INFORMATION TO USERS

Transcription

1 INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Each original is also photographed in one exposure and is included in reduced form at the back of the book. Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6" x 9" black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. A Bell & Howell Information Company 300 Nortn Z eeb Road. Ann Arbor. Ml USA 313/ /

2

3 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

4 UMI Number: UMI Microform Copyright 1995, by UMI Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. UMI 300 North Zeeb Road Ann Arbor, MI 48103

5 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 Approved by Chair, Doctoral Dissertation Committee HA QM iajl*. Members, Doctoral Dissertation Committee m. Accepted by jchair, Department of Physics, Dean, College of Arts and Sciences u

6 TABLE OF CONTENTS List of Figures List of Tables Acknowledgements vii xviii xix Chapter Page 1. Introduction Brief History of Liquid Crystal Displays Introduction to Polymer Stabilized Cholesteric Texture (PSCT) Focus of the Dissertation 5 2. Liquid Crystal Displays: Principles and Applications Properties of Liquid Crystals Nematic and Cholesteric Phases Dielectric and Optical Properties Elastic Properties Liquid Crystal Display Applications Twisted Nematic Displays Supertwist Birefringence Effect Cholesteric-Nematic Phase Change Effect Polymer Dispersed Liquid Crystals 20 iii

7 TABLE OF CONTENTS 2.3 Liquid Crystal Displays Addressing Technique Passive Matrix Display Active Matrix Display Polymer Stabilized Cholesteric Textures Materials Monomers Chiral Dopants and Liquid Crystals Cell Fabrication Photopolymerization Set-up Polymerization Process Principles and Applications of PSCT Polymer Networks in Liquid Crystals Optical and Scanning Electronic Microscopy Studies of Polymer Networks Introduction Planar Alignment (I) Planar Alignment (II) Homeotropic Alignment by Chemical Treatment Homeotropic Alignment by External Field 48 iv

8 TABLE OF CONTENTS Monomer Concentration Effect High Temperature Effect Different Monomers Frequency Effect on Curing No External Field and Surface Effect Cholesteric Liquid Crystal in an Electric Field Birefringence of Polymer Networks Introduction Theory Experimental Results and Discussions Electro-optics of PSCT Apparatus Set-up Samples Effects of Chiral Concentration Polymer Concentration-dependent Response Time Polymer Concentration-dependent Contrast Polymer Concentration-dependent Drive Voltage Polymer Concentration-dependent Hysteresis Effects of Temperature 100 v

9 TABLE OF CONTENTS 5.9 Effects of UV Intensity Wavelength Dependence Angular Transmission x 320 PSCT Projection Display Prototype Design Concept Display Fabrication System Implementation Display Characteristics Active Matrix 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

10 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 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 A diagram showing the orientation of a liquid crystal molecule Illustrations of the three different types of elastic deformation of the liquid crystals 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 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

11 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.

12 LIST OF FIGURES 12. The chemical structures of the chiral dopants: (a) R1011; and (b) CB Voltage vs. transmission curves for: (a) cholesteric liquid crystal; and (b) polymer stabilized cholesteric texture Dynamic response curves for: (a) cholesteric liquid crystal; and (b) polymer stabilized cholesteric texture SEM image of the liquid crystal free polymer network. Note the direction of the fibers running parallel to the rubbing direction 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 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 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

13 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. 23. SEM image of the liquid crystal free polymer network. The sample is 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. 24. SEM image of the liquid crystal free polymer network. The sample is 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

14 LIST OF FIGURES 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 SEM image of the liquid crystal free polymer network. The polymer 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 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 SEM image of the liquid crystal free polymer network. The polymer is BAB6 at a concentration of 2.7wt.%. The polymer appears to be a fiber like structure and exhibits some local orientation SEM image of the liquid crystal free polymer network. 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. 62 xi

15 LIST OF FIGURES 31. SEM image of the liquid crystal free polymer network. 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 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 S 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= Birefringence measurements of a BAB6/5CB sample. The curve is a fit to Eq. (4.21) with So=0.3, R=50A and cp=0.02. xii

16 LIST OF FIGURES 38. Birefringence measurements of a BAB6/5CB sample. The curve is a fit to Eq. (4.21) with So=0.3, R=50A and cp= Birefringence measurements of a BAB6/5CB sample. The curve is a fit to Eq. (4.21) with So=0.3, R=50A and cp= Birefringence measurements of a BAB6/5CB sample. The curve is a fit to Eq. (4.21) with So=0.3, R=50A and cp= 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= A diagram of the apparatus set-up for studying the electro-optic properties of the PSCT light valve 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

