Nematic Liquid Crystals Formed from Flexible Molecules: a Molecular Field Theory

Abstract

Thermotropic liquid crystal polymers may be formed by linking rigid mesogenic units, such as an azoxybenzene, with flexible spacers which are normally alkyl chains. The liquid crystalline properties of these materials are found to depend critically on the length of the alkyl chain. For example, both the nematic-isotropic transition temperature and the entropy of transition exhibit a marked alternation with the number of methylene groups in the flexible spacer, as the results for the poly α,ω-[4,4’-(2,2’-dimethylazoxyphenyl)]alkandioates given in Figs 1 and 2 clearly demonstrate.1 In consequence any molecular theory of such liquid crystal polymers must allow specifically for the effects of the flexible spacer. This is a formidable task for a polymeric system but one which may not be necessary for certain mesogenic properties of low molar mass materials are found to be analogous to those of the liquid crystal polymers. Thus the variation of the transition temperatures and entropies for the) α,ω-bis(4,4’-cyanobiphenyloxy)alkanes with the length of the flexible spacer is strikingly similar to those found for polymeric materials as the results in Figs 3 and 4 show.2 It is clearly easier to develop a molecular theory for these simpler systems and this should enable us also to understand the liquid crystal behaviour of the thermotropic liquid crystal polymers. Indeed such a theory has been produced for nematics such as the 4-n-alkyl-4’-cyanobiphenyls for which an alkyl chain is attached to a single rigid core.