images of (a, b) PLA/TPU/CNT4 and (c, d) PLA/TPU/CNT4/PEG5 ............ 93
Figure 4-6. FTIR spectra of different PLA/TPU/PEG/CNT composites ................................... 94
Figure 4-7. (a, b, & c) DSC curves; (d, e, f) TGA curves of PLA/TPU/PEG/CNT composites . 96
Figure 4-8. Viscosity of PLA/TPU/PEG/CNT composites ......................................................100
Figure 4-9. (a, b, c) Stress-strain curves; (d, e, f) storage and (g, h, i) loss modulus of different
PLA/TPU/PEG/CNT composites.............................................................................................103
Figure 4-10. Shape memory cycles of different PLA/TPU/PEG/CNT composites ...................106
Figure 4-11. (a, b) Room temperature stretched shape memory cycle of different
PLA/TPU/PEG/CNT composites; (c) Rr & Rf of under 80°C stretching; (d) Rr & Rf under room
temperature stretching .............................................................................................................108
Figure 4-12. (a) Electrical conductivity, (b) Joule heating performance and (c) Joule heating
time of different PLA/TPU/PEG/CNT composites (d) Demonstration of electrical shape recovery
ability of CNT2PEG10 sample ................................................................................................110
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Figure 5-1. (a) SWCNT film fabricated from solvent casting, and (b) flexing of SWCNT film
...............................................................................................................................................121
Figure 5-2. Dimension of U-shape actuator ............................................................................123
Figure 5-3. Cross sectional SEM images of fabricated SWCNT/PDMS ETA ..........................126
Figure 5-4. Cross sectional SEM images of fabricated SWCNT/PDMS ETA ..........................128
Figure 5-5. Actuation, tip displacement, and bending angle of PDMS ETA ............................129
Figure 5-6. Shape programming demonstration of (a) SWCNT/PDMS ETA (b) pure PDMS and
(c, d) after programming .........................................................................................................131
Figure 5-7. Stress relaxation (left) and the following strain recovery (right) of pure PDMS and
PDMS + CNT composites under different condition................................................................134
Figure 5-8. a) Isothermal of PDMS curing behaviour, b) DSC temperature ramp of different
PDMS samples, c) 2 & 4 hours 80oC curing behaviour with respect to time, and d) 2 & 4 hours
80oC curing behaviour with respect to temperature ..................................................................136
Figure 5-9. a) Storage and loss modulus of PDMS & PDMS+CNT ETA under temperature
ramping, b) derivative of the storage modulus, c) heat-and-hold experiment with respect to
temperature, and d) heat-and-hold experiment with respect to time .........................................138
Figure 5-10. NMR spectrum of PDMS ...................................................................................139
Figure 5-11. ETA actuation from a programmed curled shape and temperature changes
compared to non-programmed (flat) configuration ..................................................................141
Figure 5-12. ETA crawling soft robot and the actuation behaviour .........................................142
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Figure 6-1. Basic building blocks for 4D printing development ..............................................150
Figure 6-2. Geometry of 3D printed arc-shape component ......................................................153
Figure 6-3. Fracture surface of (a, d) PLA, (b, e) 10PEG, and (c, f) 30PEG ............................155
Figure 6-4. 3D printed fracture surface of (a, d) PLA, (b, e) 10PEG, and (c, f) 30PEG ...........156
Figure 6-5. FTIR absorption spectra of PLA/PEG composites ................................................157
Figure 6-6. (a) DSC curves of PLA/PEG composites, (b) thermal stability of PEG at 180oC, (c)
thermal degradation and (d) the derivative of thermal degradation curves ................................159
Figure 6-7. (a) Dynamic frequency sweep, (b) temperature sweep from dynamic mechanical
testing; (c) storage/loss moduli and (d) viscosity measurement ................................................162
Figure 6-8. Stress strain curves of PLA/PEG composites at (a) room temperature and (b) 70oC
...............................................................................................................................................163
Figure 6-9. 3D shape memory cycle for different PLA/PEG composites .................................165
Figure 6-10. (a) 4D printed PLA arc and (b) flattened temporary shape ...........................166
Figure 6-11. Thermal recovery of 4D printed (a) PLA, (b) 10PEG, and (c) 30PEG component
...............................................................................................................................................167
Figure 6-12. 4D printed functionally graded model.................................................................169
Figure 6-13. Thermal recovery of 4D functionally graded model ............................................169
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List of Symbols, Abbreviations and Nomenclature
CNF Carbon nanofiber
CNT Carbon nanotube
CTE Coefficient of thermal expansion
DEA Dielectric elastomer actuator
DMA Dynamic mechanical analyzer
DSC Differential scanning calorimetry
ΔHcc Cold crystallziation enthalphy
ΔHm Mleting enthalphy
EAP Electroactive polymer
EDS Energy-dispersive X-ray spectroscopy
ETA Electrothermal actuator
εm Applied strain
εp Recovry strain
εu Stress-free strain
FDM Fused deposition modeling
FGM Functionally graded material
FTIR Fourier transform infrared spectroscopy
HA Hydroxyapatite
IR Infrared radiation