17 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 BABB Plots of the rise time and decay time as a function of the polymer concentration for BAB, BABB6 and BAB Plots of the contrast as a function of the polymer concentration for BAB, BABB6 and BAB Plots of the drive voltage as a function of the polymer concentration for BAB, BABB6 and BAB Definition of AV. AV is measured at the position indicated by 50% of the transmittance. xiv

18 LIST OF FIGURES 51. A plot of the hysteresis as a function of the polymer concentration for BAB6. The chiral concentration is 2.2wt.% 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.% 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.% 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.% A diagram of the apparatus set-up for measuring the light transmission at different wavelengths 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 isriollata concentration of 2.2wt.% A plot of the contrast as a function of the wavelengths for the same sample as Fig Plots of the transmittance as a function of the incident angle for the same sample as Fig xv

19 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 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.% A plot of the hysteresis as a function of the polymer concentration for two different ramp rates: (O) 0.25 V/sec.; and ( ) V/sec 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 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) 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

20 LIST OF FIGURES 65. A schematic illustration of the projection light valve system using the polymer stabilized cholesteric textures Photograph of the complete projection light valve system Photograph of the system projecting a picture image on a wall Photograph of the system projecting a text image on a wall 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 Photograph of the PSCT projection system operating on an active matrix display based on MIM technology Photograph of a direct view PSCT display using MIM technology. 132 xvii

21 LIST OF TABLES Table Page 1. List of the monomers used in the studies Physical properties of E48 and ZLI Calculated values for Sop from the experimental values of Aiip Display characteristics of a 320 x 320 pixel prototype. 129 xviii

22 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

23 XX

24 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

25 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

26 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)

27 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

28 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

29 6 OFF STATE (OPAQUE) ON STATE (CLEAR) 11 Y i 1, 1 r I <11 l'i i i 1 'I'll'fl ' Vi!.1I Y IB.1.1 «I 14.. I 9 y v v y y v (a) (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.

30 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.

31 Chapter 2 Liquid Crystal Display-Principles and Applications 2.1 Properties of Liquid Crystal 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

32 n (a) NEMATIC (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.

33 10 Y X Figure 3. A diagram showing the orientation of a liquid crystal molecule.

34 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/p 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.

35 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 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

36 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. that, The free energy equation of a cholesteric liquid crystal has an additional term, q0, such F a - 54 lk < y-nf K J n - V x n q 0f. K. J n ^ n f ] (2.8) 2.2 Liquid Crystal Display Applications 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

37 SPLAY TWIST BEND Figure 4. Illustrations of the three different types of elastic deformation of the liquid crystals.

38 ANALYZER POLARIZER LIGHT SOURCE (b) < 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.

39 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 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 - p * 2tc (2-9)

40 17 RUBBING DIRECTION (a) ^ ^ ^ RUBBING DIRECTION RUBBING DIRECTION (b) 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.

41 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 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

42 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.

43 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 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

44 21 OFF STATE o V v V ON STATE 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.

45 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 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).

46 23 ITO electrode Glass substrate J- J- " Column electrode Pixel Row electrode i i i i i i i 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."

47 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)vl N (2.13) F+ - \ (Vr-Vcf,(N -l)vl 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

48 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 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.

49 26 X l X2 X3 Y3 Y2 Y i Pixel MOS Transistor 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.

50 Chapter 3 Polymer Stabilized Cholesteric Textures 3.1 Materials 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 BAB BABB6 BAB6 Chemical Name 4,4'-Bisacryloyloxy biphenyl 4,4'-Bis{4-[6-(acryloyloxy)hexyloxy]benzoate}biphenyl 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

51 2 8 CH2 = CHCCX, ^ } ~ tc C H = CHj (a) c h 2= c h c o 2 - ( c h 2) 6-0-^^ co2 02c^Q >-0- ( c h2 ) 6 -o2c c h = c h 2 (b) CH2 = CHCOj - ( CH2) 6 - o - ( CH2) 6 ' 0 2CCH = CH, (C) ^r\ \ /f O ft \ // o c h 3 (d) Figure 11. The chemical structures of the monomers and photoinitiator: (a) BAB; (b) BABB6; (c) BAB6; and (d) BME.

52 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: SmC I00 c 108 C N 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 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 (3.1)

53 30 (a) CH3 CHaQHaCHCH Q CN * (b) Figure 12. The chemical structures of the chiral dopants: (a) R1011; and (b) CB15.

