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POLYMER STABILIZED CHOLESTERIC TEXTURES FOR SCATTERING-MODE PROJECTION LIGHT VALVES

A dissertation submitted to Kent State University in partial

fulfillment of the requirements for the degree of Doctor of Philosophy

by

Yeuk Keung Fung

December, 1994


UMI Number: 9534493

UMI Microform 9534493 Copyright 1995, by UMI Company. All rights reserved.

This microform edition is protected against unauthorized copying under Title 17, United States Code.

UMI300 North Zeeb Road Ann Arbor, MI 48103


Dissertation written by

Yeuk Keung Fung

B.A.Sc., University of Ottawa, Canada, 1981

M.A., Kent State University, 1992

Ph.D., Kent State University, 1994

HA QMiaJL*.

Approved by

Chair, Doctoral Dissertation Committee

Members, Doctoral Dissertation Committee

m .

Accepted by

jChair, Department of Physics

, Dean, College of Arts and Sciences

u


TABLE OF CONTENTS

List of Figures vii

List of Tables xviii

Acknowledgements xix

Chapter Page

1. Introduction 1

1.1 Brief History of Liquid Crystal Displays 1

1.2 Introduction to Polymer Stabilized Cholesteric Texture (PSCT) 4

1.3 Focus of the Dissertation 5

2. Liquid Crystal Displays: Principles and Applications 8

2.1 Properties of Liquid Crystals 8

2.1.1 Nematic and Cholesteric Phases 8

2.1.2 Dielectric and Optical Properties 11

2.1.3 Elastic Properties 12

2.2 Liquid Crystal Display Applications 13

2.2.1 Twisted Nematic Displays 13

2.2.2 Supertwist Birefringence Effect 16

2.2.3 Cholesteric-Nematic Phase Change Effect 18

2.2.4 Polymer Dispersed Liquid Crystals 20

iii


TABLE OF CONTENTS

2.3 Liquid Crystal Displays Addressing Technique 22

2.3.1 Passive Matrix Display 22

2.3.2 Active Matrix Display 25

3. Polymer Stabilized Cholesteric Textures 27

3.1 Materials 27

3.1.1 Monomers 27

3.1.2 Chiral Dopants and Liquid Crystals 29

3.2 Cell Fabrication 31

3.3 Photopolymerization 32

3.3.1 Set-up 32

3.3.2 Polymerization Process 33

3.4 Principles and Applications of PSCT 34

4. Polymer Networks in Liquid Crystals 38

4.1 Optical and Scanning Electronic Microscopy Studies of Polymer Networks 38

4.1.1 Introduction 38

4.1.2 Planar Alignment (I) 40

4.1.3 Planar Alignment (II) 43

4.1.4 Homeotropic Alignment by Chemical Treatment 46

4.1.5 Homeotropic Alignment by External Field 48

iv


TABLE OF CONTENTS

4.1.6 Monomer Concentration Effect 50

4.1.7 High Temperature Effect 56

4.1.8 Different Monomers 56

4.1.9 Frequency Effect on Curing 59

4.1.10 No External Field and Surface Effect 59

4.1.11 Cholesteric Liquid Crystal in an Electric Field 63

4.2 Birefringence of Polymer Networks 63

4.2.1 Introduction 63

4.2.2 Theory 69

4.2.3 Experimental Results and Discussions 74

5. Electro-optics of PSCT 87

5.1 Apparatus Set-up 87

5.2 Samples 89

5.3 Effects of Chiral Concentration 91

5.4 Polymer Concentration-dependent Response Time 94

5.5 Polymer Concentration-dependent Contrast 97

5.6 Polymer Concentration-dependent Drive Voltage 97

5.7 Polymer Concentration-dependent Hysteresis 100

5.8 Effects of Temperature 100

v


TABLE OF CONTENTS

5.9 Effects of UV Intensity 104

5.10 Wavelength Dependence 104

5.11 Angular Transmission 107

6. 320 x 320 PSCT Projection Display Prototype 113

6.1 Design Concept 113

6.2 Display Fabrication 120

6.3 System Implementation 122

6.4 Display Characteristics 129

6.5 Active Matrix 130

7. Conclusion 134

References 138

Appendices: A Modified Bessel Functions 142

B A 320 Line Mask for 4" x 4" Substrate 144

C Schematic Diagram of the Microcontroller Board 145

D Schematic Diagram of the Row Driver Board 146

E Schematic Diagram of the Column Driver Board 147

F Schematic Diagram of the Driver Board Connection 148

vi


LIST OF FIGURES

Figure Page

1. Schematic illustration of a PSCT system. Polymer network connecting

top and bottom plate for focal-conic and homeotropic textures: (a) light

scattered by the helical structure and sample appears opaque; (b) light

passes through in field on state and sample appears clear. 6

2. Three different phases of liquid crystal: (a) nematic phase where the

director is indicated by n; (b) isotropic phase and; (c) cholesteric phase

where p is the pitch. 9

3. A diagram showing the orientation of a liquid crystal molecule. 10

4. Illustrations of the three different types of elastic deformation of the

liquid crystals. 14

5. A diagrammatic illustration of a TN display: (a) polarized light rotated

by the liquid crystal and emerging from the analyzer; (b) polarized

light undisturbed by the liquid crystal and therefore absorbed by the

analyzer. 15

6. The 270° twist of a SBE cell is shown in (b). For comparison, the twist

is 90° in (a), a typical TN cell. 17

vii


LIST OF FIGURES

7. A diagram showing the three different phases of cholesteric liquid

crystal: (a) planar texture; (b) homeotropic state; and (c) focal-conic

texture. The arrows indicate the switching between the states under an

applied field. The fastest switching occurs when the homeotropic state

relaxes directly to planar state.

8. A typical PDLC sample: (a) in the OFF state where the liquid crystal

directors of the droplets are randomly oriented resulting in an opaque

state; (b) in the ON state, the liquid crystal aligns in the electric field

direction diminishing the scattering.

9. A diagram to illustrate the dot-matrix format. Each crossover point of

the ITO electrodes is a "pixel."

10. Active Matrix Liquid Crystal Display operation using MOS (Metal-

Oxide-Semiconductor) transistors. Not shown is a common connection

to all elements. The pixel is held "ON" (or "OFF") during the time

between addressing by virtue of the charge held on the drain terminal.

To address a pixel, a voltage is applied through the appropriate row (Y's)

to "open" the gate. The voltage on the respective column (X's) will

then appear across the pixel.

11. The chemical structures of the monomers and photoinitiator: (a) BAB;

(b) BABB6; (c) BAB6; and (d) BME.


LIST OF FIGURES

12. The chemical structures of the chiral dopants: (a) R1011; and (b)

CB15. 30

13. Voltage vs. transmission curves for: (a) cholesteric liquid crystal; and

(b) polymer stabilized cholesteric texture. 36

14. Dynamic response curves for: (a) cholesteric liquid crystal; and (b)

polymer stabilized cholesteric texture. 37

15. SEM image of the liquid crystal free polymer network. Note the

direction of the fibers running parallel to the rubbing direction. 41

16. Photograph of the liquid crystal free polymer network. Picture taken

with cross polarizers. The alignment axis of the polymer network (or

rubbing direction) is at 45° to the polarizers. 42

17. Photograph of the liquid crystal free polymer network. Picture taken

with cross polarizers. The rubbing directions on the top and bottom

plates are perpendicular to each other. Since both left and right hand

twist exist, defect lines appear in the juncture of these two different

twisted structures. The dark image on the picture is the defect line

captured by the network. 44

18. SEM image of the liquid crystal free polymer network. Note the part of

polymer folded on top of the other, the fiber directions are running

perpendicular to each other. 45

ix


LIST OF FIGURES

19. SEM image of the liquid crystal free polymer network. The large

openings are created when the solvent evaporates.

20. SEM image of the liquid crystal free polymer network. The network

connects the top and bottom plates via bundles of polymer fibers.

21. SEM image of the liquid crystal free polymer network. The network

retracts in all directions revealing the fibers and the bottom plate.

22. SEM image of the liquid crystal free polymer network. The sample is

tilted at 45° to the normal. The length of the fiber is ~14pm, close to

the cell spacing of 15pm.


tilted at 45° to the normal. The large empty space forms when the

polymer network of this area stays with the other plate during the

separation.


same as Fig. 23 but viewed from above.

25. SEM image of the liquid crystal free polymer network. The polymer

concentration is 1.2wt.%. No noticeable structure is observed with this

concentration.

x


LIST OF FIGURES


is BAB6 at a concentration of 2.7wt.%. Cured with an applied field,

the polymer appears to be irregular in both shape and size. 57


is BAB at a concentration of 3.5wt.%. The sample is tilted at 45° to the

normal. The network collapses onto the plate surface and appears like a

layer of polymer "beads" stacked together. 58

28. SEM image of the liquid crystal free polymer network. The polymer is

BAB6 at a concentration of 2.7wt.%. The frequency of the applied

electric field is 20Hz. The polymer does not seem to have the fiber like

structure but appears to be a thin layer of polymer. 60


BAB6 at a concentration of 2.7wt.%. The polymer appears to be a fiber

like structure and exhibits some local orientation. 61


BAB6 at a concentration of 2.7wt.%. Chiral dopant (2.2wt.%) is added

into the nematic liquid crystal and cured without field. The polymer

fibers appear to be randomly oriented because of the helical structure of

the liquid crystal. 62

xi


LIST OF FIGURES


BAB6 at a concentration of 2.7wt.%. Chiral dopant (2.2wt.%) is mixed

with the nematic liquid crystal and cured with an applied field. The

sample is tilted at 45° to the normal. The plate surface appears to be

partitioned with numerous polymer "walls."

32. A diagram of the apparatus set-up for measuring the birefringence of

the polymer network.

33. The columnar description of the polymer fiber with radius R. The order

parameter of the fiber is Sop. The order parameter of the liquid

crystal on the polymer fiber surface is S0.

34. A plot of the birefringence of the system as a function of the

temperature for different polymer concentrations.

35. A plot of the birefringence of the polymer network in an isotropic

solvent (Octane) as a function of the temperature for different polymer

concentrations.

36. Birefringence measurements of a BAB6/5CB sample. The curve is

a fit to Eq. (4.21) with So=0.3, R=50A and cp=0.01.


a fit to Eq. (4.21) with So=0.3, R=50A and cp=0.02.

xii


LIST OF FIGURES


a fit to Eq. (4.21) with So=0.3, R=50A and cp=0.025. 82





41. Birefringence measurements of a BAB6/5CB sample. The curves

are the fits to Eq. (4.21) with So=0.3, R=10A, 25A, 40A, 50A, 60A

and 80A; and cp=0.02. 85

42. A diagram of the apparatus set-up for studying the electro-optic

properties of the PSCT light valve. 88

43. SEM image of a cell gap. The polymer is BAB6 at a concentration of

2.7wt.%. Chiral dopant (2.2wt.%) is mixed with the nematic liquid

crystal and cured with an applied field. The cell is vacuum at

0.03mTorr for 20hrs. with the temperature set at 200°C. The fiber like

structure is hardly distinguished due to the liquid crystal still trapped in

the fiber. 90

xiii


LIST OF FIGURES

44. Phase diagram of the PSCT system. In the upper section, the liquid

crystal remains in homeotropic state even after the field is removed.

The middle section is a region that the focal-conic texture of the liquid

crystal is stabilized by the polymer network. The lower section

indicates that the focal-conic texture is not stable due to insufficient

polymer content.

45. Definition of the rise time (xr) and decay time (xj). xr is the time taken

for the transmission to reach from 10% to 90%. xd is the time from

90% to 10%.

46. Plots of the response time, contrast and drive voltage as a function of

the chiral concentration. The chiral dopant is CB15 and the polymer

(1.7wt.%) is a combination of equal proportion of BAB and BABB6.

47. Plots of the rise time and decay time as a function of the polymer

concentration for BAB, BABB6 and BAB6.

48. Plots of the contrast as a function of the polymer concentration for

BAB, BABB6 and BAB6.

49. Plots of the drive voltage as a function of the polymer concentration

for BAB, BABB6 and BAB6.

50. Definition of AV. AV is measured at the position indicated by 50% of

the transmittance.

xiv


LIST OF FIGURES

51. A plot of the hysteresis as a function of the polymer concentration for

BAB6. The chiral concentration is 2.2wt.%. 102

52. Plots of rise time, decay time, contrast, hysteresis and drive voltage as

a function of the temperature. The polymer is BAB6 at a concentration

of 2.7wt.%. The chiral dopant is R1011 at a concentration of 2.2wt.%. 103

53. Plots of rise time, decay time, contrast, and hysteresis as a function of

uv intensity. The polymer is BAB6 at a concentration of 2.7wt.%. The

chiral dopant is R1011 at a concentration of 2.2wt.%. 105

54. Plots of rise time, decay time, contrast, and hysteresis as a function of

uv intensity. The polymer is BAB6 at a concentration of 2.1wt.%. The

chiral dopant is R1011 at a concentration of 2.2wt.%. 106

55. A diagram of the apparatus set-up for measuring the light transmission

at different wavelengths. 108

56. A plot of the transmittance as a function of the wavelengths in the ON

and OFF states. The polymer is BAB6 at a concentration of 2.7wt.%.

The chiral dopant i sRIOl la ta concentration of 2.2wt.%. 109

57. A plot of the contrast as a function of the wavelengths for the same

sample as Fig. 56. 110

58. Plots of the transmittance as a function of the incident angle for the

same sample as Fig. 56. 112

xv


LIST OF FIGURES

59. A diagram showing the location of the bias voltage on a voltage vs.

transmittance curve. Points A and B are considered to be the lower and

higher limits of the bias voltage for the optimum performance. The

contrast decreases with time if the voltage below that of point A is

used. If the voltage is shifted beyond point B, the ON state is stable but

rather appears to be "washed out" due to increasing amount of light

leaking from those OFF pixels. 114

60. A plot of the hysteresis as a function of the voltage ramping rate. The

polymer is BAB6 at a concentration of 2.7wt.%. The chiral dopant is

R1011 at a concentration of 2.2wt.%. 116

61. A plot of the hysteresis as a function of the polymer concentration for

two different ramp rates: (O) 0.25 V/sec.; and (□) 0.0083 V/sec. 117

62. A plot of the transmittance as a function of the time with the applied

waveform illustrated at the top of the figure. V,=75 V and V0=l 1.2V. 118

63. A plot of the transmittance as a function of the time (curve a) with

applied waveform illustrated at the top of the figure. V0=l 1.2V. The

contrast is plotted as a function of the time (curve b). 119

64. A plot of the contrast as a function of the bias voltage V0. The contrast

is measured 3 Osec. after the application of the bias voltage. 121

xvi


LIST OF FIGURES

65. A schematic illustration of the projection light valve system using the

polymer stabilized cholesteric textures. 123

66. Photograph of the complete projection light valve system. 124

67. Photograph of the system projecting a picture image on a wall. 126

68. Photograph of the system projecting a text image on a wall. 127

69. A schematic illustration of the addressing scheme. In the beginning, all

the pixels are OFF. The ON pixels in the first row will have the column

voltages in opposite phase to the row voltage. The rest of the pixels in

the same row will have both voltages in phase. All other rows will have

zero voltages. 128

70. Photograph of the PSCT projection system operating on an active

matrix display based on MIM technology. 131

71. Photograph of a direct view PSCT display using MIM technology. 132

xvii


LIST OF TABLES

Table Page

1. List of the monomers used in the studies. 27

2. Physical properties of E48 and ZLI4389. 31

3. Calculated values for Sop from the experimental values of Aiip. 79

4. Display characteristics of a 320 x 320 pixel prototype. 129

xviii


ACKNOWLEDGEMENTS

I am deeply indebted to my advisor Dr. J. William Doane for his guidance and support.

