POLYMER S ANCHORING BEHAVIOR IN LIQUID CRYSTAL CELLS. A dissertation submitted. to Kent State University in. fulfillment of the requirements for the

Transcription

1 POLYMER S ANCHORING BEHAVIOR IN LIQUID CRYSTAL CELLS A dissertation submitted to Kent State University in fulfillment of the requirements for the degree of Doctor of Philosophy by Yue Cui August

2 Dissertation written by Yue Cui B.S., Hebei University of Technology, People s Republic of China, 2008 Approved by, Chair, Doctoral Dissertation Committee Deng-Ke Yang, Members, Doctoral Dissertation Committee Philip J. Bos Hanbin Mao Robert Twieg Hiroshi Yokoyama Accepted by, Chair, Liquid Crystal Institute Hiroshi Yokoyama, Dean, College of Arts and Science James L. Blank ii

3 Table of Contents LIST OF FIGURES...X LIST OF TABLES... XXV DEDICATION...XXVI ACKNOWLEDGEMENTS... XXVII CHAPTER 1 INTRODUCTION Liquid crystal Liquid crystal phases Order Parameter Liquid Crystal physics Responsivity of Liquid Crystal Liquid crystal cell Polymer Surface Alignment and Anchoring of Nematic Liquid Crystals Liquid Crystal / Polymer Composite An Overview of Chapter 2 through Chapter References CHAPTER 2 METHODS TO MEASURE ANCHORING STRENGTHS Introduction and Review of Surface Anchoring Energy Measurements Preparation Work Polar Anchoring Strength Meaurement Simulation Method of Relaxation... 59

4 2.3.2 Optical Simulation Experiment Measurement and Fitting Result: Rubbed Bare Glass and Polyimide Azimuthal Anchoring Strength Meaurement Nematic and Chiral Nematic Boundary Conditions and TAM method Null Transmittance Derivations Null Transmittance Measurements and Azimuthal Anchoring Calculations Additional Experiment Techniques Optical Chopper and Lock-in Amplifier Eliminate Reflections Reference CHAPTER 3 ANCHORING STRENGTH VARIED BY PVA ALIGNMENT LAYER THICKNESS Introduction Preparation AFM Examinations Polar Anchoring Strengths Result Azimuthal Anchoring Strengths Result Conclusion and Discussion Reference CHAPTER 4 TEMPERATURE EFFECT ON ANCHORING STRENGTH iv

5 4.1 Liquid Crystal Properties with Varied Temperature Elastic Constants Helical Twisting Power Dielectric Constants Birefringence Polar Anchoring Strength Measurements Azimuthal Anchoring Strength Measurements Additional Measurements with PI Polar anchoring strength Azimuthal anchoring strength Characterization of PDLC Sample Preparation Switching Voltage and Response Time Measurements Conclusions and Discussions Appendix Measurements Details Polar Anchoring Strength Fittings PDLC Response Time Measurements Sample Details Reference CHAPTER 5 ADJUSTING ANCHORING STRENGTH OF POLYMERS AND THE PERFORMANCE OF PDLC Introduction of methacrylate polymers v

6 5.2 Anchoring strength of PMMA and PiBMA mixtures Preparation Polar anchoring strength measurements Azimuthal anchoring strength PDLC made from PMMA and PIBMA Anchoring strength of methacrylate polymers/copolymers Preparation Polar anchoring strength Azimuthal anchoring strength PDLC made from methacrylate polymers/copolymers Preparation Electro-optics characterization Discussion Reference Appendix: Chemicals information CHAPTER 6 ALIGNMENT BEHAVIOR OF DUAL DIRECTION RUBBED ALIGNMENT LAYER Introduction Preparation of dual-rubbed cell Observance under the microscope Measure the easy axis direction Easy axis deviation with varied rubbing combination vi

7 6.6 Derivations Polar anchoring strength Azimuthal anchoring strength Experiments with PVA alignment layers Conclusion and discussion References CHAPTER 7 ALIGNMENT LAYER S EFFECT ON LIQUID CRYSTAL Temperature dependence of pitch in thermotropic cholesteric liquid crystal Introduction Materials Results Discussion Blue phase formation and temperature range Introduction TM75A E7+chiral dopant Discussion References CHAPTER 8 ENCAPSULATED POLYMER STABILIZED CHOLESTERIC TEXTURE (EPSCT) Introduction Introduction to epoxy resins vii

8 8.1.2 Introduction to emulsion method Encapsulation by curing epoxy resin minutes epoxy DER BADGE Encapsulation by emulsion method Materials and emulsification Coating and laminating UV curing SEM Electro-optical characterization Electro-optical characterization: varying film thickness Discussion Reference CHAPTER CONCLUSION Physics aspect Application aspect CHAPTER 10 APPENDIX: CHARACTERIZING AND MODELING OF LIGHT DIFFUSING SHEET Objectives and Backgrounds Experimental results viii

9 10.3 Modeling Conclusion Acknowledgements References ix

10 LIST OF FIGURES Figure 1-1 Foreground: thermotropic liquid crystals at. From left to right CB15 ( ), E7 ( ), E44 ( ), RM257 ( ) Figure 1-2 (a) Rod-like molecule in 3D frame. (b) Average orientation of molecules. (c) Temperature dependence of order parameter for nematic liquid crystal E44 and E Figure 1-3 Liquid crystal director deformations: (a) splay; (b) twist; (c) bend Figure 1-4 (a) Cholesteric director field in planar state. (b) Cholesteric director field in focal conic state. (c) Cholesteric director field in homeotropic state. (d) Planar state under the microscope with crossed polarizers. (e) Focal conic state under the microscope with crossed polarizers. (f) Homeotropic state under the microscope with crossed polarizers Figure 1-5 (a) Main chain polymer. (b) Side chain polymer. (c) Cross-linked polymer.. 39 Figure 1-6 Chemical structure: (a)mononer RM257; (b) photo initiator BME Figure 1-7 Chemical structure: PMMA Figure 1-8 (a) Homogeneously aligned liquid crystal on rubbed polymer surface. (b)homeotropically aligned liquid crystal on rubbed polymer surface Figure 1-9 (a) Schematic side view of a cell with un-rubbed polymer alignment layer (PI2555). (b) Schematic side view of a cell with rubbed polymer alignment x

11 layer(pi2555). (c) Microscopic picture of E7 in un-rubbed cell. (d) Microscopic picture of E7 in rubbed cell Figure 1-10 Easy axis director and liquid crystal director Figure 1-11 SEM pictures of: (a) polymer network in PDLC; (b) polymer fibers in PSCT. Liquid crystals are washed away. Photo courtesy: reference Figure 1-12 Working principle of PDLC. (a) No field applied, scattering mode. (b) Apply electric field across the cell, transparent mode Figure 1-13 Working principle of PSCT. (a) Normal mode: no field, scattering mode. (b) Normal mode: with electric field, transparent mode. (c) Reverse mode: no field, transparent mode. (d) Reverse mode: with electric field, scattering mode Figure 2-1 (a) Assembled panel inch 2. (b) A separated row. (c) A separated individual cell Figure 2-2 Planar cells with PI 2555 as alignment layer under polarizing microscope. Cell gap = 5 µm Figure 2-3 (a) Homogeneous cell: anti-parallel rubbing, filled with pure nematic liquid crystal. (b) Apply AC voltage, the liquid crystals show splay deformation. (c) Top view of director in the coordinates Figure 2-4 (a) Denotation of polar angle. (b) Polar anchoring strength,. (c) Polar anchoring strength, Figure 2-5 (a) Schematic drawing of light passing through birefringent film with imaginary slices. (b) Coordinates on a slice with thickness xi

12 Figure 2-6 (a) Optical setup for polar anchoring strength measurements. (b) Cross-section view of polarizers and rubbing direction Figure 2-7 Experiment data and fitting result for PI 2555 and rubbed bare glass Figure 2-8 Chemical structure of R811/S811. The chiral center is circled out in red Figure 2-9 (a) Planar cell filled with E44. (b) Planar cell filled with chiral nematic. (c) Top view of director on two substrates Figure 2-10 Rotating both polarizer and analyzer: cross-section view of polarized light directions Figure 2-11 Rotating analyzer: cross-section view of polarized light directions Figure 2-12 (a) Optical setup for azimuthal anchoring strength measurements. (b) Crosssection view of polarizers and rubbing direction Figure 2-13 Measured transmittance vs. position of rotating analyzer. (a) rubbed bare glass. (b) PI All cells are assembled with 5 µm spacers. Legendary indicates weight percentage of chiral dopant Figure 2-14 Analyzer deviation at minimum transmittance vs. concentration of chiral dopant Figure 2-15 Open circles: experimentally measured transmittance vs. analyzer deviation. Solid line: Calculated by Jones Matrix method Figure 2-16 Actual experimental setup in polar and azimuthal anchoring strength measurements Figure 2-17 (a) Bare chopped light detection. (b) Light through crossed polarizers, with or without optical chopper+lock-in amplifier xii

13 Figure 2-18 (a) Eliminate reflection issue. (b) Calibration of light intensity while fully rotating the analyzer Figure 3-1 Chemical structure of poly vinyl alcohol (PVA) Figure 3-2 (a) 3D image of spin-coated and rubbed PVA surface. (b) Flattened (view from top) image of PVA surface Figure 3-3 Various PVA solution spin-coated film thickness. Left column: 3D image. Right column: section analysis. From top to bottom rows: 4.3%, 3.8%, 3.0%, 2.3%, 1.5%, 1.0% Figure 3-4 Measured alignment layer thickness as a function of PVA concentration in the solution for spin-coating Figure 3-5 Experimentally measured transmittance-voltage curves of the ECB cells with different alignment layer thicknesses Figure 3-6 Transmittance-voltage curves of EBC cells with various alignment layer thicknesses. Blue symbol: experimentally measured. Red line: theoretically calculated Figure 3-7 The polar anchoring strengths vs. the PVA alignment layer thickness. : quadratic term; : quartic term. The lines are guide to the eye Figure 3-8 Transmittance vs. analyzer angle twist angles of two twist cells with PVA alignment layers. Markers: experimental results, solid lines: theoretical fit. PVA alignment layer thickness, : 120 nm, : 14 nm Figure 3-9 The azimuthal anchoring strengths of the PVA alignment layers with various layer thicknesses. The line is a guide to the eye xiii

14 Figure 4-1 Calculated order parameter for liquid crystal E44, which goes to isotropic (S=0) at 105ºC Figure 4-2 Calculated elastic constants for liquid crystal E Figure 4-3 (a) Schematic drawing for HTP measurements. (b)microscopic picture of disclination lines formed by filling Cholesteric liquid crystal in between flat glass and circular wedge, both with alignment layer. (c) Linear fitting vs in order to get Figure 4-4 Measured of S811 in E44 at different temperatures. Inserted picture: S811 structure Figure 4-5 Measured of CB15 in E44 at different temperatures. Inserted picture: CB15 structure Figure 4-6 Circuit for measuring capacitance of a liquid crystal cell Figure 4-7 Measured dielectric constants of E44 as a function of temperature Figure 4-8 Optical setup for Sernamont method Figure 4-9 Measured birefringence for E44 and E Figure 4-10 Transmission as a function of voltage (up to 6 V) plots for an ECB cell with PVA alignment layer and filled with E44. Cell gap = 5 µm Figure 4-11 Fredericksz transition threshold voltage of an ECB cell as a function of temperature. Cell gap = 5 µm Figure 4-12 Transmission as a function of voltage (up to 60 V) plots for an ECB cell with PVA alignment layer and filled with E44. Cell gap = 5 µm Figure 4-13 Polar anchoring strength of PVA as a function of temperature xiv

15 Figure 4-14 Analytically calculated total twist angle through the cell as a function of analyzer deviation angle at various temperatures Figure 4-15 Calculted analyzer deviation as a function of temperature. Assume the twist angle does not change. Cell gap = 5 µm Figure 4-16 Analyzer deviation angle as a function of temperature. PVA alignment. Left: data measured from E44/S811 mixture. Right: data measured from E44/CB15 mixture Figure 4-17 Total twist through the chiral nematic cell as a function of temperature. PVA alignment. Left: data calculated from E44/S811 mixture. Right: data calculated from E44/CB15 mixture Figure 4-18 Azimuthal anchoring strength of PVA as a function of temperature. PVA alignment. Solid line is guided by eye Figure 4-19 Transmission as a function of voltage (up to 60 V) plots for an ECB cell with PI 2555 alignment layer and filled with E44. Cell gap = 5 µm Figure 4-20 Polar anchoring strength of PI 2555 as a function of temperature Figure 4-21 Analyzer deviation angle as a function of temperature. PI 2555 alignment. Left: data measured from E44/S811 mixture. Right: data measured from E44/CB15 mixture Figure 4-22 Analyzer deviation angle as a function of temperature. PI 2555 alignment. Left: data measured from E44/S811 mixture. Right: data measured from E44/CB15 mixture xv

16 Figure 4-23 Azimuthal anchoring strength of PI 2555 as a function of temperature. PVA alignment. Solid line is a guide to the eye Figure 4-24 Schematic drawing of a bipolar droplet. Black segments indicate liquid crystal director Figure 4-25 PVA-PDLC sample under microscope with crossed polarizers, same exposure time for each picture. 100µm scale bars are marked; temperatures when the picture is taken are also marked Figure 4-26 Optical setup for PDLC characterizing Figure 4-27 Transmission over voltage curves for PVA-PDLC sample. Cell gap=10µm. Legends show temperature Figure 4-28 PDLC switching voltages as a function of temperature Figure 4-29 PDLC contrast ratio as a function of temperature Figure 4-30 Voltage driving scheme for response time measurement Figure 4-31 Response time at different temperatures summary. Red data points represent on time, corresponding to left axis; green data points represent off-time, corresponding to right axis Figure 4-32 Microscopic pictures of disclination rings with S811/E44 mixture at different temperatures Figure 4-33 Mixture S811/E44, squared radius vs. numbering of the rings. Data points: experiment data. Solid black line: linear fit of the series Figure 4-34 Microscopic pictures of disclination rings with CB15/E44 mixture at different temperatures xvi

17 Figure 4-35 Mixture CB15/E44, squared radius vs. numbering of the rings. Data points: experiment data. Solid black line: linear fit of the series Figure 4-36 Fitting PVA experimental data of polar anchoring strengths at different temperatures Figure 4-37 Fitting PVA experimental data of polar anchoring strengths at different temperatures Figure 4-38 Response time measurements examples: (a). Left: turn-on time; right: turn-off time. (b). Left: turn-on time; right: turn-off time. (c). Left: turn-on time; right: turn-off time Figure 5-1 Transmittance-voltage curves of the ECB cells with PMMA/PiBMA mixed alignment layers. Open circles: experimentally measured; solid lines: fitting result Figure 5-2 The polar anchoring strengths with varied PMMA and PiBMA combination. : quadratic term; : quartic term Figure 5-3 Total twist through the ECB cells with varied PMMA and PiBMA combination Figure 5-4 Azimuthal anchoring strength with varied PMMA and PiBMA combination Figure 5-5 Experimentally measured transmittance as a function of the applied voltage on the PDLCs made from different polymer composites Figure 5-6 PDLC under AC field. Polymer in (a): PMMA; (b): PiBMA; (c): 50%PMMA+50%PiBMA. Scale bar = 100 µm. Cell gaps all 20 µm xvii

18 Figure 5-7 Transmittance-voltage curves of the ECB cells with methacrylate polymer alignment layers. Open circles: experimentally measured; solid lines: fitting result Figure 5-8 Total twist as a function of chiral dopant percentage with varied methacrylate polymer alignment layer Figure 5-9 PDLC samples made by methacrylate polymers and copolymers with arbitrary droplet size. Scale bar = 100 µm Figure 5-10 PDLC samples made from methacrylate polymers and copolymers with controlled droplet size. Scale bar = 100 µm Figure 5-11 Experimentally measured transmittance as a function of the applied voltage on the PDLCs made from methacrylate polymers Figure 6-1 Schematic drawing of rubbing directions and cell assembly. (a) Case 1: regular ECB cell with one direction rubbing on each substrate. (b) Case 2: rubbed in two directions on each substrate. First direction with parallel rubbing, second direction with anti-parallel rubbing. (c) Case 3: rubbed in two directions on each substrate. Both with anti-parallel rubbing Figure 6-2 Dual direction rubbed cells show uniform alignment.. All filled with E44. All cell gap controlled by 5 µm spacers. Polarizers make 45ºto the 2 nd rubbing direction. (a) PI 2555 alignment layer, case 2. (b) PI 2555 alignment layer, case 3. (c) PVA alignment layer, case 2. (d) PVA alignment layer, case 3. (e) PMMA alignment layer, case xviii

19 Figure 6-3 Darkness comparison.. (a) PI alignment, case 1. (b) PI alignment, case 2. (c) PI alignment, case 3. (d) PVA alignment, case 1. (e) PVA alignment, case 2. (f) PVA alignment, case Figure 6-4 Long exposure time photos when rotating the case 3 cells between fixed polarizers Figure 6-5 Schematic drawing of experimentally measured angles in TAM method Figure 6-6 Measured transmittance as a function of angles for case 1 cells. (a) Rotate P and A simultaneously, horizontal axis. (b) Rotate A only, horizontal axis and Figure 6-7 Measured transmittance as a function of angles for case 2 cells. (a) Rotate P and A simultaneously, horizontal axis. (b) Rotate A only, horizontal axis and Figure 6-8 Measured transmittance as a function of angles for case 3 cells. (a) Rotate P and A simultaneously, horizontal axis. (b) Rotate A only, horizontal axis and Figure 6-9 Director configuration on the surfaces in the three cases. Left column: substrate 1. Right column: substrate Figure 6-10 Measured easy axis direction w.r.t. second rubbing direction. Alignment material: PVA Figure 6-11 Non-uniform alignment created by dual rubbing combination 10/ Figure 6-12 (a) PI 2555 transmittance vs. voltage curves. Legend shows rubbing condition. (b) Fitted anchoring strength xix

20 Figure 6-13 (a) PVA (1.5%) transmittance vs. voltage curves. Legend shows rubbing condition. (b) Fitted anchoring strength Figure 6-14 Right-handed chiral nematic between substrates with preferred aligning direction. (a) Actual pitch. (b) Actual pitch Figure 6-15 Twisting angle of each rubbing combination. Alignment material: PVA 1.5% Figure 6-16 (a) Calculated values for and. (b) Azimuthal anchoring strength as a function of rubbing combination. Single meaning only rubbed in one direction Figure 7-1 Chemical structure of TM75A components Figure 7-2 Temperature induced pitch change in thermotropic cholesteric: TM75A (90%) with LC7029 (10%). Top row alignment layer: PI. Bottom row alignment layer: WPU Figure 7-3 Temperature denpendance of pitch with confined boundary. Material: TM75A (90%), LC7029 (10%); alignment layer: WPU. Cholesteric photos in reflection mode. Left corner photo is showing smectic A phase when temperature is lower than cholesteric phase in transmission mode Figure 7-4 Typical reflectance spectrum of a cholesteric liquid crystal. In this plot the material was TM75A (80%) with LC7029 (20%). Temperature was at. Alignment layer was PI Figure 7-5 Center wavelength of the reflection band as a function of temperature, with different alignment layer and different material concentration. TM75A was as marked in the pictures, the rest was LC Cooling rate was xx

21 Figure 7-6 Center wavelength of the reflection band as a function of temperature, with different alignment layer. TM75A was as 80% and the rest was LC7029. Cooling rate was Figure 7-7 Blue phase texture. LC material: TM75A+LC7029(10%). Alignment layer: PI. The pictures were shifted around within the cell a little bit in order to catch the end of BP phase transition Figure 7-8 Blue phase texture. LC material: TM75A+LC7029(10%). Alignment layer: PVA_1.5% Figure 7-9 Blue phase texture. LC material: TM75A+LC7029(10%). Alignment layer: WPU Figure 7-10 Temperature range of BP phase with different alignment layers. LC material: TM75A+LC7029(10%) Figure 7-11 Blue phase texture. LC material: E7+R811 (32%). Alignment layer: PI Figure 7-12 Blue phase texture. LC material: E7+R811(32%). Alignment layer: PVA_1.5% Figure 7-13 Blue phase texture. LC material: E7+R811 (32%). Alignment layer: MPU Figure 7-14 Blue phase texture. LC material: E7+R811 (32%). Alignment layer: WPU Figure 7-15 Temperature range of BP phase with different alignment layers. LC material: E7+R811 (32%) Figure 8-1 Schematic drawing of Encapsulated PSCT structure xxi

22 Figure 8-2 Chemical structure of (a) epoxide group and (b) amine group Figure 8-3 Chemical structure of a simple bisphenol A epoxy: diglycidyl ether of bisphenol A (DGEBA or BADGE) Figure 8-4 Process of crosslinking epoxy resin by diamine. (Picture courtesy from Department of Polymer Science, University of Southern Mississippi) Figure 8-5 Nematic mixed with 5 minutes epoxy as time goes. Top row: E7. Bottom row: E Figure 8-6 Pre-PSCT droplets and chunks formed with room temperature cured DER Figure 8-7 Pre-PSCT droplets and chunks formed with high temperature cured DER Figure 8-8 Pre-PSCT droplets and chunks formed with room temperature cured BADGE Figure 8-9 After UV curing, PSCT droplets and chunks formed with room temperature cured BADGE. Left: 0 V. Right: 60V AC applied Figure 8-10 Left: Mayer rod (picture courtesy from holoeast.com). Right: the dried film with pre-psct droplets and PVA environment Figure 8-11 Left: laboratory doctor blade (photo courtesy from mtixtl.com). Right: the dried film with pre-psct droplets and PVA background Figure 8-12 UV cured PSCT droplets in PVA background Figure 8-13 Manufacture process of encapsulated PSCT xxii

23 Figure 8-14 (a) Sample before curing, one hollow droplet. (b) Sample after curing, one droplet filled with fibers. (c) Sample before curing, cross section of the film. (d) Sample after curing, cross section of the film Figure 8-15 Experiment setup for EO characterization Figure 8-16 Working principle of EPSCT: scattering state and transparent state Figure 8-17 Before and after UV curing, EO characterization: (a) transmittance vs. voltage curve; (b) response time. Film thickness = 12µm Figure 8-18 Light intensity as a function of viewing angle with different samples Figure 8-19 (a) Transmittance as a function of applied voltage of the EPSCT light shutters with different film thicknesses. (b) contrast ratio vs. film thickness Figure 10-1 SEM photograph of the diffuser showing dispersed particles in resin Figure 10-2 Optical measurement setup top view Figure 10-3 Open circles: experimentally measured scattering profile measurements of the diffusing sheet. Solid lines: Fitted by our model. The vertical dashed lines indicate peak center. Viewing angle is defined with respect to the film normal Figure 10-4 Schematic showing the incident and scattering angles Figure 10-5 General picture of modeling. On the left is normal incident light; on the right is oblique incident light Figure 10-6 Geometry in the diffuser (grey box): incident light (blue) with o i 30 scattered light (green) with fixed o t 20, and varied azimuthal angle. All green dots and the blue dot are on the top surface of the box xxiii

