Low-Molecular Weight Liquid Crystals

LMWLCs are often used as models for theoretial calculations and experimental observations of the behavior of liquid-crystalline polymers. Several homologous series of mesogenic compounds are classihed as LMWLCs. Among them are 

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We are all familiar with tire tliree states of matter gases, liquids and solids. In tire 19tli century the liquid crystal state was discovered [1 and 2] tliis can be considered as tire fourtli state of matter [3].The essential features and properties of liquid crystal phases and tlieir relation to molecular stmcture are discussed here. Liquid crystals are encountered in liquid crystal displays (LCDs) in digital watches and otlier electronic equipment. Such applications are also considered later in tliis section. Surfactants and lipids fonn various types of liquid crystal phase but this is discussed in section C2.3. This section focuses on low-molecular-weight liquid crystals, polymer liquid crystals being discussed in tire previous section. 

Demus D, Goodby J, Gray G W, Spiess H-W and Vill V (eds) 1998 Handbook of Liquid Crystals Vol 2A. Low Molecular Weight Liquid Crystals I (New York Wiley-VCH)... 

The transition behavior of a number of liquid crystals with side-chain mesogens is summarized in Table 5. The most obvious feature of macromolecular liquid crystals is the frequent absence of fully ordered crystals at low temperatures. If fully ordered crystals are observed, crystallization is incomplete, i.e. the observed phase states are to be described by an area on the right side of Fig. 3. Glass transitions, which were hard to find in low molecular weight liquid crystals (see Table 3), are now prominent. 

Information on the crystal to liquid crystal transitions is scarce and is to be treated with caution since partial crystallization is prominent and polymorphism of the smectic phase is frequent. Only the data on poly(acryloyloxybenzoic acid) (entry 2 of Table 5) have been extrapolated to 100% crystallinity. As with the low molecular weight liquid crystals, the total heat of transition is lower than expected for fully ordered crystals. Various combinations of two phase structures as suggested by Fig. 3 could be produced for the poly(acryloyloxybenzoic acid)21>. 

Fig. 7. Effect of Long Alkyl Terminal Groups on the Conformation of Low Molecular Weight Liquid Crystals...

In low molecular weight liquid crystals the enthalpy of clearing is usually a small fraction, approximately 3-5 %, of that of melting In polymers, the lack of complete... 

Despite the apparent advantages of such a technique only very few references are found in the literature. This is surprising in view of the very large number of publications in recent years on liquid crystal polymers and of course also on low molecular weight liquid crystals. We wish here to illustrate this thermo-optical analysis (70A) as a tool for characterization of liquid crystal polymers. 

Because the textures of liquid crystalline polymers (LCPs) are qualitatively similar to those of low molecular weight liquid crystals, they are interpreted in the same way. However, the microscopy of LCPs is less straightforward ... 

N,N-Dimethylacetamide (DMAC). The two polymers exhibit similar conformations and both exist in anisotropic phases above a certain critical concentration. It was found that when the ternary systems were examined, two anisotropic phases exist above the critical volume fraction. Further, each of the stable anisotropic phases contain pure polymer. The authors conclude that miscibility between polymers forming the same type of mesophase isn t necessarily observed, in contrast to low molecular weight liquid crystals. 

Fully developed 3D microstructures are depicted in Figure 3.77. However, edges of the positive-resist structures are rounded in Figure 3.75. This is mainly caused by proton diffusion during the postexposure step [264], The prismatic cavities shown in Figure 3.75c may be a possible approach for switchable grating devices if they are filled with a low molecular weight liquid crystal. 

T. Kajiyama, H. Kikuchi, and K. Nakamura, Photoresponsive electrooptical effect of liquid crystalline polymer/low molecular weight liquid crystal composite system, Proc. SPIE 1911, 111-121 (1993). 

Side-Chain Polymers and Low Molecular Weight Liquid Crystals... 

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