His impeccable foresight always amazed me, but also served as inspiration to me. My

personal thanks go to Dr. Dengke Yang who taught me so much about life, in addition to all

the experimental work. I would also like to thank Elaine Landry for her help, especially with

my English, and advice in pursuing my personal goal. Although I spent very little time with

Dr. S. Zumer during his short visit to LCI, I wish to thank him for his generous and

experienced insight. I would like to thank Dr. L.-C. Chien for the initial supply of the

materials used in this research. I would also like to thank Dr. John West and Dr. Jack Kelly

for their always helpful and sincere guidance. I would also like to acknowledge Vari-Lite

Corporation and NSF ALCOM Center for supporting this research.

It was always difficult to keep my sanity while working in a lab full of equipment, I

would like to thank my fellow students X. Y. Huang and Z. J. Lu with whom I shared the

same lab, for raising my spirit. I would also like to thank Claude Boulic for bringing in

"color" and some French culture to this lab. I would like to thank Dr. Cliff Brumett, whose

condescending remarks always brought a lot of smiles, Mr. Hasan Khan, David Fredley,

Brian Cull, Merrill Groom and Brian Quinn for their friendship.

I am grateful to my family for their support and understanding. Any achievement on my

part should be credited to Dad, Mom, brother Yeuk Kin, and sister Yuet Lin whose courage

in fighting personal trauma made me value life more.

xix


XX


Chapter 1

Introduction

1.1 Brief History of the Liquid Crystal Display (LCD)

The early work on liquid crystals dates back to the 19th century. In 1888, Reinitzer^

identified the liquid crystal phase for the first time and reported his observations on the

melting behavior of cholesteryl benzoate, a cholesterol derivative, which can be found in

both plants and animals. Not long after, the first nematic liquid crystal was synthesized

by Gattermann and Ritschke.(2) In the 1960's, a group headed by Heilmeier at the David

Samoff Research Center discovered a number of electro-optic phenomena including the

dynamic scattering effect,<3) the guest-host effect,{4) and phase change effect,(S) all of

which had potential for display applications. There were, however, no suitable room

temperature nematic liquid crystals available for the application of the dynamic scattering

effect at that time. In 1969, Keller and Scheuerle at 3M discovered MBBA,(6) a room

temperature nematic with negative dielectric anisotropy, which allowed dynamic

scattering liquid crystal displays to operate in practical temperature range.

Problems already existed when the dynamic scattering liquid crystal display was first

commercialized. The display, which required relatively high voltage and power, had a

very short lifetime. In addition, its contrast was low. In 1971, Schadt and Helffich(7) in

Switzerland and Fergason(8) in the U.S. discovered the twisted nematic (TN) effect which,

1


2

because of its low power consumption, soon found application in watches, calculators,

and other electronic instruments. The advantages of the TN LCD included good contrast

in ambient light and long lifetime. The successful debut of LCD technology opened up a

whole new area of research and application. In the 1980's, the development and

application of the Supertwist Bireffingent Effect (SBE)(9) and later the Supertwist

Nematic (STN) were additional breakthroughs in liquid crystal research. In addition to

the nematic and cholesteric phases of liquid crystals, the smectic C phase was also found

to be suitable for display application. Throughout the decade, other new technologies

were developed and implemented; these included thin-film-transistor (TFT)(I0) liquid

crystal display, surface stabilized ferroelectric liquid crystal (SSFLC),(I1) and polymer

dispersed liquid crystal (PDLC).(12)

The twisted-nematic liquid crystal display (TN LCD) gradually became popular but

the demand also uncovered shortcomings: slow response time and reduced contrast if

viewed from an angle. A high degree of multiplexing was difficult since the voltage

versus transmittance curve was not steep enough. The discovery of SBE improved the

contrast and, because of the steepness of its response curve, allowed further multiplexing

in addressing for higher information display application. The SBE display, however,

required a surface pre-tilt angle of ~15° which was unacceptable to most display

manufacturers, because the high pre-tilt angle could only be obtained through the use of

silicon monoxide and not the conventional polyimide. The STN LCD became the

immediate alternative as it employed a lower pre-tilt angle. The contrast was not as good


3

as the SBE display, yet still far superior to the TN display. The switching times for both

SBE and STN displays, in the neighborhood of 200 milliseconds, were still too slow for

TV application. The switching time problem was solved by the invention of the TFT

LCD but complex manufacturing technology drove up the cost of these displays.

Typically, six masking techniques were required to fabricate an active matrix LCD,

resulting in extremely low yields. The SSFLC offers the potential for fast switching

speed, high contrast and low power consumption but is limited by three important

aspects: cell spacing, surface anchoring effects and materials. The requirement of small

cell gap (2-3 pm) and poorly controlled surface anchoring as well as the optimization of

the material properties are major hurdles.

AH of the displays mentioned above have one similarity: the use of polarizers. With

the front and back polarizers in place, the light efficiency drops significantly. In the case

of TFT LCD, the light efficiency is only 1-2%. To compensate for the low efficiency,

backlights are used and power consumption becomes an important issue once again. The

invention of PDLC's eliminated the need for the polarizers and offered the advantages of

simple fabrication and fast response time. The ability to provide high transmission means

lower power consumption. The drawback of the PDLC display is poor viewing properties

for direct view applications although use as a projection light valves has attracted many

companies to pursue its development. The pursuit of perfection in PDLC display

eventually lead to the discovery of the polymer stabilized cholesteric texture (PSCT).(I3)


4

1.2 Introduction to Polymer Stabilized Cholesteric Textures

Polymer Stabilized Cholesteric Textures (PSCT) are a new type of dispersion

combining polymer and cholesteric liquid crystal in which the textures of the liquid

crystal are stabilized as well as modified by dispersed polymer networks. These materials

have many features that are well suited for display applications:(14) excellent viewing

properties and simple fabrication techniques.

One of the many objectives of studying PSCT is to gain insights on network structure

and understand what effects networks have on liquid crystals in terms of electro-optical

characteristics and birefringence. Other objectives include the optimization of the

material and the application of these materials in a spatial light modulator for projection

light valves, direct view displays and other applications.

One of the major components in the PSCT is the cholesteric liquid crystal which may

adopt a homeotropic, focal-conic, or planar texture depending on surface and external

field conditions. The homeotropic texture is obtained by an applied field of sufficient

strength and exhibits characteristics of the homeotropic aligned nematic liquid crystal,

that is, optically transparent and has its optic axis uniformly aligned along the cell

normal. The focal-conic texture, however, is a polydomain system which is characterized

by the random orientation of the helical axis of the molecular orientation. Strong light

scattering occurs at the boundaries of these domains where there are distinct changes in

the refractive indices.(I5) The focal-conic texture can be created by cooling the isotropic

liquid crystal or by applying a small electric field to the planar texture. In either case, the


5

domain sizes are randomly distributed. The evolution of focal-conic texture is a multiple

nucleation process which usually begins from the boundary or from foreign particles

trapped inside the liquid crystal, or both. The incorporation of polymer networks provides

sites for nucleation and thus domain sizes can be controlled by network density.

Under a zero field condition, the PSCT system scatters light strongly. In the presence

of an electric field, liquid crystals with positive dielectric anisotropies align themselves

with the long axis in the direction of the field. The mismatch between the ordinary

refractive index of the liquid crystal and that of polymer networks is greatly reduced

because of the small concentration ratio of polymer to liquid crystal. The PSCT therefore

appears transparent under an applied field (see Fig. 1).

1.3 Focus of the Dissertation

The focus of this dissertation research is to understand the physics of a chiral nematic

material in which the focal-conic texture is stabilized by a polymer network in order to

develop an improved display technology. Studying the PSCT system involves:

characterizing the monomer and chiral concentration, which have strong effects on the

electro-optic response of PSCT; understanding the physics of the network; and

constructing a working prototype. To study the polymer network, the birefringence is

measured for a range of monomer concentration and experimental results are fitted with

an equation derived from the Landau-de Gennes equation. The value obtained for the

fitting parameter yields a measure of the radius of the polymer fibers. The polymer

networks formed in the liquid crystal environment are examined with optical and


6

OFF STATE (OPAQUE)

(a)

ON STATE (CLEAR)

11 Y i 1, 1

r I <11 l'i i i 1 ®' I ' l l ' f l ' V i ! . 1 I YIB.1 .1 « I 14. . I 9

y v v y y v

(b)

Figure 1. Schematic illustration of a PSCT system. Polymer network connecting

top and bottom plate for focal-conic and homeotropic textures: (a) light scattered

by the helical structure and sample appears opaque; (b) light passes through in

field on state and sample appears clear.


7

scanning electronic microscopy. The dissertation particularly addresses the effect of

liquid crystal orientation on the structure of the polymer network and offers some

descriptions on the formation of the networks.

The hysteresis exhibited by the cholesteric liquid crystal mixture is enhanced by the

polymer network. A bias voltage can be defined within the hysteresis loop such that the

application of this bias voltage keeps the sample state either ON or OFF for bistable

electro-optic application. A 320 x 320 pixel display prototype is constructed and driven

by a scheme that relies on the bias voltage concept.


Chapter 2

Liquid Crystal Display-Principles and Applications

2.1 Properties of Liquid Crystal

2.1.1 Nematic and Cholesteric Phases

The nematic liquid crystal phase is characterized by the long-range molecular

orientational order and the randomness of the positional order. A unit vector (director), n,

describes the average direction of the molecular long axes (Fig. 2a). An order parameter,

S, given by:(16)

S = - ( 3 cos20-1) (2.1)2

provides a measure of the degree of orientational order. The order parameter is a thermal

average value and the angle 0 (Fig. 3) is the angle between the molecular direction and

the director n. A value of 1 indicates that the molecules align themselves perfectly along

the director. A value of 0 represents that the molecules are randomly oriented (isotropic

state, Fig. 2b), while S—l/2 implies that the long axes of the molecules align

perpendicular to the director n.

In cholesteric liquid crystals, the direction of the molecular long axes is arranged in a

twist formation (Fig. 2c) around an axis termed the helical axis. The twisting structure is

periodic along the helical axis, and one pitch length (p0) of the structure represents the

8


(a) NEMATIC

n

(b) ISOTROPIC

• * _L — ^= - P

_i_ JL JL |(c) CHOLESTERIC

Figure 2. Three different phases of liquid crystal: (a) nematic phase where the

director is indicated by n; (b) isotropic phase and; (c) cholesteric phase where p is the

pitch.


10

X

Y

Figure 3. A diagram showing the orientation of a liquid crystal molecule.


11

director turning through 360°. Similarly, the cholesteric liquid crystal has no positional

order. With Z-axis as the helical axis, The director n can be written in Cartesian

coordinates as:

nx = cos (q0z)ny = sin(q0z) (2.2)nz = 0

where q0 =2n/p0

2.1.2 Dielectric and Optical Properties

The anisotropy of the dielectric constant in a nematic liquid crystal is described by s,

and sx in which s, and e± are the dielectric constants measured parallel and perpendicular

to the nematic director respectively. For a uniaxial liquid crystal, the difference, As, is

defined as,

Ae = e, - ex (2.3)

This value may be positive or negative; and its magnitude indicates how strong the

interaction will be between the liquid crystal and an applied electric field. The electric

field acts on the dielectric anisotropy of the liquid crystal and produces an orienting

torque. This torque vanishes as soon as the field is removed allowing the original

orientation or structure to be restored. In general, a nematic liquid crystal with positive

dielectric anisotropy aligns its long axis along the applied electric field direction. For

negative dielectric anisotropy, the axis orients perpendicular to the field.


12

The optical properties of liquid crystal also exhibit an anisotropy. The refractive

indices associated with nematic liquid crystal, n«. and r^, are determined with light

polarized parallel and perpendicular to the nematic director, respectively. The

birefringence is defined as,

= ne - n0 (2.4)

For light propagating in a uniaxial media, the effective refractive index is given by,

= /. "'"V,- <2-5>i ne cos (p +na sin <p

where cp is the angle the light wave vector makes with the director n.

It has been known that a twisted liquid crystal layer is able to rotate the plane of

polarization of light of wavelength X. This is true as long as the following inequality,

known as the " Mauguin limit ",(17) holds:

2 d An » A (2.6)

Here, d is the thickness of the liquid crystal.

2.1.3 Elastic Properties

Nematic liquid crystals can be deformed under different surface conditions and by

electric or magnetic fields. Macroscopically, the spatial extent of this deformation greatly

exceeds the molecular dimension. Thus the liquid crystal can be regarded as a continuum


13

and therefore, the deformation can be described by the continuum theory in terms of

director field. This theory was pioneered by Oseen(18) and Zocher,(19) and then later by

Frank,(20) who formulated the elastic free energy density equation of the distorted state,

F n = Vi [Kn ( y - n f + ^ (w -V x ii)2 + ^ 33(«xV x«)2] (2.7)

where Ku, K22 and K33 are elastic constants of splay, twist and bend (Fig. 4), respectively.

The free energy equation of a cholesteric liquid crystal has an additional term, q0, such

that,

F a - 54 lK „ < y-n f ♦ K J n - V x n ♦ q 0f . K . J n ^ n f ] (2.8)

2.2 Liquid Crystal Display Applications

2.2.1 Twisted Nematic Displays

One of the many display configurations that utilizes the dielectric anisotropic effect of

liquid crystal is the twisted nematic liquid crystal display (TN LCD) (Fig. 5). The

construction of a TN display usually begins with patterning ITO coated glass into the

desired format. A thin layer of orientational film is then deposited on the patterned glass,

followed by rubbing. The rubbing directions in the top and bottom plates of the cell are

perpendicular to each other. The purpose of rubbing is to exert an influence on the liquid

crystal molecules to align along the rubbing direction. After the rubbing process, spacers

are sprayed onto the surface of the orientational film. The top and bottom plates are then


SPLAY

TWIST

BEND

Figure 4. Illustrations of the three different types of elastic deformation of the

liquid crystals.


