Electrical properties, thermotropic liquid crystal polymers

Liquid crystal polymers (LCP) are polymers that exhibit liquid crystal characteristics either in solution (lyotropic liquid crystal) or in the melt (thermotropic liquid crystal) [Ballauf, 1989 Finkelmann, 1987 Morgan et al., 1987]. We need to define the liquid crystal state before proceeding. Crystalline solids have three-dimensional, long-range ordering of molecules. The molecules are said to be ordered or oriented with respect to their centers of mass and their molecular axes. The physical properties (e.g., refractive index, electrical conductivity, coefficient of thermal expansion) of a wide variety of crystalline substances vary in different directions. Such substances are referred to as anisotropic substances. Substances that have the same properties in all directions are referred to as isotropic substances. For example, liquids that possess no long-range molecular order in any dimension are described as isotropic. 

A specific property of liquid crystals is the ability to undergo structural transformadons under the effect of electric and magnetic fields. The appearance of lyo- and thermotropic mesomorphism in polymer systems also implies the possibility of controlling their structural-optical properties with external fields. This ability to alter the structure in electric and magnetic fields was one of the most convincing arguments in support of the validity of existence of polymers in the LC phase in general. 

The unique physical and chemical properties of liquid crystalline polymers make them attractive to chemists, physicists, electrical engineers, mechanical engineers and chemical engineers. We have limited ourselves here to a description of thermotropic liquid crystals—those prepared by heating certain polymers. 

This review deals with LC polymers containing mesogenic groups in the side chains of macromolecules. Having no pretence to cover the abundant literature related to thermotropic LC polymers, it seemed reasonable to deal with the most important topics associated with synthesis of nematic, smectic and cholesteric liquid crystals, the peculiarities of their structure and properties, and to discuss structural-optical transformations induced in these systems by electric and magnetic fields. Some aspects of this topic are also discussed in the reviews by Rehage and Finkelmann 27), and Hardy 28). Here we shall pay relatively more attention to the results of Soviet researchers working in the field. 

---