THERMALLY STIMULATED DEPOLARIZATION OF A COPOLYMER OF POLY(ETHYLENE-TEREPHTHALATE) AND P-HYDROXYBENZOIC ACID

A liquid crystal copolymer containing poly(ethylene terephthalate) (PET) and 0.6 mole fraction of p-hydroxybenzoic acid (PHB) was studied. There is a large amount of information available for this material, which has been obtained by techniques other than thermally stimulated depolarization (t.s.d.). Hence the usefulness of this latter method can be evaluated for the characterization of polymer liquid crystals. First, by applying the same field of 13.3 MV m-1 in each case, effects caused by varying the polarization temperature were determined. Second, for a constant polarization temperature of 90-degrees-C, the polarizing field was varied between 3.3 MV m-1 and 13.3 MV m-1. Third, for the same polarization temperature and fixed field, the storage time was varied between 0 h and 125 h. In the temperature range studied (20-190-degrees-C) the PET-rich phase contributes more to t.s.d. than the PHB-rich phase. High temperatures and strong fields produce similar effects of randomization of dipole orientations, or more generally of destruction of low-temperature structures. There is a competing effect of removal of restrictions on angular motions of dipoles along with increasing free volume. Longer storage times enable relaxation of electric stress created during electret formation. T.s.d. is one of several methods used for the determination of locations of phase transitions. However, in contrast to most other characterization techniques, t.s.d. permits the determination of effects of annealing on resulting structures at lower (e.g. ambient) temperatures. In particular, it is found that annealing shifts the cold crystallization process to higher temperatures.