ANALYZER POLARIZER

(b) <

LIGHTSOURCE

Figure 5. A diagrammatic illustration of a TN display: (a) polarized light rotated by

the liquid crystal and emerging from the analyzer; (b) polarized light undisturbed by

the liquid crystal and therefore absorbed by the analyzer.


16

put together to form a cell with its spacing maintained by the spacers. A nematic liquid

crystal doped with a small amount of chiral agent is injected into the cell in a vacuum

chamber. The liquid crystal molecules align themselves in the rubbing direction near the

aligning film on the top and bottom plates, while between the plates, a twisting

configuration forms. The function of chiral agent is to provide a sense of twist, either left

or right, to the molecules.

Polarizers with polarization axes parallel to the alignment direction of the liquid

crystal are placed on the cell. Only the part of light polarized parallel to the polarization

axis comes through the polarizer. This linearly polarized light will follow the twisting

configuration and rotate 90° before emerging from the back polarizer. A small electric

field applied to a liquid crystal of positive dielectric anisotropy causes the molecules to

align in the field direction. The polarized light that passes through this configuration

without being rotated is absorbed by the back polarizer and appears dark to the viewer.

2.2.2 Supertwist Birefringence Effect

The display configuration of the supertwist birefringence effect (SBE) (Fig. 6) requires

the liquid crystal molecules to twist 270° (a) within a range compatible to the cell gap. A

desired pitch (p) couples with a precise cell spacing (d) and this 270° twist can be

realized as,

d a- * (2-9) p 2tc


17

RUBBING DIRECTION

(a)

(b)

^ ^ ^

RUBBING DIRECTION

RUBBING DIRECTION

RUBBING DIRECTION

Figure 6. The 270° twist of a SBE cell is shown in (b). For comparison, the twist

is 90° in (a), a typical TN cell.


18

This configuration yields a steep slope on the switching curve enabling the multiplex

level to go beyond the conventional TN configuration. It is also due to this configuration

that the SBE exhibits strong interference colors. The placement of the polarizers is also

different from the TN cell. The polarization axes of both front and back polarizers are at

an angle to the alignment axis rather than parallel to it, enhancing the contrast but also

producing the color effect. One other significant difference is the pretilt angle which is

15° comparing to 0.5°~1° in the TN cell. The high pretilt angle helps to reduce the

relatively high drive voltage required for SBE.

2.2.3 Cholesteric-Nematic Phase Change Effect

A cholesteric liquid crystal can adopt two different metastable states, namely, planar

and focal conic textures (Fig. 7). With the application of an electric field, it is possible to

unwind the helical structure of cholesteric liquid crystals to form a pseudo-nematic (or

quasi-nematic) homeotropic state. This unwinding takes place at a threshold field which

was first given by de Gennes:(21)

P o \ Ae(2.10)

The corresponding rise time xon(22) and decay time xofr(23) are,

n

A (2.11) j 2 2 d P o


Figure 7. A diagram showing the three different phases of cholesteric liquid

crystal: (a) planar texture; (b) homeotropic state; and (c) focal-conic texture. The

arrows indicate the switching between the states under an applied field. The

fastest switching occurs when the homeotropic state relaxes directly to planar

state.


where r\ is the viscosity of the liquid crystal. A cholesteric liquid crystal of positive

dielectric anisotropy, originally in an optically scattering focal conic texture, can be

converted to an optically clear homeotropic state. The transformation between these two

textures by an electric field forms the basis of the phase change effect. Upon removal of

the field, it reverts to focal-conic texture in a nucleation process. The dynamics of this

phase change effect give rise to a hysteresis effect which yields larger multiplexing

capacity than the TN LCD. This kind of effect has a brightness higher than a twisted

nematic devices and makes it more attractive for use in displays.

2.2.4 Polymer Dispersed Liquid Crystals

The Polymer Dispersed Liquid Crystals (PDLC's) (Fig. 8) is a different class of

materials in which a polymer binder is used along with the liquid crystal. Instead of

utilizing the phase shifting properties of liquid crystal as in TN displays, PDLC's make

use of the scattering effect of the liquid crystal droplets formed in the polymer binder.

PDLC's are constructed in such a way that a nematic liquid crystal is dispersed in the

form of droplets with diameters of l~2pm within the polymer binder. The mismatch of

refractive indices between the liquid crystal droplets and the polymer binder causes the

light to scatter. When a voltage is applied to it, the liquid crystals inside the droplets

orient themselves along the electric field direction, the mismatch of refractive indices


21

OFF STATE

o

V v V

ONSTATE

Figure 8. A typical PDLC sample: (a) In the OFF state where the liquid crystal

directors of the droplets are randomly oriented resulting in an opaque state; (b) in

the ON state, the liquid crystal aligns in the electric field direction diminishing

the scattering.


22

becomes minimal, and the PDLC's appear clear. The PDLC's do not require surface

treatment nor polarizers. The requirement in cell spacing of PDLC's is also not stringent

as opposed to that of TN cells. The electro-optical characteristics of PDLC's depend on

droplet size, shape, nematic structure in the droplet and the polymer binder.

2.3 Liquid Crystal Displays Addressing Technique

2.3.1 Passive Matrix Display

Liquid crystal displays for simple applications are patterned in such a way that each

segment of the pattern is separated from the others and connected to the outside through

the ledge on the top plate, while on the bottom plate, there is one single pattern which

acts as the common electrode. The addressing method is extremely simple: the segment is

"on" when there is a pulse voltage applied to it, or "off" when the voltage is zero. This

method is termed "direct drive."

As the number of segments increases, the number of connections between the display

and the driving circuit also increases. When more information is needed to display in a

limited space, the number of connections becomes greater and the design of the driving

circuit becomes complicated. To ease the problem, a matrix format on the patterning,

combined with a multiplex addressing scheme, is adopted. The realization of the matrix

format is by patterning the bottom plate with rows of electrodes and the upper plate with

columns of electrodes. Each crossover point of the rows and columns is called a pixel

(Fig. 9).


23

ITO electrode

Glass substrateJ-

J- "

Columnelectrode

PixelRowelectrode

i i i i i ii i i i i i

Figure 9. A diagram to illustrate the dot-matrix format. Each crossover point of

the ITO electrodes is a "pixel."


24

A pulsed voltage Vr is applied to the first row with the rest being held at 0 volts and

voltages of different waveform are applied to the column Vc sequentially. Depending on

the desired state of the pixels, either "on" or "off1, the respective r.m.s. voltage Von and

V0ff are

on \(Vr,Vcf , { N - l ) V l

N(2.13)

F+ - \

(Vr-Vcf , ( N - l ) V l

N(2.14)

where N is the number of rows. This driving scheme has been carefully studied by

Alt-Pleshko(24) and the optimum ratio of Von/Voff is obtained as

on

off

y/N+l

y/N-l(2.15)

The key point of this driving scheme is to address one line (row) at a time. Hence, it is

too slow for video application. Furthermore, limitations occur when the ratio approaches

unity as N goes to infinity. When N becomes too great, say 100, the contrast is barely

acceptable.

Alternative driving schemes in addressing multi-lines at a time have since been

proposed and investigated by Madhusudana,(25) later by Ruckmongathan(26) and recently


by Scheffer et al.(27) These driving schemes, described by Scheffer as "active addressing",

provide better gray shade, higher contrast and good brightness uniformity, and hence are

capable of video application.

2.3.2 Active Matrix Display

The active matrix display employs one or more nonlinear circuit elements to address

each of its pixels. The ultimate benefit of this method is to hold the addressed pixel "on"

or "off' for a time longer than the pixel address time and thus the scanning limitation is

no longer a problem. A diagrammatic circuitry is shown in Fig. 10.

Similarly, the display consists of two panels of glass. The front panel is not patterned,

and acts as a ground electrode. The nonlinear elements, including diodes and transistors,

are deposited as thin films onto the other glass substrate. The technology of thin film

deposition is borrowed from wafer fabrication in the semiconductor industry; however, it

requires thousands of those nonlinear elements in one single display. Any defect in those

elements results in the loss of a pixel and therefore the production cost tends to be high.

with permission of the copyright owner. Further reproduction prohibited without permission.

26

Xl X2 X3

Y3

PixelY2

MOSTransistor

Y i

Drain Gate Source

Figure 10. Active Matrix Liquid Crystal Display operation using MOS (Metal-Oxide-

Semiconductor) transistors. Not shown is a common connection to all elements. The

pixel is held "ON" (or "OFF") during the time between addressing by virtue of the

charge held on the drain terminal. To address a pixel, a voltage is applied through the

appropriate row (Vs) to "open" the gate. The voltage on the respective column (X's)

will then appear across the pixel.


Chapter 3

Polymer Stabilized Cholesteric Textures

3.1 Materials

3.1.1 Monomers

The monomers used for Polymer Stabilized Cholesteric Textures (PSCT's) are

multifunctional monomers in concentration ranges from 1 ~ 4wt.%. These monomers,

together with the photoinitiator, the chiral dopants and the nematic liquid crystals, are the

building blocks of the PSCT system. The three monomers used for the studies are listed

below,

Abbreviation Chemical Name

BAB 4,4'-Bisacryloyloxy biphenyl

BABB6 4,4'-Bis{4-[6-(acryloyloxy)hexyloxy]benzoate}biphenyl

BAB6 4,4'-Bis[6-(acryloyloxy)hexyloxy]biphenyl

Table 1. List of the monomers used in the studies.

Each of these lab-synthesized monomers has a rigid core as its central part, and a pair

of flexible hydrocarbon tails on two ends. The structure of the central part is similar to

that of the liquid crystals, while each of the two ends consists of reactive double bonds.

Their chemical structures are illustrated in Fig. 11.

27


2 8

CH2 = CHCCX, — ^ } ~ °tCCH = CHj

(a)

c h 2= c h c o 2 - ( c h 2) 6 - 0 - ^ ^ — c o2— 0 2c ^ Q > - 0 - ( c h 2 ) 6 -o2c c h = c h 2

(b)

CH2 = CHCOj - ( CH2) 6 - o - ( CH2) 6 ' 0 2CCH = CH,

(C)

^r\ Oft\ / f

\ //o ch3

(d)

Figure 11. The chemical structures of the monomers and photoinitiator: (a) BAB; (b)

BABB6; (c) BAB6; and (d) BME.


29

The monomers are crystalline at room temperature. BAB or BAB6 does not have a

nematic phase and their melting temperature are 150°C and 80°C respectively. BABB6

exhibits the following transitions:

It does not have an isotropic state as thermal polymerization takes place at 180°C. The

photoinitiator, Benzoin Methyl Ether (BME), is capable of undergoing decomposition

into free radicals when irradiated with ultraviolet light in the region of 360nm. Its

chemical structure is illustrated in Fig. 11.

3.1.2 Chiral Dopants and Nematic Liquid Crystals

Chiral dopants are optically active substances which are added in small amounts to

nematic liquid crystalline phases to yield cholesteric phases. The chiral dopants cause the

director of the liquid crystal molecules to adopt a helically twisted orientation. The chiral

dopants used in the experiments are CB15 and R1011 (Fig. 12); both are obtained from

Merck. These two chiral dopants are right-handed twist agents and the Helical Twist

Power (H.T.P.)(28) of R1011 is approximately four times as much as CB15. The term

H.T.P. is defined as

SmC NI00°c 108°C

(3.1)


30

(a)

CH3

CHaQHaCHCH —Q — CN *

(b)

Figure 12. The chemical structures of the chiral dopants: (a) R1011; and (b)

CB15.


31

where p is the pitch of the cholesteric liquid crystal and C is the concentration of the

dopant in the mixture. One advantage of R1011 over CB15 is that it reduces the clearing

temperature of its nematic host insignificantly.

The nematic liquid crystals used in the experiments are E48 and ZLI4389, also

available from Merck. Both E48 and ZLI4389 are multi-component liquid crystals. It is

not known what the exact components and their respective percentages are as the

manufacturer does not make it public. Some of the physical properties of these two liquid

crystals are listed below:

Mixture E48 ZLI4389

Clearing Point (°C) 87 62

n* 1.7536 1.6614

An 0.2306 0.1567

As 15.14 45.6

£II 20.49 56.0

Viscosity (20°C) (mm2/s)

43.5 76

Table 2. Physical properties of E48 and ZLI4389.

3.2 Cell Fabrication

The construction of the sample cell begins with a cleaning process for the glass

substrates which have a thin layer of Indium-Tin-Oxide (ITO) deposited on them. For the

purpose of parallel alignment, the substrates are coated with a layer of polyimide and


32

rubbed in a parallel direction. In the case of homeotropic alignment, the substrates are

treated with octadecyltrichlorosilane. This is followed by the application of epoxy sealant

materials along the four edges of the substrates, leaving only a small opening for future

liquid crystal injection. Before the top and bottom plates are assembled together, glass

spacers of desired diameter are sprayed evenly on the inner surfaces of the plates to

ensure uniform cell spacing.

After the materials are mixed together, they are transferred to a shallow trough. The

trough and its contents are placed inside a vacuum chamber. An empty sample cell is

mounted over the trough with its injection hole right above the mixture. The vacuum

chamber is pumped down to about lOOmillitorr and the cell is then slowly lowered until

the injection hole is completely immersed in the mixture. Air is vented into the chamber

to push the mixture further into the sample cell. As soon as the cell is filled and the

chamber pressure is returned to normal, the cell is lifted and dismounted. The injection

hole is plugged by a epoxy sealant.

3.3 Photopolymerization

3.3.1 Set-up

A chamber is equipped with a metal-halide ultra-violet (uv) light source and a water

system that takes out most of the heat generated by the light source. The uv intensity can

be adjusted by the power supply, or simply by changing the distance from the light

source. Most of the studies are performed with uv intensity of 14mW/cm2. The intensity

of the uv light is monitored manually by a radiometer (Oriel). A voltage large enough to


33

switch the liquid crystal into homeotropic state is applied to the cell and maintained

throughout the process.

3.3.2 Polymerization Process

The polymerization process in a PSCT system can be described as a photo-initiated

polymerization process.(29) In general, polymerization is possible if the free energy

difference (AG) between monomer and polymer is negative. AG is defined as,

where AH, T and AS are enthalpy, temperature and entropy, respectively.

The polymerization process can be characterized by a sequence of events, viz,

initiation, propagation and termination. The process begins as the photoinitiator (I)

decomposes into free radicals (R-) with the exposure to ultraviolet light. The relatively

low stability of the carbon-carbon double bond on the two ends of the monomers makes it

susceptible to attack by a free radical. When free radicals are generated in the presence of

monomers, the radical adds to the double bond of monomer with the regeneration of

another radical. This is characterized as the initiation process.