24 Figure 10-7 Single incident light (0º, 30º, 60ºfrom top row to bottom row) scattered light intensity vs. scattered polar angle (0º-90º) as well as scattered azimuthal angle (0º- 360º). Left column: right after being scattered by the diffuser; right column: light coming out to the air. Note that the scales are different for each subplot xxiv

25 LIST OF TABLES Table 2-1 Polar anchoring strength: bare glass (rubbed) and PI Table 2-2 Azimuthal anchoring strength: bare glass (rubbed) and PI Table 5-1 Methacrylate polymers and copolymers (scale bar = 50 µm) Table 0-2 Pretilt angle generated by PMMA and PiBMA mixture Table 5-3 Pretilt angle generated by methacrylate polymers and copolymers Table 5-4 Polar anchoring strengths of methacrylate polymers Table 5-5 Azimuthal anchoring strengths of methacrylate polymers Table 5-6 Threshold voltage and response time of PDLC made from methacrylate polymers Table 7-1Alignment layers anchorign strength

26 For my parents and my grandma. DEDICATION

27 ACKNOWLEDGEMENTS First of all I owe my deepest gratitude to my advisor, Professor Deng-Ke Yang. He taught me great physics, guided me through my research, and always instructed me to think independently and focus on essentials. More importantly he passed on to me the attitude of being rigorous to science yet delightful to life. I cherish all the moments with him no matter when he helped me overcome research difficulties or when he encouraged me to enjoy a concert. I am indebted to Professor Peter Palffy-Muhoray, with whom I had very interesting research done and very valuable experience as his teaching assistant. It was a great pleasure and honor to work with him. I was also very fortunate to take classes from faculties in Liquid Crystal Institute and Physics Department to learn their precious quintessence of knowledge. I would like to thank Liou Qiu, Doug Bryant and Merrill Groom for their technical support and my colleagues Dr. Paul Luchette, Dr. Michele Fontana, Dr. Young-Cheol Yang, Dr. Shawn Hurley, Dr. Sarah Hicks, Dr. Rafael Zola, and soon to be Doctors Hossein Nemati, Ali Reza Moheghi, Xiaochen Zhou and Jinghua Jiang for their great help in the lab. Dr. Duncan Johnston had many discussions with me during my research and provided innovative suggestions. MFLEX UK Ltd. provided financial support for my research. My friends especially Lu Lu and my officemate Thanh Son Nguyen accompanied me through laughter, tears, and fishing time. They made my life in Kent colorful. Yue Cui Aug , Kent, Ohio

28 CHAPTER 1 Introduction The current chapter gives necessary introduction to the works covered by the current dissertation. 1.1 Liquid crystal Liquid crystal phases As suggested from its name, liquid crystals 1-3 are mesophases which possess similarities to liquid as well as crystal. Liquid is an isotropic phase, without orientational order or translational order. Crystal, on the other hand is a phase with both orientational ordered and translational ordered. In liquid crystal phase, however, molecules have partial orientational order and partial translational order. Generally speaking liquid crystals can be categorized as lyotropic, thermotropic and amphotropic. Nature has created many amazing liquid crystal structures 4. Lyotropic liquid crystal phases are self-assembled in solvent with specific concentration ranges and are widely acknowledged in everyday life and in biological systems, forming bilayers, micelles, and vesicles. Thermotropic liquid crystal phases occur in specific temperature ranges. The typical method to obtain thermotropic liquid crystal phase is to heat up a certain solid or cool down a certain liquid. Usually as temperature decreases, the material increases its degree of order and may go through different liquid crystal phases before it 28

29 29 turns to crystal. Amphotropic represent the material that exhibit both lyotropic and thermotropic properties. Physicists and chemists have dedicated in explaining the various forms, studying their properties, and synthesizing new liquid crystals 1, 2, 5-9. Engineers have developed hundreds of applications like display devices and sensors based on liquid crystal s special electro-optical properties 5, The current dissertation is display application oriented and the most commonly mentioned liquid crystal phases are nematic and cholesteric. Figure 1-1 shows several materials that can become liquid crystal at proper temperatures, i.e. thermotropic liquid crystal. CB15 16, 17 (Merck) is a pure compound and is in isotropic phase at room temperature, thus appears as clear fluid. CB15 can be served as simple picture of liquid crystal, or as dopant to modify the chirality of other chemicals. E7 (Merck) and E44 (Merck) are both mixtures in nematic phase at room temperature. The milky fluid indicates light scattering caused by random domains of the liquid crystal. This is typical for nematic liquid crystal phase in macroscopic bulk. Both E7 and E44 are widely studied in electro-optics due to their excellent electro-optical properties, as will be introduced in later chapters. RM257 18, 19 (Merck) presents as solid white powder. It is a special liquid crystal that forms crosslinked polymers. Room temperature is in the range of its crystal state.

30 30 Figure 1-1 Foreground: thermotropic liquid crystals at. From left to right CB15 ( ), E7 ( ), E44 ( ), RM257 ( ). Physicists like to think the microscopic liquid crystal mesogens as rods (calamitic liquid crystal), discs (discotic liquid crystal) and bananas (bent liquid crystal). For the materials mentioned in the current dissertation, the molecules are simplified as uniaxial rods as in Figure 1-2 (a) Order Parameter In liquid crystal studies, order parameter 1, 2 is the elementary characteristic to describe phases. Some schematic drawings help to introduce orientational order parameter. Calamitic liquid crystal molecules can be represented by rods as shown in Figure 1-2 (a). The average of oriented molecules long axis directions can be defined as liquid crystal director, denoted by a unit vector as shown in Figure 1-2 (b). In general it is specified by an orientational distribution function ( ) where. Macroscopic properties of liquid crystals depend on the orientational order of the

31 31 molecules. Therefore it is convenient to define in terms of order parameter. In the case of uniaxial nematic liquid crystal, there is no azimuthal direction preference, and the order only depends on polar angle. Also, it is necessary to define the order parameter quantitatively and be able to obtain zero value for isotropic phase and obtain non-zero values for ordered phase. The first order Legendre polynomial does not fit the requirements, but the second does. Finally the order parameter is reduced to equation (1.1). (a) (b) (c) Figure 1-2 (a) Rod-like molecule in 3D frame. (b) Average orientation of molecules. (c) Temperature dependence of order parameter for nematic liquid crystal E44 and E7.

32 32 ( ) ( ) (1.1) Temperature dependence of nematic order parameter is shown in Figure 1-2 (c). From the plot, we can see the nematic-isotropic transition happens abruptly at a certain temperature, defined as. Below, the material is in the nematic phase with nonzero order parameter. Above this temperature, the material goes to isotropic liquid phase and the order parameter is zero. Many other anisotropic properties of liquid crystal are closely related to this trend. That was about nematic order parameter. There are other forms of order parameter as well. For example, order parameter for smectic liquid crystal is defined by number density Liquid Crystal physics In order to understand and describe the unique features of liquid crystal, certain physical characteristics needed to be reinforced and theories needed to be developed. Maier-Saupe theory is based on London-Van der Waals interaction between molecules. This theory pictures the interaction between induced dipoles. Mean field theory is used to get free energy density. After minimizing free energy in equation (1.2), we get a self-consistent equation as equation (1.3) with the order parameter. Note that S depends on T, thus describing phase transition via temperature variation. ( ) ( ( ) ) (1.2)

33 33 ( ) ( ( ) ) ( ( ) ) (1.3) Note that has the same order of magnitude as. describes the strength of the interaction. Landau-de Gennes theory 25, 26 is developed from the general Landau theory and is a phenomenological way to describe phase transition. It can be derived by simplifying Taylor s expansion of Maire-Saupe free energy (1.2) to the form of equation (1.4). This equation helps to solve the order parameter numerically. ( ) ( ) (1.4) In the above equation,,, are positive parameters. At high temperatures, the minimized free energy corresponds to the isotropic phase. At low temperatures, the minimized free energy has two solutions but the stable solution is the non-zero order parameter, corresponding to the nematic phase. In Onsager theory 26, 27, we consider excluded volume for rod-like molecules as in equation (1.5). (1.5) and concentration describe the length and diameter of the molecule. With a high enough of molecules, the system can form into a nematic phase which minimizes free energy. Thus the transition concentration from isotropic to nematic can be predicted.

34 Responsivity of Liquid Crystal With external stimulus, such as temperature change, electric field, and light, liquid crystal can respond via orientational order. Elastic properties 1, 4, 13, 25 of liquid crystals are described by Oseen-Frank energy which characterizes three deformations of liquid crystal director: splay, twist, and bend as pictured in Figure 1-3. Beyond these three types of deformation, some divergence terms 27 (or surface terms) are suggested in case of high surface to volume ratio. In the current study, splay, twist and bend would be sufficient. For nematic liquid crystal with reflection symmetry, the spatial variation of the director induces elastic energy, and each distorted elastic energy can be expressed with corresponding term as in equation (1.6).,, are called the splay, twist, and bend elastic constants, with the same unit as force - Newton. Note that in equation (1.6), the is actually the energy density, i.e. the energy per unit volume, so it is measured by Joules per cubic meter or Newton per meter. (a) (b) (c) Figure 1-3 Liquid crystal director deformations: (a) splay; (b) twist; (c) bend. ( ) ( ) ( ) (1.6)

35 35 For the liquid crystals that naturally have broken reflection symmetry, the elastic terms should contain chiral elements. In case of cholesteric liquid crystal, the molecules twist in space as in Figure 1-3(b) and even periodically form a screw-like structure. In this case, the twist elastic energy is required to have an additional linear term as the addition of in equation (1.7). Such twist has a fixed pitch, i.e. the natural pitch or the intrinsic pitch. ( ) ( ) ( ) (1.7) (1.8) Liquid crystals are dielectric and diamagnetic 2, 13, 20. Dipole moments can be induced by external fields. There are distinguished dielectric permittivities and magnetic susceptibilities along the director and perpendicular to the director. Along the director, we define dielectric constant as, while perpendicular to the director we define. The dielectric anisotropy is parameter. Note that can be positive or negative., which is proved to be proportional to the order Liquid crystal s response to external field (electric or magnetic) is one unique anisotropic character and has been a key feature in liquid crystal display devices. Electric or magnetic work is presented as an additional energy term as in equation (1.9). If the electric field is a constant, the effect of electric polarization is an additional negative term in the original energy density as in equation (1.9). If the electric displacement is constant in the system, the polarization effect is a positive term in addition to the original free

36 36 energy density as in equation (1.10). In numerical simulations, a relaxation process is applied to get lowest free energy. Details about relaxation method will be introduced in Chapter 2. (1.9) (1.10) 2, 3, 8, Another widely adopted property of liquid crystals is the optical birefringence 13-15, 20, 28. There are distinctive refractive indices along the director and across the director, referred to as and respectively. Optical birefringence is thus defined as. It can be positive or negative. Polarized light propagation in such anisotropic materials leads to enormous inventions of display devices, laser and modulators, and sensors. Great tools for studying polarized light analytically and numerically have been developed. Poincarésphere, Jones Matrix method, Muller Matrix method, and Berreman 4 4 method are specifically well-accommodated into liquid crystal researches. Simulations by extended Jones Matrix method are applied in the current research. In this method, each optical element (polarizer, liquid crystal, etc.) can be represented by a 2 2 matrix which will be described in details in Chapter Liquid crystal cell Liquid crystal textures are such delicate soft matter that researches are usually carried out with the help of supporting frame or substrate. For electro-optical studies, a typical method is sandwich the material in two pieces of glass substrate with edges glued.

37 37 Glass is optically isotropic and allows high transmittance at visible range, so it is good for optical study on liquid crystals in this range. For electrical studies, electrodes can be 5, 13 implanted at the inner surface of the substrates. Good electrode candidates are required to be as transparent as possible, for example Indium Tin Oxide (ITO), metal nanowire, and graphene, among which ITO is most widely used at present. In most cases alignment layer is presented in the cell as well in order to provide uniform director field in macroscopic scale. Figure 1-4 illustrates a liquid crystal cell equipped with glass, ITO, and alignment layers, filled with cholesteric liquid crystal. Due to the presence of the alignment layer, the surface liquid crystals are stabilized in a certain direction and the bulk liquid crystal shows planar state. The cell is quite reflective in this condition. When moderate external field is applied, the liquid crystal (with positive dielectric anisotropy) will respond to the field and would like to align along the field direction. Therefore the competition of surface alignment and the external field will result in the focal conic configuration and the cell will be scattering. However if higher field is applied, the external field wins. All the director points along the field direction and the helix is untwisted as shown in Figure 1-4 (c). It is called homeotropic state.

38 38 Figure 1-4 (a) Cholesteric director field in planar state. (b) Cholesteric director field in focal conic state. (c) Cholesteric director field in homeotropic state. (d) Planar state under the microscope with crossed polarizers. (e) Focal conic state under the microscope with crossed polarizers. (f) Homeotropic state under the microscope with crossed polarizers. By designing the alignment condition and cooperating with the liquid crystal s response to external field, many light modulating devices can be achieved, including modern electronics display, smart windows. 1.2 Polymer In biological systems, DNA is a double helix shaped biopolymer 29. In every life, natural rubber, synthetic rubber, nylon, and PVC are all easily found examples of polymer. In the view of a physicist, polymer is a molecule with long chain consisting of repeating units 7, 30, 31. The units can compose into various architectures. Several common shapes are shown in Figure 1-5.

39 39 Figure 1-5 (a) Main chain polymer. (b) Side chain polymer. (c) Cross-linked polymer. Polymers can be formed by step-growth process or chain-growth process 7. In step-growth process, monomers gradually build up dimers, trimmers, and longer chains. Polymer is formed during the last reaction. In chain-growth process, an initiator is presented to start the reaction. Then the reaction goes on, picking up monomers and forming polymers throughout the process. The polymerizations in the current dissertation mostly refer to a free-radical chain-growth process: cross-linking monomers into polymer under exposure to ultra-violet (UV) light. In this process, a photo-initiator is often used to start the linking. RM257 and BME (benzoin methyl ether) are often used monomer and photo-initiator respectively.

40 40 Figure 1-6 Chemical structure: (a)mononer RM257; (b) photo initiator BME. Other than composing polymers, the current study also includes many already polymerized materials, usually linear. A typical linear polymer: PMMA (poly methyl meth acrylic) is illustrated in Figure 1-7. More chemical structures will be introduced in the following chapters. Figure 1-7 Chemical structure: PMMA. There are two distinctive thermal characteristic properties for polymers 6, 7, 32 : melting temperature and glass transition temperature. refers to the material transit from crystalline structure to liquid, which requires the exist of crystalline state in such material. This transition is first order transition. on the other hand is a second order transition, and it refers to the material changing from a brittle, glassy state to a rubbery state, i.e. it softens

41 41 the material but do not melt it. Completely amorphous polymers for example thermoplastics exhibit glass transition only. If the material shows and both, is lower than. In order to get stable polymer alignment, it is suggested to keep it under the glass transition after forming the alignment structure, especially the structure is delicate. We also need to consider dissolving polymer in solutions. If the chemical structure of a compound is given, the solubility parameter is given by equation (1.11). is the mass density of the material, is the molecular weight for monomer, and is group attraction constant of chemical group. When the polymer is fully dissolved in a good solvent, the segments will follow self-avoiding random walk. (1.11) The main interest of the current research is on the interactions and phase separations between polymers and liquid crystals, as will be introduced in 1.3 and Surface Alignment and Anchoring of Nematic Liquid Crystals Liquid crystals have partial degrees of orientational order. In real applications, it is often required to have good alignment in macroscopic range. For example in liquid crystal display devices, the liquid crystals are sandwiched between two parallel substrates which are coated with alignment layers. These special coatings can be designed to align liquid crystals in homogeneous, homeotropic (Figure 1-8), or tilted configurations. Popular industrial methods to fabricae alignment layers are mechanically rubbed polymer, photo-alignment polymer, and obliquely evaporated SiO 2 film 33, 34. Liquid crystals would

42 42 prefer to stay with the polymer either parallel to the main chain or perpendicular to it, depending on the surface coupling agents. The un-processed polymer coating is usually in random orientation in the substrate plane. Additional force is usually required to obtain uniformly aligned polymer side chain or other surface agent. Photo-aligning creates crosslinking in preferred direction by shining polarized UV light 35, 36. Rubbing is a mechanical process to comb the side chains. Rubbed polymer films are applied in the research throughout the current dissertation. Figure 1-9 depicts the difference between un-rubbed and rubbed polymer alignment layer 9, 13, 15, 37. In the un-rubbed case, liquid crystals are aligned in plane but randomly. The brush-like defects proves that. In the rubbed case, the liquid crystals are all aligned in the same direction, leaving nearly no defect. Figure 1-8 (a) Homogeneously aligned liquid crystal on rubbed polymer surface. (b)homeotropically aligned liquid crystal on rubbed polymer surface.

43 43 Figure 1-9 (a) Schematic side view of a cell with un-rubbed polymer alignment layer (PI2555). (b) Schematic side view of a cell with rubbed polymer alignment layer(pi2555). (c) Microscopic picture of E7 in un-rubbed cell. (d) Microscopic picture of E7 in rubbed cell. The interesting fact is that although rubbed polymer has been employed for decades in the billion dollar display industry, the true physical mechanism behind it has always been controversial. D. W. Berreman has proposed a model and showed the mechanism comes from rubbing created microgrooves as well as the short-range intermolecular forces 38. The liquid crystal favors to align with the geometry grooves and the preferred surface molecular aligning direction to achieve lower energy state. Suppose grooves as shown in Figure 1-8 has s sinusoidal undulation, then it can be characterized by a shape factor where represent the amplitude of the grooves and representing periodicity. Thus to achieve minimum free energy, the liquid crystal director aligned to

44 44 such structure would be ( ) ( ) Note that if dimensionless factor is small, the solution suggest the director can follow the groove structure. Kahn considered surface tension effect in determining parallel or perpendicular alignment 9, 39. Yokoyama wrote anchoring strength in terms of interfacial tension 37. Kobayashi argued the importance of polymer side chain length and branching which causes different steric interactions 40. Complex physical and chemical reasons are behind the simple technique, yet it reveals the great charm of scientific research Science is the great antidote to the poison of enthusiasm and superstition 41. In liquid crystal devices, alignment layers provide surface anchoring to the liquid crystal, this surface energy plays the role in balancing with the liquid crystal s bulk elastic energy as well as external field induced bulk energy. Bulk elastic energy is represented by Frank energy while surface anchoring energy is represented by Rapini- Papoular model 42. The liquid crystal at the surface is exposed to polymer alignment layer, so there is a preferred direction for liquid crystal, called the easy axis. However the b ulk may have other preferred directions to align, for example due to the intrinsic elastic deformation. Therefore the surface liquid crystal may deviate from the easy axis. Surface anchoring energy is related to this deviation, i.e. the stronger the anchoring, the less the liquid crystal deviate. It is convenient to define polar anchoring strength and azimuthal anchoring strength separately in order to describe the anchoring with two degrees of freedom. Figure 1-10 is a schematic drawing of easy axis and the actual director, each represented by polar and azimuthal angles. Equation (1.12) is the Rapini-Papoular (R-P) form of surface energy per unit area with the anchoring strengths as coefficients. The

45 45 equation contains only even order terms. Because the nematic liquid crystal molecules do not distinguish and. Figure 1-10 Easy axis director and liquid crystal director. ( ) ( ) (1.12) 1.4 Liquid Crystal / Polymer Composite Liquid crystal / polymer composites 13, 30, 31, refer to phase separated materials of liquid crystal and cured polymer because at room temperature they are often not miscible. It is possible to control the phase separation process, which makes the composites diversified and fascinating to various applications. There are 3 types of phase separation: thermally induced phase separation, solvent induced phase separation, and polymerization induced phase separation. Based on concentration ratio of liquid crystal and polymer in the mixture, two kinds of structures can be formed: polymer dispersed liquid crystal (comparable amount of each) and polymer stabilized liquid crystal (only <10% polymer). In the first case (PDLC), polymers weave into a porous network and liquid crystals occupy the pores.

46 46 Under the polarized optical microscope, birefringent liquid crystal droplets can be distinguished from isotropic polymers. In the second case (PSLC), polymers braid into fibrous network in the ocean of liquid crystal. Under the polarized optical microscope, in liquid crystalline phase, only liquid crystal s configuration can be observed since there is too little polymer. However when the sample is brought adjacent to the liquid crystal s isotropic temperature, polymer fibers can be identified through its scattering. Figure 1-11 SEM pictures of: (a) polymer network in PDLC; (b) polymer fibers in PSCT. Liquid crystals are washed away. Photo courtesy: reference 13. The electro-optical switching behavior of PDLC is illustrated in Figure Consider for the liquid crystal,, and for polymer. With incident light angle, the effective refractive index is defined by. When the field is off, the liquid crystal droplets orient randomly. The effective refractive index of liquid crystal does not match the polymer:. The droplets are about in diameter, comparable to visible light wavelength, so the light will be scattered and the whole device appears opaque. When the field is on, liquid crystal aligns parallel to the field. Now in normal direction, the

47 47 effective refractive index of liquid crystal is close to the polymer s refractive index: and the device appears transparent. In oblique directions, -polarized light has, but -polarized light still has. Figure 1-12 Working principle of PDLC. (a) No field applied, scattering mode. (b) Apply electric field across the cell, transparent mode. For polymer stabilized cholesteric 13, 43, 46, there are two operation modes: normal mode as in Figure 1-13 (a) and (b), and reverse mode as in Figure 1-13 (c) and (d). In normal mode, polymer network is created perpendicular to the substrate. The pitch is chosen to be around a few microns so that the focal conic state generates enough scattering. With voltage applied, the cholesteric untwists and aligns homeotropically. There is almost no scattering in all directions. In reverse mode, substrates are treated with alignment layers and polymer network is aligned parallel to the substrate. The cholesteric reflection band is chosen to be in infrared range so that without field the device is transparent in the frequency range of visible light. When field is applied, the focal conic will generate scattering effect.

48 48 (c) (a) (b) (d) Figure 1-13 Working principle of PSCT. (a) Normal mode: no field, scattering mode. (b) Normal mode: with electric field, transparent mode. (c) Reverse mode: no field, transparent mode. (d) Reverse mode: with electric field, scattering mode. To summarize, in order to utilize the liquid crystal s unique anisotropy properties, the mixtures have to be designed in such ways that liquid crystal can re-align to a distinct configuration when an external field is at presence. The elastic energy of liquid crystal itself, the interaction/anchoring between liquid crystal and polymer, the external field induced distortion of liquid crystal and possibly that of the polymer network - all these factors come to contribute in determining the stable state of the mixture.

