Side-chain liquid crystalline elastomers: the relationship between the orientational ordering of the polymer backbone and the length of the coupling chain

The influence of cross-linking on the phase behaviour of a series of side-c hain liquid crystalline elastomers has been studied. For samples cross-link ed in the temperature range corresponding to the nematic phase, the phase t ransition was shifted compared to that observed when an identical sample wa s cross-linked in the isotropic phase. This shift represented a stabilisati on of the nematic phase in the former case, in line with theoretical expect ations. By utilising a novel., slow cross-linking method, which allows the polymer backbone to take up an equilibrium conformation prior to network fo rmation, it proved possible to monitor the shifts in phase transition tempe rature as a function of the length of the methylene chain coupling the meso genic units to the polymer backbone. The results obtained are related to th e backbone anisotropy and indicate that the level of orientational order of the polymer in the nematic phase backbone increases with a reduction in th e length of the coupling chain. (C) 1999 Elsevier Science Ltd. All rights r eserved.