AG = AH - T AS (3.2)

I R-

HR- + CH2=CHX * ~ r c h 2c-

2 IX


34

During the stage of propagation, the radical formed is capable of adding successive

monomers to it and the reaction continues. The process will finally terminate when either

the supply of monomers is exhausted or the radicals react with each other with the loss of

radical activity.

The whole process is accompanied by a process of phase separation; the polymer

networks formed in the photopolymerization process separate themselves from the liquid

crystal. The rate of separation is influenced by the rate of polymerization which in turn

depends on uv intensity.

3.4 Principles and Application of PSCT

The voltage that was applied to the sample prior to polymerization kept the liquid

crystals in a homeotropic state which is believed to have an orientational effect on the

monomers, probably because the monomers have a structure similar to the liquid crystals.

Polymerization is a cross-linking process in which the monomers link each other together

to form a polymer. The monomers tend to link to one another in the direction that is

favored by the liquid crystals. Since the liquid crystals align in the direction of the

applied electric field, the polymer also grows in the direction of the electric field and

eventually connects to the surfaces of both plates. Since the monomers are distributed

evenly throughout the liquid crystals, a network of polymer is formed. Once the

polymerization is completed and the field is removed, the liquid crystal will relax back to

its helical structure and settle in domains. These domain formations are also called focal-


35

conic textures. The presence of the polymer network limits the growth of those domains

and confines them to a certain size. Since the polymer networks attach themselves to the

surfaces of the two plates, the whole structure is very stable. When a high enough electric

field is applied, the liquid crystals will switch to the homeotropic state again. A typical

electro-optical curve of a sample made solely from cholesteric liquid crystal is illustrated

in Fig. 13 a. At zero or low voltage, a great deal of light passes through the layer of

cholesteric liquid crystal, because the domain sizes of the focal conic textures are not

uniform, with some of them too large to scatter light efficiently. With an appropriate

polymer network density, these domains with too large a size will be suppressed and the

desired domain size can be maintained (Fig. 13b); hence, the scattering effect will be

maximized. Similarly, the bounce effect that symbolizes the dynamic response of a

twisted cell is also suppressed (Fig. 14 a and b). The polymer networks also give rise to

an aligning effect in the liquid crystals which will prolong the nucleation process and

result in a larger hysteresis effect. A bias voltage can be determined within the hysteresis

loop so that the homeotropic state (clear state) produced by a pulse persists under this

bias voltage. As will be demonstrated, the hysteresis is a key feature in using these

materials for displays or projection light valves.


36

0.8

0.6

0.4

C /3Z 1.0

H0.8

0.6

0.4

0.2

0.030 4010 200

APPLIED VOLTAGE (V)

Figure 13. Voltage vs. transmission curves for: (a) cholesteric liquid crystal; and

(b) polymer stabilized cholesteric texture.


37

1.0

0.8

0.6

0.4

g 0.2

E 0.0

1.0

0.8

0.6

0.4

0.2

0.00 50 100 150 200 250 300

TIME (ms)

Figure 14. Dynamic response curves for: (a) cholesteric liquid crystal; and (b)

polymer stabilized cholesteric texture.


Chapter 4

Polymer Networks in Liquid Crystals

4.1 Optical and Scanning Electronic Microscopy Studies of Polymer Networks

4.1.1 Introduction

The method of forming polymer networks in a liquid crystal environment was pioneered

by a group of researchers in Hitachi Research Laboratory. Araya et al.(30) succeeded in using

nematic liquid crystal as the polymerization solvent to produce polyacetylene. Mariani et

al.(31) used a smectic B (SB) solvent as the host medium for the polymerization of monomers.

Both results indicated that the highly ordered liquid crystal had their orientation imprinted

onto the polymer. Hikmetf32,33-34’35* later employed polymer networks in display application.

In the course of his studies, he used cholesteric liquid crystal and found that the light

reflectivity was basically preserved by the networks/36* Crawford et al.(37) applied a similar

technique by using polymer networks to capture nematic director-fields in confined spherical

droplets. To summarize, the formation of polymer in a liquid crystal medium is heavily

influenced by the liquid crystal; the final structure of the polymer formed depends on the

state of the liquid crystal. This section deals with studies of polymer networks formed in

liquid crystal environments when the state of the liquid crystal is subjected to different

surface conditions, namely, plain ITO surface, rubbed polyimide and silane treated surface;

as well as the external electric field effect.

38


39

When a pre-fabricated sample cell is filled with the mixture of photopolymerizable

monomer and liquid crystal, it exhibits a similar electro-optic effect as the cell itself filled

with liquid crystal only. Depending on the surface condition, the liquid crystal will adopt the

orientation that is imposed on it by the surface. After photopolymerization has taken place,

the polymer networks will adopt the same orientation of the liquid crystal. This is also true

for the case when an external field is applied to the liquid crystal prior to and throughout the

whole photopolymerization process.

The monomer (BAB6), liquid crystal (ZLI4389) and photoinitiator (BME) are mixed

together and then undergo cycles of heating and stirring. The purpose of heating is to

facilitate the monomer to dissolve in the liquid crystal mixture, while the stirring enables

them to form a homogeneous solution. The filling is carried out in a vacuum chamber. The

filled cells are then irradiated with uv light to initiate the photopolymerization process which

takes place at ambient temperature. After the process was completed, the two opposite edges

of each cell are removed allowing access from outside. The cell is then submerged into the

solvent, hexane, for a period of three days. During that period of time, the solvent will work

its way into the cell and gradually replace the liquid crystal. The cell is then placed in open

area to allow the solvent to evaporate. The solvent is found to be effective in removing the

liquid crystal and has little deformative effect on the polymer networks or the epoxy seal

material. Once the evaporation of solvent is completed, the cell is free of both liquid crystal

and solvent. Meanwhile, the top and bottom plates are still tightly held together by the

sealant material; the most effective way of separating the two plates is by the use of a razor


40

blade. Holding the razor blade and applying a slight pressure at the edge is enough to pry

the cell open. After they are opened, the inner surface of the plates are sputtered with a thin

layer of palladium before being examined by Scanning Electronic Microscopy (SEM).

4.1.2 Planar Alignment (I)

The sample is coated with a layer of polyimide and then rubbed in parallel direction. The

mixture, 97.0wt.% of ZLI4389, 2.7wt.% of BAB6 and 0.3wt.% of BME, shows strong

birefringence and exhibits high transmission when the alignment axis is at 45° to the

polarization axis of the two crossed polarizers. These optical characteristics persists even

after photopolymerization. The procedures of preparing the sample for observation has

already been discussed in 4.1.1, but when the top and bottom plates are separated, the

polymer network breaks away mostly from one plate leaving traces of polymer on the other.

Figure 15 is the SEM picture of the polymer network which is free of any liquid crystal

and solvent. As indicated in the picture, the direction of the fiber structure coincides with the

rubbing direction. The polymer network also appears to be much denser than it should be.

The fact is that the networks tend to retract in all directions, but particularly in the direction

perpendicular to the alignment axis (rubbing direction). The retraction along the alignment

axis is comparatively small due to the anisotropic nature of the network structure (more

about this in section 4.1.5). Nevertheless, this anisotropy can also be observed using optical

microscopy when the alignment axis is at 45° to the crossed polarizers (Fig. 16). However,

the anisotropy is hardly distinguishable when the cell is placed in the direction parallel to the

polarization axis of either polarizer, indicating that this structure is highly birefringent.


41

Figure 15. SEM image of the liquid crystal free polymer network. Note the

direction of the fibers running parallel to the rubbing direction.


42

Rubbingdirection

Figure 16. Photograph of the liquid crystal free polymer network. Picture taken

with cross polarizers. The alignment axis of the polymer network (or rubbing

direction) is at 45° to the polarizers.


43

Moreover, the fibers are sputtered with a thin layer of palladium whose thickness is

estimated at ~35nm (0.035pm), much of the details of the fibers are disguised. Hence, the

size of the fibers do not appear to be uniform.

4.1.3 Planar Alignment (II)

The sample in the experiment on planar alignment is also polyimide coated, but the

mbbing directions on the top and bottom plates are perpendicular to each other. The mixture

and the procedures are the same as 4.1.2. Here, the mixture takes up a twisted configuration

throughout the cell from one plate to the other. Since there is no chiral agent, both left-

handed and right-handed twists coexist as manifested by the presence of defect lines. When

a small electric field is applied to the mixture, these defect lines disappear. If the rate of the

electric field applied is slow enough, the defect lines are actually seen to gradually disappear.

When the field is removed, the defect lines will appear again but very likely not in the same

location. After photopolymerization, the defect lines become immobile and remain intact

even if a strong field is applied. The reason is that the polymer networks also assume a

twisted structure and the sense of twisting is influenced by the liquid crystal. The right-

handed and left-handed twisted structures of the polymer networks are connected together

at the positions where the defect lines were located. The polymer in such locations has the

defect lines "locked up" in its formation. Under a microscope, the polymer networks are

clearly marked by those formations (Fig. 17 ). Also, when the polymer is folded up as shown

in Fig. 18, the fibers in the bottom extend in the direction perpendicular to that on the top,

while those in the intermediate position point to different angles. This observation indicates


44

50|im

Figure 17. Photograph of the liquid crystal free polymer network. Picture taken

with cross polarizers. The rubbing directions on the top and bottom plates are

perpendicular to each other. Since both left and right hand twist exist, defect lines

appear in the juncture of these two different twisted structures. The dark image on

the picture is the defect line captured by the network.


Figure 18. SEM image of the liquid crystal free polymer network. Note the part

of polymer folded on top of the other, the fiber directions are running

perpendicular to each other.


46

that the twisted configuration of the liquid crystal is preserved by the network.

4.1.4 Homeotropic Alignment by Chemical Treatment

Homeotropic alignment refers to the orientation of the liquid crystal director

perpendicular to the substrate surfaces. To achieve this configuration, the substrates are spin

coated with a lwt.% octadecyltrichlorosilane in toluene. The solvent, toluene, is then

evaporated in an oven with temperature set at 100°C for a period of one hour. The mixture

and the preparation procedures are the same as before. The conoscopic studies on the sample

before and after polymerization show a well defined uniaxial cross. To prepare for SEM

studies, the cured sample is taken through the procedures described in 4.1.1. Since the

evaporation takes place while the top plate is still in place, it was thought that the detachment

of the network from the plate would occur when the plates were separated, and the network

would collapse. What is unexpected is the standing structure of the polymer network as

shown in Fig. 19. To understand what possibly happens, we first go back to the

polymerization process. The formation of polymer fibers tend to take place along the

preferred direction of the liquid crystal molecules. This can be seen in the case of parallel

alignment (Fig. 15). The same holds true for the case of homeotropic alignment where the

polymer fibers grow in the direction perpendicular to the substrates and subsequently become

strong enough to maintain the network structurally standing. The replacement of liquid

crystal by the solvent does not affect the network structure. However, the polymer fibers are

pulled away from each other in the direction perpendicular to the fibers during solvent

evaporation. As a result of pulling away, large openings form as seen in the SEM pictures.


47

Figure 19. SEM image of the liquid crystal free polymer network. The large

openings are created when the solvent evaporates.


48

Some openings are large enough to allow direct view of the bottom plate on which the

network is standing.

4.1.5 Homeotropic Alignment by External Field

In this experiment, the substrate surfaces do not have any alignment treatment. The

sample was prepared with a mixture of 2.2wt.% R1011,94.8wt.% ZLI4389,2.7wt.% BAB6

and 0.3wt.% BME. A voltage of 15V is applied to the sample and the liquid crystal

transforms itself into a homeotropic state. After photopolymerization and the removal of the

liquid crystal, the sample is examined through the edge with the two plates still together (Fig.

20). The picture clearly shows that the two plates are connected together via bundles of

polymer fibers. The appearances of polymer fibers pulling away from each other to form

those openings and the polymer fibers pulling together to form those bundles are exemplified

in the picture.

Another similar sample is held in a liquid nitrogen environment so that the contents can

be solidified. Once the contents become solid, the two plates are separated and placed in the

solvent. As the liquid crystal is being dissolved out of the networks, the solvents evaporate

leaving behind the network. Since the plates are separated with the solid contents still in

between, the separation occurs at the very interface of the plates and the solid contents.

Compared to the previous samples, the polymer fibers suffered very little tearing and even

that part of polymer formed near the immediate surface of the plate came off with minimal

damage. The polymer structure formed near the plate surface is seen as a perforated polymer

sheet. The absence of liquid crystal caused this sheet of polymer to shrink and crack. The


49

g © r s 2 ^ - ^ ! « - ^ ^ 3 ^gite^ps^jitfflggjMM• i*W <>ftr •;;■*y>.> ^>;:«■?■“ a*,tz ^ ^ .- ^ .- v ♦.

Figure 20. SEM image of the liquid crystal free polymer network. The network

connects the top and bottom plates via bundles of polymer fibers.


50

cracks also exposed the polymer fibers which are still attached to the bottom plates (Fig. 21).

When the applied voltage is raised to 100V for a similar sample, the SEM image (Fig. 22)

shows that there are fewer polymer fibers extending sidewards. Another sample with

polymer concentration of 4.5wt.% is shown in Fig. 23 where the highly dense polymer

structure is left standing on the plate. In both cases, the height of the polymer structures is

found to be at least 14pm.

4.1.6 Monomer Concentration Effect

Two mixtures of different monomer concentration, 1.2% and 4.5%, respectively, are used

in this experiment. The surfaces of the substrates are not chemically treated. A voltage of

15V is applied to the cell. Fig. 24 is the SEM picture of this highly interconnected polymer

network from 4.5% BAB6. In this case, the high concentration of monomer allowed the

formation of a stable polymer network, and the absence of liquid crystal did not have any

noticeable effect on the polymer configuration. This is in big contrast to the Fig. 19 when the

polymer fibers link up in almost all possible directions. There are also some regions where

the polymer fibers appear on both top and bottom plates, an indication that the network

breaks somewhere in the middle. The implication is that the polymer networks attached

firmly on the substrates and break off only at the weakest link. On the contrary, the polymer

formation with 1.2% BAB6 did not conform to any defined structure (Fig. 25), probably

because the polymer is spread so thin that the fibers are not strong enough to sustain a

standing structure.


51

I--------- 110nm

Figure 21. SEM image of the liquid crystal free polymer network. The network

retracts in all directions revealing the fibers and the bottom plate.


52

Figure 22. SEM image of the liquid crystal free polymer network. The sample is

tilted at 45° to the normal. The length of the fiber is ~14pm, close to the cell

spacing of 15 pm.


10pm


tilted at 45° to the normal. The large empty space forms when the polymer

network of this area stays with the other plate during the separation.


