Liquid Crystals
Liquid crystals (LCs) liquid crystals (LCs) are widely used in information-processing devices, for optical visualization optical visualization of physical influences (heat, IR, high-frequency radiation, pressure, etc.), for nondestructive testing, nondest
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5.1.1 Liquid Crystalline State ....................... 5.1.1.1 Chemical Requirements ........... 5.1.1.2 Physical Properties of Liquid Crystals ..................... 5.1.1.3 Applications of Liquid Crystals... 5.1.1.4 List of Abbreviations ................ 5.1.1.5 Conversion Factors .................. 5.1.2 Physical Properties of the Most Common Liquid Crystalline Substances ............... 5.1-2 Acids ..................................... 5.1-3 Two-ring Systems without Bridges .................................. 5.1-4 Two-ring Systems with Bridges . 5.1-5 Three and Four-ring Systems .... 5.1-6 Ferroelectric Liquid Crystals ...... 5.1-7 Cholesteryl (cholest-5-ene) Substituted Mesogens .............. 5.1-8 Discotic Liquid Crystals ............. 5.1-9 Liquid Crystal Salts ..................
941 943 943 944 944 945 946 946 947 955 964 967 968 972 973
5.1.3 Physical Properties of Some Liquid Crystalline Mixtures ...... 975 5.1-10 Nematic Mixtures .................... 975 5.1-11 Ferroelectric Mixtures .............. 975 References .................................................. 977
5.1.1 Liquid Crystalline State Liquid crystals represent an intermediate state of order (mesophase) between crystals and liquids. Crystals have a three-dimensional long-range order of both position and orientation (Fig. 5.1-1a). Liquids, in contrast, do not show any long-range order (Fig. 5.1-1b). In plastic crystals (disordered crystals, Fig. 5.1-1c), positional order is maintained, but orientational order is lost. In mesophases, imperfect long-range order is observed, and thus they are situated between crystals and liquids. The reasons for the formation of a mesophase can be the molecular shape or a microphase separation of amphiphilic compounds. More than 100 000 individual liquid crystals have been prepared until now [1.1–4]. About 2000 of them have been tested for physical properties and technical applications [1.5–14]. These materials can be classified
by chemical structures and physical characteristics (see Table 5.1-1). Generally, molecules of liquid crystalline substances have the following shapes:
• • •
rod-like molecules, which form calamitic liquid crystals (nematicand smectic phases); disk-like molecules, which form discotic liquid crystals (discoid nematic and discotic phases); amphiphilic compounds, which form layered columnar or cubic phases in the pure state and in solution.
The simplest and most widespread liquid crystalline phase is the nematic phase. The molecules are statistically distributed within the medium, but the long axes are orientated in one direction, the director (Fig. 5.1-2a). A special class of nematic phases is the cholesteric phase
Part 5 1
Liquid Crystal 5.1. Liquid Crystals
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Part 5
Special Structures
Part 5 1.1
Table 5.1-1 Classifications of liquid crystals
Shape Phase structure Mesophase units Mesophase origin
a)
Rod-like molecules Calamitic liquid crystals Thermotropic liquid crystals Amphiphilic liquid crystals
Disk-like molecules Discotic liquid crystals Lyotropic liquid crystal
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