Dielectric relaxation spectra of two 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 structure 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 strength and alignment with temperature. The speed of motion of the mesogens is compared between different members of the copolymer series by normalising the experimental temperature by either the glass transition temperature 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 of motion due to greater mesogen mobility if the data is compared at temperatures normalised by the clearing point. However, if the temperature is normalised by the glass transition, the peak frequencies of motion are much closer (although the material with a longer spacer group still moves slightly faster), indicating the important role the motion of the polymer backbone plays in determining the mesogenic mobility. The activation energy of motion is found to be less for the copolymer with the longer spacer length and consequently in the copolymers with greater content of the long spacer unit side chain.