54


same as Fig. 23 but viewed from above.


55

Figure 25. SEM image of the liquid crystal free polymer network. The polymer

concentration is 1.2wt.%. No noticeable structure is observed with this

concentration.


56

4.1.7 High Temperature Effect

In the experiment on the high temperature effect, the substrate is polyimide coated and

rubbed in parallel directions. The mixture and most of the procedures are similar to that of

3.1 except the ambient temperature in which the photopolymerization takes place. The

temperature is maintained at 70°C, well above the isotropic temperature of the liquid crystal.

An electric field is also applied to the cell. The polymer formed in this temperature is shown

in Fig. 26. As the picture shows, chunks of polymer of different sizes and irregular shape

appear all over the substrates; the formation does not exhibit any degree of orientation. The

random orientation of the isotropic liquid crystal prior to photopolymerization accounts for

the lack of ordering in the monomers. Since there is no preferred direction on the part of the

liquid crystal, the polymer chain grows in every possible direction once the polymerization

process begins.

4.1.8 Different Monomers

The polymer network formed with BAB is illustrated in Fig. 27. The length of the flexible

parts of the BAB molecule is somewhat shorter than that of BAB6. The BAB network

therefore appears to be formed with beads of polymer stringing together. Although the

sample is cured with an applied electric field, it does not appear to form an anisotropic

network. The network shown is free of any liquid crystal or solvent, it definitely shrinks in

all directions. It is therefore difficult to determine whether there is previously an anisotropic

network. On the other hand, the BABB6 molecule is longer than that of BAB6 and its

network appears to have similar fiber type structure.


57

Figure 26. SEM image of the liquid crystal free polymer network. The polymer is

BAB6 at a concentration of 2.7wt.%. Cured with an applied field, the polymer

appears to be irregular in both shape and size.


58


BAB at a concentration of 3.5wt.%. The sample is tilted at 45° to the normal. The

network collapses onto the plate surface and appears like a layer of polymer

"beads" stacked together.


59

4.1.9 Frequency Effect on Curing

Curing takes place in the presence of an applied electric field with frequency of 20Hz; the

resulting network is shown in Fig. 28. The polymer appeared to form a carpet-like structure,

a sharp contrast to the one cured at 500Hz (Fig. 20). At low frequencies, the interaction

between the ionic impurities (including the radicals from the decomposition of

photoinitiator) and the external field becomes large enough to destabilize the polymerization

process. A previous study about frequency effects in PDLC's shows that frequency higher

than 400Hz is enough to overcome the instability created by the ionic impurities. In fact, the

use of high frequency in PSCT's produces the same structure as those with 500Hz.

4.1.10 No External Field and Surface Effect

In the experiment of no external field and surface effect, the glass surfaces are not treated

for any kind of alignment, nor is there any applied electric field. The sample is cured and the

polymer assumes whatever the liquid crystal configuration is during polymerization. The

polymer fiber formed in either the nematic (Fig. 29) or cholesteric (Fig. 30) liquid crystal

environment do not conform to any pattern mentioned previously. The one formed in

nematic liquid crystal adopts some of the local molecular orientation but the overall

randomness is preserved. In the one formed in cholesteric liquid crystal, because of the

twisting nature of the liquid crystal, the polymer fibers are also intertwined. It is also because

of the randomness of the helical axis of the cholesteric liquid crystal, the polymer fibers

appear to be a bunch of fibers mingled together.


60


BAB6 at a concentration of 2.7wt.%. The frequency of the applied electric field is

20Hz. The polymer does not seem to have the fiber like structure but appears to

be a thin layer of polymer.


6 1


BAB6 at a concentration of 2.7wt.%. The polymer appears to be a fiber like

structure and exhibits some local orientation.


62

---- 1lpm

Figure 30. SEM image of the polymer networks free of liquid crystals. The

polymer is BAB6 at a concentration of 2.7wt.%. Chiral dopant (2.2wt.%) is

added into the nematic liquid crystal and cured without field. The polymer fibers

appear to be randomly oriented because of the helical structure of the liquid

crystal.


63

4.1.11 Cholesteric Liquid Crystal in an Electric Field

The glass substrates of the experiment on a cholesteric liquid crystal in an electric field

are not treated for any form of alignment. The cholesteric liquid crystal is mixed with the

monomer, BAB6, and the photoinitiator. A sufficiently high electric field is applied to the

sample to achieve a homeotropic state prior to polymerization. This condition is thought to

be similar to that of 4.1.5; however, the two networks are not the same. Instead of a full

length of network standing on the substrate, only a small portion of it remains, not just on

one plate, but on both plates as seen from the SEM picture taken with the sample tilted at 45 °

to the normal (Fig. 31). The height of the part left on the plates ranges from 1 to 3pm.

Taking the cell spacing of 15pm into consideration, a large amount of the polymer network

is not accounted for. It is speculated that this part of the polymer network is not as strong as

those in 4.1.5 due to the twisting nature of the cholesteric liquid crystal. During the winding

process, the cholesteric liquid crystals may have exerted some forces on the networks and

therefore weakened the network structure. This structure is washed away in the course of

solvent evaporation. The remaining structure also appear to be partitions erected over the

whole surface of the plates; it is not yet clear how the structure is formed.

4.2 Birefringence of Polymer Networks

4.2.1 Introduction

Scanning Electronic Microscopy (SEM) studies of the polymer networks provide some

idea how the liquid crystal orientation influences the network structure. However, the effects

of the networks on the liquid crystals have yet to be examined. Hikmet has shown that, for


64


BAB6 at a concentration of 2.7wt.%. Chiral dopant (2.2wt.%) is mixed with the

nematic liquid crystal and cured with an applied field. The sample is tilted at 45°

to the normal. The plate surface appears to be partitioned with numerous polymer

"walls."


65

various concentrations of monomers, the temperature dependence for both ordinary and

extraordinary indices of refraction changes after photopolymerization.(36) The result is the

existence of so-called "residual birefringence"(38) beyond the nematic-isotropic (N-I) phase

transition of the liquid crystal. The order parameter of the liquid crystal derived from the

refractive index measurement as well as the birefringence measurement shows that a high

degree of ordering still exists well above the N-I phase transition temperature.(38-39,40,41’42) This

ordering is further confirmed by measurements using the method of infrared dichroism.(43)

The findings are characterized as evidence of two fractions of liquid crystal co-existing in

the polymer network, one bound by the network and the other not. Above the N-I

temperature, the unbound fraction evolves into the isotropic phase while the bound fraction

is kept oriented by the anisotropic network.

The ordering of liquid crystal near a surface at N-I temperature was studied by Sheng

using Landau-de Gennes theory.(44) Miyano used birefringence measurements to investigate

the aligning forces that are responsible for the wall-induced ordering at the N-I phase

transition. Crawford et al.(4S,46) studied this ordering effect in submicrometer cylindrical

channels via NMR. The measurements revealed the order of the first molecular layer at the

cavity wall. In a later experiment, Crawford et al. performed birefringence measurements on

the polymer network formed with the monomer BAB.(47) The measurements were made

above the N-I phase transition point of the liquid crystal and the results were used to estimate

the internal surface area of the network and its order parameter. In their studies, a one

dimensional equation with a single fit parameter was employed to fit the experimental data.


66

The fit was reasonably well considering that it was only a one-dimensional equation. The

value obtained for the fit parameter translated into a size of approximately 1 nm for the

radius of a column of polymer fibers. With molecular radius between 5-7A, this number

implied that each column was made up of 1 to 2 molecules. It seems unreal that the polymer

fiber is one big long molecule. In fact, the description of the polymer networks being made

up of polymer fibers can be substantiated by the images depicted in SEM pictures. Since the

surface of the fiber can be a surface with curvature, its ordering effect is not as direct as the

flat surface. Hence, an equation of higher dimension may need to accommodate the ordering

effect induced by a surface with curvature.

This section is devoted to the birefringence measurements of the polymer network for

various concentrations in order to make a better estimate of the column radius.

The monomer BAB6, with concentration ranging from l-4wt.% and the photoinitiator

benzoin methyl ether (BME) with 0.1-0.4 wt.%, are dissolved into the nematic liquid crystal

4'-pentyI-4-cyanobiphenyI (5CB). The cells are made of 1/8 inch thick glass coated with

polyimide and rubbed in the parallel direction. The cell spacing is controlled by mylar

spacers of ~29pm. The N-I phase transition temperature TNI of bulk 5CB is 34.7°C. The

mixture is injected into the cell and irradiated with uv light at ambient temperature for one

hour.

The optical system used to measure the birefringence consists of a He-Ne laser light

source, a spatial filter, a pair of collimated lenses, pin hole, polarizers, hot stage equipped

with temperature controller, analyzer and a photodetector. These parts are assembled together


67

and illustrated in Fig. 32. The polarizer and analyzer are positioned at 90° to each other with

the sample cell sandwiched between them. The orientation of the liquid crystal is at 45° to

both the polarizer and analyzer axis. The experiment is conducted at a temperature range of

34°C to 105°C. The intensity of light transmitted through the sample is related to the

birefringence of the polymer network in the isotropic liquid crystal by the following

equation:(48)

I = / 0sin( ) sin2(j> (4.1)A

where I0 is the intensity of the incident light, I is the intensity of the transmitted light, d is the

cell spacing, X is the wavelength of the laser light, An is the birefringence of polymer

networks in isotropic liquid crystal, and <j> is the angle that the network orientation made with

the polarization axis of the polarizers. Since that angle is 45°, the equation can be simplified

to I=I0sin2(7;dAn/A.). After some manipulation, the birefringence can therefore be calculated

from An=(A./rcd)arcsin[(I/I0)'/l].

In order to determine the birefringence of the polymer network, the whole sample cell is

dipped into the solvent octane for an extended period of time. Eventually the solvent

displaces the liquid crystal and the openings are sealed up with epoxy sealant to prevent the

solvent from evaporating. The birefringence measurement is performed again with the same

temperature range, this time with the solvent still inside the cell. The same equation for the

birefringence calculation is applied. Because of the isotropic nature of the solvent, the

birefringence obtained can be attributed only to the polymer network. The two sets of results


68

Sample PhotodetectorAperture

Alignment axis \ of Polymer \

Analyzer

________Temperature Chamber

Polarizer

He-Ne Laser

Figure 32. A diagram of the apparatus set-up for measuring the birefringence of

the polymer network.


69

are compared and the differences represent the birefringence induced by the polymer

network. This induced birefringence can be regarded as the result of the aligning force

effected by the polymer networks. An approach similar to that used by Crawford et al. is

employed, that is, the birefringence is expressed in terms of the order parameter. This

equation is fitted into the experimental data and the fitting parameters are the radius of the

fiber column and the order parameter of the liquid crystal on the surface of the column.

4.2.2 Theory

In the vicinity of the phase transition temperature, the Landau-de Gennes theory(49,50)

stipulates that the free energy density is an analytical fiinction of the order parameter (S). In

the context of the order parameter, its value is significantly less than 1; the function is

expanded in a power series of this parameter. The equilibrium value of this order parameter

is then the value that minimizes the free energy. The free energy density function in the

absence of an external magnetic or electric field is expressed as,

/ - f 0 . | a (T -rc)S 2 - T f tS 3. i c s - 4 . i i ( V 5 ) 2 (4.2)

where f0 is the free energy density of the isotropic phase independent of S, and the gradient

term represents the spatial variation of the order parameter. Tc* is a temperature slightly

below the phase transition temperature (Tc) of the system, which in this case is the liquid

crystal. The coefficients a, B, C and L are material parameters that can be extracted directly

from the experiments. In the PSCT system, it is believed that the anisotropic polymer

network is made up of columns of fibers and each column is a collection of polymer fibrils.


70

The director orientation of the nematic liquid crystal surrounding each of the columns will

have a rotational symmetry and translational symmetry along the axis of the columns (Fig.

33). Also, at a temperature above the N-I transition, the order parameter is very small. The

terms of power higher than the quadratic term will be discarded. Hence, the free energy

density function is written as,

/ . / o.Ia(r-r;)s2, iL (|? )2 (4.3)2 2 dr

The corresponding energy function per unit length is given, in cylindrical coordinates, by

F = 2tcfJR—a(T-T')S 2+—£(— )2

L 2 2 drrdr (4.4)

A quantity called the "correlation length"(50) is defined as,

c2 L= ----------- (4.5)

a{T-Tc)

such that any fluctuation that occurs over a distance % measured from the surface of the

polymer will be in phase. Multiplying both the numerator and denominator with Tc*, one

obtains,

T'c

r ca (T-T'c)0 c (4.6)

2 Tc= I 2 ---° (T-K)


71

Polymer Fibrils

00

0 \\

Liquid ^ _Crystal <T ^ 0 Q o op o £

PolymerFiber

0

0 o - r t RA 0 0 .0U o ° / o O / o \ o n f t

™ ° i o y n - ! o 0 y,0 0 ° U n r0 " 0 °0 0 °

O & Q : O 0 0

o o o

R r

Figure 33. The columnar description of the polymer fiber with radius R. The

order parameter of the fiber is Sop. The order parameter of the liquid crystal on

the polymer fiber surface is S0.


72

where £02 = L/Tc*a is called bare correlation length. A typical value for is 6.5A.(S,) To

further simplify the equation, dimensionless variables such as,

rx = —

I <4-7>u = — S

are substituted into Eq. (4.4). Here, S0 is the order parameter of the liquid crystal on the

surface of the fiber column. Since the term f0 does not depend on the order parameter S, it

is neglected. The energy function F becomes,

F - i t L S g f ’ l u ^ f i x d x (4.8)

Minimization of the free energy with respect to S requires that the Euler equation be

satisfied, that is,

— = 0 (4.9)du dx du/dx

where

<& = [« 2+(— )2] x (4.10)dx

and


73

- 2xudu

± { J ± - ) , 2— * 2 x ^ ^ ~dx du/dx dx dx2

(4.11)

one obtains,

2 x u - 2 — - 2 x — = 0 (4.12)dx 0*2

After some rearrangements, one obtains,

32k 1 du A u = 0 (4.13). + -

0x2 X dx

The solutions of this partial differential equation are called modified Bessel functions. Apply

the following boundary conditions,

i Ru = 1 =* x = —I (4.14)

u = 0 =>• x = °°

The solution is therefore,

u(x) K o WR (4.15)


74

(K0's are described in the Appendix A). The important point about the solution is that the

order parameter is now expressed in terms of R and S0. This order parameter will then be

used in the calculation of the birefringence of the polymer network.

