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The dielectric properties of biaxially stretched polyethylene terephthalate (PET) films of thickness and 68% degree of crystallinity were investigated by means of dielectric relaxation spectroscopy in the frequency range Hz and the temperature range C. Differencial scanning calorimetry (DSC), in the range C, was employed to investigate the thermal properties of the PET samples. Besides measuring the relaxation associated with the glass transition and the secondary 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 analysed within the complex permittivity formalism and discussed in terms of interfacial Maxwell - Wagner - Sillars polarization, the peak, conductivity 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, indicating that the charge-carrier transport mechanism is governed by the motion of the polymeric chains. On the basis of the temperature dependence of the DC conductivity PET is classified as a fragile system.

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Effects of confinement of the 4.0 nm pores of Vycor glass on the a relaxation associated with the glass transition were investigated for lhree glassforming liquids, glycerol. propylene glycol and propylene carbonate, by dielectric relaxatioo spectroscopy. The relaxation shifts lo lower temperaNres in the isochronal (constant-frequency) measurements and to higher frequencies in the isothermal measurements for the confined liquids as compared to the bulk. Confinement induces a broadening of the relaxation and a change of its shape. The results =e discussed in relation to the characteristic length of the glass transition (cooperativity length).