DIELECTRIC-RELAXATION SPECTRA OF 2 SIDE-CHAIN LIQUID-CRYSTALLINE HOMOPOLYMERS AND THEIR COPOLYMERS

Dielectric relaxation spectra have been measured for a complete series of siloxane-based side chain Liquid crystalline copolymers (with two mesogenic side groups of similar structure but having flexible spacer groups of different length) and the effect of the copolymer chemical s tructure on thermal properties and dielectric motion is presented. The qualitative behaviour observed in the spectra of two of the polymers with regard to their change in peak height of the loss relaxation with temperature is discussed in terms of influences on relaxation strengt h and alignment with temperature. The speed of motion of the mesogens is compared between different members of the copolymer series by norma lising the experimental temperature by either the glass transition tem perature of the material or its clearing point (liquid crystalline to isotropic temperature). The effect of an increased content of the side group with a very long flexible spacer is to increase the frequency o f motion due to greater mesogen mobility if the data is compared at te mperatures normalised by the clearing point. However, if the temperatu re is normalised by the glass transition, the peak frequencies of moti on are much closer (although the material with a longer spacer group s till moves slightly faster), indicating the important role the motion of the polymer backbone plays in determining the mesogenic mobility. T he activation energy of motion is found to be less for the copolymer w ith the longer spacer Length and consequently in the copolymers with g reater content of the long spacer unit side chain. Copyright (C) 1996 Elsevier Science Ltd