DIELECTRIC RELAXATIONS AND PHASE-TRANSITIONS IN THERMOTROPIC POLYMER LIQUID-CRYSTALS - POLY(N-ETHYLENE OXIDE TEREPHTHALOYL BIS(4-OXYBENZOATE))S

The relaxations occurring in poly(triethylene oxide terephthaloyl bis( 4-oxybenzoate)), PTrETOB, were studied by thermally stimulated depolar ization currents technique (TSDC), and dynamic mechanical analysis (d. m.a.), and the results compared with those obtained by differential sc anning calorimetry, thermo-optical analysis and wide angle X-ray diffr action. The low temperature relaxation observed by TSDC and d.m.a. is a complex band and as the temperature increases there are three very s harp and intense peaks which are attributed, by comparing the results of the various techniques used here, to the primary glass transition o f the amorphous zones of the material, to an additional crystallizatio n process and to the crystal-liquid crystal transition. Our results sh ow the existence of a phase transition from this smectic mesophase to a nematic one at 210 degrees C and a transition to the isotropic melt at 250 degrees C. The direct signal analysis was applied to the low te mperature spectrum in order to decompose the complex band in elementar y curves characterized by Arrhenius relaxation times. It was found tha t 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 i n this range. Similar results were obtained for the polyester with fou r oxymethylenic units in the spacer. These results can be understood i f the mechanisms responsible for this low temperature relaxation are v ery localized motions of the COO groups of the mesogenic unit, whose p arameters are not affected by changes in the length of the spacer. Cop yright (C) 1996 Elsevier Science Ltd.