49 An Overview of Chapter 2 through Chapter 9 The current dissertation starts out from the surface anchoring strength studies, and then relates that to PDLC performances. Along the way, interesting factors that affect anchoring strengths are discovered and are explained physically. Afterward, alignment layers with known anchoring strengths are introduced in different liquid crystal applications. Besides, an encapsulated PSCT structure is achieved with the help of the famous polymer PVA throughout this dissertation. In chapter 2, methods to measure anchoring energy are described. The polar anchoring strength is measured by a high-field method and a higher order term in R-P expression is introduced. The azimuthal anchoring strength is measured by twisting angle method. In chapter 3, poly vinyl alcohol (PVA) is used as the alignment material and its anchoring strengths are studied with varied layer thickness. The main result is quantitatively proving thicker alignment film induce higher anchoring strengths. In chapter 4, the temperature s effect on the anchoring strengths is studied, based on PVA alignment layer. In this chapter, many liquid crystal properties will also be characterized. In chapter 5, a group of polymers/copolymers (families of poly methyl methacrylate, PMMA) is studied. They have the same main chain but different side chains. Anchoring strengths of each material and even some mixed materials are measured with the same method as in chapter 2. Meanwhile PDLC samples are made

50 50 with these polymers. The experiments help understand how PDLC s electro-optical performance is related to the anchoring strength of the polymers. In chapter 6, alignment layers rubbed in two orthogonal directions are created. The particular alignment directions are measured and explained by introducing higher order term in R-P azimuthal anchoring strength. Anchoring strengths are measured respectively. Different combinations of rubbing are studied separately to get the trend of easy axis direction and magnitude of anchoring strength. Different materials are used to for comparison. In chapter 7, strong, medium, weak alignment materials are employed to study the alignment layer s effect on special liquid crystals: the pitch change in thermotropic cholesteric liquid crystals and the morphology of blue phase liquid crystals. In chapter 8, encapsulated polymer stabilized cholesteric texture (EPSCT) is manufactured successfully. The mixture is sandwiched between substrates (can be flexible), with PVA encapsulated PSCT droplets. PVA shell is thin and droplets are large (>10 µm). This structure combines the advantages of PDLC and PSCT, being selfadhesive as well as possessing good viewing angle. Chapter 9 is the summary of this dissertation. It concludes important results from chapter 2-8 in physics aspect as well as in application aspect. There are also suggestions for further research and modifications to the manufacture approaches. References 1. P. G. De Gennes and J. Prost, The Physics of Liquid Crystals. (1993).

51 51 2. S. Faetti, Physics of liquid crystal materials. (Gordon and Breach Publisers, 1991). 3. J. L. Fergason, G. H. Brown, G. J. Dienes and M. M. Labes, Liquid crystals. (1966). 4. A. Jakli and A. Saupe, One- and Two-Dimensional Fluids: Properties of Smectic, Lamellar and Columnar Liquid Crystals. (Taylor & Francis, 2010). 5. L. M. Blinov and V. G. Chigrinov, Electroptical Effects in Liquid Crystal Materials. (1994). 6. J. Brandrup and E. H. Immergut, Polymer handbook. (Interscience Publishers, 1966). 7. J. M. G. Cowie and J. M. K. G. Cowie, Polymers: Chemistry and Physics of Modern Materials. (Nelson Thornes, 1991). 8. P. G. d. G. a. J. Prost, The Physics of Liquid Crystals 2nd ed ed. (Oxford University, Oxford, 1993). 9. A. A. Sonin, The Surface Physics of Liquid Crystals. (Gordon and Breach Publishers, 1995). 10. L. M. B. a. V. G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials. (New York, Springer, 1994). 11. G. Crawford, Flexible Flat Panel Displays. (Wiley, 2005). 12. I. Sage and B. Buhadur, Liquid Crystals-Applications and Uses. (1990). 13. D.-K. Yang and S.-T. Wu, Fundamentals of liquid crystal devices. (John Wiley & Sons, Ltd., 2006).

52 P. Yeh and C. Gu, Optics of Liquid Crystal Displays. (Wiley, 2009). 15. H. Yokoyama, Handbook of Liquid Crystal Research. (Oxford University, Oxford, 1997). 16. C. Hunte, U. Singh and P. Gibbs, J. Phys. II France 6 (9), (1996). 17. J. Leys, C. Glorieux, M. bbenhorst and J. Thoen, Liquid Crystals 34 (6), (2007). 18. P. A. Kossyrev, J. Qi, N. V. Priezjev, R. A. Pelcovits and G. P. Crawford, Applied Physics Letters 81 (16), (2002). 19. K. Sandomirski, S. Martin, G. Maret, H. Stark and T. Gisler, (Bibliothek der Universität Konstanz, Konstanz, 2004). 20. P. Palffy-Muhoray, Liquid Crystal Materials Lecture notes (2008). 21. G. R. LUCKHURST and C. ZANNONI, Nature 267 (5610) (1977). 22. W. Maier and A. Saupe, Z. Naturforsch. 13a (1959). 23. W. Maier and A. Saupe, Z. Naturforsch. 14a (1959). 24. W. Maier and A. Saupe, Z. Naturforsch. 15a (1960). 25. D. Allender, Liquid Crysta Physics Lecture Notes (2009). 26. J. V. Selinger, Soft Matter Lecture Notes (2008). 27. M. Kleman and O. D. Laverntovich, Soft Matter Physics: An Introduction. (Springer, 2003). 28. K. Roch Chan Yu, J.-F. Brun, B. Duponchel, M. Ismaili and F. Roussel, Journal of Applied Physics 108 (11), 6p (2010).

53 V. I. Salyanov, O. V. Kondrashina, M. A. Lagutina, A. A. Gasanov, V. N. Nikiforov, V. I. Borshchevskii, K. A. Dembo and I. V. Reshetov, Doklady Biochemistry & Biophysics 402 (1-6), 3p (2005). 30. J. L. West, R. B. Akins, J. Francl and J. W. Doane, Applied Physics Letters 63 (11), (1993). 31. J. L. Fergason, SID Int. Symp. Dig., (1985). 32. M. A. Meyers and K. K. Chawla, Mechanical Behavior of Materials. (Cambridge University Press, 2009). 33. T. Uchida, Surface Alignment of Liquid Crystals in Liquid Crystals Applications and Uses. (World Scientific, Singapore, 1990). 34. J. V. Gandhi, X. D. Mi and D. K. Yang, Physical Review E 57 (6), (1998). 35. S.-K. Park, U.-S. Jung, S.-B. Kwon, M. Yi, T. Ahn, J.-S. Kim, Y. Kurioz and Y. Reznikov, Journal of the Society for Information Display 18 (3), (2010). 36. M. Schadt, H. Seiberle and A. Schuster, Nature 381 (6579) (1996). 37. H. Yokoyama, Molecular Crystals and Liquid Crystals 165 (1988). 38. D. W. Berreman, Molecular Crystals and Liquid Crystals 23 (3-4), (1973). 39. F. J. Kahn, Applied Physics Letters 22 (8), (1973). 40. H. Fukuro and S. Kobayashi, Molecular Crystals and Liquid Crystals 163, 157 (1988).

54 A. Smith, The Wealth of Nations. (W. Strahan and T. Cadell, London, Scotland, 1776). 42. A. Rapini and M. Papoular, J. Phys. Colloques 30 (1969). 43. D.-K. Yang, L.-C. Chien and J. W. Doane, Applied Physics Letters 60 (25), (1992). 44. M.-K. Seo, M. Han and J.-R. Lee, Optical Materials 21 (1-3), 5p (2003). 45. P. S. Drzaic, Liquid Crystal Dispersions. (World Scientific Publishing Company Incorporated, 1995). 46. S. P. Hurley, Dissertation, Kent State University, 2010.

55 55 CHAPTER 2 Methods to Measure Anchoring Strengths This chapter focuses on the methods to measure the anchoring strengths. First introductions of various methods will be reviewed and briefly discussed, and then specific methods that are used in the current research will be introduced. The polar anchoring strength of the alignment layer was measured using high field method and the azimuthal anchoring strength was measured using twist angle method. 2.1 Introduction and Review of Surface Anchoring Energy Measurements The interaction between the liquid crystal molecules and a solid substrate encloses a fundamental problem for researchers. As introduced in chapter 1, to describe this behavior, A. Rapini and M. Papoular 42 proposed an widely used energy form which depends on a phenomenological parameter W. However no information about the interaction is given. The interaction of the substrate and the liquid crystal molecules has been intensively studied, by taking the Van-der Walls and short-range interactions to describe the anchoring behavior. Most of the liquid crystal displays need alignment layers which pre-determine the orientation of liquid crystal. The anchoring strength and the easy direction of the alignment layers are of great importance for fundamental science and practical applications 8, 9, 39. Alignment layers are usually made from polymers, such as polyimide and polyvinyl alcohol 47, 48 _ENREF_5, which can be dissolved in solvents, spin coated on solid substrates and rubbed. Strong anchoring enables fast relaxation from field-driven

56 56 states, while weak anchoring makes possible to have high contrast ratio under low driving voltages 13, 49. Many researches developed methods to change the anchoring strength such that in manufacture processes the desired anchoring can be tailored to fit the application. Different polymers are capable of align liquid crystal at different angles with respect to the interface, depending on the interaction between the polymer and the liquid crystal. In a display, or just a cell in the lab, the polar angle of liquid crystal at the surface is defined as the pretilt angle. Common alignment layers in display provide either homogeneous/planar alignment (with pretilt angle ~ a few degrees) or homeotropic alignment (with pretilt angle close to 90º). Crystal rotation method 50 is an accurate method to determine pretilt angle, capable of distinguishing 0.1º. It involves a thick homogeneous cell and a pair of polarizers. The theory based on refraction is quite straight forward, but the experiment does require high accuracy of rotation angle of the cell. We can also obtain pretilt angle by measuring the capacitance under external field, knowing dielectric anisotropy or diamagnetic anisotropy. Polar anchoring strength measurements are usually designed to capture the critical moment of transition with the help of external field and known dielectric/diamagnetic anisotropy of liquid crystal. In other words, utilize the balance between external field and the surface anchoring. The problem is that the anchoring only affects the surface liquid crystals, so the method has to eliminate the bulk transition part first in order to observe the surface transitions. Yokoyama-van Sprang technique _ENREF_10 has been widely accepted because it introduced a high-electric-field method, which studies the field range where bulk alignment is already finished and only surfaces are transiting.

57 57 Applying the same idea of high electric field (up to 100V but usually higher than 50V over a 5µm cell), the method used in this dissertation involves accurate measurement of transmittance. Azimuthal anchoring strength measurements 54 is however, difficult to carry out with the similar help of external field, because the in-plane twist need to be balanced with a large in-plane field (larger than most labs can provide, either electric or magnetic). Researchers have been designing surface orientation that deviates from the easy axis _ENREF_ Faetti developed an optical method which requires a reflectometric apparatus setup, wedge cell, and oblique incident light. In the current study, a twist angle method ia applied, in which a liquid crystal cell is made to generate surface distortions and measures optical retardation of the cell. 2.2 Preparation Work With the methods applied in the current research, electrically controlled birefringent (ECB) cells are needed. They were made in the onsite cleanroom with all necessary LCD prototype facilities. To make an ECB cell, glass substrates are cleaned, spin-coated with polyimide or other polymers. For polyimides, the substrates need to be baked at certain temperature for proper imidization 6, 7. For polymers that do not need further chemical transition, low temperature baking is also applied in order to remove solvent and moisture. After baking, two substrates are rubbed and assembled with polymer coating facing inside and making sure the rubbing directions are anti-parallel. In the experiments, in order to keep consistency among multiple cells, inch 2 glass substrates were assembled first, as shown in Figure 2-1(a), and then cut in a

58 58 programmed fashion. Individual cells are obtained by separating the panel carefully. In this way, the cells are made sure to have consistent coating layer, rubbing direction. (a) (b) (c) Figure 2-1 (a) Assembled panel inch 2. (b) A separated row. (c) A separated individual cell. Alignment is considered as good if the cell filled with nematic liquid crystal is uniform under polarizing microscope. This uniformity should not be changed even with applied electric field or with varied temperature, although birefringence will change which will be revealed from the color change. Figure 2-2 Planar cells with PI 2555 as alignment layer under polarizing microscope.figure 2-2 shows example of planar cells from the same panel. In all the pictures, rubbing direction is at 45ºin between crossed polarizers. Comparing (a) and (b), the chiral dopant R811 makes the liquid crystal mixture have a different retardation than the pure nematic E44 cell. Comparing (a), (c), and (d), from the same cell, the ITO electrode covered area is showing different retardation as the applied voltage varies. If the voltage is high enough, it will be totally dark.

59 59 (a) (b) (c) (d) Figure 2-2 Planar cells with PI 2555 as alignment layer under polarizing microscope. Cell gap = 5 µm. 2.3 Polar Anchoring Strength Meaurement In polar anchoring strength measurement, a high-field method is applied, which includes measuring light transmission as a function of voltage across the cell 14, 62. The stronger the surface is, the harder it can anchor the liquid crystal at the interface. In other words, the transmission at high voltage is sensitive to surface anchoring Simulation Method of Relaxation In the ECB cell, the nematic liquid crystal molecules are aligned homogeneously in the absence of applied voltage because the PVA alignment layers on the inner surfaces of the cell generate almost zero pretilt. The easy direction of the alignment layers has close to polar angle and azimuthal angle. When an external field is applied, the

60 60 liquid crystal will be tilted away from the easy direction. When the voltage is applied cross the cell, the generated electric field tends to tilt the liquid crystal ( ) toward the direction normal to the cell plane while the alignment layers tend to keep the liquid crystal in the cell plane 13, 49. The polar angle at the surface is determined by the balance between the electric and surface forces. There is no azimuthal torque with or without field, so the azimuthal angle through the whole cell is always the same as easy direction. At one substrate, the surface energy per unit area can be written as: (2.1) Note that in the above equation, a quartic term is kept besides the commonly used quadratic term. This is because when very high field is applied, the surface tilt angle will be high, too. Keeping to the 4 th order term is necessary and suffcient for the current study. Due to the boundary condition imposed by the alignment layer, the director in bulk is not uniform as shown in Figure 2-3 (b). The bulk free energy of the liquid crystal per unit area is given by [ ( ) ( ) ( ) ( ) ] (2.2) where is the cell thickness and is the electric field. The liquid crystal director is described by. Under a given electric field, the polar angle at the surfaces of the alignment layers and in the bulk of the cell is determined by minimizing the total free energy given by

61 61 * ( ) ( ) + [ ] (2.3) [ ] Because the top and bottom alignment layers are the same,. (a) (b) (c) Figure 2-3 (a) Homogeneous cell: anti-parallel rubbing, filled with pure nematic liquid crystal. (b) Apply AC voltage, the liquid crystals show splay deformation. (c) Top view of director in the coordinates. With the given cell properties, a 1-dimensional relaxation process 13, 14, 62 is carried out to obtain the director configuration through the cell. Figure 2-4 (b) at 0 V, all liquid crystals lie along easy axis, ( ). As voltage increases, the middle directors start to tilt to the electric field first, and then the surface directors start to tilt. At 30 V, all directors are in homeotropic state already. In (c), which depicts the similar picture but with a relatively stronger anchoring strength, the surface liquid crystal layers are more difficult to tilt. Even 50 V is just enough to tilt the surface liquid crystals to 70º. However, by comparing the curves at 3V, V, and 10V in (b) and (c), we see that the director configurations in bulk are very similar. This suggested that the bulk change does

62 62 not depend on the surface properties much, and we should really eliminate the bulk effect to study the surface. (a) (b) (c) Figure 2-4 (a) Denotation of polar angle. (b) Polar anchoring strength,. (c) Polar anchoring strength, Optical Simulation 10, 13, 14, Extended Jones Matrix method is applied to calculate the light transmission through the optical components involved. It takes into account the reflections and refractions but not multiple reflections. It is suitable for both normal and oblique incident light. Un-polarized light goes through the polarizer (at ), the liquid

63 63 crystal cell (at ), and then the analyzer (at ). The polarizer and analyzer are considered as 2 separate layers outside the liquid crystal, and liquid crystal cell is sliced into imaginary layers as in Figure 2-5. The birefringence properties and directors are represented by the Jones matrix of each layer. (a) (b) Figure 2-5 (a) Schematic drawing of light passing through birefringent film with imaginary slices. (b) Coordinates on a slice with thickness. The idea of this method is start out with a vector which represents the incident light, and then multiply by matrices which represent each optical layer, finally get the exit light as a vector again. In equation (2.4), ( ) is called the Jones Matrix for layer, and includes retardation of this layer. ( ) is a rotation matrix, used to switch between lab frame and principal frame. [ ( ) ( ) ( )] (2.4)

64 Experiment (a) (b) Figure 2-6 (a) Optical setup for polar anchoring strength measure ments. (b) Crosssection view of polarizers and rubbing direction. In the optical measurement, the rubbing direction in LC director plane makes 45 o to the crossed polarizer and analyzer. When an incident light with wavelength propagates through the LC cell, the transmittance is given by ( ) (2.5) where and are the ordinary and extraordinary refractive indices of the LC, respectively. is the polar angle of liquid crystal. In the experimental, the transmission vs. voltage curve was measured with high precision with the help of a pair of Glenn-Thompson polarizers. The nematic liquid crystal used was E44 (from Merck). The elastic constants of E44 are,, and. The dielectric anisotropy is and birefringence is at room temperature. AC voltage of 1 khz square wave was applied to the cell. A He-Ne laser light with the wavelength of 633 nm was used to illuminate the cells at normal incidence. A photo-diode was used to measure

65 65 the light intensity. The possible effect of the polymer alignment film s birefringence difference due to thickness was cleared by coupling an empty cell to the reference light intensity Measurement and Fitting Result: Rubbed Bare Glass and Polyimide The fitting program is a combination of relaxation process and optical calculation process. Adjustable parameters are only anchoring strengths (quadratic term and quartic term coefficients). Two cases were shown in the Figure 2-7: rubbed bare glass, PI PI 2555 is a commonly used planar alignment polyimide and is supposed to have strong aligning effect. Rubbed bare glass means there is no alignment but velvetrubbings is applied to the surface. If the glass substrate is not rubbed, some flow alignment can still exist, but the cell is not uniform. The rubbed glass substrates do show stable aligning effect similar to polymer, although weak. All measured cells were assembled with 5 µm spacers. The cell with rubbed polymer has stronger anchoring, thus the surface liquid crystal is tilted less by the electric field. Therefore it has more birefringence effect and so the transmittance is higher. The best fitting results for the two alignments are: Table 2-1 Polar anchoring strength: bare glass (rubbed) and PI 2555 Alignment ( ) ( ) Bare glass, rubbed PI

66 66 Figure 2-7 Experiment data and fitting result for PI 2555 and rubbed bare glass. 2.4 Azimuthal Anchoring Strength Meaurement In order to measure the azimuthal anchoring, we applied the idea of balancing the anchoring against the liquid crystal s in-plane twisting, i.e. the anchoring strength can be calculated from the resistance produced by the alignment layer in the azimuthal d irection. Adding a chiral dopant into the liquid crystal can accomplish the task. O one hand tahe doped chiral dopant causes the liquid crystal to twist through the cell; on the other hand the alignment layers tries to anchor the liquid crystal in the rubbing direction Nematic and Chiral Nematic As used in the polar anchoring strength measurements, E44 is the nematic host in the azimuthal anchoring strength measurements. Pure E44 will be referred to as pure nematic in the following context. Chiral dopants R811 and S811 have similar structures but opposite handness. Both are soluble in E44. The helical twisting power 65 to E44 is

67 67 and respectively. Doping R811 or S811 into E44 with mass concentration will induce natural pitch (or intrinsic pitch) as calculated by equation (2.6) The chiral nematic will form in this pitch if there are no restrictions such as boundary conditions or external field. (2.6) Figure 2-8 Chemical structure of R811/S811. The chiral center is circled out in red. (a) (b) (c) Figure 2-9 (a) Planar cell filled with E44. (b) Planar cell filled with chiral nematic. (c) Top view of director on two substrates Boundary Conditions and TAM method Provided boundary, i.e. alignment film, will change the twist through the cell. Free energy per unit area can be written as in equation (2.7). Note that the easy direction

68 68 is along. Total twist angle across the cell in the bulk term and surface deviated angle, need to be adjusted so that total free energy will be lowest. As the result of the competition, in the bulk the liquid crystal twists less than the intrinsic twist angle. If the two surfaces are the same then. Also. ( ) ( ) can be found by minimizing the free energy with respect to (2.7). For the case of small angle : ( ) (2.8) By the acquisition of twist angle, the azimuthal anchoring strength can be obtained. This method is called twist angle method (TAM) Null Transmittance Derivations First consider the polarizer and the analyzer are fixed at perpendicular directions and rotate simultaneously. Prove that when polarizer bisects the total twist, there is minimum transmission. As pictured in Figure 2-10, angle is between rubbing direction and polarizer.

69 69 Figure 2-10 Rotating both polarizer and analyzer: cross-section view of polarized light directions. Incoming light to the liquid crystal is decomposed into two components: e component parallel to the director at the bottom and o component perpendicular to the director: ( ) and ( ). Because ( ) ( ) (Mauguin condition) is satisfied, when the light propagates through the LC cell, these two components follow the twist as the liquid crystal director. Thus they remain parallel and perpendicular to the director, respectively. Their phase changes are, however, different. The e component becomes ( ) and the o component becomes ( ). The sum of their projections along the analyzer is ( ) ( ) ( ) ( ) (2.9) The transmittance is given by (2.10)

70 70 ( ) ( ) ( ) The angle m that gives the minimum transmittance is found by ( ) (2.11) Therefore gives minimum transmittance ( ). Figure 2-11 Rotating analyzer: cross-section view of polarized light directions. Note that this is the method to find the rubbing direction. Now that the statement in the beginning of this section is proved, in the measurement of the twist angle, the polarizer is fixed in the direction parallel to the rubbing direction as shown in Figure The analyzer is in the direction making the with the orthogonal direction of the polarizer. The electric field of the incident light entering the cell is in the polarizer s direction. Light is decomposed to and. The exit light from the liquid crystal has the e component and the o

71 71 component. The sum of their projections along the analyzer is ( ) ( ( )) The transmittance is given by ( ) ( ) (2.12) ( ) ( ) (2.13) The angle m that gives the minimum transmittance is found by ( ) ( ) (2.14) ( ) ( ) (2.15) Null Transmittance Measurements and Azimuthal Anchoring Calculations The pitch of the chiral nematic is chosen in such a way that, in order to avoid over-90º-twist through the cell. The cell is still rubbed in anti-parallel directions. One possible error is the rubbing direction is not perfectly 180ºaway. This error is caused by assembling the cell while can be controlled within 3º. Therefore calibration is achieved by a reference cell, which is from the same panel of the sample cell but filled with pure nematic.

