DIELECTRIC-RELAXATION SPECTROSCOPY OF POLYETHYLENE TEREPHTHALATE (PET) FILMS

The dielectric properties of biaxially stretched polyethylene terephth alate (PET) films of 6 mu m thickness and 68% degree of crystallinity were investigated by means of dielectric relaxation spectroscopy in th e frequency range 10(-2)-10(6) Hz and the temperature range 20-190 deg rees C. Differencial scanning calorimetry (DSC), in the range 50-300 d egrees C, was employed to investigate the thermal properties of the PE T samples. Besides measuring the alpha relaxation associated with the glass transition and the secondary beta relaxation, special attention has been paid to the investigation of DC-conductivity-related effects. They give rise to high dielectric permittivity values and dielectric losses at low frequencies and high temperatures. The results are analy sed within the complex permittivity formalism and discussed in terms o f interfacial Maxwell-Wagner-Sillars polarization, the rho peak, condu ctivity relaxation, space-charge polarization, electrode polarization and DC conductivity. DC conductivity values determined from frequency plots of the AC conductivity follow the Vogel-Tamman-Fulcher equation at temperatures higher than the glass transition temperature, indicati ng that the charge-carrier transport mechanism is governed by the moti on of the polymeric chains. On the basis of the temperature dependence of the DC conductivity PET is classified as a fragile system.