Dielectric relaxation in a thermotropic side chain liquid crystalline polymer with nematic reentrant phase

Abstract

Dielectric spectroscopy of liquid crystalline cyanobiphenyl polyacrylates (CBPA6) with six methylene groups in the side chain has been investigated in the frequency range of 30 Hz–13 MHz and in the temperature interval from 100 to 470 K. From the temperature dependence of ε′(T), the anomalies corresponding to sequential phase transitions of liquid crystalline mesophases were revealed. The observed α- and δ-relaxation processes were described by the Vogel–Fulcher–Tammann law. It was found that δ-relaxation exhibits a multistage slowing behavior with decreasing temperature. The existence of a reentrant nematic (Nre) phase upper glassy phase shows us the special conditions for glass transition: the enhancement of the degree of cooperativity in mesogenic subsystems due to corrugation effects results in interaction of the side chain modes with a mixture of segmental and normal modes of the backbone, leading to a step-like anomaly of ε′(T) at the glass transition.