.IES ON LIQUID CRYSTALS Volume 1

LIQUID CRYSTAL DISPERSIONS

Paul S. Drzaic Raychem Corporation

№ World Scientific • Singapore • New Jersey • London • Hong Kong

Contents Preface yii

C red i t s ix

Introduction 1 T h e P D L C p a r a d i g m 3

H i s t o r y 4

Scope o f t h e b o o k 7

References 9

Recipes 11 Polymer primer 14

Polymer structure and physical properties 14 Polymer chemistry 17

Emulsion methods for PDLC films 22 Polyvinyl alcohol systems 23 Latex-based systems 28 Other water-based systems 29

Phase separation methods for PDLC films 30 Polymerization-induce phase separation (PIPS) 33

Photoinitiated PIPS systems 33 Thiol-ene systems 33 Acrylate systems 38 Variations in photocured PDLC systems..42 Polymer network systems 43

Thermally initiated PIPS systems 47 Electron beam—initiated PIPS systems 50

Thermally-induced phase separation (TIPS) 50 Solvent-induced phase separation (SIPS) 51

Other methods for PDLC formation 51 Coatings, substrates, and film manufacture 54 Aspects of film structure 55

Film thickness 56 Microscopy of PDLC films 58 Liquid crystal domain size 59 Liquid crystal solubility in the polymer matrix 66

Liquid crystal/polymer phase separation 75 Phase diagrams for liquid crystal/monomer mixtures 76 Phase separation in the binodal and spinodal regimes 81 Energetics and kinetics of PDLC film formation 88

References 92

LIQUID CRYSTAL DISPERSIONS

Nematic configurations within droplets 99 Factors controlling nematic alignment 101

Theoretical considerations of droplet structures 103 Alignment and elastic properties of liquid crystals (inset) 106 Defect structures I l l Defect structures in nematic liquid crystals (inset) 112

Analysis of director configurations 115 Optical microscopy 115

Calculation of droplet patterns 118 Nuclear magnetic resonance 121

Catalog of droplet structures 125 Parallel wall alignment 127

Bipolar configuration 127 Concentric {aka toroidal or axial) structure 131 Twisted bipolar 136

Cholesteric droplets with parallel wall alignment 139 Radial line defect (spherulite structure) 140 Diametrical defect structure 142 Double-radial defect 143 Planar structures 143 Escaped defect structures in cholesteric systems 144 Focal conic textures 145

Perpendicular anchoring 145 Radial and twisted radial 145 Axial (equatorial) 147 Escaped radial 148

Cholesteric droplets with perpendicular alignment 149 Configurations with weak surface anchoring 150

Transformations between droplet structures 154 Radial-Axial 154 Bipolar - Radial 160 Bipolar — Twisted bipolar and Bipolar - Concentric 162 Bipolar with twist wall 164 Cholesteric radial line (spherulite) - planar transition 166 Cholesteric spherulite helical unwinding 166 Radial - escaped radial transition 167 Negative - positive helical twist transitions 167 Temperature-driven transitions in radial droplets 168

Assorted topics in confined systems 168 Surface transitions 169 Cylindrical structures 169 Microporous structures 172 Shape distortions in fluid-suspended droplets 173

References 174

CONTENTS xiii

4 | Electro-optical effects 183 Light scattering 185

Macroscopic properties 185 Measurement of light scattering 187 Film scattering profiles 190 Wavelength effects in light scattering 194 Multiple scattering effects 196 Light scattering at high fields 197 Light scattering and droplet director configuration...199

Models for light scattering of nematic droplets 200 Anomalous diffraction approximation 203

Radial droplets 208 Bipolar droplets with internal curvature .210 Experimental results 211

Rayleigh-Gans approximation 214 Other light scattering models 220 Light scattering at high droplet concentrations 221 Other aspects of PDLC light scattering 226

Reverse mode devices 226 Multiple scattering effects 227 2-D vs. 3-D randomized alignment 228 Depolarization effects 228 Scattering in the infrared 229

Dichroic effects in PDLC films 229 Emulsion-type dichroic PDLC films 230 Phase separation-type dichroic PDLC films 234

PDLC film operating voltage 235 Definition of reorientation field 238

Hierarchical order parameters 240 Factors controlling reorientation fields 242

Droplet size and shape 242 A model based on shape anisotropy 244 Film conductivity and dielectric constant 246 Surface anchoring and alignment 249 Alignment by fields during cure 252 Polymer composition 253

Swi t ch ing speeds 257 Defining switching speeds 257 Experimental PDLC response times 259 Numerical models for PDLC response times 264 Means of improving response times 267

xiv LIQUID CRYSTAL DISPERSIONS

Hysteres is a n d m e m o r y effects 269 Hysteresis effects 270

Sources of hysteresis 271 Reduction of hysteresis 274

Memory effects 275

P D L C electrical p roper t i e s 278 Equivalent circuit (impedance) analysis 280

3-element model 281 Film capacitance 284 Distributed models 287

Conductivity and depolarization fields 289 Square wave excitation of PDLC films 295

Some miscellaneous electro-optical effects 296 Reverse mode devices 296 Polarization-dependent scattering in sheared films 298 Nonlinear optical effects 300

Chiral nematic dispersions 303 Dispersions with negative dielectric anisotropy liquid crystal 304 Optics of cholesteric materials 305 Dispersions with positive dielectric anisotropy liquid crystal 309 Flexoelectric effect in dispersed cholesteric systems 310

Mesogenic polymer networks 311 Nematic fluids with mesogenic polymer networks 313 Morphology of mesogenic network composites 314 Electro-optics of mesogenic network/nematic composites 316 Mesogenic networks containing cholesteric liquid crystals 317

Monostable devices 318 Bistable devices 319 Multiple reflection wavelengths 320

Other network systems 321 Polymer stabilized display devices 321 Side chain liquid crystal polymer composites 322

Chiral smectic dispersions 323 Chiral smectic materials 323 Some conventional chiral smectic device modes 327 Chiral smectic dispersions 331

Linear SmC* devices 331 Bistable SmC* devices 335 Antiferroelectric SmC* devices 336 Electroclinic effects in dispersed SmA* systems 338

References 338

CONTENTS XV

5 | Applications 353 Comparison of TN and PDLC devices 354 Architectural windows 357

Scattering at zero field 359 Maximizing film scattering 359 Red-bleedthrough in PDLC films 361

Haze at high fields 365 Standard PDLC films 365 Other low-haze PDLC films 368

Energy control 370

Pro jec t ion displays 371 Screen luminance and display efficiency 372 Projector designs 373 Active matrix electronics (inset) 377 Spatial light modulators 380 Scattering considerations in projection displays 382 PDLC material properties in projection displays 384 Alternative PDLC systems in projection displays 390

Direct-view scattering-mode displays 392 Reflective display illumination and contrast ratios 392 Displays with rear reflectors 395 Transmissive display designs 398 Scattering displays incorporating polarizers 399

Dichroic dye-based displays 399 Transmission properties of dichroic displays 400 Scattering properties in dichroic displays 404 Applications of dichroic displays 406

Direct-view cholesteric displays 407 Reflective color cholesteric displays 407 Multiplexing drive schemes for cholesteric displays 408

Multiplexing with a large hysteresis loop 408 Multiplexing with bistable materials 410

Miscellaneous applications 413 References 416

| Color Plates 423

| Index 425