Molecular dynamics in liquid-crystalline side chain polymers studied by dielectric relaxation spectroscopy: A comparative study

Abstract

A series of liquid-crystalline side chain copolymers with different main chains have been studied by the dielectric method in a maximum frequency range of 9 decades. Oriented samples were used throughout. The data were analysed in terms of the Havriliak-Negami and Fuoss-Kirkwood formulae for the relaxation functions. Two well separated dispersion regions with their strengths depending strongly on the macroscopic orientation were found. The low frequency or δ-relaxation shows a marked change in its curve form and width with different main chain structure, its strength being determined by the longitudinal dipole moment of the mesogenic unit. The high frequency relaxation shows a more complicated dependence of its characteristic parameters on the molecular structure. In some cases a decomposition into two underlying relaxations was successfully attempted. We discuss the models for molecular motions developed for low molecular weight liquid crystals and for amorphous polymers, in order to explain the behaviour of the different dispersions found.