IPMC Ionic polymer metal composite
MAG-PLA Maleic anhydrided-grafted poly(lactic acid)
MEMS Microelectromechanical systems
MMT Montmorillonite
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MWCNT Multi-walled carbon nanotubes
NMR Nuclear magnetic resonance spectroscopy
OCL Oligo(ɛ-caprolactone)diol
ODX Oligo(p-dioxanone)diol
PAC Printed active composite
PAE Polyamide elastomer
PE Polyethylene
PEB Poly(ethylene-co-butylene)
PCL Poly(ϵ-caprolactone)
PDLLA Poly(D,L-lactide)
PDMS Polydimethylsiloxane
PEG Poly(ethylene glycol)
PLA Poly(lactic acid)
PLLA Poly(L-lactide)
PMMA Poly(methyl methacrylate)
POA Poly(octylene adipate)
PPDL Poly(ω-pentadecalactone)
PU Polyurethane
PVAc Poly(vinyl acetate)
PVDF Poly(vinylidene fluoride)
Rf Shape fixing ratio
Rr Shape recovery ratio
SBS Poly(styrene-butadiene-styrene)
SEM Scanning electron microscopy
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SLA Stereolithography
SMA Shape memory alloy
SME Shape memory effect
SMP Shape memory polymer
SWCNT Single-walled carbon nanotube
TEM Transmission electron microscopy
TGA Thermogravimetric analyzer
TPU Thermoplastic polyurethane
Tcc Cold-crystallization tempearture
Tg Glass transition temperature
Ti Initial degradation temperature
Ti, max Maximum weight loss slope
Ti, max, 1 First maximum weight loss slope
Ti, max, 2 Second maximum weight loss slope
Tm Melting temperature
Tpermanent Permanent temperature
Ttrans Transitional temperature
t1/2 crystallization half time
UV Ultraviolet
WPU Waterborne polyurethane
wt% Weight percent
XRD X-ray diffraction
Xc% Percent crystallinity
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Chapter 1
Introduction
1.1 Preamble
The development of actuators can be traced back to the early industrial revolution when it was
discovered that steam power could be harvested to power a variety of mechanical structures and
devices. In comparison to human or animal muscles, steam-powered machines do not experience
fatigue and could achieve much higher efficiency. Inventors and engineers all over the world have
since designed and built machines that can perform various motions to do work like lifting,
rotating, and re-positioning. Parallel to the evolution of machining, the use of electrical energy has
played a significant role in the progression of engineering development. Electricity can be easily
transmitted through a conductive material or stored as an electrical potential by electrochemical
reactions. Electricity has replaced most of the massive steam engines and has become the primary
energy source powering our society today.
The use of more complex and efficient actuators has allowed for machines to become faster and
more powerful, with precisions beyond what biological muscles can achieve. However, these
design-specific actuators often perform poorly when removed from their typical operation modes
to carry out other discontinuous or non-repetitive tasks. Despite the great force output and response
speed, traditional motors and actuators face a series of challenges in the progress of mimicking
human motions. Complex transmission systems and designs are also required for the construction
of muscle-like actuators. These unavoidable secondary systems increase the overall
weight/bulkiness and add to the manufacturing cost. Researchers have expressed interest in
overcoming these challenges by replacing the bulky electrical and mechanical systems with bio-
inspired designs. It was discovered that certain groups of material could change their physical
dimensions when external signals, such as electrical or thermal stimuli, are applied to them. The
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behavior and response of these materials are extremely similar to biological tissues, and these so-
called “artificial muscle” materials would be suitable for biomedical applications where muscle-
like motion is required.
There were several challenges posed during the development of material-based actuators. These
range from the understanding of fundamental material properties to manufacturing and
implementing the smart materials. The purpose of this research is to provide insight into the
essential microscale material parameters and their implication on the macroscopic actuator
behaviour. Chapter 1 starts by introducing different categories of smart active material. Their
history and corresponding actuation mechanisms are discussed in detail. The motivations and
objectives of the research are also addressed. Finally, the chapter concludes with a brief layout of
the following chapters.
1.2 Definition of soft robotics smart active materials
Soft robotics is an emerging research field and can be defined as the practices of replacing
traditional motors/actuators with low compliance materials to generate motions [1-3]. These
materials are able to undergo large amount of dimensional or geometrical changes when subjected
to a proper stimulus, which can vary from electricity, light, heat, or even solvent induced chemical
reactions [4-6]. Compared to conventional robotics systems which are usually mechanically
driven, these biomimicking or bio-inspired technologies, or artificial muscle materials, are highly
flexible and can be easily manipulated to suit the desirable functionalities that range from wearable
textiles to industrial applications. The greatest advantages offered by these materials are flexibility
and adaptability, which make them favorable in applications related to biomimicking, and
complex/non-repeatable tasks. One of the currently research focuses of soft robotic materials is
the development of pneumatic based actuator [7-9]. These actuators have the capability to generate
motions by changing the amount of air or liquid pressure within a flexible tentacle-like polymeric
based tube that is designed to either to expand or contract in order to respond to the pressure
difference. It has been demonstrated that such materials have the ability to grip and manipulate
complex features and tools [4, 10, 11].
