Combined dielectric spectroscopy and thermally stimulated currents studies of the secondary relaxation process in amorphous poly(ethylene terephthalate)

The dielectric secondary beta relaxation process of poly(ethylene terephthalate) has been investigated by dielectric spectroscopy (DS) and thermally stimulated currents (TSC). To analyze the broad and asymmetric DS spectra, without previous assumptions on the shape and the number of overlapped loss components constituting the beta relaxation, a description in terms of a distribution of relaxation times (DRT) has been used. The Tikhonov regularization method has been used to extract the DRT from DS data. At lower frequency, the TSC technique revealed two components in the beta relaxation process. The TSC spectra are experimentally resolved into elementary Debye processes by using the fractional polarization method. A set of elementary relaxation times with relaxation strength Delta epsilon(i) as statistical weight of TSC-DRT have been calculated. The temperature shifts of the DRT obtained from both dielectric techniques are analyzed. Whereas the maximum and the lower time wing of the DRT obtained from DS correspond to the beta(2) process resolved by TSC and associated to the noncooperative motions of carbonyl groups, the upper time wing of DRT extracted from DS is in accordance with the beta(1) process resolved by TSC and assigned to the cooperative motions of phenyl rings. (C) 1999 John Wiley & Sons, Inc.