4.2.3 Experimental Results and Discussions

The birefringence of the system can be considered as the sum of two contributions:

A n = A np + A n lc (4.16)

where Anp and Anlc are the birefringence of the polymer network and the liquid crystal,

respectively. The liquid crystalline order only exists around the fiber and is subject to the

aligning force that is induced by the fiber. It has been stated that the polymer network is

constructed with columns of polymer fiber; the birefringence of the network can therefore

be described as the total birefringence of all the fiber columns. For a perfectly oriented fiber

(order parameter = 1), the birefringence is An,*,. Taking 1 as the number density of the fibers

in a system with the order parameter of each individual column being equal to Sop, An,, is

given by,

J[* 2 it f R

& np o S opr d r d Q (4 ‘1 ?)

0 Jo

or,

Ah = l n R 2A n S m (4.18)p po op v

Similarly, An,c can be written as,


75

A nlc = / A nlcoS0u(r)rdrdd Jo Jr

= 2 n lA n lc0S0 u(r)rdr= 2 * /A « /cA

= 2 n lA n lc0S0E,2j R u(x)xdx(4.19)

2 n lA n lcoS J

where Anlco is the birefringence of the perfectly oriented liquid crystal. Putting them together,

the birefringence of the system, An, is,

Taking a unit length into consideration, the product ;rR2l yields the volume fraction of the

polymer fibers in the system. Making the approximation that the volume fraction is very

close to the mass fraction, one can replace the volume fraction with the mass fraction which

is just the concentration of the polymer, cp. The approximation is justified as the density of

the polymer is close to unity. Hence,


(4.20)

(4.21)

76

The birefringence of the system, An, is measured experimentally at different temperatures

for various polymer concentrations (Fig. 34). The birefringence decays very rapidly near the

N-I transition temperature and slowly as the temperature was increased further. The

birefringence of a similar sample with the liquid crystal (5CB) but without the monomer

vanishes rapidly beyond the N-I transition temperature. The measured birefringence is

therefore believed to be induced by the polymer network. This effect of the network on the

liquid crystal is described as the aligning force of the network.

The birefringence of the polymer network, An^pSopAnp,,, is also measured experimentally

for various polymer concentrations (Fig. 35). In both figures, the birefringences clearly

increased with increasing polymer concentration. The term,

is calculated by adopting the polynomial approximations^ for the Ko's. A computer program

written in BASIC is set up to do the calculation. The values of cp are just the concentrations

of polymer. The other parameters, Tc* = 307°K(51) and Anlco = 0.35{52) are used in the

calculations. The results of the calculation indicate that the fitting parameters, R = 50A and

So=0.3, yield reasonably good results for all five concentrations. This value of R is equivalent

to approximately 10~15 times the molecular radius. Realistically, it is quite possible that

these many molecules cling together to form a column. The value, So=0.3, agrees very well

with Sheng's finding.(42) The fitted curves and the experimental data for various polymer


(4.22)

77

0.02

Wu§O 0.01

A A

□ □A 4%

Oo,

° o 2.5%

1%

0.0020 30 40 50 60 70 80 90 100 110 120

TEMPERATURE (°C)

Figure 34. A plot of the birefringence of the system as a function of the

temperature for different polymer concentrations.


78

0.003

oo oo oO ° 4%

□ □□□

a 2.5%.0.002

o .C<3

0.001

0.00020 30 40 50 60 70 80 90 100 110 120

TEMPERATURE (°C)

Figure 35. A plot of the birefringence of the polymer network in an isotropic

solvent (Octane) as a function of the temperature for different polymer

concentrations.


79

concentrations are shown in Figs. 36-40. The fittings could be further improved if a

distribution function of R were used. Using the same equation and parameters, the

birefringence for different R is calculated and illustrated in Fig. 41.

The values of An^ and Sop in Eq. (4.18) are yet to be determined. In fact, it is impossible

to determine what the values are with the present experimental data. It is, however,

interesting to estimate Sop using a reasonable assumption for the value of An,*,. Here, both the

polymer and 5CB have two biphenyl rings in the center of their molecules, but 5CB has an

extra cyano group on one of its tails. A value, Anp^O.3, used to scale to that of 5CB

(Anlco=0.35) is considered to be quite reasonable. Using this value, Anp^O.3, one can

calculate Sop for perfectly oriented polymer networks. The calculated Sop for various polymer

concentrations are tabulated below,

Polymer Concentration Sop(35°C)

1.0wt.% 0.1843

2.0wt.% 0.2150

2.5wt.% 0.3000

3.0wt.% 0.3077

4.0wt.% 0.2382

Table 3. Calculated values for Sop from the experimental values of An,,

One might expect to have a constant Sop, but the order parameter fluctuates with polymer

concentration. This can be understood as follows: at the concentrations of lwt.% and 2wt.%,

the polymer networks are not strong enough to sustain any disturbances that are introduced


80

0.02

< 0.01

0.0030 40 50 60 70 80 90 100 110

TEMPERATURE (°C)

Figure 36. Birefringence measurements of a BAB6/5CB sample. The curve is a fit

to Eq. (4.21) with So=0.3, R=50A and cp=0.01.


81

0.02

0.01

0.0030 40 50 60 70 80 90 100 110

TEMPERATURE (°C)




82

0.02

o.oi

OoO o

0.0030 40 50 60 70 80 90 100 110

TEMPERATURE (°C)




83

0.02

% 0.01

o oO o

0.0030 40 50 60 70 80 90 100 110

TEMPERATURE (°C)




84

0.02

0.01

OO

0.0020 30 40 50 60 70 80 90 100 110 120

TEMPERATURE (°C)




85

0.015

0.010

G<3

0.005

R=25

R=40

R=50 R=60 R ' 80

0.000100 12060 8020 40

TEMPERATURE (°C)

Figure 41. Birefringence measurements of a BAB6/5CB sample. The curves are

the fits to Eq. (4.21) with So=0.3, R=10A, 25A, 40A, 50A, 60A and 80A; and

cp=0.02.


86

by both the liquid crystal and the solvent. Part of the network may have broken off and

detached from anything inside the cell. The concentration, cp, therefore does not reflect the

true concentration of the polymer at the time of measurement. As a matter of fact, the SEM

picture (Fig. 25) does show a lack of a complete polymer network for low polymer

concentration. At 4wt.% concentration, the polymer network is highly interconnected, but

the columnar description of the network is far from the real situation as it can be seen from

the SEM pictures. The many branches may have compromised the measurements. The

concentrations of 2.5wt.% and 3wt.% produce the two largest values of Sop. The polymer

networks are probably strong enough and consist of fewer branches. Nevertheless, the

assumption of polymer network being constructed by columns of fibers provides the basis

for the theoretical approach which closely approximates the real situation.


Chapter 5

Electro-optics of PSCT

5.1 Apparatus Set-up

The equipment used to perform the measurement is set up and shown diagrammatically

in Fig. 42. The light beam coming from the He-Ne laser has a wavelength of632.8nm. This

beam of light passes through a spatial filter and a pair of converging lenses before emerging

as a collimated beam. The beam size is reduced to a smaller one by a pinhole placed in front

of the sample. The light transmitted through the sample passes through another converging

lens before being collected by the photodetector. The angular measurement is performed with

the sample placed inside a transparent container filled with glycerine. The light always enters

glycerine at the same location before it reaches the sample which can be positioned at

different angles to the incoming beam. This arrangement will minimizes the Fresnel effect

that usually occurs at an air-to-glass interface.

Measurements of the temperature dependence are conducted with the sample placed

inside a temperature chamber. The temperature is monitored and controlled by an Instec

temperature controller. The voltage supplied to the sample is generated through an Analogic

Polynomial Waveform Synthezier 2020 and amplified with the Kepco BOP500M amplifier.

The frequency of the waveform generated, unless otherwise stated, is 2kHz. The light

collected by the photodetector is converted to DC voltage and read by the Keithley 194A

87


88

Photodetector

Converging Lens _

Sample

Aperture

ConvergingLensesSpatial

Filter

HP 54501A OscilloscopeHe-Ne

Laser

KEPCO BOP 500M Amplifier

O O

ANALOGIC 2020WaveformSynthesizers

KEITHLEY 194A High Speed Voltmeter

□ □

GATEWAY2000 486DX/33 PC

Figure 42. A diagram of the apparatus set-up for studying the electro-optic

properties of the PSCT light valve.


89

high speed voltmeter. The outputs of both the Kepco amplifier and the photodetector are

connected to the HP54501A oscilloscope. The waveform generator, the high speed

voltmeter, the oscilloscope and the 486 computer are all connected through IEEE protocol.

5.2 Samples

Fabrication of samples is described in 3.2. The cured mixture appears to be light

scattering and very viscous. Prolonged uv exposure does not change any of that. Heating the

cured sample up to 200°C at high vacuum does not seem to cause any melting or major

damage to the network. At this high temperature and vacuum, most of the liquid crystals

evaporate. The portion that does not evaporate is trapped in the polymer. Under the SEM,

the polymer appears differently (Fig. 43).

A light pressure applied to the surface of the cured sample will squeeze the liquid crystal

radially outward, just like a TN cell the liquid crystal will restore itself to the original texture

once the pressure is removed. In PSCT, the appearance can only be restored through the

application of a pulse. If the applied pressure is too high, the restoration will not materialize,

an indication that the networks have been disrupted.

The concentrations for both chiral dopant and monomer have enormous effect on the

electro-optical characteristics of the PSCT. Too few monomer will render the display electro-

optically unstable, while too much monomer will limit the electro-optic function of the liquid

crystal. On the other hand, the PSCT is not scattering enough with too little chiral dopant.

Especially when the polymer concentrations exceed certain limits, the high polymer content

is capable of keeping the cholesteric liquid crystal in the homeotropic state even if the


90

Figure 43. SEM image of a cell gap. The polymer is BAB6 at a concentration of

2.7wt.%. Chiral dopant (2.2wt.%) is mixed with the nematic liquid crystal and

cured with an applied field. The cell is vacuum at 0.03 mTorr for 20 hrs. with the

temperature set at 200°C. The fiber like structure is hardly distinguished due to

the liquid crystal still trapped in the fiber.


91

electric field is removed. The PSCT will appear clear and stay in this condition indefinitely.

Samples with too few monomer will not maintain the same kind of scattering as time elapses.

A phase diagram for different polymer and chiral concentrations is shown in Fig. 44.

5.3 Effects of Chiral Concentration

In small concentration, the pitch (p) of a cholesteric liquid crystal is inversely proportional

to the chiral concentration (C),(28,53)

P “ (5.1)

However, the threshold field (EJ is proportional to the pitch inversely, as seen from equation

2.10. This implies that Ec is proportional to the concentration of chiral dopants,

Ec « C (5.2)

For a constant polymer concentration, the rise time, the decay time, the contrast and the drive

voltage are measured with various chiral concentrations. The rise time (xr) is defined as the

time elapsed for the light transmission to reach 90% of the maximum from its 10%.

Similarly, the decay time (tj) is defined as the time elapsed for the transmission to reach 10%

from its 90% (Fig. 45). The contrast is defined as,

where Ton and T0fr are the transmittances in ON and OFF states, respectively. The drive

voltage is defined as the voltage required to achieve 90% of the maximum transmission.


92

0s

gHHHiIOgoPim

oPx

5

STABLE AND CLEAR

4

3

STABLE AND SCATTERING

2

1

UNSTABLE

02.5 3.01.5 2.01.0

CHIRAL CONCENTRATION (%)

Figure 44. Phase diagram of the PSCT system. In the upper section, the liquid

crystal remains in homeotropic state even after the field is removed. The middle

section is a region that the focal-conic texture of the liquid crystal is stablilized by

the polymer network. The lower section indicates that the focal-conic texture is

not stable due to insufficient polymer content.


93

TRANSMITTANCE

90%

10%TIME

H Trh ~ H Td I*-

Figure 45. Definition of the rise time (xr) and decay time (xd). xr is the time taken

for the transmission to reach from 10% to 90%. xd is the time from 90% to 10%.


94

In all the experiments involving different chiral concentrations, no matter what the

polymer is, the results seem similar. Shown in Fig. 46 are the results from a combination of

BAB and BABB6 , and various CB15 concentrations. The results show that the chiral

concentration has a stronger impact on the decay time than on the rise time. This can be

understood by the fact that the rise time is more dependent on the applied voltage. The

maximum contrast is recorded with 9wt.% chiral concentration indicating that the

corresponding domain sizes provide the maximum scattering efficiency. The drive voltage

increases almost linearly with respect to the chiral concentration, an indication that agrees

well with the prediction Ec oc C.

5.4 Polymer Concentration-dependent Response Time

In measuring polymer concentration-dependent response time, a gated pulse of duration

100ms at 40Vims is applied to each of the samples made from the three different monomers

for various concentration but fixed chiral concentration. For both BAB and BAB6 , the ratio

of E48 to CB15 is 91:9. In BAB6 , the ratio of ZLI4389 to R1011 is 97.75:2.25. The

corresponding rise time and decay time versus polymer concentration are shown in Fig. 47.

It can be seen in 2.2.3 that the rise time is a function of the applied voltage, but the results

here indicate that there are some deviations from that. The same is true for the decay time

which changes with the polymer concentration. It is particularly obvious with BAB6 . For

some reason that are not quite clear at this moment, the two polymers, BAB and BABB6 did

not appear to be as well behaved as BAB6 .


95

50

40c/a

30

2 0RISE TIME N DECAY TIME1 0

0150

125H

100

£1O<

O>>2Q

25

2 0

15

10

5

00 3 6 9 12 15

CHIRAL CONCENTRATION (%)

Figure 46. Plots of the response time, contrast and drive voltage as a function of

the chiral concentration. The chiral dopant is CB15 and the polymer (1.7wt.%) is

a combination of equal proportion of BAB and BABB6 .


96

40-o-

30BAB

2 0

V-1 0

02.01.0 1.50.0 0.5

40BABB6

30

2 0

1 0

04.01.5 2.5 3.0 3.52.0

100BAB6

80

60

40

2 0

03.52.5 3.02.0

POLYMER CONCENTRATION (%)° RISE TIME v DECAY TIME

Figure 47. Plots of the rise time and decay time as a function of the polymer

concentration for BAB, BABB6 and BAB6 .


97

5.5 Polymer Concentration-dependent Contrast

The contrast for all the samples made from the three different monomers is plotted against

various polymer concentration (Fig. 48). The proportion of chiral dopants to nematic liquid

crystals are the same as for 5.4. All three monomers show the same feature, that is, the

contrast peak at a certain concentration. Ton for various concentrations remains quite stable

while Tofr shows a slight increase from its minimum with increasing polymer concentration.

A larger value of Toff is also recorded at lower polymer concentration in which the polymer

networks tend to be less dense and the domain size is large. Larger domain size means fewer

domains are present for light scattering. At higher concentration, the domain size is either

smaller than or comparable to the wavelength of the light and hence less effective in light

scattering. In both cases, the contrast decreases as Tofr increases.

