Chiral nematic LCPs

The chiral nematic state, first observed in cholesteryl derivatives, was later detected in other chiral mesogens, and can also he induced hy adding small chiral molecules to a host nematic TCP. Chiral nematic LCPs have thus heen synthesized as side-chain polymers hy introducing a chiral unit in the tail moiety of mesogens or hy copolymerizing cholesterol-containing monomers with another potential mesogenic monomer. Examples [4] of these types are shown as structures (V) and (VI) ... 

In general, chiral nematic polymer liquid crystals (LCP) cannot form monodomains in which the rodlike polymers have a spatially uniform orientation within the sample. Typically, because of the high density of orientational defects, the LCPs are textured, with a distribution of polymer orientation. Microscopically, the polymer chains have a preferred orientation with a relatively narrow distribution around the average orientation. Macroscopically, the variation in space of the orientation results in a domain structure. Defects and orientational variations give rise to the polydomain texture and the overall LCP sample may be randomly ordered (Fig. 3). 

The development of storage and loss moduli after flow cessation is a useful tool to analyze structural relaxations on LCPs. Upon flow cessation, the flow-induced orientation is lost. The evolution of the moduli with time is because of the reformation of a chiral nematic phase that had become nematic under flow. In Fig. 10, the relaxation in complex modulus with time for various AEC samples is shown. 

Like the other side-chain LCPs described in the previous section, these materials also oflfer the possibility of locking the chiral nematic phase into the glassy state by rapid cooling to temperatures below Tg. This leads to a preservation of the structure and its reflected color. With suitable systems, the process can thus be used to produce stable and light-fast monochromatic films. 

Interference and angle-dependent color effects can also be achieved by layers or particles based on liquid crystal polymers (LCP) Such effects can, for example, be produced by small plate-like substances which consist of an LCP material itself, or by small platelets which are uniformly coated with a cross-linked liquid crystalline polymer in a chiral-nematic arrangement. ... 

A unidirectionally aligned nematic or smectic liquid crystal phase can act as an optical retardation film. The same principle occurs with well-aligned LCP films which are being explored as compensation films for STN displays. Chiral nematic films also offer unique optical properties and in a polymeric form could he useful in converting ordinary light into circularly polarized light. 

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