72 72 The experiment setup is similar to the one in polar anchoring measurements, but with different polarizer directions. Horizontally polarized light enters the cell, and will be re-directed by the liquid crystal. The analyzer is rotatory within the plane perpendicular to the light traveling direction. The rotation can be controlled by a step motor with precision at 0.1º. A calibration is plotted in Figure 2-18 (b) when no sample cell is at present. While rotating analyzer, the detector records the transmission and the null position can be found by a parabola fitting. For each alignment layer, identically prepared cells were used. Figure 2-14 Showed direct measurements of rubbed bare glass and PI In each subplot, there are two curves indicating two chiral dopant concentrations. (a) (b) Figure 2-12 (a) Optical setup for azimuthal anchoring strength measurements. (b) Cross-section view of polarizers and rubbing direction.

73 Figure 2-13 Measured transmittance vs. position of rotating analyzer. (a) rubbed bare glass. (b) PI All cells are assembled with 5 µm spacers. Legendary indicates weight percentage of chiral dopant. 73

74 74 Figure 2-14 Analyzer deviation at minimum transmittance vs. concentration of chiral dopant. Although the total twist angle can be calculated directly from the experimentally measured analyzer deviation, it is still worth checking the equations. According to equation (2.15), with given cell configuration and incident light information, transmittance can be simulated. In Figure 2-15, 2 samples of fitting experimental data are demonstrated. Figure 2-15 Open circles: experimentally measured transmittance vs. analyzer deviation. Solid line: Calculated by Jones Matrix method.

75 75 The results of azimuthal anchoring strength calculated by equation (2.15) are listed in the following table. Table 2-2 Azimuthal anchoring strength: bare glass (rubbed) and PI 2555 Alignment ( ) Bare glass rubbed 1.0 PI Additional Experiment Techniques Optical Chopper and Lock-in Amplifier The transmittance measurements have to be as accurate as possible. On one hand, the high field method requires high accuracy when measuring transmittance on the order of 10-2 % when the anchoring is weak. On the other hand, when rotating the analyzer, although it is not hard to get a parabola transmittance and obtain the minimum position, it is physically meaningful to distinguish the actual minimal transmittance. Equation (2.15) suggests that transmittance is related to the total twist: the more twist, the higher minimal transmittance. Also in Figure 2-13 this difference is also presented: with the same bare glass, the 0.217% chiral dopant induced smaller twist than the 0.415% chiral dopant, hence the 0.217% curve has smaller minimal than the 0.415% curve; similar behavior in PI 2555 case.

76 76 Therefore in the experiment setup, besides using Glenn-Thompson polarizers, an optical chopper and a lock-in amplifier are introduced. The actual data acquisition system is depicted in Figure Lock-in amplifier s frequency is synchronized with the optical chopper. The transmittance detector (photo diode) is connected to the input of the lock-in which can take up to 1 volt. This signal is amplified and fed into NI DAQ input I2. The real-time reference detector can eliminate the laser s fluctuation, and is fed into DAQ input I1. If voltage is needed, DAQ is responsible to provide 0-10 V and it will be amplified up to 100 V by another amplifier (left out in the picture). All inputs, outputs of DAQ card are controlled by LabVIEW programs. Figure 2-16 Actual experimental setup in polar and azimuthal anchoring strength measurements. Without the optical chopper/lock-in amplifier, with the Glenn-thompson polarizers, the detector can still see some ambient room light (from neighboring working sections) and it is actually pretty sensitive. The top signal in Figure 2-17 (b), the

77 77 operating frequency of ambient room light can be detected, since the optical chopper s frequency is set far away from that. After inserting the chopper and the lock-in, the signal is much more uniform and the fluctuation is reduced to within 1 mv. (a) (b) Figure 2-17 (a) Bare chopped light detection. (b) Light through crossed polarizers, with or without optical chopper+lock-in amplifier Eliminate Reflections A problem was discovered when measuring azimuthal angles. Rotating the analyzer 360ºdid not provide symmetric peaks. This was due to reflections going on in between polarizers. To get rid of this effect, the analyzer is tilted a little w.r.t. incident light direction, and the first reflected light will hit a piece of black paper. A good calibration is obtained as in Figure 2-18.

78 78 (a) (b) Figure 2-18 (a) Eliminate reflection issue. (b) Calibration of light intensity while fully rotating the analyzer. Reference 1. A. Rapini and M. Papoular, J. Phys. Colloques 30 (1969). 2. A. A. Sonin, The Surface Physics of Liquid Crystals. (Gordon and Breach Publishers, 1995). 3. P. G. d. G. a. J. Prost, The Physics of Liquid Crystals 2nd ed ed. (Oxford University, Oxford, 1993). 4. F. J. Kahn, Applied Physics Letters 22 (8), (1973). 5. M. R. Costa, R. A. C. Altafim and A. P. Mammana, Molecular Crystals and Liquid Crystals 393 (1), (2003). 6. P. Vetter, Y. Ohmura and T. Uchida, Japanese Journal of Applied Physics 32 (9A), L1239 (1993).

79 79 7. D.-K. Yang and S.-T. Wu, Fundamentals of liquid crystal devices. (John Wiley & Sons, Ltd., 2006). 8. D.-K. Yang, J. Opt. A: Pure Appl. Opt. 5, 402 (2003). 9. T. J. Scheffer and J. Nehring, Journal of Applied Physics 48 (5), (1977). 10. H. Yokoyama and H. Van Sprang, Journal of applied physics 57 (10), (1985). 11. T. Rasing and I. Musevic, Surfaces and Interfaces of Liquid Crystals. (Springer, 2004). 12. H. Yokoyama, Molecular Crystals and Liquid Crystals 165 (1), (1988). 13. Y. Sato, K. Sato and T. Uchida, Japanese Journal of Applied Physics 31 (5A), L579 (1992). 14. Tadashi?Akahane, Hideki?Kaneko and Munehiro?Kimura, Japanese Journal of Applied Physics 35 (8R), 4434 (1996). 15. Ying Zhou, Zhan He and Susumu Sato, Japanese Journal of Applied Physics 36 (5R), 2760 (1997). 16. Y. Zhou, Z. He and S. Sato, Japanese Journal of Applied Physics 38 (8R), 4857 (1999). 17. J. G. Fonseca and Y. Galerne, Applied Physics Letters 79 (18), (2001). 18. S. Faetti and G. C. Mutinati, Physical Review E 68 (2), (2003). 19. S. Faetti and P. Marianelli, Physical Review E 72 (5), (2005). 20. S. Faetti, K. Sakamoto and K. Usami, Physical Review E 75 (5), (2007).

80 J. M. G. Cowie and J. M. K. G. Cowie, Polymers: Chemistry and Physics of Modern Materials. (Nelson Thornes, 1991). 22. J. Brandrup and E. H. Immergut, Polymer handbook. (Interscience Publishers, 1966). 23. P. Yeh and C. Gu, Optics of Liquid Crystal Displays. (Wiley, 2009). 24. P. Bos, Liquid Crystal Displays Lecture Notes (2009). 25. B. Max and E. Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light. (Cambridge University, 1997). 26. B. D. Guenther, Modern Optics. (Wiley, 1990). 27. L. M. B. a. V. G. Chigrinov, Electrooptic Effects in Liquid Crystal Materials. (New York, Springer, 1994). 28. M. J. Cook and M. R. Wilson, The Journal of Chemical Physics 112 (3), (2000).

81 CHAPTER 3 Anchoring Strength Varied by PVA Alignment Layer Thickness Solid poly vinyl alcohol (PVA) was dissolved in a thinner and spin-coated on glass substrates. We found that the anchoring strength of the PVA alignment layer can be greatly varied by changing the film thickness 66. Both the polar and azimuthal anchoring strengths increased with increasing film thickness, but they had different film thickness dependences. We also found that the quartic term in the Rapini-Popular expression of the anchoring energy is important at high anchoring strength Introduction Poly vinyl alcohol is a water-soluble resin, with the chemical structure shown in Figure 3-1. Liquid crystal can be homogeneously aligned very well by PVA, and the pretilt angle is rather low 68. It was used in alignment and anchoring strength studies extensively Figure 3-1 Chemical structure of poly vinyl alcohol (PVA). PVA has been used as surface alignment layers in liquid crytal cells due to its uniform aligning effect and non-toxic character. Also, many commercial PDLC light 81

82 82 shutters are made of water-soluble polymers. Therefore in this chapter, the well-known PVA will start its journey through the dissertation and play the role of reference and comparison. 3.2 Preparation The polyvinyl alcohol used in the experiment was provided by MFLEX UK Ltd.: PVA MS-88 (15% dissolved in aqueous solution). The original PVA solution was diluted by de-ionized water. The weight concentrations of PVA in the final solutions used for spin-coating were: 4.3%, 3.8%, 3.0%, 2.3%, 1.5%, and 1.0%, respectively. The solutions were spin-coated 72 on inch2 indium tin oxide (ITO) patterned glass substrate at a speed of 1500 rpm for 30 seconds. The substrates with the PVA coating were pre-baked at 90 C for 1 minute, and then oven baked at 120 C for about 1.5 hours. Afterward, the alignment layers were rubbed with velvet under fixed pressure and direction. Two substrates with the same alignment layer were anti-parallel assembled to make one panel of cell with UV glue. The cell gap was controlled by 5µm spacers. The large cell was then cut into 20 single cells in a programmed fashion. This process ensured that all the single cells had the same alignment layer and rubbing direction. The cell gap of each single cell was measured to be between 4.85 and 4.95 microns. Liquid crystal was filled into the cells at isotropic phase to avoid flow alignment. 3.3 AFM Examinations The PVA alignment layer thickness was measured by atomic force microscopy (AFM). Empty cells were opened to keep consistency as later measurements. Figure

83 83 3-2(a) shows a sample surface of a rubbed PVA surface and the flattened picture (b) shows the grooves produced by the rubbing more clearly. Rubbing (a) Figure 3-2 (a) 3D image of spin-coated and rubbed PVA surface. (b) Flattened (view from top) image of PVA surface. To measure the thickness of the polymer film thickness, each alignment layer sample was cut through and scraped by a surgical blade. Therefore a step was created with one side of the polymer shaved and the other side left on the substrate. AFM pictures illustrating this measurement are shown in Figure 3-3. The measured thicknesses are plotted in Figure 3-4, from which we can conclude that the alignment layer thickness decreased more than linearly with decreasing PVA concentration in the aqueous solution. This is due to the fact that when the solution s PVA concentration was reduced, the viscosity of the solution was also decreased, and therefore a thinner layer of solution was dispensed on the substrate during spinning, combining with the result from the solvent s evaporation. The depth of the grooves is larger on the thicker PVA films, since the thinner layer is baked harder. (b)

84 84 (a) (b) (c) (d) (e) (f)

85 85 (g) (h) (i) (j) (k) (l) Figure 3-3 Various PVA solution spin-coated film thickness. Left column: 3D image. Right column: section analysis. From top to bottom rows: 4.3%, 3.8%, 3.0%, 2.3%, 1.5%, 1.0%.

86 86 Figure 3-4 Measured alignment layer thickness as a function of PVA concentration in the solution for spin-coating. 3.4 Polar Anchoring Strengths Result The method has been described in chapter 2 in details. The experimental results are plotted in Figure 3-5. The anchoring strength of the alignment layers were obtained by fitting the experimentally measured transmittance-voltage curves of the ECB cells. When only the 1 st term of the Rapini-Papoular Equation was used, the fitting was not good, because the polar angle became large (close to 90 o ) under high voltages. When both terms were used, the theoretically calculated results agreed with the experimental ones very well as shown in Figure 3-6. The best-fitted polar anchoring strengths are shown in Figure 3-7. Both the quadratic and quartic anchoring coefficients increased

87 87 significantly with the alignment layer thickness, with same order of magnitude approximately the same trend. The anchoring strength was on the order of J / m, where the large end agreed with other reported measurements of PVA with cyano-biphenyl LC interface prepared with similar conditions 48, 73, 74. Figure 3-5 Experimentally measured transmittance-voltage curves of the ECB cells with different alignment layer thicknesses.

88 88 Figure 3-6 Transmittance-voltage curves of EBC cells with various alignment layer thicknesses. Blue symbol: experimentally measured. Red line: theoretically calculated. Figure 3-7 The polar anchoring strengths vs. the PVA alignment layer thickness. : quadratic term; : quartic term. The lines are guide to the eye.

89 Azimuthal Anchoring Strengths Result Figure 3-8 shows a typical transmittance as a function of the analyzer angle of two cells. The chiral dopant concentration was %, which produced an intrinsic pitch of 46 m. When the thickness of the alignment was 14 nm, the angle of the analyzer for the minimum transmittance was 5.16 o. The optical retardation was nh / / From equation (2.15), the twist angle was found to be 5.1 o. Using this twist angle and equation (2.13), the transmittance vs. the analyzer angle was calculated and plotted with solid lines in Fig. 8, which gratifyingly fit the experimental result. From equation (2.8), the azimuthal anchoring strength was found to be 2.3 m J /. When the thickness of the alignment was nm 120, the azimuthal anchoring was stronger. The angle of the analyzer for the minimum transmittance decreased to 1.33 o. The twist angle was found to be 1.3 o. Similarly, the azimuth anchoring strength was found to be J / m. When the anchoring was strong, the twist angle was small and relative experimental error could be large. In order to improve the accuracy, the concentration of the chiral dopant was increased to 0.415%. The azimuthal anchoring strengths of the PVA alignment layers with various layer thicknesses are shown in Figure 3-9. The anchoring strength was varied in the range of J / m, with significant increase with the alignment layer thickness and then tend to saturate with layer thickness of about 100 nm. As reported by other researchers 54, 74, 75, PVA s azimuthal anchoring strength was in the same range as our result.

90 90 Figure 3-8 Transmittance vs. analyzer angle twist angles of two twist cells with PVA alignment layers. Markers: experimental results, solid lines: theoretical fit. PVA alignment layer thickness, : 120 nm, : 14 nm Figure 3-9 The azimuthal anchoring strengths of the PVA alignment layers with various layer thicknesses. The line is a guide to the eye.

91 Conclusion and Discussion It has been demonstrated that both polar and azimuthal anchoring strengths of polyvinyl alcohol alignment layer can be changed by varying the layer thickness. They were increased with increasing layer thickness, but their thickness-dependences were different: the azimuthal anchoring strength increased more drastically at thin alignment layers than polar anchoring strength. The polar anchoring is resulted from the anisotropic inter-molecular interaction between the liquid crystal and the polymer 9, 33. When both anisotropic directions of the liquid crystal and polymer are in the same direction, the interaction energy is low. When the liquid crystal deviates from the parallel direction, the interaction energy increases, which is described by the surface anchoring energy. Anisotropy of liquid crystal is an intrinsic property, while the anisotropy of the polymer is produced by the mechanical rubbing which causes the polymer chains to align in the same direction. Because the inter-molecular interaction is short-range, the increase of the anisotropic interaction can be explained by that the polymer chains are aligned better in thicker films. For the azimuthal anchoring, besides the anisotropic inter-molecular interaction, the microgrooves created by the rubbing also contribute 38, When the director is parallel to the grooves, there is no elastic distortion of the liquid crystal. When the director is no longer parallel to the grooves and varies spatially, it results in an elastic energy that is counted as a part of the azimuthal anchoring energy. The elastic energy depends on the spatial variation rate and depth of the grooves. When the PVA alignment layer is thicker, not only the polymer chains are more ordered, but also the grooves are

92 92 deeper. Both effects together will make the azimuthal anchoring strength increases faster with the alignment layer thickness in the beginning than the polar anchoring strength. Control of the surface anchoring strength through the variation of the layer thickness is reliable and repeatable. This method of generating alignment layers with different anchoring strength provides another perspective in basic research and practical applications. Reference 1. R. Petkovšek, M. Čopič and J. Pirš, Journal of Applied Physics 99 (4), - (2006). 2. S. Faetti and V. Palleschi, J. Physique Lett. 45 (7), (1984). 3. T. J. S. a. J. Nehring, J. Appl. Phys. 48, 1783 (1977). 4. J.-H. Son and W.-C. Zin, Applied Physics Letters 97 (24), - (2010). 5. Z. G. M. Jiao, Q. Song, and S.-T. Wu, Appl. Phys. Lett. 92, (2008). 6. M. R. P. J. B. M. J. O'Callaghan, J. of Appl. Phys. 107, (2010). 7. C. J. Lawrence, Physics of Fluids ( ) 31 (10), (1988). 8. P. Vetter, Y. Ohmura and T. Uchida, Japanese Journal of Applied Physics 32 (9A), L1239 (1993). 9. W. C. M. B. Feller, and Y. R. Shen, Phys. Rev. A 43 (1991). 10. S. K. T. Ohide, and S. Kobayashi, Mol. Cryst. Liq. Cryst. 164, 91 (1988). 11. M. Reznikov, P. J. Bos and M. J. O'Callaghan, (2010). 12. Y. Sato, K. Sato and T. Uchida, Japanese Journal of Applied Physics 31 (5A), L579 (1992).

93 T. Uchida, Surface Alignment of Liquid Crystals in Liquid Crystals Applications and Uses. (World Scientific, Singapore, 1990). 14. A. A. Sonin, The Surface Physics of Liquid Crystals. (Gordon and Breach Publishers, 1995). 15. D. W. Berreman, Molecular Crystals and Liquid Crystals 23 (3-4), (1973). 16. D. W. Berreman, J. Opt. Soc. Am. 63 (11), (1973). 17. D. W. Berreman, J. Opt. Soc. Am. 62 (4), (1972). 18. J. B. Fournier and P. Galatola, Physical Review E 60 (2), (1999). 19. J. Elgeti and F. Schmid, Eur. Phys. J. E 18 (4), (2005). 20. E. S. Lee, P. Vetter, T. Miyashita, T. Uchida, M. Kano, M. Abe and K. Sugawara, Jpn. J. Appl. Phys. 32 (Part 2, No. 10A) (1993). 21. J.-i. Fukuda, M. Yoneya and H. Yokoyama, Physical Review Letters 98 (18), (2007).

94 CHAPTER 4 Temperature Effect on Anchoring Strength Researches on temperature s effect on the anchoring strengths are carried out. The prediction is that as the temperature goes high, both polar and azimuthal anchoring strength will decrease 1. The challenge in this part of study is the lack of knowledge of basic properties of liquid crystal under different temperatures. This chapter introduces both experimental as well as analytical methods to study the effect of temperature on the anchoring strength. The experimental setup and process for measurements have been modified to be precise enough. In order to make sure the result is generally applicable, we used a variety of alignment layers, liquid crystals and chiral dopants. Then we also made a PDLC samples with PVA to check its performance under different temperatures, in order to relate to the anchoring strengths change. We measured response time, and switching voltages. First, the studies focused on PVA alignment layers and nematic liquid crystal E44, since PVA has been a starting point for water-soluble polymers (especially polyurethanes used in PDLC manufactures). Then, alternate alignment layer and liquid crystal/chiral dopant mixtures were also studied. The PDLC characterization is carried out with PVA and E44 in order to compare the anchoring strength s effect at various temperatures. 94

95 Liquid Crystal Properties with Varied Temperature As the study goes on, it is discovered that the material s properties can be crucial to the final result 2-5. Therefore characterizations of the materials are performed and will be introduced first. To fit polar anchoring strength, elastic properties (splay constant), dielectric properties and optical retardation of the ECB cell will be needed. In the azimuthal anchoring strength measurement, twisting elastic constant and helical twisting power of the materials are required. It is important to determine these factors before carrying out the analysis Elastic Constants Many factors of liquid crystal are related to order parameter. Therefore we start out from the order parameter s dependence of temperature, well described by: ( ) For E44 (cyanobiphenyl 3 structure),,. Actually starting from, isotropic wetting is already observed under the microscope. In the current study all the measurements were took up to. Elastic constants are involved in the T-V measurement and simulations, as well as in obtaining twist angle. According to Maier-Saupe theory 6-8, the orientation part of the intermolecular interaction energy is proportional to the square of orientational order parameter, ( ), as in Figure 4-2.

96 10-12 J/m2 96 E44 order parameter temperature (ºC) Figure 4-1 Calculated order parameter for liquid crystal E44, which goes to isotropic (S=0) at 105ºC. 30 Elastic constant E44 K11 E44 K22 E44 K temperature (ºC) Figure 4-2 Calculated elastic constants for liquid crystal E44

97 Helical Twisting Power One method of determining helical twisting power 9-11 is to create Granjean fringes with known concentration of the chiral dopant. The space between adjacent fringes directly relate to the pitch. We use a circular wedge (a small lens, with a diameter of 10 mm) to measure the pitch directly, and then calculate the HTP. The idea is demonstrated in Figure 4-3(a). Cover the lens over a glass substrate, with chiral nematic liquid crystal sandwiched in between them. Both lens s bottom and glass substrate s top are spincoated with strong polyimide and rubbed in anti-parallel directions, so that the cholesteric liquid crystal will be forced to form whole numbers of 1/2 pitch. Circular disclination lines will appear as a boundary of two zones with half pitch difference. Figure 4-3(b) is a microscopic photo of the rings of disclination lines. The lens s radius of curvature (R in Figure 4-3(a)) is known. We can measure several consecutive ( ) disclination line s radius (r n in Figure 4-3(b) and (c)). By geometric calculations, it cen be found out that is proportional to the order of the rings Thus vs can be fitted by a straight line with slope as in Figure 4-3(c). Then we can derive that the natural pitch P 0 is proportional to 1/R. (4.1) Then HTP is calculated as in equation: (4.2) where c is concentration of chiral dopant.

98 r n^2 98 (a) (b) (c) n 25C Linear (25C) Figure 4-3 (a) Schematic drawing for HTP measurements. (b)microscopic picture of disclination lines formed by filling Cholesteric liquid crystal in between flat glass and circular wedge, both with alignment layer. (c) Linear fitting vs in order to get.

