Influence of molecular orientation on polarization phenomena in polyethylene terephthalate films

A comparative study of the amorphous and crystallized unoriented polyethylene terephthalate (PET), uniaxially and biaxially oriented PET films has been performed by thermally stimulated current (TSC) spectroscopy. The study of TSC peak observed around the glass transition temperature (T_g) has been undertaken. Below T _g a relaxation mode that seems to be a precursor of the glass transition is observed. The shift of the T_g peak towards higher temperature under stretching and crystallization indicates a stiffening of the amorphous chains. The analysis of the fine structure of the T_g peak has been done by using fractional polarization. The various elementary peaks correspond to relaxation times obeying a compensation law. At the compensation temperature T_c all the relaxation times become equal. The difference (T_c-T_g) is higher for the biaxially oriented PET. This behavior has been assigned to the existence of an important crystalline phase consecutive to biaxial orientation. This phase modifies the relaxation processes involved in the amorphous phase