5.6 Polymer Concentration-dependent Drive Voltage

In this experiment, a voltage of continuous sine wave with frequency 2kHz is applied to

the sample stepwise at 0.5V every 2 seconds. The liquid crystal mixtures are the same as for

5.4. The voltage is first ramped up and then down. For constant chiral concentration, the

respective drive voltage for various polymer concentrations is plotted in Fig. 49. Similar to

the results obtained earlier, it lacks consistency. Both BAB and BAB6 showed a decrease of

drive voltage at increasing polymer concentration, but not for BABB6 . The decrease of

voltage can be attributed to the aligning effect of the polymer networks although this is not

quite apparent in the case of BABB6 .


98

125

100

BAB

1.5 2.00.5 1.00.0200

Oo BABB6

3.5 4.01.5 2.0 2.5 3.0125

100

BAB6

3.53.02.0 2.5

POLYMER CONCENTRATION (%)

Figure 48. Plots of the contrast as a function of the polymer concentration for

BAB, BABB6 and BAB6 .


99

2 0

BAB

2.01.00.0 0.525

2 0

15

1 0

5 BABB6

03.5 4.02.5 3.01.5 2.0

25

2 0

15

1 0

BAB65

03.53.02.0 2.5


Figure 49. Plots of the drive voltage as a function of the polymer concentration

for BAB, BABB6 and BAB6 .


100

5.7 Polymer Concentration-dependent Hysteresis

The hysteresis exhibited by PSCT with applied voltage ramping up and down can be

observed in Fig. 12b. In order to compare this effect with different polymer concentrations,

the width of the hysteresis is used as a measure to determine if there is any change with

respect to changing polymer concentration. This width can be considered as the voltage

difference between the ramp up and ramp down curves. The voltage difference, AV, is

defined as the difference of the two voltages taken at the mid-point of the ramp up curve (V t)

and the ramp down curve (V4), between the maximum and minimum transmission, that is,

AV=Vt - Vj, (Fig. 50). The plot of AV versus polymer concentration for BAB6 is shown in

Fig. 51, clearly, AV increases with increasing polymer concentration. Since AV depends on

both Vt and V4, , a closer examination of the curves reveals that the ramp up curve remains

basically in place in the region of concentrations considered; the increase of AV mostly

comes from the shift in position of the ramp down curve towards the low voltage side. This

shift is due to the delay in the relaxation process of the liquid crystal, a result of stronger

aligning force due exclusively to higher polymer concentration.

5.8 Effects of Temperature

The PSCT sample in measuring the effects of temperature is made up of 2.7wt.% BAB6 ,

2.2wt.% R1011,0.3wt.% BME and 94.8wt. % ZLI4389. The cured sample is placed inside

an Instec heating stage. Measurements are performed at different temperatures and the results

are shown in Fig. 52. The drive voltage differs in approximately IV over a course of 36°C,

a not very significant change. The AV, though, changes from 6V at room temperature to 2V


101

TRANSMITTANCE

AV

VOLTAGE

AV = V t - Vj,

Figure 50. Definition of AV. AV is measured at the position indicated by 50% of

the transmittance.


102

12.5

10.0

^ 7.5>>< 5.0

2.5

0.03.53.02.52.0


Figure 51. A plot of the hysteresis as a function of the polymer concentration for

BAB6 . The chiral concentration is 2.2wt.%.


103

40RISE TIME DECAY TIME -30

2 0

1 0

0150125

52 100

w%So>

18

15AVDRIVE VOLTAGE

1 2

96

30

20 25 30 35 40 45 50 55

TEMPERATURE (°C)

Figure 52. Plots of rise time, decay time, contrast, hysteresis and drive voltage as

a function of the temperature. The polymer is BAB6 at a concentration of

2.7wt.%. The chiral dopant is R1011 at a concentration of 2.2wt.%.


104

at 50°C. A significant portion of this difference, 3V, comes from the shift of the ramp down

curve. The rise time and decay time also decrease steadily, while the contrast remains quite

stable below 40 °C. On the contrary, the similar cell filled only with cholesteric liquid crystal

exhibits poor contrast over the same temperature range. Also, the drive voltage and AV

fluctuate with temperature, and the response was never well defined due to relatively high

transmission in OFF state.

5.9 Effects of UV Intensity

It is noted in 3.3.2 that the rate of polymerization is very much dependent on the intensity

of uv light. Reducing the intensity results in fibers with larger diameter. It is speculated that

a sudden blast of high UV intensity causes photopolymerization to take place almost

instantly, creating a network which consists of large numbers of fine polymer fibers. Two

monomer concentrations are studied and the results are shown in Figs. 53 and 54. The value

of AV for both cases shows consistently that the hysteresis is smaller for samples with high

uv exposure. Similar results are also obtained for rise time and decay time. It is believed that

the large amount of polymer fibers provide numerous nucleation sites for the liquid crystal

when the electric field is removed. This results in faster decay time and small hysteresis. It

is also the anisotropic formation of the network that the risetime is also shorter. However,

the effect of uv intensity becomes insignificant at the range above 1 0mW/cm2.

5.10 Wavelength Dependency

The electro-optical measurements are performed with a light source of wavelength


105

C/3

£WSH-<H

50

40

30

2 0

10

0

1 ------1------ 1------1------1------1------r

0 RISE TIME

* DECAY TIMEj i i i____i—

1 0

8

6

4

2

00 5 10 15 20 25 30 35 40

UV INTENSITY (mW/cm2 )

Figure 53. Plots of rise time, decay time, contrast, and hysteresis as a function of

uv intensity. The polymer is BAB6 at a concentration of 2.7wt.%. The chiral

dopant is R1011 at a concentration of 2.2wt.%.


106

tO£

40

30

2 0

1 0

/■—s.

><1

- I rt~ S ^ -

RISE TIME

DECAY TIME

Z 140 OU 120

8

6

4

2

00 5 10 15 20 25 30 35 40

UV INTENSITY (mW/cm2 )

Figure 54. Plots of rise time, decay time, contrast, and hysteresis as a function of

uv intensity. The polymer is BAB6 at a concentration of 2.1wt.%. The chiral



107

632.8nm. The results shown in previous sections are therefore good for that frequency. It is

of practical importance to have at least some idea how the PSCT system performs in a wide

spectrum of light in terms of transmission. A simple experimental setup including a Newport

780 white light source and a PR704 spectrophotometer from Photo Research is illustrated

in Fig. 55. The sample is filled with a mixture similar to the one in section 5.8. The barium

sulphate plaque has a diffuse reflectance of approximately 98% through the visible and near

infrared bandwidth. The optical head of the spectrophotometer is oriented at 45° to the

surface of the plaque. Also, the optical head has to be close, but not close enough to cast a

shadow on the plaque surface, so that the aperture is smaller than the image of the plaque in

the viewfinder of the meter. The results of the transmission of the sample with respect to ON

and OFF states are shown in Fig. 56. The corresponding contrast is shown in Fig. 57. It is

necessary to point out that the collection angle of the spectrophotometer is somewhere

between 2° and 10°. Hence, the value of contrast at a wavelength around 630nm is nowhere

near 1 0 0 as compared to the previous results.

5.11 Angular Transmission

Angular transmission measurements demonstrate that the PSCT is capable of high

transmission at wide viewing angles. The same mixture as 5.8 is used. The apparatus set-up

is basically similar to Fig. 42 except that the sample is placed inside a transparent container

filled with glycerine. The glycerine has almost the same index of refraction as the glass.

Light enters the glycerine always at normal incidence and the sample inside can be

positioned at different angles to the normal. This arrangement greatly reduces the effect of


108

WHITE LIGHT SOURCEAPERTURE

SAMPLE

BARIUMSULPHATE

SPECTRO - PHOTOMETER

Figure 55. A diagram of the apparatus set-up for measuring the light transmission

at different wavelengths.


109

1.0

ON STATE0.8

Wy 0.6

0.2

OFF STATE

0.0700 800 900500 600300 400

WAVELENGTH (nm)

Figure 56. A plot of the transmittance as a function of the wavelengths in the ON

and OFF states. The polymer is BAB6 at a concentration of 2.7wt.%. The chiral



110

50

40

30

2 0

1 0

0700 800 900500 600300 400

WAVELENGTH (nm)

Figure 57. A plot of the contrast as a function of the wavelengths for the same

sample as Fig. 56.


I l l

light bending when entering the sample. The measurement for both ON and OFF states are

shown in Fig. 58. The high transmission in ON state, especially at large angle, is clearly not

disrupted by the presence of the network.


112

1.0

0.8

0.6

0.4

0.2 ON State

« 0.05 / 3 0.02£

0.01

OFF State

0.0080 -60 -40 -20 0 20 40 60 80

ANGLE (° )

Figure 58. Plots of the transmittance as a function of the incident angle for the

same sample as Fig. 56.


Chapter 6

320 x 320 PSCT Projection Display Prototype

6.1 Design Concept

The conventional projection system that employs liquid crystal devices usually requires

a light source with high intensity. The high intensity offsets the tremendous light loss

resulting from absorption of the polarizers. A scattering type liquid crystal display such as

that described in this dissertation needs no polarizers and has the advantage of utilizing the

light source more efficiently to improve the brightness of the projected image. High

resolution liquid crystal devices used in projection systems are either a passive or an active

matrix type. An active matrix requires a transistor switch at each pixel site and therefore

considerably adds to the cost of a display. One of the most important features of the PSCT

system is the broad hysteresis loop which allows the use of a passive matrix considerably

simplifying and reducing the cost of the display. A bias voltage (V0) can be applied within

the hysteresis loop to yield a bistable condition. If the display is in an ON state, it will stay

on indefinitely as long as the bias voltage is applied; the same is true for the OFF state. Using

the voltage versus transmittance curve, it is possible to determine a range into which the bias

voltage should fall to give that kind of bistability (Fig. 59). If V0 < VA, it will not sustain the

ON state because the voltage is not high enough to keep the liquid crystal in the homeotropic

state. The elastic force of the liquid crystal gradually overcomes the electric force and the

113


114

TRANSMITTANCE

VOLTAGE

Figure 59. A diagram showing the location of the bias voltage on a voltage vs.

transmission curve. Points A and B are considered to be the lower and higher

limits of the bias voltage for the optimum performance. The contrast decreases

with time if the voltage below that of point A is used. If the voltage is shifted

beyond point B, the ON state is stable but rather appears to be "washed out" due

to increasing amount of light leaking from those OFF pixels.


115

liquid crystal begins to relax back to the helical structure. If V0 > VB, the ON state is

maintained but the OFF state is not opaque enough, hence the system will suffer the loss of

contrast. In the OFF state, the magnitude of V0 begins to have some re-orientational effect

on the liquid crystal and the scattering effect is therefore reduced.

The voltage ramp rate is a factor in deciding the size of AV and therefore the effectiveness

of V0. As indicated in Fig. 60, AV decreases with a decreasing voltage ramp rate. However,

even at an extremely slow rate, AV will not vanish. This means that the hysteresis will still

be present and V0 can still be defined. In general, for a constant chiral concentration, the

hysteresis increases with polymer concentration. The plot of AV versus polymer

concentration was already shown in Fig. 51; but this plot is repeated in Fig. 61 with an

additional curve obtained from a different voltage ramp rate for comparison.

In order to demonstrate the effectiveness of this bias voltage, a 50ms wide square wave

of 75V is applied to a cell containing 2.7wt.% BAB6 ,2.2wt.% R1011,0.3wt.% BME and

94.8wt.% ZLI4389. The liquid crystal is switched into the homeotropic state allowing the

transmission to reach to 90%. The voltage is then switched from 75 V to a bias voltage V0 of

11.2V as shown in the top of Fig. 62. At this bias voltage, the liquid crystal remains in the

homeotropic state and the transmission is still at 90% as shown in the bottom of Fig. 62. If

on the other hand the voltage is increased from zero to 11.2 V as shown in the top of Fig. 63,

the liquid crystal remains in the focal-conic texture and the transmission is minimal (curve

a in the bottom part of Fig. 63). The corresponding contrast between the ON and OFF state

is shown as curve b in the same figure. This contrast is well over 100 and remains stable for


116

10

8

6

4

2

00.2 0.30.0 0.1

VOLTAGE RAMPING RATE (V/sec)

Figure 60. A plot of the hysteresis as a function of the voltage ramping rate. The

polymer is BAB6 at a concentration of 2.7wt.%. The chiral dopant is R1011 at a

concentration of 2 .2 wt.%.


117

12.5

10.0

7.5>

I 5.0

2.5

0.03.52.5 3.02.0


Figure 61. A plot of the hysteresis as a function of the polymer concentration for

two different ramp rates: (O) 0.25 V/sec.; and (□ ) 0.0083 V/sec.


118

%5O>

co

1.0

0.8

0.6

0.4

0.2

0.0 L o ° -

0 4020 3010

TIME (sec)

Figure 62. A plot of the transmittance as a function of the time with the applied

waveform illustrated at the top of the figure. V, = 75 V and V0 = 11.2V.


9999999999999999

119

g

O

so>■Mili n i

2001.0

1600.8Wo120 23£ 0.6

H

g 0.4

400.2

0.030 4020100

TIME (sec)

Figure 63. A plot of the transmittance as a function of the time (curve a) with the

applied waveform illustrated at the top of the figure. V0 = 11.2V. The contrast is

plotted as a function of the time (curve b).


120

the whole measurement period. In fact, the bistability lasts for days without any sign of

degradation. The effect of bias voltage V0 on the contrast is shown in Fig. 64. Here, the same

voltage waveform is used but V0 will be varied. The contrast is measured 30 seconds after

the application of V0. A maximum contrast of 120 is achieved with a bias voltage of 10.5V.

However, this contrast decreases with time as the initial high transmission decreases

gradually with the application of the bias voltage. A slightly higher bias voltage is therefore

preferred to maintain a stable transmission.

6.2 Display Fabrication

To demonstrate the effectiveness of a scattering-mode shutter for projection applications,

a high resolution 320 x 320 pixel shutter on a 4 x 4 in. substrates (80 dots per inch) was

constructed. The patterning of ITO is similar to that used in the LCD industry for making a

dot-matrix display. This normally begins with a photolithographic process: (i) The substrates

are spin coated with a thin layer of photoresist and then placed under a mask which is

mounted on an exposure unit. The unit is designed to produce highly collimated uv light so

that the exact dimensions of the image on the mask can be transferred to the coated

substrates, (ii) The coated substrates are then dipped into a developing solution. For positive

photoresist, the part of the photoresist that is exposed to uv light will be dissolved away

while those not exposed adhere to the ITO of the substrates, (iii) The substrates are then

placed in an acid solution to etch away the part of ITO that is not covered by the photoresist,

(iv) After the etching process, the remaining photoresist is then stripped off by a solvent. A

patterned ITO is then formed on the substrates (see Fig. 9 and also the mask in Appendix B).


