Polymers in the Liquid Crystal State

7 POLYMERS IN THE LIQUID CRYSTAL STATE 2.7.1 Liquid Crystal State 

The liquid crystal state (LCS) shows order in one or two dimensions it lacks the three-dimensional long-range order of the crystalline state. LCS has characteristics intermediate between those of the crystalline and the disordered amorphous states. These phases are called liquid crystals because many of them can flow like ordinary liquids but they display-birefringence and other properties characteristic of crystalline soHds. In liquid crystal phases the molecules can move but the orientational order is conserved in at least ne direction. The LCS can be displayed by small molecules and by polymersj but in both cases a characteristic chemical structure is needed. The existence of the liquid crystal state is related to the molecular asymmetry and the presence of strong anisotropic intermolecular interactions (19-21). Thus, molecules with a rigid rod structure can form highly ordered 

Poly(p-phenylene terephthalamide (Kevlar) (b) copolyester of p-hydroxybenzoic acid and biphenylene terephthalate (Xydar) (c) copolyester of p-hydroxybenzoic acid and 2,6-hydroxynaphthoic acid (Vectra). 

Liquid crystals are classified into two groups known as thermotropics and lyotropics. Thermotropics are those that are formed in the melting of crystalline solids, and they can remain in the liquid crystal mesophase without decomposition, passing to the isotropic liquid state when subsequently heated. As their mesophases are turbid, the temperature at which the transition to the isotropic liquid phase takes place is called the clearing temperature. Lyotropic LCs form mesophases in concentrated solution when the concentration exceeds a critical value. 

Thermotropic main-chain LC polymers are copolymers or terpoly-mers, generally polyesters and polyamides. Homopolymers have such high melting temperatures that they decompose before melting. There are several ways to lower their melting temperatures and be able to use them as thermotropics. 

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