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The fundamental definition of smart materials is the ability to respond to environmental changes
[12]. By using this special characteristic, the behaviour of the material can be controlled by
manipulating the material properties. The smart materials can then be implemented into a wide
range of applications in healthcare, biomedical, structure, and aerospace sectors [13-17]. One of
the most popular applications is to employ smart active material as artificial muscles [18-20].
Compared to traditional motors, material-based actuators offer advantages such as superior
flexibility, better biomimicry, lower cost, and higher power-to-weight ratio. These properties make
them more suitable for many applications involving lightweight prosthetics and soft robotics [20-
23]. Furthermore, some of these actuators have even demonstrated performances that surpass
biological muscles [9, 18].
Figure 1-1 shows the categories that the active material can be placed based on their actuation
mechanisms. Their microstructure and properties can respond to various stimuli, such as light
(optically active), heat (thermally active), and electric fields (electroactive) [24-27]. Deformation
or geometrical changes can be observed once the material has been subjected to the appropriate
stimuli; hence, their suitability as an actuator. The variation in actuation mechanisms can also
affect their actuation performance. For instance, the deformation produced by ceramic based
ferroelectric or piezoelectric actuators are highly accurate and controllable but occur on the
microscale level [28]. In comparison, dielectric elastomer actuators (DEA) can produce record-
breaking strain but require an extremely high electric field for actuation [29, 30]. Conductive
polymers or ionic polymer metal composites (IPMC) only require a minimum amount of electrical
power, but the actuation can only be observed when the material is present in an ionic-rich liquid
environment [31, 32]. Consequently, the choices of material-based actuation are usually
application dependent and must consider the possible and available stimuli.
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Figure 1-1. Smart active materials and their corresponding mechanics.
Smart Active Materials
Optically Active Chemically Active Thermally Active Magnetoactive Electroactive
MagnetostrictiveShape Memory EffectIonic Electronic
Dielectric Elastomer
Shape Memory
Polymer (SMP)
Shape Memory Alloy
(SMA)
FerrorelectricIonic Polymer Metal
Composites (IPMCs)Conductive Polymer
Polymeric Gels Chiral Materials
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1.3 Design of electrically and thermally active materials
Over the past few decades, the development of smart materials had focused on the enhancement
of electrical and thermal activation. The input voltage/current can be easily controlled by
laboratory-scale instruments, while the variation in environmental temperature can be adjusted in
an air oven or water bath. Furthermore, the electrical energy that is imparted can be converted into
thermal energy through the Joule heating phenomenon. Among the thermally activated
mechanisms, the shape memory effect and thermal expansion dominate as they can produce the
best actuation performance. The following sections focus on shape memory polymers and
electrothermal actuators, as well as possible manufacturing procedures for these materials.
1.3.1 Shape Memory Polymer
The shape memory effect (SME) is defined as the material’s ability to change back to a predefined
shape after deformation. For the recovery to take place, an external stimulus is necessary. This
stimulus can be heat, light, electrical, or magnetic fields [33-39]. Two of the most common shape
memory materials are shape memory alloys (SMAs) [40-42] and shape memory polymers (SMPs)
[35, 43, 44]. The actuation mechanism of SMA is driven by the phase transformation between
three crystalline structures: twinned martensite, detwinned martensite, and austenite. These
transformations can be induced by temperature differences or applied stress. For example,
austenite can be transformed into detwinned martensite by stretching, and the strain recovery can
take place after unloading at high temperature. The advantages of SMA are their high stress (>200
MPa), high strain rate (300%/s), and large strain (>5%) [45]. However, the major drawback of
SMAs is the significant hysteresis during phase transformation and their limited lifetime;
activation strain can drop from 5% to 0.5% after millions of cycles [46, 47].
In contrast to the SMA, SMPs are constructed from two distinct soft and hard polymer phases. The
status of the SMP is determined by the transitional temperature (Ttrans). Below Ttrans, the SMP is
rigid. When SMPs are heated above Ttrans, the material is in the soft/rubbery state. In such a state,
the SMP can transform into a new configuration by applied stress or deformation on the material.