54 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) n* An As II Viscosity (20 C) (mm2/s) Table 2. Physical properties of E48 and ZLI 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

55 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 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

56 33 switch the liquid crystal into homeotropic state is applied to the cell and maintained throughout the process 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, AG = AH - T AS (3.2) 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. I R- R- + CH2=CHX * ~ r c h 2c- 2 I X H

57 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-

58 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.

59 C /3 Z 1.0 H APPLIED VOLTAGE (V) Figure 13. Voltage vs. transmission curves for: (a) cholesteric liquid crystal; and (b) polymer stabilized cholesteric texture.

60 g 0.2 E TIME (ms) Figure 14. Dynamic response curves for: (a) cholesteric liquid crystal; and (b) polymer stabilized cholesteric texture.

61 Chapter 4 Polymer Networks in Liquid Crystals 4.1 Optical and Scanning Electronic Microscopy Studies of Polymer Networks 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, *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

62 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

63 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) 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.

64 41 Figure 15. SEM image of the liquid crystal free polymer network. Note the direction of the fibers running parallel to the rubbing direction.

65 42 Rubbing direction 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.

66 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 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 Here, the mixture takes up a twisted configuration throughout the cell from one plate to the other. Since there is no chiral agent, both lefthanded 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 righthanded 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

67 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.

68 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.

69 46 that the twisted configuration of the liquid crystal is preserved by the network 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 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.

70 47 Figure 19. SEM image of the liquid crystal free polymer network. The large openings are created when the solvent evaporates.

71 48 Some openings are large enough to allow direct view of the bottom plate on which the network is standing 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

72 49 g r s 2 ^ - ^! «- ^ ^ 3 ^ gite^ps^jit fflggjmm i*w <>f tr ;; *y>.> ^>;:«? a*,t z ^ ^.- ^.- 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.

73 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 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.

74 51 I nm Figure 21. SEM image of the liquid crystal free polymer network. The network retracts in all directions revealing the fibers and the bottom plate.

75 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.

76 10pm Figure 23. SEM image of the liquid crystal free polymer network. The sample is 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.

77 54 Figure 24. SEM image of the liquid crystal free polymer network. The sample is same as Fig. 23 but viewed from above.

78 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.

79 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 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.

80 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.

81 58 Figure 27. SEM image of the liquid crystal free polymer network. The polymer 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.

82 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 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.

83 60 Figure 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.

84 61 Figure 29. SEM image of the liquid crystal free polymer network. The polymer is BAB6 at a concentration of 2.7wt.%. The polymer appears to be a fiber like structure and exhibits some local orientation.

85 lpm 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.

86 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 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 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

87 64 Figure 31. SEM image of the liquid crystal free polymer network. 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 sample is tilted at 45 to the normal. The plate surface appears to be partitioned with numerous polymer "walls."

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

89 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 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

90 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

91 68 Aperture Alignment axis \ of Polymer \ Sample Photodetector Analyzer Temperature Chamber Polarizer He-Ne Laser Figure 32. A diagram of the apparatus set-up for measuring the birefringence of the polymer network.

92 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 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.

93 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 = 2tc f JR a(t-t')s 2+ ( )2 L2 2 dr A quantity called the "correlation length"(50) is defined as, rdr (4.4) 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 (T-K)

94 71 Polymer Fibrils \ \ Liquid ^ _ Crystal <T ^ 0 Q o op o U n 0 o - r t RA U o / o O / o \ o n ft io yn-!o0 y, r 0 " O & Q : O 0 0 o o o Polym er Fiber 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.

95 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, r x = 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 - itl 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, where = 0 (4.9) du dx du/dx and <& = [«2+( )2] x (4.10) dx

96 73 du - 2xu ± { J ± - ), 2 * 2 x ^ ^ ~ dx du/dx dx dx2 (4.11) one obtains, 2 x u 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, u = 1 i =* x = R I (4.14) u = 0 => x = The solution is therefore, u(x) K o W R (4.15)

97 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 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 = lnr2a n S m (4.18) p po op v Similarly, An,c can be written as,

98 75 A nlc = / Jo Jr A nlcos0u(r)rdrdd = 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, (4.20) 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, (4.21)

99 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, (4.22) 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

100 W u O 0.01 AA Oo, A 4% o 2.5% 1% TEMPERATURE ( C) Figure 34. A plot of the birefringence of the system as a function of the temperature for different polymer concentrations.