99 -HTP(µm -1 ) % by weight of S811 can be fully dissolved in E44. A drop of the mixture is carefully sandwiched between lens and flat glass. The measured result is shown in Figure 4-4. Then the similar experiments are carried out for CB15 (8.99% by weight) in E44. CB15 is a right-handed chiral dopant (chemical structure inserted in Figure 4-5). Both mixtures show decreasing behavior when increasing temperature. Details of the measurement pictures and fittings can be found in the appendix temperature (ºC) Figure 4-4 Measured picture: S811 structure. of S811 in E44 at different temperatures. Inserted

100 HTP (µm -1 ) temperature (ºC) Figure 4-5 Measured picture: CB15 structure. of CB15 in E44 at different temperatures. Inserted Dielectric Constants Dielectric constant and were measured by the voltage drop when our ECB cell was connected in series with a standard capacitor as pictured in Figure 4-6. Knowing the active area and the cell thickness, we can figure out the dielectric constant. Freedericksz transition voltage is critical 12, 13. To keep the liquid crystal in homogeneous state, the voltage applied must not exceed the Freedericksz threshold voltage. Figure 4-6 Circuit for measuring capacitance of a liquid crystal cell. The capacitance of the ECB cell can be calculated as:

101 101 where is the voltage drop on commercial capacitor, and is the voltage drop on our ECB cell. Total voltage is a sinusoidal wave with frequency of 1kHz. Adjusting and will help properly measure. For example, at, when the applied voltage is ( ), the liquid crystal is in the homogeneous state. While applied voltage is ( ), the liquid crystal is in homeotropic state. Therefore we use the following conditions for measuring and respectively: When measuring,, ( ), 1kHz When measuring,, ( ), 1kHz eps_para 25 eps_perp temperature (ºC) Figure 4-7 Measured dielectric constants of E44 as a function of temperature Birefringence The retardation of the LC cell is crucial to the azimuthal angle measurements. Therefore it s a good idea to actually measure and. There are many methods to measure birefringence or refractive indices separately. The most sensitive and convenient

102 102 technique is Sernamont method 5. It s sensitive because it can distinct a retardation ( ) of 3.5nm by 1 degree rotation of analyzer. It s convenient because only a quarter-waveplate (QWP) need to be added to the existed experiment setup, shown in Figure 4-8. The input polarizer is set parallel to the QWP, and the ECB cell is oriented at 45 to the polarizer. The analyzer is then rotated to find the null or maximum transmission position. The LC cell s phase angle is two times of the angle between the maximum transmission analyzer direction and the input polarizer direction. In the real experiment, I measured the minimum transmission analyzer direction (because it s more precise), and compensated 90ºto get maximum transmission analyzer direction. Figure 4-8 Optical setup for Sernamont method. ECB cells with PI 2555 alignment layers are filled with pure E44 and pure E7 respectively for the measurements. Result shown in Figure 4-9 is very similar to the references 14, 15.

103 E44 E temperature (ºC) Figure 4-9 Measured birefringence for E44 and E Polar Anchoring Strength Measurements As before, an ECB cell is characterized by measuring transmission vs. applied AC voltage. Figure 4-10 shows the transmittance when applied voltage is within 6 Volts. As temperature is varied, the initial transmittance at 0 V changes as well. This is due to the liquid crystal s birefringence change. The Fredericksz transition voltage is also decreased when temperature is increased, as plotted in Figure 4-11.

104 Fred. Trans. Voltage (V) Transmittance (%) RT 35C 45C 55C 65C 75C 85C Voltage (V) 95C Figure 4-10 Transmission as a function of voltage (up to 6 V) plots for an ECB cell with PVA alignment layer and filled with E44. Cell gap = 5 µm Temperature (ºC) Figure 4-11 Fredericksz transition threshold voltage of an ECB cell as a function of temperature. Cell gap = 5 µm.

105 Transmitance (%) 105 In order to find out the polar anchoring strength, the high electric field method is applied here again. The results are plotted in Figure The best fitted anchoring strength is shown in Figure Detailed fitting results will be found in the appendix. Both the quadratic term coefficient and the quartic term coefficient decrease as the temperature gets higher. has drastic decrease after 95ºC while decrease more significantly after 65ºC Voltage (V) Figure 4-12 Transmission as a function of voltage (up to 60 V) plots for an ECB cell with PVA alignment layer and filled with E44. Cell gap = 5 µm.

106 Wp J/m Wp1 Wp Wp J/m Temperature (ºC) Figure 4-13 Polar anchoring strength of PVA as a function of temperature. 4.3 Azimuthal Anchoring Strength Measurements In chapter 2, the analytical solution of total twisting angle according to analyzer angle is provided by equation (2.15). The cell thickness is determined before filling in liquid crystal. At each temperature, birefringence is measured already. Therefore total twist angle vs. analyzer angle map is plotted in Figure In this figure, the cell thickness is 5 µm. By taking an overall look at these curves, it can be discovered that as the temperature increases, the curves first expand horizontally, and then shrink toward the center very fast.

107 107 Figure 4-14 Analytically calculated total twist angle through the cell as a function of analyzer deviation angle at various temperatures. Assume there is such a cell that its twist does not change with temperature. In reality, this situation can be an ECB cell filled with pure nematic liquid crystal. However the rubbing direction of the top substrate is not exactly anti-parallel to that of the bottom substrate due to experimental error, so the nematic will have a small twist (usually within a few degrees). Consider the liquid crystal is E44, fix the twist from bottom to top substrate, use the measured birefringence of E44 under various temperatures, apply to equation (2.15), and then we can plot the expected analyzer deviation angle as a function of temperature as in Figure Note that there is a non-monotonic change. In fact, if the cell gap is changed, the minimum of the curve will locate at other temperatures.

108 108 Figure 4-15 Calculted analyzer deviation as a function of temperature. Assume the twist angle does not change. Cell gap = 5 µm. In the experiments, this non-monotonic behavior had been a troubling issue until all the properties of the liquid crystal were figured out as temperature changes. With PVA alignment layer, multiple cells (from the same batch) were filled with 2 mixtures of chiral nematic materials respectively: E44 with 0.217% S811 (intrinsic pitch 46.1 µm -1 at room temperature), E44 with 0.395% CB15 S811 (intrinsic pitch 29.8 µm -1 at room temperature). As before, the rule is to keep the natural twist less than 90ºin the 5 µm cells. Directly measured analyzer deviation is plotted in Figure Then calculations were done to find out the twist angle as in Figure Finally the anchoring strength was obtained, shown in Figure All the measurements suggested that at lower temperatures, the measured analyzer deviation was small; therefore the anchoring strength results have larger standard deviation. At higher temperatures, the measured angles were relatively larger; therefore the results were much closer to each other. In addition, according to the data

109 Total twist 2φs (deg) Total twist 2φs (deg) Analyzer deviation α (deg) Analyzer angle α (deg) 109 series, the anchoring strength does not change a lot before 50ºC, while drop down linearly after that S811 data 1 S811 data Temperature (ºC) CB15 data Temperature (ºC) Figure 4-16 Analyzer deviation angle as a function of temperature. PVA alignment. Left: data measured from E44/S811 mixture. Right: data measured from E44/CB15 mixture S811 data Temperature (ºC) CB15 data Temperature (ºC) Figure 4-17 Total twist through the chiral nematic cell as a function of temperature. PVA alignment. Left: data calculated from E44/S811 mixture. Right: data calculated from E44/CB15 mixture.

110 Azimuthal anchoring strength ) S811 data J/m S811 data 2 CB15 data Temperature (ºC) Figure 4-18 Azimuthal anchoring strength of PVA as a function of temperature. PVA alignment. Solid line is guided by eye. 4.4 Additional Measurements with PI 2555 More experiments were carried out to catch the general trend of the anchoring strength s temperature dependence. As a standard strong planar alignment material, PI 2555 was made into alignment layers of ECB cells, and its anchoring strengths measurements at room temperature were presented in chapter Polar anchoring strength The transmissions vs. voltage curves were recorded at various temperatures, and fitting plots can be found in the appendix as well. The anchoring strengths as the fitting parameters were plotted in Figure Comparing the anchoring strengths with those of PVA, they do not have exactly the same trend. However for both alignment layers, the fitted parameters all decrease with temperature. From room temperature to close to,

111 Transmission (%) 111 of PI alignment layer dropped about 7 times. of PVA alignment layer dropped about 5 times. This is also comparable Temperature (ºC) Figure 4-19 Transmission as a function of voltage (up to 60 V) plots for an ECB cell with PI 2555 alignment layer and filled with E44. Cell gap = 5 µm.

112 112 Wp J/m Wp1 Wp Wp J/m Temperature (ºC) Figure 4-20 Polar anchoring strength of PI 2555 as a function of temperature Azimuthal anchoring strength In the azimuthal anchoring strength measurements, PI 2555 cells filled with E44 with 0.217% S811 and E44 with 0.395% CB15 respectively. As in the PVA case, we plot the directly measured analyzer deviation angle, calculated total twist angle, and the final azimuthal anchoring strength respectively in Figure 4-21, Figure 4-22, and Figure Note that Figure 4-23 suggests that before 45ºC, the anchoring strength change is not obvious while after that, it decreases more drastically.

113 Total twist 2φs (deg) Total twist 2φs (deg) Analyzer deviation (deg) Analyzer deviation (deg) E %S Temperature (ºC) 1.40 E %CB Temperature (ºC) Figure 4-21 Analyzer deviation angle as a function of temperature. PI 2555 alignment. Left: data measured from E44/S811 mixture. Right: data measured from E44/CB15 mixture E %S Temperature (ºC) 2.5 E %CB Temperature (ºC) Figure 4-22 Analyzer deviation angle as a function of temperature. PI 2555 alignment. Left: data measured from E44/S811 mixture. Right: data measured from E44/CB15 mixture.

114 Azimuthal anchoring strength J/m E %S811 E %CB Temperature (ºC) Figure 4-23 Azimuthal anchoring strength of PI 2555 as a function of temperature. PVA alignment. Solid line is a guide to the eye. 4.5 Characterization of PDLC PVA provide planar alignment to the liquid crystal, therefore the liquid crystal droplets form bipolar structures When there is no external field, the symmetry axis is pointing to random directions. When there is external field, the liquid crystals will tend to align to the field, so is the symmetry axis.

115 115 Figure 4-24 Schematic drawing of a bipolar droplet. Black segments indicate liquid crystal director Sample Preparation The goal is to make PDLC with PVA and E44 at weight ratio of 50%:50%. 15% PVA aqueous solution and E44 are made into emulsions{yang, 2006 #6} by stirring the mixture at a speed of 3200 rpm. This will make sure the formed droplet size around a few microns and very stable. Use a clean razor blade to coat the emulsion on ITO substrate and let the coating dry for 1 hour in a ventilating hood. Then laminate a top substrate while the coated bottom substrate is heated to about 50ºC. The warmth helps to achieve more uniform lamination. 10 µm spacers are dispensed in the mixture and therefore will be in the coating too. Final cell gap is usually within µm. Microscopic pictures of heating up such a PDLC cell are shown in Figure 4-25.

116 116 Figure 4-25 PVA-PDLC sample under microscope with crossed polarizers, same exposure time for each picture. 100µm scale bars are marked; temperatures when the picture is taken are also marked Switching Voltage and Response Time Measurements The characterization of PDLC usually involves the switching voltage, response time, contrast ratio, etc. Here, for the PVA/E44 PDLC sample, switching voltage and response times are measured. Experimental setup is as depicted in Figure Sample cell is secured on heating stage, which is controlled by computer with precision of 0.1ºC. Laser beam shine through the cell, and get scattered by the cell. The scattered light is collected within 4º, collimated to the detector by 2 proper lenses.

117 117 Figure 4-26 Optical setup for PDLC characterizing The transmittance vs. voltage curve can provide switching voltage. For a single curve in Figure 4-27, there is minimum light transmittance, defined as opaque state, and also maximum light transmittance (saturated part), defined as transparent state. If scale from opaque to transparent as 0-100%, then at 10% of that, the corresponding voltage is denoted as. Similarly, at 90% of the scale, the corresponding voltage is denoted as. The switching voltage is defined as the same as. There is also a saturation voltage, which is defined as. The T-V curves for each temperature are plotted in Figure In Figure 4-27, at 0 V, the transmittance for different temperature is different is because of the birefringence of the liquid crystal is reducing with temperature (i.e. and are approaching closer to each other), therefore the refractive index-mismatch between PVA and the droplet becomes less thus scattering effect is decreased. From the curves, and can be obtained, and they are shown in Figure Not only the switching voltage, but also the measured contrast ratio (Figure 4-29) is decreasing as the sample is heated up.

118 V-10 (V) V-90 (V) 118 Figure 4-27 Transmission over voltage curves for PVA-PDLC sample. Cell gap=10µm. Legends show temperature V V temperature (ºC) Figure 4-28 PDLC switching voltages as a function of temperature.

119 Contrast ratio temperature (ºC) Figure 4-29 PDLC contrast ratio as a function of temperature. For each temperature, the response time is measured by applying pulse like in Figure A preparation stage of 50 ms is applied first (in my case is 0 V), then the saturation voltage is applied for 500 ms and removed immediately after that. The saturation voltage is determined by the T-V curves. Turn-on time is defined by the rising time from 10% to 90% of the opaquetransparent scale, while the turn-off time is defined by the falling time from 90% to 10%. The details of a few typical measurements can be found in the appendix. The summary of the turn-on time and turn-off time are plotted in Figure 4-31.

120 On-time (ms) Off-time (ms) 120 Figure 4-30 Voltage driving scheme for response time measurement on (10-90) off (90-10) Figure 4-31 Response time at different temperatures summary. Red data points represent on time, corresponding to left axis; green data points represent off-time, corresponding to right axis. 4.6 Conclusions and Discussions Two common planar alignment materials were measured: PVA and PI Both polar and azimuthal anchoring strengths of the two materials showed decrease with

121 121 temperature. The polar and azimuthal anchoring suggest different trend of decrease for each alignment. For the polar anchoring strength, of the two materials do not have exact the same trend. The PVA alignment layer showed faster drop in the range of 65-90ºC, while the PI alignment layer drop faster in about 40-75ºC. Over the whole range of our measurement from room temperature to the clear point, of PVA becomes 5 times lower and of PI becomes 7 times lower. For the azimuthal anchoring, both alignment materials indicated that before a certain temperature, the anchoring strength does not change a lot while after this temperature it drops down fast and linearly. All the measurements were taken with E44 as the major liquid crystal, and for both PVA and PI 2555, this special temperature is similar, thus this special temperature can be related to E44 itself. The characterization of PDLC demonstrated decreasing driving voltage, contrast ratio as well as response time. Driving voltage for PDLC depend on the cell thickness, the droplet size and shape, elastic constant of the liquid crystal, and dielectric anisotropy of the liquid crystal. In our case, the change of the cell thickness, droplet size and droplet shape can be neglected when varying temperature. Therefore the driving voltage for strong anchoring strength is dependent on: ( ) ( ) (4.3) where is the order parameter. However both measured and show quicker decreasing behavior than equation (4.3). This is due to the weaker anchoring strength

122 122 when the temperature gets higher. When the anchoring strength is medium, or the extrapolation length ( ) is comparable to droplet size, the surface energy plays a role. The switching voltage will be dependent on: ( ) (4.4) where and are constants. From the anchoring strengths measurements, drops down faster than or. Therefore it will result in faster drop than the strong anchoring case. Contrast ratio is related to scattering efficiency. The refractive index of polymer network is close to the ordinary refractive index of liquid crystal. ( ) ( ) (4.5) Comparing the parabola fitting of for E44 and the trend line in measured contrast ratio result (Figure 4-29), the equation (4.5) relation can be confirmed. The response time for a fixed sample is related to the viscosity and elastic constant. ( ) (4.6) Thus the response time will decrease very fast due to the exponential decay with temperature, which is just what we see from our measurements.

123 Appendix Measurements Details In the twisting analyzer measurements, two chiral nematic materials were made: E44 with S811 and E44 with CB 15. Figure 4-32 Microscopic pictures of disclination rings with S811/E44 mixture at different temperatures.figure 4-32 and Figure 4-34 show the mixtures in between lens and flat glass. Disclination lines formed by quantized pitch were observed clearly. For each temperature, counting rings and measuring the diameter of each ring, the resulted data can be fitted by a straight line, of which the slope is used to calculate the according to equation (4.1) and (4.2).

124 124 25ºC 35ºC 45ºC 55ºC 65ºC 75ºC 85ºC 90ºC Figure 4-32 Microscopic pictures of disclination rings with S811/E44 mixture at different temperatures.

125 r n^ y = x y = 11.49x y = x y = x y = x n Linear (25) Linear (45) Linear (55) Linear (75) Linear (85) Figure 4-33 Mixture S811/E44, squared radius vs. numbering of the rings. Data points: experiment data. Solid black line: linear fit of the series. Figure 4-34 Microscopic pictures of disclination rings with CB15/E44 mixture at different temperatures.

126 r n^ y = 60.22x y = x y = 52.03x y = 50.75x y = x n 25C 45C 65C 85C 95C Linear (25C) Linear (45C) Linear (65C) Linear (85C) Linear (95C) Figure 4-35 Mixture CB15/E44, squared radius vs. numbering of the rings. Data points: experiment data. Solid black line: linear fit of the series Polar Anchoring Strength Fittings The following figures give the exact fitting result for temperature dependence of the polar anchoring strengths of PVA and PI 2555, respectively.

127 127 30ºC 35ºC 45ºC 50ºC 30ºC 65ºC 75ºC 50ºC 85ºC 96ºC 99ºC 100ºC 101ºC Figure 4-36 Fitting PVA experimental data of polar anchoring strengths at different temperatures.

128 128 25ºC 35ºC 45ºC 55ºC 65ºC 70ºC 75ºC 80ºC 85ºC 90ºC 95ºC Figure 4-37 Fitting PVA experimental data of polar anchoring strengths at different temperatures.

129 PDLC Response Time Measurements Sample Details Here are some examples of response time measurements for PVA alignment layer. (a) (b) (c) Figure 4-38 Response time measurements examples: (a). Left: turn-on time; right: turn-off time. (b). Left: turn-on time; right: turn-off time. (c). Left: turn-on time; right: turn-off time.

130 130 Reference 1. S. K. H. Yokoyama, and H. Kamei, J. Appl. Phys. 61, 4501 (1987). 2. H. Yokoyama, Handbook of Liquid Crystal Research. (Oxford University, Oxford, 1997). 3. D.-K. Yang and S.-T. Wu, Fundamentals of liquid crystal devices. (John Wiley & Sons, Ltd., 2006). 4. P. G. d. G. a. J. Prost, The Physics of Liquid Crystals 2nd ed ed. (Oxford University, Oxford, 1993). 5. A. Jakli and A. Saupe, One- and Two-Dimensional Fluids: Properties of Smectic, Lamellar and Columnar Liquid Crystals. (Taylor & Francis, 2010). 6. W. Maier and A. Saupe, Z. Naturforsch. 15a (1960). 7. W. Maier and A. Saupe, Z. Naturforsch. 14a (1959). 8. W. Maier and A. Saupe, Z. Naturforsch. 13a (1959). 9. T. Nakagiri, H. Kodama and K. K. Kobayashi, Physical Review Letters 27 (9), (1971). 10. A. Saupe, Angewandte Chemie International Edition in English 7 (2), (1968). 11. I. G. Christyakov, Soviet Physics Uspekhi 9 (1967). 12. V. Fréedericksz and A. Repiewa, Z. Physik 42 (7), (1927). 13. V. Freedericksz and V. Zolina, Transactions of the Faraday Society 29 (140), (1933).

131 S.-T. Wu, A. M. Lackner and U. Efron, Applied Optics 26 (16), (1987). 15. J. S. Gwag, I.-Y. Han, C.-J. Yu, H. C. Choi and J.-H. Kim, Optics Express 17 (7), (2009). 16. M.-K. Seo, M. Han and J.-R. Lee, Optical Materials 21 (1-3), 5p (2003). 17. J. L. Fergason, SID Int. Symp. Dig., (1985). 18. G. P. Crawford and S. Žumer, Liquid Crystals in Complex Geometries: Formed by Polymer and Porous Networks. (Taylor & Francis, 1996).

132 132 CHAPTER 5 Adjusting Anchoring Strength of Polymers and the Performance of PDLC It is interesting to study how the combination of planar/homeotropic anchoring affects the PDLC s performance. Poly methyl methacrylate (PMMA) provides planar alignment 1, while poly isobutyl methacrylate (PiBMA) provides homeotropic alignment 2. The two of them have similar structures except for the side chain, so the study began with mixing PMMA and PiBMA at different ratios. For each mixture, the anchoring strength and the resulted PDLC performance were compared. Our new target is to characterize what will be the result for the mixed polymers. However, when making the PDLC with temperature induced phase separation method, it was noticed that the PDLC form two regions 3-5 _ENREF_3: PMMA-rich region and PiBMA-rich region. To solve this issue, instead of mixing two polymers, we found copolymers which have two alternating units I one polymer chain. The anchoring strengths of each polymer/copolymer were measured and the corresponding PDLC performance was studied. The results suggest that with proper design of polymer side chain, the driving voltage can be largely decreased without hurting the contrast ratio or the response time. 5.1 Introduction of methacrylate polymers The family of polymers acrylates called by chemists (or acrylics called by the rest of the world) is involved in the current chapter. For example, PMMA is a clear plastic and is widely used as glass substitute in everyday life, because it has moderate properties and is economical, without harmful bisphenol-a. It is also helping a lot of scientific

133 133 researches, such as PMMA microspheres as colloidal particles 6. Another example, PiBMA is not as commonly acknowledged as PMMA in everyday life, but can have more astonishing applications, like in preventing uranium pollution 7. PiBMA beads carrying functional groups has been designed and manufactured to absorb heavy metal ions. As thermoplastic 8, PMMA and it families are also widely used in scientific investigations, because they can be thermally recycled repeatedly. A list polymers and copolymers which all contain methacrylate main chain were employed in the current chapter. The chemical structures, the glass transition temperature and abbreviations are shown in Table 5-1. Large PDLC droplets were intentionally made in order to identify the alignment effect. From the microscopic pictures (with crossed polarizers) in the table, we can clearly identify PMMA, PBMA and their copolymer all provide planar alignment, PiBMA gives homeotropic alignment, while the copolymer PBMA-co-iBMA droplets do not as neatly show distinctive bipolar or radial or axial configuration.

134 134 Table 5-1 Methacrylate polymers and copolymers (scale bar = 50 µm) Name Chemical structure PDLC droplet under crossed polarizers (cooled at 1ºC /min) (ºC) Abbrev iation Sym bol in figur es Poly(methyl methacrylate) 114 PMMA M Poly(methyl methacrylateco-butyl methacrylate), m/n=85/ PMMA -co- BMA (85/15) MB Poly(butyl methacrylateco- methyl methacrylate), m/n unknown 52 PBMA- co- MMA BM Poly(butyl methacrylate) 15 PBMA B Poly(butyl methacrylateco-isobutyl methacrylate) 35 PBMAcoiBMA BI Poly(isobutyl methacrylate) 55 PiBMA I

135 Anchoring strength of PMMA and PiBMA mixtures Preparation In the first stage, PMMA and PiBMA were mixed at different ratios. Thus there were 5 samples of alignment layers as listed in Table 5-2, in which pretilt angles are also shown. The pretilt angles were determined by measuring the capacitance of a corresponding ECB cell with known thickness and electrode area. To get uniform, smooth alignment layer out of the acrylates, they were dissolved in toluene (rather than chloroform since the latter evaporates too fast to get uniform coating) and then spincoated on to the substrates. Then the assembled ECB cells were filled with E44 which has dielectric constants, at room temperature under 1 khz AC field. The capacitance of the cell is given by, where is ITO area, and is the cell gap. The effective dielectric constant is given by. 100 mv AC field was applied in the measurements. Interestingly, the PiBMA alignment layer in the ECB cell provided a large tilt homogeneous alignment, instead of homeotropic alignment. Table 5-2 Pretilt angle generated by PMMA and PiBMA mixture Polymer composition Pretilt angle (ºC ) 100%PMMA 2 75%PMMA, 25%PiBMA 3 50%PMMA, 50%PiBMA 10 25%PMMA,75%PiBMA %PIBMA 40

136 Transmittance (% ) Polar anchoring strength measurements Transmission vs. voltage curves were obtained as in Figure 5-1 and the fitting results of polar anchoring strength are diagrammed in Figure %PMMA 25%PIBMA75%PMMA 50%PIBMA50%PMMA fitting fitting fitting 75%PIBMA25%PMMA fitting 1 100%PIBMA fitting Voltage (V) Figure 5-1 Transmittance-voltage curves of the ECB cells with PMMA/PiBMA mixed alignment layers. Open circles: experimentally measured; solid lines: fitting result.