The decrease in temperature to below Ttrans “fixes” the SMP into a new/temporary shape because
of the hard/glassy state. If heat is applied to the “fixed” SMP, the material state returns to the
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rubbery state, recovering the overall geometry [35, 44, 48]. As a result, crosslinked polymers or
semi-crystalline are good candidates for SMP application. The crosslinked regions or “nodes”
serve as the hard phase for memorizing the shape while the amorphous regions are responsible for
the shape recovery process once the temperature is above Ttrans [44, 49]. To characterize the shape
memory performance, two parameters, shape recovery ratio (Rr) and shape fixing ratio (Rf), are
often used. Rr is associated with the ability to maintain its temporary shape while the latter is used
to determine the recovery ability. These parameters can be calculated by using Equation (1.1) and
Equation (1.2), as shown below:
𝑅𝑟 =𝜀𝑚 − 𝜀𝑝
𝜀𝑚 (1.1)
𝑅𝑓 =𝜀𝑢𝜀𝑚
(1.2)
where εm is the amount of strain applied, εp is the strain after shape recovery, and εu is the strain
after removing the deformation stress. For high-performance SMPs, both Rr and Rf approach
100%, which indicate that the material can maintain its temporary shape (Rf = 100%) and fully
recover to its permanent shape (Rr = 100%)
Compared to crosslinked thermoset polymers, thermoplastics have the advantage of remoldability
and reusability. This can be achieved because their primary polymer chains are not chemically
crosslinked [50]. Also, their shape recovery behaviours can be easily tailored by adjusting the two
different polymeric matrices [51]. Because of its biocompatibility, polylactic acid (PLA) is one of
the most studied semi-crystalline thermoplastic SMPs. The shape memory of polylactic acid was
first reported by Zhang et al. in 2009, during the touching process [52]. The group reported that
the ductility of PLA can be significantly enhanced by blending with an elastomer (5.1% to 161.5%
and 194.6% with 5 and 10 wt% of elastomer). Along with the increase in stretchability, the SME
can also be observed as PLA no longer exhibits the brittle fracture behaviour. The phrase
“polymeric blend SMP” can be used to describe this multi-phase system since the fabricated SMP
is constructed from two immiscible polymers [44, 51]. PLA-based SMPs can be created from
blending PLA with polyvinyl acetate (PVAc) [44], polyethylene glycol (PEG) [53, 54], or
thermoplastic polyurethane (TPU) [55-59].
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1.3.2 Electrothermal actuator
The operating principle of Electrothermal actuator (ETA) is governed by the thermal expansion of
the material. The kinetic energy of individual molecules increases and vibrate more at higher
temperatures. These vibrations impose larger separations between molecules leading to an
observable volumetric expansion. One of the earliest applications to utilizing such an effect was a
micro-sized actuator made from silicon for microelectromechanical systems (MEMS) [60, 61] and
in situ microscopy testing for nanomaterials [62, 63]. To induce actuation, resistive Joule heating
is commonly employed. As the degree of thermal expansion of silicon can be predicted with high
accuracy, micro or even nanoscale movement can be precisely controlled by these actuators.
Several analytical models and studies have demonstrated that the output stress and strain for
silicon-based thermal actuator can be accurately described [60, 62, 64]. According to these models,
both the actuator geometry and material properties play important roles in determining actuation
behaviour. For example, the electrical resistivity and thermal conductivity describe the electro-
thermal relationship and energy conversion efficiency while the material’s Young’s modulus and
coefficient of thermal expansion (CTE) is related to actuation behaviour. Although the MEMS
thermal actuator can be considered as a well-established field, the micro/nanoscale displacements
are too small for other applications.
Researchers have also demonstrated that certain polymeric composites systems possess the same
actuation properties as silicon-based thermal actuators. Compared to silicon-based ETA, these
polymeric based materials have a much greater CTE and can produce a significant amount of
deformation. Since most of the polymers are electrical insulators, the ETA composites are
constructed with reinforcement fillers that are highly conductive. An applied voltage to the
composite material provides Joule heating to the polymer matrix through the conductive fillers,
thereby activating the matrix. The very first polymeric based ETA was proposed by Chen et al. in
2008; the group utilized Joule heating to drive the thermal expansion of an elastomer based
composite containing polydimethylsiloxane (PDMS) and 5 wt% of carbon nanotubes (CNTs) [65].
Under 52V or 1.5V/mm electric field, the composite was able to reach a maximum strain of 4.4%
after a temperature increase to 180°C. In comparison to other electroactive polymers reported in
the literature, ETAs demonstrate large deformation, fast response time, and low electrical field
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requirement. It has been identified that CTE controls the overall ETA deformation performance;
nevertheless, there are very limited studies that focus on enhancing the CTE of the bulk polymer.
An alternative CTE enhancement approach was proposed by Samel et al. that uses expandable
microspheres, which can undergo a large amount of volumetric increase after heating [66, 67]. The
initial designs proposed by the group can be implemented in microfluidic systems as micro-valves
or pumps; however, the major disadvantage of the composite is that it can only provide one-time
actuation due to the irreversible expansion. Despite this, an astonishing volumetric expansion of
over 270% was reported from this design. One-time expansion actuators should not be overlooked
as they might be useful for specific applications, such as single-use smart bandages or fasteners.
Compared to the single layer CNT/PDMS film proposed in [65], Zeng et al. coated pure PDMS
onto a PDMS/CNT film and created a double layer structure. This structure enables bending
motion due to the different CTE of the two layers [68]. To further increase the actuation
performance, proposed strategies such as fiber alignment can be implemented. Chen et al.
demonstrated that by replacing randomly distributed CNT with ultra-aligned CNT sheets or “bulky
paper”, the voltage required for actuation was significantly reduced [69]. The same research group
also showed that the actuation behaviour has a strong relationship with the orientation of the bulky
paper: if the electrical current is applied in the fiber direction, less heat and actuation motion is
generated because of the lower resistance. These examples demonstrate the potential of using
polymeric ETA materials for artificial muscle applications due to their low voltage requirement
and high displacement/force response.