101 oo oo o O 4% a 2.5%. o. C < 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.

102 79 concentrations are shown in Figs 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.% wt.% wt.% wt.% wt.% 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

103 < 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.

104 TEMPERATURE ( C) Figure 37. Birefringence measurements of a BAB6/5CB sample. The curve is a fit to Eq. (4.21) with So=0.3, R=50A and cp=0.02.

105 o.oi Oo O o TEMPERATURE ( C) Figure 38. Birefringence measurements of a BAB6/5CB sample. The curve is a fit to Eq. (4.21) with So=0.3, R=50A and cp=0.025.

106 % 0.01 o o Oo TEMPERATURE ( C) Figure 39. Birefringence measurements of a BAB6/5CB sample. The curve is a fit to Eq. (4.21) with So=0.3, R=50A and cp=0.03.

107 OO TEMPERATURE ( C) Figure 40. Birefringence measurements of a BAB6/5CB sample. The curve is a fit to Eq. (4.21) with So=0.3, R=50A and cp=0.04.

108 G < R=25 R=40 R=50 R=60 R' 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.

109 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.

110 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

111 88 Photodetector Aperture Sample Converging Lens _ He-Ne Laser Spatial Filter Converging Lenses HP 54501A Oscilloscope KEPCO BOP 500M Amplifier O O ANALOGIC 2020 Waveform Synthesizers KEITHLEY 194A High Speed Voltmeter GATEWAY DX/33 PC Figure 42. A diagram of the apparatus set-up for studying the electro-optic properties of the PSCT light valve.

112 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 electrooptically 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

113 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.

114 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 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 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.

115 92 5 0s 4 STABLE AND CLEAR g HH Hi 3 IO go Pi m 2 STABLE AND SCATTERING o Px 1 UNSTABLE 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.

116 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%.

117 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 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.

118 95 50 c/a RISE TIME N DECAY TIME H O < O > > 2 Q 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.

119 BAB V- -o BABB BAB 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.

120 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.

121 BAB O o BABB BAB POLYMER CONCENTRATION (%) 3.5 Figure 48. Plots of the contrast as a function of the polymer concentration for BAB, BABB6 and BAB6.

122 BAB BABB BAB POLYMER CONCENTRATION (%) 3.5 Figure 49. Plots of the drive voltage as a function of the polymer concentration for BAB, BABB6 and BAB6.

123 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

124 101 TRANSMITTANCE AV VOLTAGE AV = V t - Vj, Figure 50. Definition of AV. AV is measured at the position indicated by 50% of the transmittance.

125 ^ 7.5 > > < POLYMER CONCENTRATION (%) 3.5 Figure 51. A plot of the hysteresis as a function of the polymer concentration for BAB6. The chiral concentration is 2.2wt.%.

126 RISE TIME DECAY TIME w % S o > AV DRIVE VOLTAGE 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.%.

127 104 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 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 0 mw/cm Wavelength Dependency The electro-optical measurements are performed with a light source of wavelength

128 105 C/3 W SH-< H r 0 RISE TIME * DECAY TIME j i i i i 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.%.

129 106 to I r t~ S ^ - RISE TIME DECAY TIME Z 140 O U / s. > < 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 dopant is R1011 at a concentration of 2.2wt.%.

130 nm. 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 as compared to the previous results 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

131 108 APERTURE WHITE LIGHT SOURCE SAMPLE BARIUM SULPHATE SPECTRO - PHOTOMETER Figure 55. A diagram of the apparatus set-up for measuring the light transmission at different wavelengths.

132 ON STATE W y OFF STATE 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 dopant is R1011 at a concentration of 2.2wt.%.

133 WAVELENGTH (nm) Figure 57. A plot of the contrast as a function of the wavelengths for the same sample as Fig. 56.

134 I ll 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.

135 ON State «0.0 5 / OFF State ANGLE ( ) Figure 58. Plots of the transmittance as a function of the incident angle for the same sample as Fig. 56.

136 Chapter 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

137 114 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.

138 115 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

139 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.%.

140 > I POLYMER CONCENTRATION (%) 3.5 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 ( ) V/sec.

141 118 % 5 O > co L o 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.

142 119 g O s o> i n i Mil W o 0.6 H g 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).

143 120 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).

144 O 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.

145 122 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 The interface part is connected to the computer via a RS232 serial port. The commands

146 123 column driver 320 x 320 pixels display row driver column driver Figure 65. A schematic illustration of the projection light valve system using the polymer stabilized cholesteric textures.

