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.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