Recent Successes in Structural Studies of Thermotropic Liquid Crystalline Polymers

Abstract

Liquid crystals are widespread in nature and are synthesized in large amounts for various practical applications which are associated with the unique properties inherent in these compounds. These include the following: practically instant change of the macroproperties (color, light transmission, etc.) under the action of small gradients of stress, electric, magnetic, and thermal fields, and also with a change of chemical composition of the environment [1-7]. A suitable form of visual response to external actions, high sensitivity, and a wide range of “memory” times ensure practically unlimited fields of application for liquid crystal (LC) compounds. There are optical memory; light filters; fast-response optoelectronic gates; polarizers (nematic and smectic LC); temperature, stress, pressure, and chemical composition indicators; TV receivers; displays, and light panels (cholesteric and smectic LC). Due to such diverse fields of application, LC systems should meet a number of design requirements. For example, a thin film having selective reflectance is an optimum form for LC temperature indicators used in medicine and technology. It is obvious that a LC itself cannot meet this requirement. Therefore, in practice, liquid carriers of mesomorphic properties are enclosed in various sealed cells or microcapsules pressed between thin polymer films [3-10]. The practical application of such systems reveals a number of disadvantages, including low stability to stresses and low durability [8, 9]. The problems associated with the interaction of components in polymer–low molecular LC blends have not yet been studied thoroughly. Only two facts concerning the structural state of polymer–liquid crystals systems have been reliably established [9, 11–14]. First, the stability of polymer films containing liquid crystals becomes higher with decreasing gas permeability of the polymer and with the addition of a stabilizer. Second, forced blending of low molecular weight liquid crystals with a polymer results in a complete phase separation of finely dispersed liquid crystals. Microregions containing liquid crystal compound are equal to a few microns in diameter. The orientation of such a polymer film with LC microcapsules contributes to spontaneous orientation of liquid crystals in anisodiametric microcapsules due to the interaction of LC molecules with their walls [1, 12]. A higher degree of LC order in oriented films is used in the production of temperature indicators [1].