147 124 Figure 6 6. Photograph of the complete projection light valve system.

148 125 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 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.

149 126 ggjjjfrmll Figure 67. Photograph of the system projecting a picture image on a wall.

150 127 ffimoae Stabilizep Textur (PSCT) Instilt Figure. Photograph o f the system projecting a text image on a wall. 6 8

151 Vo 0 -Vo +V0 -Vo u n n 0 o v0 n -Vo u +Vo n a +Vo [J 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.

152 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

153 130 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 o f 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

154 131 Figure 70. Photograph of the PSCT projection system operating on an active matrix display based on MIM technology.

155 132 Siffcdii<^t::view normal mode Universitat Stuttgart Kent State University Figure 71. Photograph of a direct view PSCT display using MIM technology.

156 133 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.

157 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

158 135 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

159 136 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

160 137 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.

161 References 1. F. Reinitzer, Monatsh. Chem., 9,421 (1888). 2. L. Gattermann and A. Ritschke, liber Azoxyphenolather Ber. Dt. Chem. Ges., 23,1738 (1890). 3. G. H. Heilmeier, L. A. Barton and L. A. Zanoni, Appl. Phys. Lett., 13,46 (1968); Proc. IEEE, 56, 1162 (1968). 4. G. H. Heilmeier, J. A. Castellano, L. A. Zanoni, Mol. Cryst. Liq. Cryst., 8,293 (1969). 5. G. H. Heilmeier and J. E. Goldmacher, Proc. IEEE, 57,34 (1969). 6. H. Kelker, B. Scheurle, R. Hatz and W. Barton, Angew. Chem., 82,984 (1970). 7. M. Schadtand W. Helfrich, Appl. Phys. Lett., 18,127 (1971). Swiss Patent Nr of 4 December J. Fergason, U. S. Patent Nr of 9 February T. J. Scheffer, J. Nehring, M. Kaufmann, H. Amstutz, D. Heimgartner and P. Elgin, SID 1985 Digest o f Technical Papers'SMI, 120 (1985); Appl. Phys. Lett., 4 5,1021 (1984). 10. S. Morizumi, K. Oguchi, T. Misawa and R. Araki, SID 1984 Digest o f Technical Papers, 316 (1984). 11. N. A. Clark, M. A. Handschy and S. T. Lagerwall, Mol. Cryst. Liq. Cryst., 94, 213 (1983). 12. J. W. Doane, N. A. Vaz, B.-G. Wu and S. Zumer, Appl. Phys. Lett., 48,269 (1986). 13. D. K. Yang, L.-C. Chien and J. W. Doane, Proc. o f the International Display Research 138

162 139 Conference, 49 (1991); Appl. Phys. Lett., 60, 3102 (1992). 14. Y. K. Fung, D. K. Yang, J. W. Doane and Z. Yaniv, Proc. o f the 13th International Display Research Conference, 157 (1993). 15. L. M. Blinov, Electro-optical and magneto-optical properties o f liquid crystals (John Wiley & Sons Limited, 1983). 16. A. Saupe and W. Maier, Z Naturforsch., 16a, 816 (1961). 17. C. Mauguin, Bull. Soc. Frang Mineral. Cryst., 34, 71 (1911). 18. C. W. Oseen, Trans. Faraday Soc., 29, 883 (1933). 19. H. Zocher, Trans. Faraday Soc., 29,945 (1933). 20. F. C. Frank, Discuss. Faraday Soc., 2 5,19 (1958). 21. P. G. de Gennes, Solid State Comm., 6,168 (1968). 22. M. Kawachi, K. Kato and O. Kogure, Jpn. J. Appl. Phys., 1 6,1263 (1977). 23. M. Kawachi, K. Kato and O. Kogure, Jpn. J. Appl. Phys., 1 6,1673 (1977). 24. P. Alt and P. Pleshko, IEEE Trans. ED21,146 (1974). 25. N. V. Madhusudana and T. N. Ruckmongathan, Liquid Crystals, Proc. o f Int. Conf, Ed. S. Chandrasekhar, Heyden, 499 (1980). 26. T. N. Ruckmongathan and N. V. Madhusudana, Proc. o f SID, 24,259 (1983). 27. T. J. SchefFer and B. R. Clifton, SID 1992 Digest o f Technical Papers XXIII, 228 (1992). 28. R. Hochgesand, H. J. Plach and I. C. Sage (E. Merck, 1989). 29. F. W. Billmeyer, Textbook o f Polymer Science (John Wiley & Sons Limited, 1984).