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1.3.3 4D printing and 4D active materials
4D printing is a relatively novel fabrication concept which combines smart active materials and
3D printing technologies [70-73]. The 3D printing techniques enable the creation of complex
geometries, while the printed active materials allow the fabricated components to be capable of
responding to external stimuli. The 4th dimension is described as “time” as the shape or geometry
components can be designed to shift or move in the desired fashion. Similar to non-printed smart
materials, a proper environmental change, such as changes in temperature [74-76] or water
presence [70, 77, 78], is required to trigger the motion.
One of the earliest 4D printing reports was provided by Ge et al., who used the phrase “printed
active composites” (PACs) to describe the materials [79]. The group combined polymer fibers with
shape memory properties and an ultraviolet (UV) curable elastomer matrix. As the fibers and
elastomer matrix have different properties, mechanical deformation can be applied to the printed
composites, and the thermal difference can be used to recover the PAC to its original shape.
4D printing technologies consists of four fundamental elements: a 3D printing platform, printable
active materials or composites, the desired activation mechanism, and lastly, an external stimulus
for initiate the response of the material [73]. These elements have a strong correlation and are
interdependent. It is important to identify a proper 3D printer platform that can be used to print the
active material. However, not all active materials are 3D printable, and their activation
mechanisms can also vary. Lastly, the fabrication or 3D process can also influence the material
properties, and these changes might also impact the actuation behaviours of the printed 4D
material. As a result, all four elements need to be carefully considered.
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1.4 Motivation and Objectives
Both SMP and ETA are well-established research topics from literature and have the potential to
be implemented in a variety of fields, such as artificial muscles or soft robotic applications.
Nevertheless, these material-based actuators are still in the research and development phase. To
prepare the material-based actuator for real-world applications, several factors, such as material
performances, fabrication processes, and implementation abilities, need to be carefully considered.
Before the material-based actuators can be used to replace traditional electronic-actuators, they
need to demonstrate a vast increase in actuation performances. These performances are often
characterized into possible deformation, maximum force output, and response time. For material-
based actuators, the actuation behaviour strongly depends on the material properties. By
understanding these properties, it is possible to achieve a significant amount of deformation. This
research investigated two distinct material-based actuators, the shape memory polymer (SMP)
composites and polymeric based electrothermal actuator (ETA). Each study presented in this
research started from presenting a desired property for enhancement and how such improvement
can be achieved by inducing changes in material formulation. Due to these changes, the actuation
behaviour can be controlled, and a more effective material-based actuator can be created.
Conventionally, shape memory polymer needs to be heated to a temperature higher than its Ttrans
before deformation can be applied to change its geometry into its temporary shape. Below Ttrans,
the SMP cannot be re-shaped easily due to its high stiffness. Because of this property, the usability
of SMP is limited. It is proposed that a more efficient SMP can be created if the material can be
directly deformed under room temperature condition, thus bypassing the first heating cycle. The
stiffness and mechanical properties highly depend on the formulation of the composite. As a result,
it is essential to understand the basic material properties such as crystallinity, thermal behaviour,
and the mechanisms within the soft/hard phase interaction region to fabricate highly controllable
SMP composites with room temperature deformability. The soft/hard phase interaction can be
modified by using nanoparticles such as nanoclay, while the crystalline regions can be further
strengthened with an annealing effect.
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One of the biggest challenges for polymeric blend SMP is its high activation temperature. For
example, PLA/TPU blend SMP has an activation temperature close to 70°C as the Ttrans is
controlled by the glass transition temperature (Tg) of the PLA. The high activation temperature is
not suitable for applications related to the biomedical field. One of the desirable criteria for SMP
to be utilized in medical application is a recovery or activation temperature close to human body
temperature. In related work, crosslinked and network-based SMPs show domination over
polymeric based SMP as their Ttrans can be controlled by varying the degree of crosslinking. As
the initial recovery temperature is determined by Tg, the activation behaviour can be controlled by
using the plasticizing effect. The polymeric chain movement can be enhanced with plasticizer, and
a reduction in Tg can be expected. Because of this effect, an SMP with low activation temperature
can be created.
A rise in environmental temperature is a necessity to initiate the recovery behaviour for thermally
activated SMP. Such an effect is difficult to induce in real-world applications. Instead, the increase
in temperature can be induced by Joule heating, which converts electrical energy into thermal
energy. A common method to facilitate this is to incorporate conductive nanoparticles, such as
carbon nanotubes (CNTs), into the SMP system. However, CNT reinforced SMPs usually suffer
low tensile strain due to the high stiffness of CNT. The low tensile strain also limits the recovery
performance of SMP. It is proposed that the plasticizing effect can be used to enhance the recovery
performance as it promotes the PLA/TPU interaction. The novel SMP would possess the same
advantages as the previous studies (room temperature deformability and low actuation
temperature) while still being able to be activated by an electrically induced Joule heating effect.
