Influence of molecular orientation on polarization phenomena in polyethylene terephthalate films

The influence of uniaxial and biaxial orientation on polarization phenomena of polyethylene terephthalate (PET) films has been investigated. Thermally stimulated current (TSC) spectroscopy has been applied to the study of the kinetics of polarization phenomena in unoriented and in uniaxially and biaxially oriented PET films. Around 50°C, a relaxation mode that seems to be a precursor of the glass transition is observed. This sub-T_g mode has been attributed to free volume diffusion. Around the glass transition temperature (T_g) a relaxation mode is observed in all samples: in unoriented PET, it is located at 82°C, while in uniaxially oriented PET, is is shifted to 85°C, and in biaxially oriented PET, it reaches 100°C. These complex relaxation modes have been analyzed by using the technique of fractional polarizations, and the kinetics of the elementary depolarizations has been characterized by activation enthalpies and entropies. Around the crystallization temperature, a spontaneous TSC peak due to pyrocurrent is observed in unoriented and uniaxially oriented PET. It has been associated with the dielectric manifestation of the crystallization