163 K. Araya, A. Mukoh, T. Narahara and H. Shirakawa, Chem. Lett., 1141 (1984). 31. P. Mariani, B. Samori, A. S. Angeloni and P. Ferrati, Liquid crystals, 1, 327 (1986). 32. R. A. M. Hikmet, J. Appl. Phys., 68,4406 (1990). 33. R. A. M. Hikmet, Liquid Crystals, 9,405 (1991). 34. R. A. M. Hikmet, Mol. Cryst. Liq. Cryst., 213,117 (1992). 35. R. A. M. Hikmet and B. H. Zwerver, Liquid Crystals, 13, 561 (1993). 36. R. A. M. Hikmet and B. H. Zwerver, Mol. Cryst. Liq. Cryst., 200,197 (1991). 37. G. P. Crawford, R. D. Polak, A. Scharkowski, L.-C. Chien, J. W. Doane and S. Zumer, J. Appl. Phys., 7 5,1968 (1994). 38. H. A. Van Sprang, Mol. Cryst. Liq. Cryst., 97,255 (1983). 39. K. Miyano, Phys. Rev. Lett., 24, 349 (1974). 40. K. Miyano, J. Chem. Phys., 71,4108 (1979). 41. J. C. Tarczon and K. Miyano, J. Chem. Phys., 73,1994 (1980). 42. P. Sheng, Phys. Rev. A., 2 6,1610 (1982). 43. V. D. Neff, Liquid Crystals and Plastic Crystals, Vol. 2, edited by G. W. Gray and P. A. Winsor (Ellis Horwood), 231 (1974). 44. P. Sheng, Phys. Rev. Lett., 3 7,1059 (1976). 45. G. P. Crawford, R. Stannarius and J. W. Doane, Phys. Rev. A., 44,2558 (1991). 46. G. P. Crawford, D. K. Yang, S. Zumer, D. Finotello and J. W. Doane, Phys. Rev. Lett., 66,723 (1991). 47. G. P. Crawford, A. Scharkowski, Y. K. Fung, J. W. Doane snd S. Zumer, submitted for

164 141 publication. 48. M. Bom and E. Wolf, Principles o f Optics, 4th Edition (Pergamon Press, 1970). 49. P. G. de Gennes, The Physics o f Liquid Crystals (Clarendon Press, Oxford, 1974). 50. Introduction to Liquid Crystals, Edited by E. B. Priestley, P. J. Wojtowicz and Ping Sheng (Plenum Press, New York, 1975). 51. H. J. Coles, Mol. Cryst. Liq. Cryst., 49 (Letters), 67 (1978). 52. R. G. Horn, J. de Phys., 39, 105 (1978). 53. G. Durand, L. Leger, F. Rondelez and M Veyssie, Phys. Rev. Lett., 22,227 (1969). 54. Handbook o f Mathematical Functions, Edited by M. Abramowitz and I. A. Stegun (Dover Publications, Inc., New York, 1972). 55. M. Pfeiffer, Y. Sun, D. K. Yang, J. W. Doane, W. Sautter, V. Hochholzer, E. Ginter, E. Lueder and Z. Yaniv, SID 1994 Digest o f Technical Papers, XXV, 837 (1994). 56. A. Jakli, L. Bata, K. Fodor-Csorba, L. Rosta and L. Noirez, Liquid Crystals, 17, 227 (1994).

165 APPENDIX A Modified Bessel Functions The differential equation, d 2y 1 <fy ( - 1 d x 2 x dx m 2 2 )y = o 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 The boundary conditions require thaty be finite at the origin and be 0 as x -» oo. Using polynomial approximaiton, the solution is given as, for 0 < x < 2 K0(x) = -In (x/2) I0(x) (jc /2) (x/2 ) (x!2) (x/2f (jc/2) (x/2)

166 143 for 2 < x < oo jcv 0(jc) = (2 /x) (2 /x) (2 /x) (2/x) (2/x) (2/x) 6 where < x< (x) = (x/3.75) (x/3.75) (x/3.75) (x/3.75) (x/3.75) (x/3.75) <x < co x'v/0(x) = (3.75/x) (3.75/x) (3.75/x) (3.75/x) (3.75/x) (3.75/x) (3.75/x) (3.75/x) 8

167 APPENDIX B A 320 Line Mask for 4" x 4" Substrate