Thermotropic liquid crystals principle

The subject of liquid crystals has now grown to become an exciting interdisciplinary field of research with important practical applications. This book presents a systematic and self-contained treatment of the physics of the different types of thermotropic liquid crystals - the three classical types, nematic, cholesteric and smectic, composed of rod-shaped molecules, and the newly discovered discotic type composed of disc-shaped molecules. The coverage includes a description of the structures of these four main types and their polymorphic modifications, their thermodynamical, optical and mechanical properties and their behaviour under external fields. The basic principles underlying the major applications of liquid crystals in display technology (for example, the twisted and supertwisted nematic devices, the surface stabilized ferroelectric device, etc.) and in thermography are also discussed. 

Order and Mobility are two basic principles of mother nature. The two extremes are realized in the perfect order of crystals with their lack of mobility and in the high mobility of liquids and their lack of order. Both properties are combined in liquid crystalline phases based on the selforganization of formanisotropic molecules. Their importance became more and more visible during the last years in Material science they are a basis of new materials, in Life science they are important for many structure associated functions of biological systems. The main contribution of Polymer science to thermotropic and lyotropic liquid crystals as well as to biomembrane models consists in the fact that macromolecules can stabilize organized systems and at the same time retain mobility. The synthesis, structure, properties and phototunctionalization of polymeric amphiphiles in monolayers and multilayers will be discussed. 

In order to understand the basic principles of operation of the many different kinds of LCDs being developed and/or manufactured at the present time, it is necessary to briefly describe the liquid crystalline state and then define the physical properties of direct relevance to LCDs. First, the nematic, smectic and columnar liquid crystalline states will be described briefly. However, the rest of the monograph dealing with liquid crystals will concentrate on nematic liquid crystals and their physical properties, since the vast majority of LCDs manufactured operate using mixtures of thermotropic, non-amphiphilic rodlike organic compounds in the nematic state. 

As a result, not all the compounds discussed in the following are already tested for both, lyo- and thermomesomorphism. Thus, in some cases their amphotropic behavior is not yet proved but nevertheless likely. On the other hand, it becomes more and more clear that the strong separation of both kinds of liquid crystallinity is fading [65]. The amphiphilic/amphotropic compounds seem to open a view on an uniform field of liquid crystals, regarding both lyo-and thermotropic behavior of compounds together as one intrinsic principle of nature. 

A common feature of different types of liquid crystal polymers (LCPs), e.g., thermotropic side-chain or main-chain (either stiff or with flexible spacers) polymers, is their slowed-down dynamics compared to low molecular weight liquid crystals (LCs). Often polymers can be quenched to a glassy state in which the liquid-crystalline order is preserved but motions are completely frozen out. Such liquid-crystalline glasses provide a unique opportunity to determine, in principle, the full orientational distribution function, whereas only its second moment is available from motionally averaged NMR spectra. Thus LCP studies have made fundamental contributions to LC science. 

Like the liquid crystal phase sequences in which different thermotropic phases tend to occur as the temperature is increased, the usual phase sequence for a surfactant-solvent system with increasing surfactant concentration in the solvent is as follows micellar, cubic, hexagonal, lamellar, inverse hexagonal, inverse cubic, and inverse micellar. For any given system, not all phases will necessarily occur. Figure 3.6 demonstrates the principle behind this sequence with the solvent colored purple. A combination of volume fraction and molecular geometry drives phase formation. 

For the macroscopic orientation of lyotropic elastomers in mechanical fields the same principles as for thermotropic elastomers can be applied. In some ways the situation is even simpler as the rod-like micelles of the hexagonal phase are compatible with an overall prolate network chain conformation. In addition, the lamellar phase built up from disk-like micelles requires an overall oblate chain conformation to form a liquid single-crystal hydrogel (LSCH) [98]. 

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