121

O

150

120

90

60

30

020155 100

BIAS VOLTAGE V0 (V)

Figure 64. A plot of the contrast as a function of the bias voltage V0. The contrast

is measured 30 sec. after the application of the bias voltage.


122

However, the design of the mask has to take the photopolymerization process into

consideration. Since the monomers under the pixel elements have to be cured with the liquid

crystal in the homeotropic state, connections must be made between the electrodes and the

power supply. For a 320 x 320 pixel display, it becomes too cumbersome to connect all the

electrodes to the power supply. Hence, the mask is designed in such a way that the electrodes

are connected together. This mask is used for both top and bottom plates. The cell spacing

is maintained by 15 pm glass spacer. A uv based epoxy is used for the edge seals. The cell

is filled with a PSCT material of 3wt.% BAB6 , 2.2wt.% R1011, 0.3wt.% BME and

94.8wt.% ZLI4389. The filling process is similar to that described in 3.2. After the

polymerization process, the part of the glass that has ITO connecting all the electrodes is

scribed and removed.

6.3 System Implementation

The projection light valve system consists of a conventional projector, a computer, a

PSCT display and its peripheral electronics. The overhead projector provides the light source

and optics for the light valves. The electronics of the display include the drivers from OKI

and a microcontroller from Siemens. Both parts and other related components were

originally designed and successfully deployed for a dot-matrix LCD and are readily available

in the market. The driver board and the interface board were built by The Catchpole

Corporation, which also provided the software for the system. The final assembly is

illustrated in Fig. 65; and a photograph of the actual set-up is shown in Fig. 6 6 .

The interface part is connected to the computer via a RS232 serial port. The commands


123

column driver

column driver

320 x 320 pixels

display

Figure 65. A schematic illustration of the projection light valve system using the

polymer stabilized cholesteric textures.


row driver

124

Figure 6 6 . Photograph of the complete projection light valve system.


125

and the input of the characters can be processed through the keyboard of the computer;

however, any graphic or image has to be first drawn up in a bit-mapped format (.bmp) before

downloading into the microcontroller. A photograph of this system projecting a picture

image on a wall is shown in Fig. 67; another photograph of the same system projecting a text

image is shown in Fig. 6 8 .

Addressing the display is done one row at a time. The bias voltage applied is the same for

all the columns, except that the phase of each individual column may be positive or negative.

The same bias voltage, +V0, is also applied to the row , but the phase is always positive. If

the voltage in the column is +V0, then the voltage across the pixel will be zero and the pixel

remains OFF. If the voltage of the same column changes phase, then the voltage across the

pixel will become 2V0 and the pixel will be switched to ON state. When the address begins,

the columns are addressed simultaneously while the first row is held at V0 and the rest of the

rows are held at 0 V. Once the addressing of the columns is completed, the voltage of the

first row will return to 0. At this point, all the column voltages are either -V0 or +VQ. A

voltage of V0 is then applied to the second row and the column voltages will simply change

phase to turn the pixels either ON or OFF. The changes in phase will not have any effect on

those previously addressed pixels since the liquid crystal responds only to root-mean-square

(RMS) voltage. The same procedures were repeated for the third row and so on until all the

rows are addressed. After all the rows are addressed, the voltages in all the rows will be 0,

and the voltages in all the columns are held at either +V0 or -V0. This addressing scheme is

shown schematically in Fig. 69.


126

ggjjjfrmll

Figure 67. Photograph of the system projecting a picture image on a wall.


127

ffimoae

Stabilizep Textur

(PSCT)Instilt

Figure 6 8. Photograph of the system projecting a text image on a wall.


0

0

0

-Vo +V0 -Vo

+Vo

0

0

□u

n n

□

+v0

n-Vo

u

+Vo

no -----

+Vo|

[J

a□

0

EVENT SEQUENCE

Figure 69. A schematic illustration of the addressing scheme. In the beginning, all the

pixels are OFF. The ON pixels in the first row will have the column voltage in opposite

phase to the row voltage. The rest of the pixels in the same row will have both voltages

in phase. All other rows will have zero voltages.


129

6.4 Display Characteristics

The prototype display has a panel size of 140mm x 140mm. The module which has the

driver boards mounted on the edges is 250mm wide and 250mm long. It can be easily placed

on a over-head projector without any adjustment. The major characteristics of the prototype

are tabulated below,

Projector brightness 246 Cd/m2

Display brightness 176 Cd/m2

Display area 104mm x 104mm

Pixel Density 320 x 320

Pixel pitch 0.325mm

Pixel size 0.3mm x 0.3mm

Writing time 2 0 sec.

Contrast 15 :1

Drive voltage 26V

Table 4. Display characteristics of a 320 x 320 pixels prototype.

Because the response time of the display for a 26V drive is 60ms to achieve the ON state and

24ms to achieve the OFF state, a writing time of 20sec. is the minimum time required to

address 320 lines. In order to be useful for many applications, this address rate needs to be

improved. One way to improve it is to use an erase-write mode where all pixels are erased

simultaneously to the clear state (60ms). Then the image is written. Since the response time

to the OFF state is faster than to the ON state, one can then write at a rate of 7.7 sec for 320

lines. While this is an improvement still faster rates mean that one must improve the


130

response time of the material. Increasing the thickness of the cell may help. Substantial

improvements for video rates requires the active matrix described below.

The contrast of the display can be substantially improved by reducing the etched area

between the ITO columns and rows. The void of applied electric field prior to polymerization

in these areas causes the liquid crystal to exist in a planar state. The polymer network formed

in these areas therefore exhibits different structure from that formed in the pixel areas.

Hence, little light scattering occurs at these areas resulting in a substantial amount of light

passing through that areas. That is why a contrast ratio of 15:1, far lower than is expected

without this problem. Another serious problem is the structure between the ITO line acts as

a nucleation and alters the shape of the hysteresis loop which affects the contrast.

6.5 Active Matrix

In a continuing effort to increase the display addressing rate, an active matrix display

based on metal-insulator-metal (MIM) technology has been successfully constructed with

this material (Figs. 70 and 71).(55) The chiral dopant and monomer were R1011 and BAB6

respectively, the same as used in these experiments. The nematic liquid crystal is TL203,

manufactured by BDH. This liquid crystal differs from those used in these studies, especially

in their electrical properties. It is imperative for liquid crystal used for an active matrix LCD

to have large resistivity in order to achieve a high holding ratio. The resistivity of TL203 is

at least two order of magnitude greater than E48 or ZLI4389.

The prototype creating the image shown in Fig. 70 was made from a MIM cell developed

at the University of Stuttgart in Professor Ernst Luders laboratory. This 96 x 128 pixel cell


131

Figure 70. Photograph of the PSCT projection system operating on an active

matrix display based on MIM technology.


132

Siffcdii<^t::viewnormal mode

Universitat Stuttgart Kent S ta te University

Figure 71. Photograph of a direct view PSCT display using MIM technology.


133

with an area of 30mm x 40mm was filled with a mixture using the TL203 material described

above. A binary drive circuit also developed at the University of Stuttgart was used to drive

the display.


Chapter 7

Conclusion

Polymer networks formed in a liquid crystal environment with low monomer

concentration are studied systematically by means of scanning electronic microscopy (SEM),

optical birefringence and electro-optic response. The orientation of the network becomes

anisotropic through controlling the liquid crystal orientation during polymerization. Through

different surface conditions of the cell wall and application of electric fields, different

orientations of the director field are created, resulting in various polymer network

configurations.

A considerable amount of work involving SEM studies of polymer network is reported

in this dissertation. The network suffers some minor disruption when the liquid crystal has

to be removed from the sample cell in preparation for the SEM studies, but the overall

anisotropic structure remains intact and the resulting photographs are instructive. Of

principal interest are networks formed under homeotropic alignment. The homeotropic state,

created either by perpendicular surface alignment and by the electric field, gives rise to an

erect structure. The application of high voltage prior to and throughout the polymerization

process produces a finer structure.

To demonstrate the importance of liquid crystal orientation on the outcome of the final

polymer structure, polymerization is carried out at high temperature in the isotropic phase

134


135

where the polymer does not exhibit definable structure. The result is an isotropic structure

with zero birefringence. Also, low frequency fields cause instabilities in the liquid crystal;

the polymer network formed under such fields, similarly, shows no ordered network structure

at all.

Birefringence measurements of networks formed in the nematic phase then heated beyond

the N-I transition temperature indicate that there are significant amounts of birefringence still

present long after the liquid crystal turns isotropic. The amount of birefringence changes with

polymer concentrations and exists as a result of the aligning effect of the ordered polymer

network on the liquid crystal. The birefringence decays very rapidly within a small range

close to the N-I transition temperature, then levels off as the temperature further increases.

This birefringence is a sum of two contributions, the polymer network and the liquid crystal

in the vicinity of and ordered by the network. The birefringence of the polymer network is

measured with the liquid crystal being replaced with an isotropic solvent. In this case, the

results are temperature independent but change with polymer concentration.

The contribution of the liquid crystal to the system birefringence can be related to the

dimension of the network based on a model in which the polymer network is made up of

columns of polymer fibers. This model makes use of the simplified version of the Landau-de

Gennes equation in relating the order parameter to the coherence length. The relationship of

the order parameter and the coherence length is used to derive an equation which describes

the birefringence of the system. The fit of this equation to the experimental data yields a

radius of 5nm for the column, a reasonable value believed to be more realistic than


136

Crawford's(47) earlier estimates of lnm. In fact, the value obtained for the diameter of a

column by actually measuring the SEM image yields a size of approximately lOOnm in

diameter. By subtracting the thickness of 70nm due to the palladium coating, the actual

radius becomes 15nm, closer to the 5nm value. Recent small angle neutron scattering studies

of polymer networks report values of 30nm for the fiber radius.(56)

In the electro-optical characteristics of the PSCT system, the presence of the polymer

network does not present any adverse effect on the transmission of the light in the powered

state of a cell, mainly because the concentration of the polymer is very low and light

scattering by the network is minimal. In the unpowered state, the focal-conic texture is found

to be stabilized by the polymer network. The network is also necessary for the large

hysteresis exhibited by the system which allows the development of bistability in display

application. The network, however, has the undesirable effect of increasing both the rise time

and decay time which affect the writing speed of a display.

The electro-optical characteristics are also found to be influenced indirectly by the uv

light intensity. At an intensity below 10mW/cm2, the response time, contrast and drive

voltage all change with uv intensities. At an intensity above 10mW/cm2, not many changes

are observed; the polymer chain grows longer at low uv intensity while the polymer chain

tends to be short at high uv intensity. The polymer network also changes the temperature

dependency of the cholesteric liquid crystal which is not as pronounced as it might be,

probably because the network limits the change of the pitch length.

The application of the PSCT scattering-mode technology to a 320 x 320 pixel projection


137

light valve proves to be successful and promising for certain commercial and scientific

applications. The fabrication of the display is simple: it does not require an active matrix and

the electronic hardware requires no custom design. When the light valve is put into

operation, it delivers an image with very high brightness and clarity. The projection device

is not without its problems, however: the seemingly high drive voltage limits the drive

electronics to costly chips; the extended rise time and decay time produce a long writing time

for the display. These are not limiting features however for such applications as high

intensity beam shaping for stage lighting, or for image selection in telescopes, or spatially

controlled lighting of X-ray photograph, or certain direct view and projection display

applications which require infrequent updating of the images. These are a few applications

under commercial development. In our spatial light modulator, the areas between the

electrodes were made too large (~25pm) allowing too much light to pass through in the off

state, resulting in a lower contrast ratio. In addition, the polymer network formed in these

area does not have the same orientation as that formed in the pixel area, causing the

hysteresis to reduce in size and creating a problem in the determination of the bias voltage.

In commercial application, this interpixel spacing needs to be limited to 5~I0pm. To

alleviate the drive voltage problem, liquid crystal material of low drive voltage can be used.

The active addressing method should be considered so that the writing time can be further

reduced. With all these improvements in place, a powerful and elegant but cost effective

display is available for numerous applications.


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APPENDIX A

Modified Bessel Functions

The differential equation,

d 2y 1 <fy — -♦ ---- — + ( - 1m 2

2)y = o

d x 2 x dx x

has two real and independent sets of solutions. The first solution exhibits asymptotic

behaviour for large values of x, and is finite at the origin. The solution is called the modified

Bessel functions of the first kind. The second solution behaves asymptotically near the origin

and approaches zero as x —> oo. It is called the modified Bessel function of the second kind.

When m is reduced to 0, the equation becomes,

which is exactly the same as Eq. 4.13. The boundary conditions require thaty be finite at the

origin and be 0 as x -» oo. Using polynomial approximaiton, the solution is given as,

K0(x) = -In (x/2) I0(x) - 0.57721566 + 0.42278420 (jc /2)2+ 0.23069756 (x/2 )4+ 0.03488590

(x!2)6+ 0.00262698 (x/2f+ 0.00010750 (jc/2) 10 + 0.00000740 (x/2) 12


for 0 < x < 2

142

143

for 2 < x < oo

jcV £ 0(jc) = 1.25331414 - 0.07832358 (2 /x) + 0.02189568 (2 /x) 2 - 0.01062446 (2 /x)3 +

0.00587872 (2/x)4 - 0.00251540 (2/x)5 + 0.00053208 (2/x)6

where

-3.75 < x < 3.75

70(x) = 1 + 3.5156229 (x/3.75)2 + 3.0899424 (x/3.75)4 + 1.2067492 (x/3.75)6 + 0.2659732

(x/3.75) 8 + 0.0360768 (x/3.75) 10 + 0.0045813 (x/3.75) 12

3.75 <x < co

x'V/0(x) = 0.39894228 + 0.01328592 (3.75/x) + 0.00225319 (3.75/x)2 - 0.00157565 (3.75/x)3

+ 0.00916281 (3.75/x)4 - 0.02057706 (3.75/x) 5 + 0.02635537 (3.75/x) 6 - 0.01647633

(3.75/x) 7 + 0.00392377 (3.75/x) 8


APPENDIX B

A 320 Line Mask for 4" x 4" Substrate

144


00000102020001000100000200010200010200020200020001020002000002000100

APPENDIX C

Schematic Diagram of the Microcontroller Board

Hh“

145


OEC

OU

PlIH

t CA

P1

PM IM

, UB

, U*

. U7

, UJ

» AM

O U

tt

APPENDIX D

Schematic Diagram of the Row Driver Board

146


APPENDIX E

Schematic Diagram of the Column Driver Board

b = _ lU = 2 - J

l = S S _ Ji = . - J

G - T - I J -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

147


APPENDIX F

Schematic Diagram of the Driver Board Connection

148


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