The polymeric based ETA reported in the literature is often constructed by coating a thick layer of
thermally expandable thermoset material (such as polydimethylsiloxane, PDMS) onto a
conductive buckypaper made of conductive nanoparticles. ETA can only be activated from a flat
configuration because of this fabrication process. To further widen the possible applications, the
initial actuation configuration needs to be programmed into different shapes. By utilizing post-
curing and thermally induced stress relaxation behaviour, the ETA can be trained to remember a
certain geometry and actuated from such a state. This is suitable for the ETA to be directly
fabricated into a usable shape for soft robotic applications.
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4D printed materials are often fabricated from UV-curable composites as such materials can be
easily formulated. In compassion, thermoplastic materials need to be fabricated using fused
deposition modeling (FDM). Despite the low market cost and convenience, little studies focus on
4D printing development of FDM 4D printing technologies. A huge advantage of FDM is its ability
to create functional graded material (FGM). Due to the different material composition in each
layer, the 4D printed FDM can be designed with specific actuation behaviour such as localized
deformation under different temperature condition.
1.4.1 Objectives
The primary objective for this research is to investigate novel composites materials that can be
implemented as a material-based actuator with the potential of being used in soft robotic
applications. To achieve this goal, two different thermal active materials, ETA and SMP, are
investigated. In this context, the objectives and sub-objectives are defined as the following:
➢ Shape memory polymers with room temperature deformability
➢ Shape memory composites with a low activation temperature
➢ Electroactive room temperature deformable SMP with a low activation temperature
➢ Electrothermal actuator and its shape programming ability
➢ 4D printing of a localized actuation surface utilizing a functional graded layer
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1.5 Organization of the Thesis
The research was divided into three main parts: the development of novel SMP composites
(Chapter 2, 3, and 4), ETA performance enhancement (Chapter 5), and futuristic 4D fabrication
method (Chapter 6). In each chapter, a specific active material was studied with a focus on targeted
performance enhancement. The enhancement was achieved by establishing hypotheses from
literature reported behaviour, such as nanoparticle-induced crystallinity changes and plasticizing
effect. A parametric study was conducted to verify the hypotheses and actuation motions were
demonstrated.
Room temperature deformable SMP composites were fabricated by fine-tuning the crystallinity of
PLA/TPU shape memory composites using montmorillonite (MMT) nanoclay. The results indicate
that the incorporation of MMT can improve the compatibility of the two immiscible polymers.
Also, the presence of MMT affects crystallinity and improves mechanical properties. The room
temperature deformability was demonstrated by applying uniaxial stretching deformation to the
SMP composites. With 1 wt% MMT nanoclay, a 95% recovery ratio was achieved. The SMP
composites also exhibit good fixing ability higher than 95% as the plastic deformation was applied
at room temperature condition.
Low activation temperature polymeric blend SMP was created by utilizing the plasticizing effect.
Small amounts of poly(ethylene glycol) (PEG) were used as a plasticizer, which can lower the Tg
of the SMP composites. Due to this incorporation, significant changes in morphologies and thermal
behaviour were reported. The SMP performances (activation temperature, shape fixing ability,
shape recovery ratio, and recovery time) were improved due to the induced plasticizing effect.
With 5 wt% PEG, the transitional temperature of PLA/TPU was decreased from the typical 75°C
to 50°C while a high 85% recovery ratio was maintained. An improvement of 37.5% shorter
recovery time can also be observed with the SMP containing 10 wt% PEG at 50°C environment.
These results indicated that plasticizing is an effective method for altering the polymer chain
structures and can be used to develop fast response SMPs with low activation temperature.
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An electroactive polymeric based hybrid SMP was developed by incorporating single-walled
carbon nanotube (SWCNT) into PEG plasticized PLA/TPU composites. Conventionally, the CNT
conductive fillers have a negative impact on SMPs due to their high tensile strength. Thus, a
reduction in the elongation is often observed with CNT reinforced SMP, which worsen shape
recovery performance. Given that plasticizer can be included to alter the microstructure of the
SMPs, a positive effect on SMP composites with conductive fillers can be expected. The PEG can
lower the activation temperature while enhancing dispersion of SWCNT. Also, it was observed
that the presence of SWCNT can stabilize the SMP system by providing additional mechanical
support. By carefully controlling the formulation of the composite, an electroactive hybrid SMP
can be created by optimizing the amount of SWCNT and PEG plasticizer.
As polymeric ETA has high potential to be used in futuristic soft robotic applications, and it is
important that ETA with different geometries can be fabricated. Previous ETA studies strongly
focused on increasing electrical conductivity and Joule heating ability by super-aligning carbon
nanotube (CNT) sheets. The resulting ETA was often formed into a flat configuration. This study
presents the programmability of ETA. The composites material can be programmed into the
desired configuration by utilizing an induced stress relaxation mechanism and post-secondary
curing. It was demonstrated that an ETA can be programmed into a curled resting state. In such a
state, the actuator can achieve a bending angle greater than 540°. The shape programming feature
also enabled tailoring of the actuator configuration to a specific application, such as a bioinspired
crawling soft robot that mimics the motion of an inchworm.
