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

The complex β dielectric relaxation process in amorphous poly(ethylene terephthalate) (PET) is investigated by thermally stimulated depolarization currents (TSDC) and isothermal depolarization currents (IDC) techniques. TSDC and IDC data are analyzed in the framework of the Ngai coupling model in the [−150; −40°C] temperature range using the Marchal method and involving a Kohlrausch–Williams–Watts (KWW) functional form of the polarization time decay. The temperature dependence of the characteristic τKWW relaxation time and of the βKWW stretching parameter of the KWW relaxation function derived from both depolarization current methods for the β relaxation of PET are determined and then compared. The very good agreement observed between the two sets of experimental stretching parameters determined by TSDC and IDC confirms the high potentiality of the TSDC technique to study complex relaxation processes as such as the β relaxation of PET. The two different overlapped relaxation processes involved in the β relaxation have been confirmed throughout the determination of two different temperature behaviors of the βKWW parameter. Each process of complex β relaxation is characterized by a low and nearly temperature-independent KWW stretching parameter. We show that the βKWW value of the high-temperature component, involving phenyl group motions, is slightly lower than the one corresponding to the low-temperature process assigned to the motions of carbonyl groups.