137 Quadratic term coefficient ( 10-4J/m2) Quartic term coefficient ( 10-4J/m2) Wp1 Wp PiBMA concentration Figure 5-2 The polar anchoring strengths with varied PMMA and PiBMA combination. : quadratic term; : quartic term Azimuthal anchoring strength The reference cells were filled with pure E44 to determine actual easy axis, and the sample cells were filled with chiral nematic made from S811 (0.22% by weight) and E44. The total twist angles through 5 µm cells were measured and calculated as in Figure 5-3. The azimuthal anchoring strengths are plotted in Figure 5-4.

138 Azimuthal anchoring strength( 10-6 J/m2 ) Twist angle (º) PiBMA concentration Figure 5-3 Total twist through the ECB cells with varied PMMA and PiBMA combination PiBMA concentration Figure 5-4 Azimuthal anchoring strength with varied PMMA and PiBMA combination.

139 Transmittance (%) PDLC made from PMMA and PIBMA We used PMMA and PIBMA composites to make PDLC with thermally induced phase separation (TIPS) method 9, 10. Five polymer mixtures are constructed as before. Then mix each of them with liquid crystal E44 at a ratio of 50%:50%. In TIPS process, first, the polymer (or mixed polymer) and the liquid crystal were dissolved in solvent, chloroform. Then the mixture was dropped on a glass substrate and dried. 20 µm spacers were added as well in order to keep the sample thickness uniform. After the solvent was evaporated, a top substrate can be laminated. To help the lamination, temperature was raised to about 100ºC keep the mixture in isotropic state. Pressure was also applied during lamination. Each PDLC sample was applied voltage and the transmission was recorded as in Figure %PMMA 25%PiBMA75%PMMA 50%PiBMA50%PMMA 75%PiBMA25%PMMA 100%PiBMA Voltage (V) Figure 5-5 Experimentally measured transmittance as a function of the applied voltage on the PDLCs made from different polymer composites.

140 140 However, microscopic pictures (Figure 5-6) indicate that when both PMMA and PiBMA exist in the sample, they phase-separate into two regions. One region switches similarly as PiBMA, and the other region as PMMA. The transmission curves were measured at specifically chosen areas which covered both regions. This phenomenon is not favored for characterizing electro-optical properties of the PDLCs or getting uniformly good performing PDLCs. Also, PMMA and PiBMA have very different glass transition temperature ( ), which makes it hard to get the same droplet size in two regions. Therefore further pursuing of copolymers was carried out. Figure 5-6 PDLC under AC field. Polymer in (a): PMMA; (b): PiBMA; (c): 50%PMMA+50%PiBMA. Scale bar = 100 µm. Cell gaps all 20 µm.

141 Anchoring strength of methacrylate polymers/copolymers Preparation Not all polymers or copolymers are good alignment materials, yet sometimes we can find ways out of it. The methacrylate copolymers were not able to align nematic liquid crystals very well when they were spin-coated, pre-baked and assembled directly. The solution was baking them in the oven for a couple of hours. The idea is to soften the polymers and let them smooth out on the substrates. All ECB cells were assembled with 5µm spacers. Pretilt angles were measured, with known ITO area and cell gap. It was noticed that the pretilt angle of PiBMA in this case was smaller than it was before. This was due to the baking process. Table 5-3 Pretilt angle generated by methacrylate polymers and copolymers Polar anchoring strength Polymer/copolymer name Pretilt angle (º) PMMA 1 PMMA-co-BMA (85/15) 4 PBMA-co-MMA 8 PBMA 10 PBMA-co-iBMA 11 PiBMA 16 Transmission vs. voltage curves were obtained as in Figure 5-7 and the fitting results of polar anchoring strength are diagrammed in Table 5-4.

142 Transmittance (%) I fit BI fit B fit BM fit MB fit M fit Voltage (V) Figure 5-7 Transmittance-voltage curves of the ECB cells with methacrylate polymer alignment layers. Open circles: experimentally measured; solid lines: fitting result. Table 5-4 Polar anchoring strengths of methacrylate polymers Polymer/copolymer ( 10-4 J/m 2 ) ( 10-4 J/m 2 ) PMMA PMMA-co-BMA (85/15) PBMA-co-MMA PBMA PBMA-co-iBMA PiBMA

143 total twist angle (degree) Azimuthal anchoring strength The reference cells were filled with pure E44 to determine actual easy axis, and the sample cells were filled with chiral nematic made from S811 (0.22% and 0.42% by weight,) and E44. The total twist angles through 5 µm cells were measured and calculated as in Figure 5-8. The azimuthal anchoring strengths are plotted in Table percentage of chiral dopant (%) M MB(85/15) BM(unknown) B BI(50/50) I Figure 5-8 Total twist as a function of chiral dopant percentage with varied methacrylate polymer alignment layer. Table 5-5 Azimuthal anchoring strengths of methacrylate polymers Polymer/copolymer ( 10-6 J/m 2 ) PMMA 9.3 PMMA-co-BMA (85/15) 13.0 PBMA-co-MMA 0.2 PBMA 1.5 PBMA-co-iBMA 2.1 PiBMA 0.1

144 PDLC made from methacrylate polymers/copolymers Preparation Each polymer was mixed with E44 to make PDLCs. The ratio of polymer was fixed at 50%. TIPS method was applied for all. The phase separation result depends on the cooling rate. As shown in Table 5-1, the large droplets were obtained by cooling down at 1ºC/min. However in real PDLC light shutters, small droplets (~1µm) were desired in order to get enough scattering. In Figure 5-9, the samples were removed from the heating stage directly to room temperature. Due to different glass transition temperature, the formed droplets have different average sizes between samples. On the other hand, the electro-optical characterization will depend on the droplet size_enref_ For example, with strong anchoring condition, threshold field is ; with weak anchoring condition,. For the current study, we want to compare the different polymer s anchoring effect to the electro-optical properties. It is necessary to eliminate the droplet size effect. Varying the cooling rate, droplets sizes can be adjusted to be similar even resulting from various polymers. In some cases, the cooling rate was increased by moving the sample directly from heating stage to a freeze bag. After many trials, PDLC samples with similar droplet sizes were realized as in Figure 5-10.

145 145 Figure 5-9 PDLC samples made by methacrylate polymers and copolymers with arbitrary droplet size. Scale bar = 100 µm. Figure 5-10 PDLC samples made from methacrylate polymers and copolymers with controlled droplet size. Scale bar = 100 µm.

146 Transmittance (%) Electro-optics characterization The characterization involves typically threshold voltage and response time. Each PDLC light shutter was applied voltage and its transmittance within a collection angle of 4ºwas recorded. From Figure 5-11, threshold voltage and can be figured out, as listed in Table 5-6. The opaque states of all samples were similar, thanks to the droplet size control Vrms (V) M MB BM B BI I Figure 5-11 Experimentally measured transmittance as a function of the applied voltage on the PDLCs made from methacrylate polymers. Response time was measured in the same way as described in chapter 4. For each sample, the pulse amplitude was chosen to be. The results were also listed in Table 5-6.

147 147 Table 5-6 Threshold voltage and response time of PDLC made from methacrylate polymers Polymer/copolymer (V) (V) (ms) (ms) PMMA PMMA-co-BMA (85/15) PBMA-co-MMA PBMA PBMA-co-iBMA PiBMA Discussion Methacrylate polymers and copolymers can provide quite distinctive alignments to liquid crystals, not just the difference of tilt angle, but also the difference of strength. Studying the effects thoroughly will help us select the desired polymer for applications or guide the synthesizing of new polymers. First the study was based on mixing PMMA and PiBMA. The polymers were dissolved well in solvent, and the solution was spin-coated. Such surface alignment was uniform and the anchoring strength was obtained. The polar anchoring strength showed monotonic change with the concentration variation, indicating the more branchy side chains can cause the out-of-plane alignment weaker. On the other hand, the azimuthal anchoring strengths showed almost the reverse effect regarding the amplitude of the strength vs. concentration of PiBMA. In this case the branchy side chains secured the in-plane alignment better. The next step was intended to characterize the corresponding PDLC performance. Comparing the PMMA PDLC with the PiBMA PDLC, it suggested

148 148 that the switching voltage depend on polar anchoring strength more significantly than azimuthal. However during the thermotropic phase separation process, not just the liquid crystal phase separate into droplets, but also the polymers phase separate into two regions, which are large enough to affect the electro-optical performance. Then the whole group of methacrylate polymers and copolymers were tested. They are all able to form uniform PDLC films. After careful adjustment, the average droplet size was similar from sample to sample. Anchoring strengths and PDLC performance were measured and analyzed. The results include surface polar anchoring strength, surface azimuthal anchoring strength, PDLC threshold voltage, turn-on time, and turn-off time. From the pretilt angle measurements, it can be seen that PMMA homogeneously aligned the nematic; PBMA also homogeneously aligned, but with higher pretilt; PiBMA produced the largest tilt angle. The copolymers showed intermediate pretilt angle in between its two separate pure components. The baking process made the alignment film produced lower pretilt than no baking. The anchoring strengths are then measured. The polar and azimuthal anchoring strengths have the same trend. The azimuthal anchoring strength of PMMA film and PiBMA film had different result comparing with the no baking film, indicating the baking process changes the property of the aligning film. PBMA has lower anchoring strength than PMMA. It may be due to PBMA s very low. The anchoring strengths of the copolymers are intermediate. The only exception is the PMMA-co-BMA (85/15), which showed surprisingly higher polar and azimuthal anchoring strength than everyone else.

149 149 In the PDLC characterization, the switching voltage and turn-off time very much depend on the anchoring strength. For higher anchoring strength, it is harder to align the liquid crystals along the electric field. When switching off the voltage, the stronger anchoring polymer means a stronger restoring force and hence pulls back the liquid crystal faster. The turn-on time was not clearly related to the anchoring strength based on our result. Reference 1. B.-G. Wu, J. H. Erdmann and J. W. Doane, Liquid Crystals 5 (5), (1989). 2. E. A. Thomas, T. A. Zupp, J. E. Fulghum, D. S. Fredley and J. L. West, Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals 250 (1), (1994). 3. R. A. Jones, Soft condensed matter. (Oxford University Press, 2002). 4. M. Kleman and O. D. Laverntovich, Soft Matter Physics: An Introduction. (Springer, 2003). 5. J. V. Selinger, Soft Matter Lecture Notes (2008). 6. P. Pusey and W. Van Megen, Nature 320 (6060), (1986). 7. T. Caykara, S. S. Alaslan, M. Guru, H. Bodugoz and O. Guvan, Radiat. Phys. Chem. 76 (2007). 8. S. A. Baeurle, A. Hotta and A. A. Gusev, Polymer 47 (17), (2006).

150 G. P. Crawford and S. Žumer, Liquid Crystals in Complex Geometries: Formed by Polymer and Porous Networks. (Taylor & Francis, 1996). 10. P. S. Drzaic, Liquid Crystal Dispersions. (World Scientific Publishing Company Incorporated, 1995). 11. D.-K. Yang and S.-T. Wu, Fundamentals of liquid crystal devices. (John Wiley & Sons, Ltd., 2006). 12. Z. Zheng, F. Guo, Y. Liu and L. Xuan, Appl. Phys. B 91 (1), (2008). 13. M. Mucha, Progress in Polymer Science 28 (5), (2003).

151 151 Appendix: Chemicals information Name Vendor CAS Number Average Mol Wt (ºC) Poly(methyl methacrylate) Sigma Aldrich Poly(methyl methacrylate-co-butyl methacrylate), m/n=85/15 Sigma Aldrich Poly(butyl methacrylate-comethyl methacrylate), m/n unknown Sigma Aldrich Poly(butyl methacrylate) Sigma Aldrich Poly(butyl methacrylate-coisobutyl methacrylate) Sigma Aldrich Poly(isobutyl methacrylate) Sigma Aldrich

152 152 CHAPTER 6 Alignment Behavior of Dual Direction Rubbed Alignment Layer In this chapter, polymer alignment films were rubbed in two orthogonal directions successively. Such process results in an easy axis along neither the first nor the second rubbing direction, but in between them. If the second rubbing gets stronger, the final easy axis is discovered to be closer to the second rubbing direction. The experimental results were analyzed and the physical reason behind this phenomenon was explained by introducing a higher order term in azimuthal anchoring strength. Different alignment materials were applied to confirm the theory. 6.1 Introduction Rubbed polymer surface show anisotropic characters, which aligns the anisotropic liquid crystals in a preferred direction. For the study in this chapter, all rubbed polymer layers provide planar alignment. Therefore the main issue of the easy axis here would be its in-plane direction. In the substrate plane, the mechanical rubbing process usually aligns the polymer main chain. However the easy axis can be parallel to the rubbing direction or perpendicular to it depending on how the side-chain aligns with respect to main-chain. In some cases it even can be controlled by proper pre-treatment or selection of liquid crystals. In our research, the polymers in use were all able to align liquid crystals in the rubbing direction. In the previous chapters, the anchoring strengths were studied based on ECB cells in which the two substrates were rubbed in an anti-parallel fashion. Note that the anti-

153 153 parallel is defined according to the assembled cell, so are the other rubbing notations. This feature assures the initial nematic liquid crystals align homogeneously throughout the cell, with the opposite pretilt at two surfaces. In many popular display modes, rubbed polymer alignment layers are assembled in intellectual ways to generate the desired initial liquid crystal configurations. For example, in twisted nematic (TN) mode, the liquid crystals at the two surfaces align 90ºto each other, and the rest in the bulk just twist along from the bottom to the top. Two polarizers are added at the outside of two substrates as light transmission valves. Cell gap is controlled to satisfy Gooch-Tarry first minimum condition so that the light can closely follow the director and exit linearly. Another example is the optically compensated bend (OCB) mode, which is often introduced by comparing to the ECB mode. In the OCB mode (or pi-cell), the two surfaces generate the same pretilt by being rubbed in a parallel way. The significant benefit of such configuration is the fast switching speed. The above examples are based on one easy axis direction on each substrate. Previous works have been done to create two degenerate easy axes. Durand et al 1-4 created unique alignment by obliquely SiO evaporation. The controlled thickness provides twofold degenerate orientation for MBBA (a nematic liquid crystal). Bistable orientation can be achieved by applying an external horizontal electric field above the threshold. In the current chapter, dual rubbing directions were generated on each single substrate. The easy axis was measured optically and was confirmed to be in between the two rubbing directions. Further study showed the easy axis depends on the strength of the

154 154 two rubbing direction. To explain such phenomena, a higher order term in the azimuthal anchoring strength was introduced. Experiments were carried out as before, but more work was done on relating the anchoring strengths of the two directions. 6.2 Preparation of dual-rubbed cell The cells were very similar to an ECB cell, except for the more complex rubbing process. Figure 6-1 illustrates the rubbing directions and the assembly method in three cases in comparison. Note that although in the picture only one cell is shown, in reality the substrates were assembled with 20 identical cells in one panel and then single cells were separated from the panel. Case 1(single direction anti parallel rubbing) was used as a reference. Case 2 (two direction rubbing, one set parallel, one set anti-parallel) was at first an experimental error during the rubbing process, but it turns out to provide valuable information in determining the easy axis. Case 3 (two direction rubbing, both sets antiparallel) was the main object of the study. Again, note that the parallel and antiparallel are defined according to the assembled cell. A rubbing strength parameter has been proposed, involving the number of times a location has been rubbed, the dimension of the roller/ block, and the relative speed of the movement, the effective contact density, the contact area, and the yield stress of the alignment material. If the only variable is the rubbing time, then approximately the rubbing strength is proportional to it. (6.1) In our experiments, the same velvet-covered block was used and it was pushed horizontally, so that the pressure was fixed and we keep track of rubbing times. In this

155 155 way, we were able to compare the rubbing strength of the two directions. For the regular ECB cells, the rubbing time was fixed at 10. For the other two cases, the first rubbing time was fixed to be times, while the second rubbing time was varied.

156 156 flip (a) flip (b) flip (c) Figure 6-1 Schematic drawing of rubbing directions and cell assembly. (a) Case 1: regular ECB cell with one direction rubbing on each substrate. (b) Case 2: rubbed in two directions on each substrate. First direction with parallel rubbing, second direction with anti-parallel rubbing. (c) Case 3: rubbed in two directions on each substrate. Both with anti-parallel rubbing.

157 Observance under the microscope Cells were picked from each batch, filled with pure E44 and observed under polarized microscope. Each cell was rotated under the microscope while the polarizers were at fixed angles. When the 2 nd rubbing direction (R2) was 45ºto the polarizers, a bright view (not necessarily brightest) can be observed. All the cells produced good alignment for nematic liquid crystal. The dual rubbing cases were shown in Figure 6-2. (a) (b) (c) (d) (e) Figure 6-2 Dual direction rubbed cells show uniform alignment.. All filled with E44. All cell gap controlled by 5 µm spacers. Polarizers make 45ºto the 2 nd rubbing direction. (a) PI 2555 alignment layer, case 2. (b) PI 2555 alignment layer, case 3. (c) PVA alignment layer, case 2. (d) PVA alignment layer, case 3. (e) PMMA alignment layer, case 3.

158 158 Former experience of single rubbing told that when the polarizer is parallel to the rubbing direction the analyzer is perpendicular to it, we see minimum transmission. So this was the initial conjecture of the dual rubbed cells as well. The case 2 cells did show lowest transmission, but not very dark comparing to the other two. Figure 6-3 shows the darkness difference among three cases when the rubbing direction and polarizers were the same. (a) (b) (c) (d) (e) (f) Figure 6-3 Darkness comparison.. (a) PI alignment, case 1. (b) PI alignment, case 2. (c) PI alignment, case 3. (d) PVA alignment, case 1. (e) PVA alignment, case 2. (f) PVA alignment, case 3. In case 3 cells, no matter which alignment material was used, there was always an even darker state when the cell was rotated away by a small angle. Extra-long exposure time was used to show this feature in Figure 6-4.

159 159 These observations strongly suggested that the second rubbing did not fully erase the first one. Further measurements were necessary to determine the actual easy axis. (a) (b) (c) (d) (e) (f) Figure 6-4 Long exposure time photos when rotating the case 3 cells between fixed polarizers Measure the easy axis direction Previously in chapter 2, the method of finding easy axis was introduced. A homogeneous cell (filled with pre nematic) is set in between two crossed polarizers and the two polarizers are rotated simultaneously. The minimum transmission was proved to

160 160 happen when the polarizer bisect the azimuthal angles from the two substrates. In this step the measured angle is in Figure 6-5. Then fix the polarizer at this angle, and rotate the analyzer only. This time the minimum happens when the analyzer is perpendicular to the exit plane director, i.e. the exact easy axis of the exiting substrate. In this step the measured angle is and in Figure 6-5. Note that and are angles with respect to. In the frame (set on the cell), they are noted as and respectively. Figure 6-5 Schematic drawing of experimentally measured angles in TAM method.

161 161 For the single rubbing case 1, as a reference, the measurement was plotted in Figure 6-6. Figure 6-6(b) showed that during the first step, rotating polarizer (P) and analyzer (A) together, the minimum was very close to the direction ( ). During the second step, fixing P and rotating A only, the deviation was still small (within ) and can be considered as experimental imperfection. (a) β β (b) Figure 6-6 Measured transmittance as a function of angles for case 1 cells. (a) Rotate P and A simultaneously, horizontal axis. (b) Rotate A only, horizontal axis and. For the dual rubbing case 2, when rotating P and A together, the minimum still happens approximately at in lab frame. However when fixing P and rotating A, the two substrates showed clear deviation in opposite directions. There was a twist

162 162 deformation through the cell. Figure 6-7 (a) and (b) shows the measurements of the two steps respectively. (a) β β (b) Figure 6-7 Measured transmittance as a function of angles for case 2 cells. (a) Rotate P and A simultaneously, horizontal axis. (b) Rotate A only, horizontal axis and. For the dual rubbing case 3, as in Figure 6-8 (a), when rotating P and A together, in both PI and PVA cells, obvious deviation was detected. In PI cell it was and in PVA it was. The when P was fixed and A was rotated, the detected minimum in Figure 6-8 (b) stays in the almost the same direction at both sides of the substrates. The liquid crystal did not have twist deformation through the cell, but just all rotated to a new direction.

163 163 (a) β β (b) Figure 6-8 Measured transmittance as a function of angles for case 3 cells. (a) Rotate P and A simultaneously, horizontal axis. (b) Rotate A only, horizontal axis and. Both alignments layers PI and PVA showed similar results. To sum up, director configurations are depicted in Figure 6-9.

164 164 Case 1 Case 2 Case 3 Figure 6-9 Director configuration on the surfaces in the three cases. Left column: substrate 1. Right column: substrate Easy axis deviation with varied rubbing combination The deviation of easy axis was observed to be in between the two rubbing directions. Additional cells were made to study how the deviation would depend on the rubbing combination. As a good alignment material, our friend PVA (1.5%) were coated and rubbed. The first rubbing was fixed at, and the second one was varied as. The results plotted in Figure 6-10 suggested the second rubbing

165 165 was re-writing the alignment more and more with the increase of the applied rubbing times. Figure 6-10 Measured easy axis direction w.r.t. second rubbing direction. Alignment material: PVA. In the case of PI 2555 alignment layer, the easy axis of rubbing and combination, the easy axis is away from the 2 nd rubbing direction. It was also tried out that if there is a limitation of how much the second rubbing can affect the easy axis. It was found that, there is still rewriting mechanism, but when, the resulted cell showed non-uniform alignment as in Figure 6-11.

166 166 Figure 6-11 Non-uniform alignment created by dual rubbing combination 10/ Derivations The results from the last two sections suggested easy axis configurations as sketched in Figure 6-9. The second rubbing did not totally wash away the first rubbing s anchoring effect. In the sense of surface aligning, the case 2 actually have the same effect as the case 3. Case 3 forms a homogeneous cell with deviated easy direction ( ), so the main study will be focused on this configuration. In the Rapini-Papoular expression, the surface azimuthal anchoring strength part at one substrate is characterized by 5-9 (6.2) where the footnote on, means the first substrate. If two aligning forces are created on one substrate, then equation (6.2) becomes ( ) (6.3)

167 167 where the footnote on and means the anchoring strength created by the first and second rubbing direction respectively. In case of two identical surfaces ( ) with consideration of bulk elastic energy, the total energy per unit area is [ ( ) ] ( ) (6.4) With pure nematic,. Minimizing the free energy per unit area, ( ) ( ) (6.5) Equation (6.5) indicates there is only minimum at or, which contradict with the experimental result. Therefore a higher order term is needed in the original expression to explain the non-trivial solution. Rewriting the surface anchoring energy at one substrate, equation (6.3) becomes ( ) ( ) (6.6) where the first footnote or on,,, means anchoring strength along first or second direction rubbing, and the second footnote or distinguishes the second order term or the forth order term coefficient. Therefore the total energy per unit area is

168 168 [ ( ) ( ) (6.7) ] ( ) Now the minimization of free energy per unit area will give: ( ) ( ) ( ) ( ) (6.8) Besides, there also exists non-trivial solution by condition ( ) ( ) (6.9) Since anchoring strength has to be positive, there is solution when ( ) and ( ). To be convenient, we name the quantities as: ( ) and ( ). If we fill the cell with chiral nematic, then the bulk twist term is no longer zero. Suppose the easy axis is already determined along, then minimize the free energy per unit area w.r.t. the twist angle (note that and ). With known variables ( ), we can solve for and.