4D printing is the combination of smart active materials and 3D printing techniques. Give that an
external stimulus is required, shape memory effect was commonly employed as the activation
mechanism. Current studies primarily focus on SMPs constructed from photopolymerizable
composites. In comparison, fused deposition modeling (FDM) can produce 3D printed structures
from thermoplastic material and can be used to create functionally graded layers. This study
demonstrates that functionally graded PLA 4D structure with different degrees of plasticization
can be fabricated from the FDM technique. In each layer, a different amount of PEG plasticizer
was incorporated into the PLA, which modified the polymer chain motions. Due to the difference
in plasticization, each layer can be actuated under different temperatures, and a 4D material with
localized actuation ability was created.
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1.6 Contribution
The major contribution of this thesis is the development of novel material-based actuation
composites that can be implemented in futuristic soft robotics or artificial muscle applications.
New composite hybrids were developed using the knowledge gained from the literature and
combined with novel polymeric mechanisms. These mechanisms range from microscopic
crystallization behaviour to novel 3D fabrication techniques. To summarize, the main
contributions are:
➢ Development of room temperature deformable SMP via tailoring crystalline phases of the
SMP with MMT nanoclay
➢ Development of low Tg and close-to-body actuation temperature SMP via induced PEG
plasticizing effect
➢ Development of electroactive SMP with both room temperature deformability and low
activation temperature
➢ Investigation of shape programming ability of ETA via the induced stress relaxation effect
and soft robotics implementation
➢ Demonstration of 4D printed functionally graded hybrid with localized actuation ability
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Chapter 2
Room Temperature Deformable Shape Memory Composite with Fine-
tuned Crystallization Induced via Nanoclay Particles 1
It is known that particular types of semi-crystalline/elastomer polymer blends exhibit shape
memory effects (SME) due to the dispersion of two immiscible phases. In this study, the crystal
structure of polylactic acid (PLA)/ thermoplastic polyurethane (TPU) based shape memory
polymer (SMP) is altered by incorporating small amounts of montmorillonite (MMT) nanoclay.
The results indicate the incorporation of MMT can improve the compatibility of the two different
polymers. Moreover, the presence of MMT affects the total crystallinity of the SMP and improves
mechanical properties. Lastly, uniaxial stretching deformation can be applied to the SMP at room
temperature conditions while maintaining its shape memory properties. With 1 wt% MMT
particles, the recovery ratio (Rr) was nearly 95%, which indicated a strong recovery effect. The
shape fixing ratio (Rf) remained above 95% for all composites due to plastic deformation applied
at room temperature.
1 The content of this chapter has been published in the Journal of Polymer Science Part B: Polymer
Physics;
Sun, Y., Cai, S., Ren, J. and E. Naguib, H. (2017), Room temperature deformable shape memory composite with fine‐
tuned crystallization induced via nanoclay particles. J. Polym. Sci. Part B: Polym. Phys., 55: 1197-1206.
doi:10.1002/polb.24370. Reproduced by permission from John Wiley and Sons (License # 4617680192905).
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2.1 Introduction
Shape memory polymers (SMPs) are a special class of polymers that have the ability to maintain
one or more temporary shapes and revert back to their initial shape once exposed to an external
stimulus [1]. Such stimuli can be light, heat, or solvent-induced chemical reactions [2-7]. The
shape memory effect (SME) of different types of SMPs have been well documented over the past
few decades, with thermally activated SMPs being studied the most [4]. Thermally activated SMPs
usually consists of two different phases: a soft and thermally reversible phase responsible for
maintaining a temporary shape, and a hard phase that has the ability to “memorize” a permanent
shape. Furthermore, the shape recovery proccess of thermaly activated SMPs can be characterized
by two critical temperatures: the permanent temperature (Tperm) for programming the permanent
shape of the SMP, and the transition temperature (Ttrans) for triggering the recovery from a
temporary shape back to its permanent shape. For physically cross-linked thermoplastic polymers
or cross-linked block copolymers, Tperm is determined by the degree of cross-linking at the
molecular level. As a result, the melting temperature (Tm) of the polymers can be utilized for
programing its permanent shape [4].
When a SMP is heated above its Ttrans, a large drop in the elastic modulus of the soft phase can be
observed. Under such condition, deformation can be applied to the SMP to form its temporary
shape. This temporary shape can then be fixed by reducing the temperature of the SMP to below
its Ttrans so that the micro-Brownian movement within the polymer chain can be frozen. By re-
heating the SMP in its temporary shape to a temperature above Ttrans, the SMP would then be able
to recover back to its permanent shape. As a result, it is common to select the soft phase based on
a polymer with a glass transition temperature (Tg) equivalent to the desired Ttrans [8]. Due to the
special properties of SMPs, it is proposed that they can be used in a wide variety of applications
such as actuator textiles [9, 10], aerospace [11], and biomedical devices [12, 13]. Lan et al.
demonstrated a prototype solar panel hinge that can be activated by SMP composites [14]. Bye et
al. developed a morphing SMP skin that has to potential to be used in shape shifting unmanned air
vehicles [15]. Xue et al. fabricated a fast self-expandable SMP stents that can be used as biomedical
insert [16].
