Secondary dielectric beta-relaxation in amorphous poly(ethylene terephthalate): combined thermally stimulated and isothermal depolarization current investigations

The complex beta dielectric relaxation process in amorphous poly(ethylene t erephthalate) (PET) is investigated by thermally stimulated depolarization currents (TSDC) and isothermal depolarization currents (IDC) techniques. TS DC and IDC data are analyzed in the framework of the Ngai coupling model in the [-150; -40 degreesC] temperature range using the Marchal method and in volving a Kohlrausch-Williams-Watts (KWW) functional form of the polarizati on time decay. The temperature dependence of the characteristic tau (KWW) r elaxation time and of the beta (KWW) stretching parameter of the KWW relaxa tion function derived from both depolarization current methods fur the beta relaxation of PET are determined and then compared. The very good agreemen t observed between the two sets of experimental stretching parameters deter mined by TSDC and IDC confirms the high potentiality of the TSDC technique to study complex relaxation processes as such as the beta relaxation of PET . The two different overlapped relaxation processes involved in the beta re laxation have been confirmed throughout the determination of two different temperature behaviors of the beta (KWW) parameter. Each process of complex beta relaxation is characterized by a low and nearly temperature-independen t KWW stretching parameter. We show that the beta (KWW) value of the high-t emperature component, involving phenyl group motions, is slightly lower tha n the one corresponding to the low-temperature process assigned to the moti ons of carbonyl groups. (C) 2001 Elsevier Science Ltd. All rights reserved.