Polymer stabilized cholesteric liquid

G. A. Held, L. L. Kosbar, I. Dierking, et al., Confocal microscopy study of texture transitions in a polymer stabilized cholesteric liquid crystal, Phys. Rev. Lett., 79, 3443 (1997). 

Ma, J., Zheng, Z., Liu, Y., Xuan, L. Electro-optical properties of polymer stabilized cholesteric liquid crystal film. Chin. Phys. B 20, 024212 (2011)... 

Nematic gels are very interesting systems, thus deserving further study. Actually, these systems are being studied experimentally for applications. Examples are polymer dispersed liquid crystal displays are sometimes dispersed not in a polymer, but in a polymer network. Displays by means of the polymer stabilized cholesteric texture change, are also achieved in crosslinked systems. In addition, the chiral smectic phase has been obtained in such systems as well. Other types of liquid crystal gels have been applied or are expected to be applied in such devices. 

D.-K. Yang, L.-C. Chien, and Y. K. Fung, Polymer stabilized cholesteric textures materials and applications, in Liquid crystals in complex geometries, ed. G.P. Crawford and S. Zumer (Taylor Francis, London, 103-143,1996). 

Another saturated tetrahydrofuryl core has found application as a component of liquid crystals. Cholesteric liquid crystal polymers are useful as photostable UV filters in cosmetic and pharmaceutical preparations for the protection of human epidermis and hair against UV radiation, especially in the range 280-450nm <2000DEP19848130>. Fused bifuran 81 is a suitable monomer for the preparation of these desired polymers as it contains the requisite characteristics of having more than one chiral, bifunctional subunit type which is capable of forming a cholesteric liquid crystal phase with a pitch of <450 nm. It also contains an achiral aromatic or cycloaliphatic hydroxyl or amino carboxylic acid subunit, achiral aromatic or cycloaliphatic dicarboxylic acids, and/or achiral aromatic or cycloaliphatic diols or diamines. Polymers prepared from suitable monomers, such as diol 81, can also be used as UV reflectors, UV stabilizers, and multilayer pigments. 

The following protocols (6-10) describe the synthesis of some cholesterol-based acrylates and their photopolymerization in an aligned cholesteric phase. The protocols utilize a modification of a system previously described by Shannon. 5 6 ip ie absence of a diacrylate comonomer, the cholesteric phase produced initially on copolymerization is not stable and reverts to a smectic phase on a single cycle of heating and cooling. In the presence of the diacrylate the first-formed phase is stable. This is one example of how crosslinking can stabilise the liquid crystal phase in liquid crystalline elastomers, others include, the so-called, polymer-stabilized liquid crystals and those described in the later protocols. 

Electro-Optic Properties of Polymer Stabilized Liquid Crystals. Polymer networks have been used to stabilize many of the liquid crystal display states in various types of displays quite advantageously. In this section, we present some recent work on correlating the material properties of the liquid crystal/polymer network composite to the electro-optic properties of the flat-panel displays specifically cholesteric texture displays (75) and simple nematic birefringent type displays (7(5). 

K. Sasagawa, H. Fujikake, H. Sato, M. Omodani, Cholesteric liquid crystal devices stabilized by polymer walls and networks. lEICE Trans. Electron. J92-C(l), 26-31 (2009)... 

Because of the additional translational order, the dislocations can exist in the cholesteric and smectic liquid crystals, which makes the texture of these liquid crystals even more complicated. Each liquid crystal phase shows characteristic textures and thus the optical texture becomes an important means to differentiate the phase of the liquid crystals. Liquid crystalline polymers have the same topologically stable defects as small molecular mass liquid crystals do, but the textures may be different due to the difference in the energetic stability of the same topological defects in both low molecular mass and polymeric liquid crystals (Kleman, 1991). In Chapter 3 we will discuss the textures in detail. 

Although instances of lyotropic PLCs predate studies of thermotropic PLCs, as they involved solutions of comparatively esoteric species — virus particles and helical polypeptides — studies of these liquid crystals were isolated to a few laboratories. Nevertheless, observations on these lyotropic PLCs did stimulate the first convincing theoretical rationalizations of spontaneously ordered fluid phases (see below). Much of the early experimental work was devoted to characterizing the texture of polypeptide solutions. (23) The chiral polypeptides (helical rods) generate a cholesteric structure in the solution the cholesteric pitch is strongly dependent on polymer concentration, dielectric properties of the solvent, and polymer molecular weight. Variable pitch (<1 - 100 pm) may be stabilized and locked into the solid state by (for example) evaporating the solvent in the presence of a nonvolatile plasticizer.(24)... 

The third section of the book addresses recent research efforts in making polymer-dispersed liquid crystals consisting of nematic or cholesteric low- molecular-mass liquid crystals in flexible-chain polymers or LCPs. These S3rstems are prepared by polymerization of reactive monomers in the presence of Uquid ciystads stabilized by flexible-chain polymer or LCP. Special attention has been paid to the use of these materials in display and electro-optical devices. 

There is a third problem for which chirality information is of current interest anisotropic phases are often stabilized by chiral structures. Apart from chiral structures with enantiomorphic crystals of chiral compounds, suprastructural chirality exists in liquid crystal phases built up by chiral molecules as in the cholesteric phases and the smectic C phases. Even liquid crystalline phases with suprastructural chirality originating in achiral, so-called banana-shaped molecules, seem to be possible. Anisotropic polymer films with chiral structures have been found. It can be anticipated that chiroptical spectroscopy with anisotropic chiral systems will lead to new questions and answers. 

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