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SMPs can be further classified into two distinct group: covalently/physically cross-linked polymer
network structures [17-19] and polymer blends [4, 20]. For network based SMPs, the hard phases
are usually in droplet form and act as the cross-linking nodes while the soft phases act as the
switching phase. In contrast, polymer blend SMPs consists of two different polymers that are
immiscible: a crystalline polymer that acts as the hard phase, and an amorphous polymer that acts
as the switching phase. Examples includes poly(ω-pentadecalactone) (PPDL)/poly(ϵ-
caprolactone) (PCL) copolymer [21], polyurethanes (PU)/PCL [22], poly(styrene–butadiene–
styrene) tri-block copolymer (SBS)/PCL [20], polyvinylidene fluoride (PVDF)/polyvinylacetate
(PVAc) [4], and polylactic acid (PLA)/PVAc [4]. Compared to network SMPs, polymer blend
based SMPs are easier to fabricate. Furthermore, their activation and shape recovery mechanism
can be tailored by carefully designing the ratio of amorphous/crystalline polymer [20]. Recently,
PLA has become one of the most popular polymer choices for the crystalline polymer in SMP
composites due to its mechanical properties and biocompatibility[23-25]. One of the earliest
studies on PLA based SMPs was reported by Zhang et al. in regards to the toughening process of
a PLA matrix [26]. The purpose was to modify the brittleness of PLA by blending it with
polyamide elastomer (PAE), which is a polymer that has both polyether and polyamide groups.
Previous research have demonstrated that the polyether groups in PAE have good compatibility
with PLA, while the polyamide groups form good interfacial polymer-polymer interactions with
PLA [27]. By incorporating 5 and 10 wt% of PAE into the PLA matrix, the elongation at break
increased from 5.1% to 161.5% and 194.6% respectively. Moreover, the PLA/PAE blend also
exhibited SME properties: 92% recover ratio (Rr) was reported for the 10 wt% PAE sample after
100% stretching. Ever since this discovery, intensive studies on PLA based SMP composites have
been conducted. For instance, Yuan et al. fabricated a fully bio-based SMP by combining PLA,
natural rubber, and thermoplastic vulcanizates together [28]. Huang et al. verfied that SMP
behaviour can be observed from polylactide-b-poly(ethylene-co-butylene)-b-polylactide (PLA–
PEB–PLA) triblock copolymers and that its perfomance can be controlled by adjusting the
crystallizability of the PLA segments [29]. Samuel et al. reported a novel SMP composite that has
multi-shape memory capability by integrating poly(L-lactide) (PLLA) with poly(methyl
methacrylate) (PMMA) [30].
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In order to induce SME in PLA based materials, the incorporation of soft segments is required,
which can done by blending in thermoplastic polyurethane (TPU) [23-25, 31, 32] and polyethylene
glycol (PEG) [33, 34]. TPU is widely selected due to its biocompatibility and the fact that it can
be easily blended within the PLA matrix. Since SMP performance and behavior highly depends
on its crystalline structure and hard/soft phase interface, therefore it is essential to understand basic
polymer properties including crystallinity and the mechanisms within the soft/hard phase
interaction region. It is long known that the polymer’s crystalline structure can be altered
dramatically through the incorporation of compatibilizers or coupling agents. Recently, Zare
evaluated the SME of several different nanoparticle enhanced SMP composite systems and the
work indicated that polymer-particle interfacial regions play an important role in SMP activation
performance [35]. Montmorillonite (MMT) is a commonly used filler due to its large surface area
and high aspect ratio of its laminar structure [36, 37]. Zou et al. showed that octadecylammonium-
treated MMT filler has good compatibility when blend with TPU; an increase in mechanical
property was reported with only 3 wt% filler content [38]. Kim et al. intercalated sodium-modified
MMT particles with PEG polymer segments and found that the segments behave as physical cross-
linkers in the poly(ethyl methacrylate) matrix [39]. More recently, Kelnar et al. investigated the
influence of organophilized MMT when blended with PLA/TPU composites [40]. The group
reported that the TPU dispersion in the PLA matrix is dramatically improved in addition to
enhancements in crystallinity and polymer-polymer compatibility.
Despite the advances in research related to different SMP composite systems and filler-controlled
polymer integration, there are only a few studies that focus on adapting nanoparticle induced
crystalline/amorphous changes and applying such alteration for fine-tuning SMP behaviour. For
instance, Tan et al. reported that MMT can be used to improve the mechanical properties of
polyurethane-epoxy composites, but critical SME parameters such as glass transition temperature,
storage modulus, and shape recovery ratio remained the same [41]. In this study, we report the