169 169 [ ( ) ( )] [ ( ) ( ) ( ) ( )] (6.10) ( ) Solving and will verify the deviation of the easy axis. Then with the newly defined easy axis, we define azimuthal anchoring strength as quadratic term coefficient. With known, we can solve for by minimizing equation (6.11). ( ) ( ) (6.11) 6.7 Polar anchoring strength The polar anchoring strength of single direction rubbed PI, PVA have been presented in the former chapters. The dual direction rubbed cells were studied, with PI and PVA (1.5%) respectively, as shown in Figure 6-12 and Figure 6-13.

170 Wp1 (*10-4 J/m 2 ) Wp2 (*10-4 J/m 2 ) Wp1 (*10-4 J/m 2 ) Wp2 (*10-4 J/m 2 ) Wp1 Wp2 10/10 single 10 Rubbing times m/n Figure 6-12 (a) PI 2555 transmittance vs. voltage curves. Legend shows rubbing condition. (b) Fitted anchoring strength Wp1 Wp2 10/6 10/9 10/12 10/15 single Rubbing times m/n Figure 6-13 (a) PVA (1.5%) transmittance vs. voltage curves. Legend shows rubbing condition. (b) Fitted anchoring strength. 6.8 Azimuthal anchoring strength The goal of the following experiments in the azimuthal anchoring study is: make cells with configuration of case 3; fill the cells with pure nematic and chiral nematic respectively; measure the directors on the surface; solve for quantities and ; redefine easy axis and solve for the anchoring strengths w.r.t. the newly defined easy axis.

171 171 In chapter 2 where azimuthal anchoring strength measurements were introduced, it was specially mentioned that the designed chiral nematic material should have a natural pitch satisfying (a) in Figure If. This condition ensures that the twist in the cell would be like, it may have configuration (a) or configuration (b). When is approaching, configuration (b) will win. The cells in {} were filled in with {%} R811. Two regions are clearly observed. (a) (b) Figure 6-14 Right-handed chiral nematic between substrates with preferred aligning direction. (a) Actual pitch. (b) Actual pitch. {cell with two regions} In the calculations of actual twisting angle and the anchoring strength, the above two cases do have subtle difference. Using the same method as in chapter 2, we can derive the actual twisting angle from the experimentally measured analyzer deviation. The result for configuration (b) can be written as: (6.12)

172 172 where is the surface azimuthal angle, w.r.t. the easy axis, and. Just as a reminder, the configuration (a) result is: (6.13) Then, in configuration (a) the actual total twist would be, while in configuration (b) the actual total twist would be. The strong alignment layers bound the surface liquid crystals so much that the chosen twisting configuration always suit for having small azimuthal angles deviated from the easy axis. With the above acknowledgement, we are prepared to understand the same case with the dual-rubbed cells. From the experiment point of view, these two configurations can be identified by the sign of measured. For example, the material is right-handed liquid crystal, and the laser shine through the cell from bottom substrate to top substrate. We define the counter-clockwise direction is positive, and clock-wise direction is negative. Then the liquid crystal director at the top substrate will be positive for the case and negative for the case. With these understanding, we are ready to challenge for obtaining large azimuthal angles. The following steps are a guide of how azimuthal anchoring strength is measured. 1). Set polarizer and analyzer in orthogonal position without sample. 2). Insert a cell (dual-rubbed, both anti-parallel) filled with nematic liquid crystal. Second rubbing direction is parallel to the polarizer.

173 173 3). Rotate polarizer and analyzer simultaneously and find minimum transmittance position. Get easy axis deviation. 4). Fix polarizer along easy axis. Insert a cell filled with chiral nematic. 5). Rotate analyzer and find minimum transmittance position. Get surface angle with respect to the easy axis. 6). Calculate and by two groups of measurements with designed distinctive natural pitch. 7). Re-define anchoring strength w.r.t. to measured easy axis. Calculate the anchoring strength by. 6.9 Experiments with PVA alignment layers Dual-rubbed PVA easy axis deviation has been shown in Figure In order to get the twisting angle as large as possible, we kept R811 concentration as high as 0.827% and 0.914%, which would result in natural pitch of 12.1 µm and 10.9 µm. The cell gap was controlled by 5 µm spacers as before. However it is important to measure each cell gap before filling in liquid crystals, because the calculation can be very sensitive to cell gap. In Figure 6-15, the actual total twist of the corresponding cell is plotted as a function of rubbing combination. In the end the single direction anti-parallel rubbed result was shown as reference.

174 Twist angle (deg) _6 10_9 10_15 single Rubbing times m/n Figure 6-15 Twisting angle of each rubbing combination. Alignment material: PVA 1.5%.

175 Wa1 (*10-5 J/m 2 ) C1, C2 (*10-5 J/m 2 ) 175 (a) 10 C1=W11+2W12-W21 8 C2=W22+W /6 10/9 10/12 10/15 Rubbing times m/n (b) /6 10/9 10/12 10/15 single Rubbing times m/n Figure 6-16 (a) Calculated values for and. (b) Azimuthal anchoring strength as a function of rubbing combination. Single meaning only rubbed in one direction Conclusion and discussion Rubbing the polymer surface in orthogonal directions successively induced an easy axis shift from the rubbing direction. Cells were assembled with two pairs of antiparallel rubbing or one pair parallel one pair anti-parallel. Nematic and chiral nematic liquid crystals were introduced to the cells to make optical measurements. Our experimental result showed that the increase of the second rubbing strength helps to bring the easy axis toward its own direction.

176 176 Employing a higher order term to the surface energy density term is necessary to explain this easy axis shifting behavior. After defining the newly measured easy axis, anchoring strength measurements were carried out. The results indicate both polar and azimuthal anchoring strength were relatively weak comparing to the single direction rubbed alignment. Note that there is a lower bound limitation of second rubbing times, i.e., it cannot be less than 3 since that will cause non-uniformity in the final cell. Increasing second rubbing times will increase anchoring strengths in both in-plane and out-of-plane directions. When it is sufficiently high (in our case 15 times), the anchoring strength is comparable to the single-direction rubbed cell and meanwhile the easy axis is very close to the second rubbing direction. This suggest that the first rubbing is mostly washed away at this point. References 1. R. Barberi, M. Boix and G. Durand, Applied Physics Letters 55 (24), (1989). 2. R. Barberi, M. Giocondo and G. Durand, Applied Physics Letters 60 (9), (1992). 3. R. Barberi, M. Giocondo, P. M. Lagarde and G. Durand, Applied Physics Letters 62 (25), (1993). 4. G. Barbero, N. V. Madhusudana, J. F. Palierne and G. Durand, Physics Letters A 103 (8), (1984). 5. A. Rapini and M. Papoular, J. Phys. Colloques 30 (1969). 6. Y. Cui, R. S. Zola, Y.-C. Yang and D.-K. Yang, Journal of Applied Physics 111, (2012). 7. Y. J. Kim, Z. Zhuang and J. S. Patel, Applied Physics Letters 77 (4), (2000).

177 C.-Y. Huang, C.-H. Lin, J.-R. Wang, C.-W. Huang, M.-S. Tsai and A. Ying-Guey Fuh, Journal of applied physics 92 (12), (2002). 9. C.-Y. Huang, Y.-S. Huang and J.-R. Tian, Japanese journal of applied physics 45, 168 (2006).

178 178 CHAPTER 7 Alignment Layer s Effect on Liquid Crystal People care the most about a homogeneous alignment layer which can provide a uniform environment for the liquid crystals at ease, or further align additional elements in bulk with sufficient treatment. Usually standard alignment materials are already chosen and researches are focused on varying the liquid crystal configurations or field architectures. If we think the other way around, there are opportunities for the alignment layers to create diverse applications. In this chapter, various liquid crystal behaviors due to alignment layer s effect will be described. 7.1 Temperature dependence of pitch in thermotropic cholesteric liquid crystal Introduction The unique reflective character of cholesteric liquid crystal has led to plenty of sensor applications. Some are based on the electric response; some are based on the mechanical response; while others are based on the temperature response. Thermotropic cholesteric changes its natural pitch according to the temperature 1, so it is made into thin, light, and very handy temperature field indicators or detectors 2 for precision applications, or extended applications such as optical filters 3, 4. The alignment layers can affect how the pitch changes with temperature 5. For a homogeneous (planar) alignment layer, the pitch changes discontinuously, while for a

179 179 homeotropic alignment layer, the pitch changes continuously. These have been proved both by experiment and by simulation. In the case of homogeneous alignment, the discontinuity is a result of strong boundary conditions. The cholesteric has to yield to the condition that can fit integer numbers of half pitch between the substrates. So now the new question is if the pitch would show gradual change with a weaker boundary Materials A thermotropic cholesteric liquid crystal TM75A (Merck) was employed in the study. TM 75A is the host, and in addition liquid crystal LC7029 was added to adjust the temperature observance range. TM75A is composed of several compounds, and they share the similar structures as in Figure 7-1. LC7029 is a negative nematic liquid crystal provided by the Air Force Research Laboratory. Figure 7-1 Chemical structure of TM75A components. Three kinds of alignment layers were prepared: PI 2555, MPU, WPU representing strong, medium, and weak anchoring strength respectively. MPU and WPU are polyurethane with experimentally confirmed medium and weak anchoring strengths. The anchoring strengths of the 3 materials are listed in Table 7-1.

180 180 Table 7-1 Alignment layers anchorign strength Material PI MPU WPU Results The mixture of TM75A and LC7029 was filled into ECB cells composed of variant alignment layers. The sample cell was secured on a hot stage, under the microscope. We use a spectrometer to measure the reflection band of the cholesteric, meanwhile taking photos. Figure 7-2 shows the difference of pitch change when raising the temperature. With PI strong alignment (Figure 7-2 top row), the color of one region (bounded by oily streak lines) change discontinuously, and soon all of the regions flip to the new color. However, with WPU weak alignment (Figure 7-2 bottom row), the color changes continuously in one region and all the regions change simultaneously. Figure 7-3 shows the whole picture of pitch change with temperature. A typical measurement of the spectrums is like shown in Figure 7-4.

181 181 Figure 7-2 Temperature induced pitch change in thermotropic cholesteric: TM75A (90%) with LC7029 (10%). Top row alignment layer: PI. Bottom row alignment layer: WPU. Figure 7-3 Temperature denpendance of pitch with confined boundary. Material: TM75A (90%), LC7029 (10%); alignment layer: WPU. Cholesteric photos in reflection mode. Left corner photo is showing smectic A phase when temperature is lower than cholesteric phase in transmission mode.

182 Reflectance (%) Wavelength (nm) Figure 7-4 Typical reflectance spectrum of a cholesteric liquid crystal. In this plot the material was TM75A (80%) with LC7029 (20%). Temperature was at. Alignment layer was PI Figure 7-5 shows TM75A mixed with different concentrations of LC7029. The less TM75A in the mixture, the narrower the observance temperature range is. Accordingly, we choose a proper concentration of the added LC7029. In Figure 7-6, the concentration of LC7029 was fixed at 20%. The measurements with different alignment layers were presented. As the standard strong anchoring material, PI 2555 resulted in very strictly quantized pitch change. When a medium anchoring strength material was used, some discrete oblique slopes depicted the pitch changing behavior, instead of the flat plateau as in the case of strong alignment. While with the weak anchoring material, there was only a continuous change, not even oblique slopes.

183 183 PI 2555 PI 2555 TM75A 90% TM75A 50% WPU TM75A 90% WPU TM75A 50% Figure 7-5 Center wavelength of the reflection band as a function of temperature, with different alignment layer and different material concentration. TM75A was as marked in the pictures, the rest was LC Cooling rate was.

184 184 PI 2555 TM75A 80% MPU TM75A 80% WPU TM75A 80% Figure 7-6 Center wavelength of the reflection band as a function of temperature, with different alignment layer. TM75A was as 80% and the rest was LC7029. Cooling rate was.

185 Discussion With adjusted alignment, it is possible to achieve discrete temperature indicator or continuous indicator. Further technique can make it into corresponding sensors. 7.2 Blue phase formation and temperature range Introduction Blue phase liquid crystal 6 has been developed into display panels 7. It is fastresponding and provides wide viewing angle. Usually alignment layer is not even required for a blue phase display device 8. During the study of thermotropic liquid crystal with our alignment layers, blue phase was identified, and interestingly, distinct blue phase formation was observed. Still, we have from strong (PI), medium (PVA_1.5%, MPU) and weak (WPU) anchoring materials presented. The blue phase liquid crystals (BPLC) were two kinds: TM75A (90%) + LC7029 (10%); E7 (68%) + R811 (32%) TM75A For the thermotropic cholesteric, we made composite of TM75A (90%) with LC7029 (10%). The mixture was filled into cells with PI, MPU, and WPU alignment layers respectively. Each sample was cooled down from isotropic state at a cooling rate of. Figure 7-7, Figure 7-8, and Figure 7-9 shows the corrsponding BP at different temperatures with the 3 alignment layers. Figure 7-10 summarizes the temperature range of the observed BP phase.

186 186 Figure 7-7 Blue phase texture. LC material: TM75A+LC7029(10%). Alignment layer: PI. The pictures were shifted around within the cell a little bit in order to catch the end of BP phase transition. Figure 7-8 Blue phase texture. LC material: TM75A+LC7029(10%). Alignment layer: PVA_1.5%.

187 Temperature (ºC) 187 Figure 7-9 Blue phase texture. LC material: TM75A+LC7029(10%). Alignment layer: WPU. TM75A(90%) +LC7029(10%) PI 2555 PVA1_9 PUD 027 Alignment layers start end Figure 7-10 Temperature range of BP phase with different alignment layers. LC material: TM75A+LC7029(10%).

188 E7+chiral dopant For the thermotropic cholesteric, we made composite of E7 (68%) and R811 (32%). The mixture was filled into cells with PI, PVA (1.5%), MPU, and WPU alignment layers respectively. Each sample was cooled down from isotropic state at a cooling rate of. Figure 7-11, Figure 7-12, Figure 7-13, and Figure 7-14 shows the corresponding BP at different temperatures with the 3 alignment layers. Figure 7-15 summarizes the temperature range of the observed BP phase. Figure 7-11 Blue phase texture. LC material: E7+R811 (32%). Alignment layer: PI.

189 189 Figure 7-12 Blue phase texture. LC material: E7+R811(32%). Alignment layer: PVA_1.5%. Figure 7-13 Blue phase texture. LC material: E7+R811 (32%). Alignment layer: MPU.

190 Temperature (ºC) 190 Figure 7-14 Blue phase texture. LC material: E7+R811 (32%). Alignment layer: WPU. E7 32%R PI 2555 MPU PVA1_9 WPU Alignment layer start end Figure 7-15 Temperature range of BP phase with different alignment layers. LC material: E7+R811 (32%).

191 Discussion It has been confirmed by both BPLC composites that with weak anchoring material, the platelets are reflecting multiple colors as we see in most non-alignment BP I textures. Also the reflection band shifted a lot over the BP temperature range. With strong anchoring materials, it showed very uniform platelets and narrow reflection. It can be concluded that a preferred pitch of the BP structure exists with presence of a strong, uniform surface alignment. It is also called the pinning effect 9. Our experiments indicate that the pinning effect depends on the anchoring strength of the alignment layers. At first it was interesting to see that the BP temperature range seems to be increasing with weaker alignment layer for the TM75A (90%). However this is not always true. We carried out experiments with TM75A (80%) formed BP, and the PI alignment layer cell showed BP range while the WPU cell showed range. References 1. F. Zhang and D. K. Yang, Liquid Crystals 29 (12), (2002). 2. I. Sage and B. Buhadur, Liquid Crystals-Applications and Uses. (1990). 3. J. Adams, W. Haas and J. Dailey, Journal of Applied Physics 42 (10), (1971). 4. H. Itoh, S. Orii and T. Satoh, (U.S. Patent and Trademark Office., Washington DC, 1991), Vol. U.S. Patent No. 5,037, J. V. Gandhi, X. D. Mi and D. K. Yang, Physical Review E 57 (6), (1998).

192 A. Jakli and A. Saupe, One- and Two-Dimensional Fluids: Properties of Smectic, Lamellar and Columnar Liquid Crystals. (Taylor & Francis, 2010). 7. H. Lee, H.-J. Park, O.-J. Kwon, S. J. Yun, J. H. Park, S. Hong and S.-T. Shin, SID Symposium Digest of Technical Papers 42 (1), (2011). 8. J. Yan, Z. Luo, S.-T. Wu, J.-W. Shiu, Y.-C. Lai, K.-L. Cheng, S.-H. Liu, P.-J. Hsieh and Y.-C. Tsai, Applied Physics Letters 102 (1), (2013). 9. H.-Y. Liu, C.-T. Wang, C.-Y. Hsu and T.-H. Lin, Appl. Opt. 50 (11), (2011).

193 193 CHAPTER 8 Encapsulated Polymer Stabilized Cholesteric Texture (EPSCT) This chapter will shift away from the polymer alignment topics. Instead, complex bulk polymer network will be included. As introduced in chapter 1, porous polymer network filled with liquid crystal is referred to as polymer dispersed liquid crystal 1-3 (PDLC), while polymer fibers formed in the liquid crystal host is referred to as polymer stabilized cholesteric texture 4-7 (PSCT). It was also introduced that PDLC is compatible with roll-to-roll process but have limited viewing angle. PSCT performs very well even at high viewing angles, but is not quite applicable for roll-to-roll. The current study is about combining the merits of the two structures together, and develop an industrial- facilityfriendly device. 8.1 Introduction A desired structure to combine the merits from PDLC and PSCT is proposed as drawn in Figure 8-1. We manufacture large droplets (diameter >10µm) which contains PSCT mixture inside and encapsulated by a thin polymer wall. Therefore, the excellent electro-optical advantage and the required viscosity for roll-to-roll process can both be achieved. This device is named as encapsulated PSCT (EPSCT). Two major mechanisms were considered in our research to form such large capsules: epoxy binders and emulsions. Working principles of the two will be introduced in and Note that thermoplastic can be also applied to achieve successful encapsulation 8.

194 194 Figure 8-1 Schematic drawing of Encapsulated PSCT structure Introduction to epoxy resins From a chemist s view, cross-linking epoxy 9 can take place in by various mechanisms under various designed conditions. For an experimental physicist, the process is down to practical usage conditions. Epoxy adhesives usually come with two parts 9 : one is prepolymer equipped with epoxide groups (Figure 8-2); the other one is the curing agent or crosslinker or hardener. The curing takes place at certain temperature range with the presence of the crosslinker, or at a high enough temperature witho ut the need of crosslinker. The two raw parts are both soft matter, while the mixed and cured resultant is hard. (a) (b) Figure 8-2 Chemical structure of (a) epoxide group and (b) amine group. For electro-optical devices, the chosen epoxy resin has to be electrically resistive, chemically resistive, and optically transparent after cured. For industrial applications, the

195 195 chosen epoxy should also be hardened within reasonable time period (preferably ~ 1 hour). It is also preferred that curing can take place at room temperature. Bisphenol A epoxy resin (Figure 8-3) is the most common kind of all. Curing agent with amine functional group Figure 8-2 is capable of curing the resin at room temperature. The curing process of epoxy is described in Figure 8-4. Figure 8-3 Chemical structure of a simple bisphenol A epoxy: diglycidyl ether of bisphenol A (DGEBA or BADGE). Figure 8-4 Process of crosslinking epoxy resin by diamine. (Picture courtesy from Department of Polymer Science, University of Southern Mississippi) It is useful to introduce the definition of equivalent weight 10 ( ) and the calculations for the curing agent weight. For epoxy resins, the Epoxy Equivalent Weight

196 196 ( unit: g/mol) is defined as the weight in grams of resin containing 1 mole equivalent of epoxide. For the amine hardener, the Amine Hydrogen Equivalent Weight ( unit: g/mol) is grams of hardener containing 1 equivalent of N-H groups. To calculate optimized amount of curing agent is needed (the stoichiometric ratio of curing agent and resin), is defined as the parts per hundred parts resin by weight Introduction to emulsion method In principle, producing PDLCs by emulsion method 11 can be achieved with any water-soluble polymers 12. However it is not easy to balance the requirements that is optically transparent and will lead to stable, separated droplets for PDLC. PVA was the first introduced into making PDLC emulsions 11. As a representative for water-soluble polymers, it is employed in our research for the encapsulations. To form a PVA based PDLC system, aqueous PVA solution (concentration 10%- 20%) and nematic liquid crystal are stirred at a high speed (~ ). The resulted PDLC droplets are desired to be ~ 1 µm in diameter for good scattering state. Then the mixture is uniformly coated on a substrate and allowed to dry. Finally a top substrate is laminated. In the current study, the key difference from a regular PDLC is to form large capsules, preferably > 10 µm. This requires efforts to form stable large capsules as well as proper coating technique to reduce coalescing.

197 Encapsulation by curing epoxy resin Different kinds of epoxy resins were tried in order to form large, stable drop lets. However there were issues with keeping the droplets from merging into each other. Even more, the pre-psct mixture lost its own monomer as well as the designed pitch minutes epoxy The most commonly used epoxy in a science lab is the 5-minutes epoxy or the A-B glue. It provides robust bonding to glass and flexible substrates in a short time period (~10 minutes). It is transparent with a tint of yellow after curing, and shows excellent resistivity. For industrial applications, the processing time limits the curing time. This is preferably to be about a few hours. When the epoxy is mixed with 90% nematic liquid crystal, it takes up to 50 minutes. The first step is to form the large droplets. Nematic liquid crystals rather than pre- PSCT mixture (as introduced in chapter 1) was mixed with each epoxy components (named as A and B) separately. Therefore we get 2 isotropic mixtures: 1) E44 (or E7) 10% by weight and epoxy A 90% by weight; 2) E44 (or E7) 10% by weight and epoxy B 90% by weight. Note that we kept the epoxy ratio low intentionally to get large droplets and thin networks. A piece of clean glass with uniformly distributed 9 µm spacer was prepared. One droplet of each homogeneous mixture were settled on the glass and stirred by toothpick rapidly for 1 minute. The stirring process was performed on a hot stage to ensure the mixtures were mixed well. Then a cover glass was laminated and pressed in order to push out the bubbles and obtain uniform cell thickness. The cell was allowed to cure at room temperature while microscopic pictures were taken as shown in Figure 8-5.

