Dielectric relaxations and phase transitions in thermotropic polymer liquid crystals: poly(N-ethylene oxide terephthaloyl bis(4-oxybenzoate))s

Abstract

Abstract The relaxations occuring in poly(triethylene oxide terephthaloyl bis(4-oxybenzoate)), PTrETOB, were studied by thermally stimulated depolarization currents technique (TSDC), and dynamic mechanical analysis (d.m.a.), and the results compared with those obtained by differential scanning calorimetry, thermo-optical analysis and wide angle X-ray diffraction. The low temperature relaxation observed by TSDC and d.m.a. is a complex band and as the temperature increases there are three very sharp and intense peaks which are attributed, by comparing the results of the various techniques used here, to the primary glass transition of the amorphous zones of the material, to an additional crystallization process and to the crystal-liquid crystal transition. Our results show the existence of a phase transition from this smectic mesophase to a nematic one at 210°C and a transition to the isotropic melt at 250°C. The direct signal analysis was applied to the low temperature spectrum in order to decompose the complex band in elementary curves characterized by Arrhenius relaxation times. It was found that the low temperature band covers a wide energy range from 5.1 to 15.9 kcal mol −1 and that the pre-exponential factor is almost constant in this range. Similar results were obtained for the polyester with four oxymethylenic units in the spacer. These results can be understood if the mechanisms responsible for this low temperature relaxation are very localized motions of the COO groups of the mesogenic unit, whose parameters are not affected by changes in the length of the spacer.