198 198 Figure 8-5 Nematic mixed with 5 minutes epoxy as time goes. Top row: E7. Bottom row: E44. During the first few minutes of the curing, liquid crystal phase separated into droplets. As the resin bonding goes on, the liquid crystal was separated more and the droplets merged into each other. In the end there was no desired droplet left. It was also noticed that the E7 samples were cured faster than E44 samples. The 5 min epoxy is made from bisphenol A epoxy with low EEW ( ) so that it can be cured fast. In addition, there is even accelerator in the resin to make sure the reactions take place fast enough. In our case the fast bonded epoxy generates small droplets of nematic and leaves the major liquid crystal separated. Therefore a higher EEW resin or a resin without accelerator will be needed.

199 DER 732 DER, as short for Dow Epoxy Resin, refers to a catalog of materials. From the catalog, DER732 was identified as a good candidate since its EEW= Also, it is designed to be show flexibility after cured. This feature is good for flexible display. Triethylenetetramine (TETA) was used as the curing agent. E7, BLO06, BLO36 were made into pre-psct respectively. The pre-psct mixture was designed to have µm pitch (chiral dopant R811), 3% monomer (RM257) and 0.3% photo-initiator (BME). The ratio between pre-psct and epoxy (including resin and curing agent) was kept as 80%: 20%. It turns out that curing at room temperature takes up to 2 days to fully cure the samples. And the samples were not uniform as large chunks of cholesteric can be observed in Figure 8-6. But in the droplets, the cholesteric pitch was observed to be much more than a few microns. Raising the curing temperature was tried out as shown in Figure 8-7, which did help reducing curing time to a few hours. However the liquid crystal in the droplets still lost the chirality.

200 200 Figure 8-6 Pre-PSCT droplets and chunks formed with room temperature cured DER C 85C Figure 8-7 Pre-PSCT droplets and chunks formed with high temperature cured DER732. So DER732 is not a good option. One problem is the curing time is too long. The other problem is that the chiral dopant may be diffused away from the original cholesteric or chemical reactions happened, where both cases result in reduced chirality BADGE BADGE (EEW= ) is the simple epoxy, also the majority of 5-min epoxy. We use pure BADGE and TETA as resin and curing agent. Pre-PSCT mixtures were

201 201 made as before with nematic, chiral dopant, monomer and photo-initiator. The nematic was staring E7, BLO06 and BLO36 respectively. The ratio between pre-psct and epoxy (including resin and curing agent) was kept as 80%: 20%. For the BLO06 and BLO36 samples, no large chunks of liquid crystal were found. So UV curing was applied to polymerize the monomers in the droplets. Figure 8-8 Pre-PSCT droplets and chunks formed with room temperature cured BADGE.

202 202 Figure 8-9 After UV curing, PSCT droplets and chunks formed with room temperature cured BADGE. Left: 0 V. Right: 60V AC applied. However the same problems exist. The stabilized curing time of epoxy was still too long, due to the low concentration of the resin monomer. The PSCT become less chiral and was no polymerization happened inside the droplets. 8.3 Encapsulation by emulsion method Emulsion method was successfully applied to form large PSCT droplets. There were several key points in the manufacturing process in order to get stable large droplets and uniform coating. Electro-optical characterization of the final film was carried out Materials and emulsification The pre-psct mixture was prepared by E44 (87%), R811 (9.7%), RM257 (3%), BME (0.3%). We were aiming at making large PSCT droplets (preferably diameter~10µm) with thin PVA wall outside, in order to avoid undesired light scattering caused by too many LC/PVA interfaces. Thus the dry PVA s amount was always

203 203 controlled to be as low as possible. With the developing of our experiments, the lowest dry PVA percentage was 14% in the final dried emulsion. Lowering than 14% was not successfully encapsulated. One other thing needed to be mentioned here is the addition of surfactant. In many trials, we saw the enhancement of mechanical strength of the emulsions due to surfactant. For the PVA emulsion without surfactant, although the emulsion itself was being even more stable than we needed, the coating has been a pain. The emulsion does not wet either glass or PET film very well. After adding the surfactant dodecanol, the emulsion is composed of 46% water, 8.1%PVA, 2.7% dodecanol, 43.2% pre-psct mixture. PSCT mixture therefore stands for 80% of the final dried emulsion. In order to control the droplet size in a relatively large range, a magnetic bar was used. The stirring was kept at 500rpm for no more than 15 seconds Coating and laminating One of the most popular coating methods, Mayer rod was the first candidate to coat a uniform film on the substrate. It is a stainless steel rod that is wound tightly with stainless steel wire. The rod is used to control the coating thickness and remove excess coating material. The wet thickness after doctoring is controlled by the diameter of the wire. The dry thickness is determined by the solids concentration of the coating solutions.

204 204 Rod moving direction Figure 8-10 Left: Mayer rod (picture courtesy from holoeast.com). Right: the dried film with pre-psct droplets and PVA environment. The rod I used will produce 24.5 µm thick wet films. In my case, the dried films were measured to be µm, determined by profilometer. The dried coating under the microscope is shown in Figure Although the coating was pretty uniform and the droplets were not merging into each other (thanks to the surfactant), they were being too far away from each other. This is due to the viscosity of the large droplets being higher than the rod desired. So the watery part of the wet coat can spread out but the droplets cannot. Then the coating tool was changed to doctor blade. There is a blade with two micrometers to control the distance between the bottom edge of the blade and the substrate. The coating is preferably done in a fast movement. In my experiment, I vary the height to obtain different film thickness. The film has pretty good coverage all over the substrate as shown in Figure The wet film was allowed to dry for at least 2 hours in ventilating, UV-blocked environment.

205 205 Figure 8-11 Left: laboratory doctor blade (photo courtesy from mtixtl.com). Right: the dried film with pre-psct droplets and PVA background. For the laminating part, we first tried glass substrates for both top and bottom, but the laminating become difficult, because the top glass is too hard to adhere to the dry film, and more pressure will cause too much crushing/damaging of the encapsulated cholesteric. Then we used PET films to do coating and laminating. These were good to sandwich the coating properly, but was easy to get shortening at the edges, since we don t have patterning on the PET film so it s covered by ITO everywhere. In the end we decided to use a glass substrate to do the coating, and then laminate with a PET film UV curing After the film was dried, the encapsulated PSCT needed to be cured with an a voltage. We measured transmission vs. voltage of the sample before we cured it, making sure the saturation voltage (typically>50v). Higher voltage is better for aligning the monomer during their transition to polymer, and thus clearer transparent state will be assured for the obtained normal-mode PSCT inside PVA. As long as the sample does not get shortened, we kept the voltage up to 90V (AC, 1 khz) during curing. The UV

206 206 intensity was 1.4 mw/cm 2, and the curing time was 30 minutes. After curing, the sample is as shown in Figure Inside the droplets, focal conic domains were observed. Schematic drawings in Figure 8-13 summarize the whole manufacture process. Figure 8-12 UV cured PSCT droplets in PVA background. Figure 8-13 Manufacture process of encapsulated PSCT.

207 SEM SEM photos were taken for samples before curing and after curing. We make samples with flexible substrates and freeze the sample with dry ice before cutting it. After the cutting, the sample was immersed in hexane thus liquid crystal and monomers would be washed away, however polymer will stay. In Figure 8-14 (a) and (c) the compartments are hollow. In Figure 8-14 (b) and (d) there are polymer fibers growing in the compartments. (a) (b) (c) (d) Figure 8-14 (a) Sample before curing, one hollow droplet. (b) Sample after curing, one droplet filled with fibers. (c) Sample before curing, cross section of the film. (d) Sample after curing, cross section of the film.

208 Electro-optical characterization In this section, EO characterizations will be demonstrated. As a light shutter or a potential display device, transmission as a function of voltage and response time were measured for before UV curing and after UV curing cells. Experiment setup is pictured in Figure Figure Figure 8-15 Experiment setup for EO characterization. The basic working principle of the encapsulated PSCT (EPSCT) film is shown in

209 209 Figure 8-16 Working principle of EPSCT: scattering state and transparent state. T-V curve of before UV curing and after UV curing in Figure 8-17 indicates that after UV curing the contrast ratio was improved by 4 times. Transition voltage dropped a little, and the turn-on time was reduced a lot due to the vertically aligned polymer fiber. These features are in accordance with normal mode PSCT 13. (a)

210 210 (b) ON OFF 100 time (ms) before curing after curing Figure 8-17 Before and after UV curing, EO characterization: (a) transmittance vs. voltage curve; (b) response time. Film thickness = 12µm. Viewing angle was also improved comparing to PDLC made from PVA. In addition to the original optics setup, a cylinder of glycerol was employed. The sample was immersed in the glycerol, which has matching refractive index as glass, to avoid reflections at air/glass interface. 12 µm EPSCT cell were compared with PDLC and PSCT with the same thickness. The PDLC was made by emulsion of 15% PVA solution and E44. The weight ratio of the dry PVA and E44 was 50%: 50%. The emulsification was accomplished by stirring at 3000 rpm. The final droplet size in the PDLC was around 1 µm. The PSCT was made by the same mixture as used in EPSCT. The resulted domain size was also around 1 µm.

211 211 Transmittance (%) Angle (degree) regular PSCT Encaps. PSCT regular PDLC Figure 8-18 Light intensity as a function of viewing angle with different samples Electro-optical characterization: varying film thickness Coating thickness is related to solvent s percentage, height setting of doctor blade, and even the speed of pulling the blade. The thickness could be achieved in the range of 6-20µm. Different films with varied thickness were made and their EO response were compared in Figure The driving voltage dropped with film thickness so did the contrast ratio. The sample with 10 µm thickness had a balanced performance: relatively low driving voltage and nice scattering and transparent states.

212 Transmittance (%) um 8um 10um 12um 15um Contrast ratio Voltage (V) thickness (um) Figure 8-19 (a) Transmittance as a function of applied voltage of the EPSCT light shutters with different film thicknesses. (b) contrast ratio vs. film thickness. 8.4 Discussion Epoxy resins were tried out to form large PSCT droplets embedded in the cured resin. But the resulted. Then we were able to make a PVA encapsulated PSCT light shutter by emulsion method. Both the transparent state and the scattering state worked well. The driving voltage was reduced by controlling thickness while the contrast ratio remains good. Viewing angle was enhanced a lot compared to conventional PDLC. Reference 1. G. Crawford, Flexible Flat Panel Displays. (Wiley, 2005). 2. D. R. Cairns, S. P. Gorkhali, S. Esmailzadeh, J. V. Vedrine and G. P. Crawford, Journal of the Society for Information Display 11 (2) (2003). 3. J. W. Doane, N. A. Vaz, B.-G. Wu and S. Zumer, Applied Physics Letters 48 (4), (1986).

213 D.-K. Yang and S.-T. Wu, Fundamentals of liquid crystal devices. (John Wiley & Sons, Ltd., 2006). 5. G. P. Crawford and S. Žumer, Liquid Crystals in Complex Geometries: Formed by Polymer and Porous Networks. (Taylor & Francis, 1996). 6. D.-K. Yang, L.-C. Chien and J. W. Doane, Applied Physics Letters 60 (25), (1992). 7. J. L. West, R. B. Akins, J. Francl and J. W. Doane, Applied Physics Letters 63 (11), (1993). 8. Y. Cui, C. Zhang and D.-K. Yang, (2012). 9. C. A. May, Epoxy Resins: Chemistry and Technology. (Marcel Dekker, 1988). 10. Dow, in Dow Liquid Epoxy Resins (1999). 11. J. L. Fergason, SID Int. Symp. Dig., (1985). 12. P. S. Drzaic, Liquid Crystal Dispersions. (World Scientific Publishing Company Incorporated, 1995). 13. S. P. Hurley, Dissertation, Kent State University, 2010.

214 214 CHAPTER 9 Conclusion The current dissertation is based on electro-optics study on liquid crystal displays, focused on the interaction between liquid crystal and polymer, and aimed at optimizing performance of display devices. Many attentions were paid to the surface interactions between polymer and liquid crystal. Anchoring strength was the key characterization parameter in my study. Measurements and simulations were done in order to compare the effect of material property (chapter 3 and 5), manufacture technique (chapter 2 and 6), and temperature (chapter 4). The studies not only involve physical understanding but also can help incorporating with different kinds of display devices (chapter 5 and 8). 9.1 Physics aspect The whole dissertation is supported by characterization of polymer alignment films, liquid crystal properties, and device performances. Popular characterization methods were included such as high-field method, twisting analyzer method, Sénarmont method, etc. However during my research, additional equipment was presented besides the required optics in order to carry out electro-optical response study or high precision measurements. The experimental setups and the characterizing techniques can be useful for future electro-optical researches. The main issue discussed in this dissertation is about polymer s aligning effect in liquid crystal devices. Varied anchoring strength was produced by alignment films with

215 215 the same material but different thicknesses, and different increasing trends were discovered for polar and azimuthal anchoring strength respectively (chapter 3). With fixed alignment film, anchoring strength variation was confirmed to be decreasing with raised temperature (chapter 4). Through the study on a list of polymers with side chain of the alignment material plays a role in the corresponding material s anchoring strength (chapter 5). These result suggested the importance of anisotropic intermolecular force between the polymer and the liquid crystal as well as the microgrooves created by the rubbing process. Rapini-Papoular equations usually well describe the anchoring strength. However it has been demonstrated that in certain cases Rapini-Papoular equations with a higher order term can better describe the anchoring behavior. One case is during the study of the balancing between the polar anchoring strength and a high electric field across the cell (chapter 2 and 3). The other case is in the study of complex azimuthal anchoring behavior created by multi-rubbing (chapter 6). 9.2 Application aspect For industrial applications, the high-field method and the twisting analyzer method (chapter 2) are plausible and convenient, and the results are reproducible. So they are suitable for investigating proper alignment film materials and check the versatility in various environments, which is especially crucial for rugged devices. Also the techniques of varying anchoring strength of the same material by film thickness (chapter 3) or shifting the easy axis on the same film sheet (chapter 6) are convenient, low-cost ways of creating novel alignments. It is worth to mention that the study has already provided the

216 216 lab colleagues with proper alignment materials 98 along with the experience of obtaining uniform alignment film of various materials. Surface aligning effects are also related to the electro-optical performance of bulk liquid crystal/polymer networking. The anchoring strength indicated the bulk polymer network in PDLC under varied temperature (chapter 4). It was also discussed that how polymer side-chain structure affects the surface anchoring strength and the correlation to the PDLC performance (chapter 5). This information was used in the selection of proper polymer in the large droplet encapsulation (chapter 8). Encapsulation of large PSCT droplets is a novel invention which combines the merits of PDLC and PSCT by introducing a complex structure. The result device demonstrate good viewing angle and can be manufactured by roll-to-roll method. However due to the limitation of laboratory equipment, the droplets were not very uniform in size. There are still small droplets (~ 1 µm) formed around the large ones (~10 µm), which causes unwanted light scattering at encapsulation walls. For industrial applications, if the droplets can be filtered before coating to the substrate, the resulted device should have even better performance.

217 217 CHAPTER 10 APPENDIX: Characterizing and Modeling of Light Diffusing Sheet The current work is on the scattering profile of diffusing sheet for flat panel displays. This piece of work does not fit into the general topic of this dissertation. Therefore it is put in the end as an appendix. Experiments were carried out to measure the angular dependence of the outcoupled light intensity of a diffuser. A phenomenological model was developed which can calculate the thickness and angular dependence of scattered light intensity of arbitrary incident light propagating through a diffuser in variable environments. The results will provide guidance to the understanding and improving of flat panel displays such as LCD and OLED Objectives and Backgrounds Diffusing sheet (or diffusion film) is essential for all flat panel displays. For instance, in liquid crystal display (LCD) back-light systems i, diffusing sheet is usually employed in order to even out the light directly emitted by CCFL or LED units. It is well-known that in LCD, 20-40% of the power is consumed by backlight unit (BLU) 1, 2. Thus improvement on the backlighting efficiency has always been highly desired. As another example, a newly emerging technology is organic light-emitting diode (OLED) display 3, which also needs diffuser to improve its lighting efficiency. Light extraction efficiency of OLED without any surface modifications can be as low as 20% due to absorptions, surface plasmon, and wave-guiding. Wave-guided mode in organics/ito and in substrate

218 218 can take response for 35% and 20% light loss, respectively 3, 4. Diffusing sheet can reduce the total internally reflection resulted from substrate-air interface. A good understanding of the scattering profile of diffusers is essential for flat panel displays. Experimental measurements of diffusing sheets have been carried out in manufacture industry to optimize the light extraction efficiency, but not much theoretical study has been done to understand the mechanism, which refers to a very complex optics field: multi-scattering 5-9. We developed straight-forward model which can characterize real diffusers in variable display environments. Our model is based on Gaussian distribution. It enables the calculation of the parameters, coefficient and standard deviation, from the film thickness and incident light angle. It can also generate the angular profile of outcoming light intensity under arbitrary incident light conditions Experimental results Common diffusing sheets have either a grid structure or dispersed particle in plastic resin. In this paper, we study the latter kind. The thickness of the film is 800 µm. Imbedded particles shown in SEM picture (Figure 10-1) are around 2 µm.

219 219 Figure 10-1 SEM photograph of the diffuser showing dispersed particles in resin. Angular light distribution of the diffusing sheet was measured by the optical set up as sketched in Figure A He-Ne laser (633nm) was used as the incident light. It was collimated. The diffuser was immersed in glycerol, whose refractive index was 1.47, contained in a cylinder. The diffuser film can be rotated such that the incident angle of the light with respect to the normal of the film can be varied. Because the refractive index of the glycerol was very close to that of the film, the reflected light intensity does not change (and thus the incident light intensity into the film was a constant) when the film was rotated. The detector was controlled by a rotation stage and could be rotated independently. The full collection angle of the detection was 5.0º.

220 220 Figure 10-2 Optical measurement setup top view. Experimental data was plotted in Figure For normal incident light, the scattering profile is a symmetric Gaussian distribution. For oblique incident light, as the incident light increases, the peak value of the scattering profile decreases and the width of the peak increases. This can be explained by that the path length increases as the incident angle is increased, as shown in Figure In addition, two very important features were observed. First the scattering profile is no longer symmetric with respect to the incident angle. The right side of the scattering peak decreases more dramatically. Second the intensity peak is no longer in the incident direction, but shifts toward smaller angles. For example, for the light incident at 40º, the scattering intensity peak is at 32º. These features are due to the facts that there are multiple scattering and the path lengths inside the film are different for the light scattered in different directions. We will explain this effect in more details in the modeling section.

221 221 Incident angle: Figure 10-3 Open circles: experimentally measured scattering profile measurements of the diffusing sheet. Solid lines: Fitted by our model. The vertical dashed lines indicate peak center. Viewing angle is defined with respect to the film normal. In our measurement, only the polar angle of the detection was varied, but not the azimuthal angle. The latter is very difficult to change in our experimental setup. For normal incident light, the scattering intensity has the azimuthal symmetry that the intensity is independent of the azimuthal angle. For oblique incident light, the azimuthal symmetry does not exist any more, because the path length inside the film of the scattered light depends on the azimuthal angle.

222 222 Figure 10-4 Schematic showing the incident and scattering angles Modeling In our modeling, the diffuser is placed on top of a glass substrate. Light is from the glass substrate, propagates through diffuser and then come out to air. The thickness of the diffuser is d o. In the lab frame, the incident light direction (inside the diffuser) is described by the polar and azimuthal angles, ), and the scattering direction ( i i (inside the diffuser) is described by (, ) as shown in Figure The polar angles are defined with respect to the normal of the film. Due to symmetry, we only have to consider the case 0. The scattering azimuthal angle is defined with respect to the i plane defined by the film normal and incident light. The path length (effective thickness) inside the diffuser is

223 223 d d o /cos (10.1) which changes with the incident angle. Therefore the scattering profile depends on the incident angle. For normal incident light ( 0), the scattering profile is described by the Gaussian distribution function i I 1 2 / 2 2 (, ) e o (10.2) 2 o For oblique incident light, it is assumed that the scattering profile is described by the Gaussian distribution I(, ) A ( ) 2 / 2 2 e i (10.3) 2 in which intensity A and the standard deviation are functions of the incident and scattering angles. We depict the general picture of our diffusing sheet model in Figure 10-5, which describes difference between normal incident light and oblique incident light. For convenience, we introduce angle t in the picture, which is the scattering polar angle in the local frame of the incident light. is related to by t i. The scattering t azimuthal angle is t in the local frame (not shown in the figure). As we know, each incident ray is scattered to all directions including polar and azimuthal angles variations.

224 224 There are multiple scattering in the diffuser, namely, light is scattered multiple times. Light scattered into different directions have different path lengths. The path length is not only a function of the polar angle but also of the azimuthal angle. As shown in Figure 10-6, incident angle is 30º, and if we fix the scattering polar angle at 20ºin the local frame, and vary the azimuthal angle in the local frame, path length (effective thickness) changes. As shown on the right side of Figure 10-5, for the scattered ray 1 and 2, the path lengths are different although they have the same t. Ray 2 has to go through longer path in the diffuser and will be scattered more on its way out, and its intensity should be lower. This explains the asymmetric Gaussian in the experimental result. The path length is a function of the incident polar angle, scattering polar angle and azimuthal angle. Figure 10-5 General picture of modeling. On the left is normal incident light; on the right is oblique incident light.

225 225 d d( i, t, t ) (10.4) which is calculated by geometry. The intensity the Gaussian scattering profile is given by A exp( c d) (10.5) where c is a constant. This is a 1 st order approximation based on single scattering. The width of the Gaussian peak depends only on the path length along the incident direction and given by [ d( i, t 0)] (10.6) o Figure 10-6 Geometry in the diffuser (grey box): incident light (blue) with i 30 o scattered light (green) with fixed t 20, and varied azimuthal angle. All green dots and the blue dot are on the top surface of the box.

226 226 By fitting the experimental results, we obtained c 0.4 (10.7) d (10.8) The calculated profiles are also shown in Figure They agree well with the experimental results. Using the above model, we study the scattering of single incident light at three incident polar angles. The results are shown in Figure The incident polar angles are 0º, 30º, 60º, respectively, while incident azimuthal angles are at 90º. The left column shows the scattering profiles before the refraction at the interface between the diffuser and air. The subplots show that for light with larger incident angle, it is scattered into wider angles with lower peak intensity. The right column shows the scattering profile in the air after the refraction. It can be seen that the Gaussian peak becomes lower and distributions gets wider.

227 Figure 10-7 Single incident light (0º, 30º, 60ºfrom top row to bottom row) scattered light intensity vs. scattered polar angle (0º-90º) as well as scattered azimuthal angle (0º-360º). Left column: right after being scattered by the diffuser; right column: light coming out to the air. Note that the scales are different for each subplot. 227