Cold crystallization effects in PET and PEN by TSDC, DSC and X-ray diffraction

A comparative study of the different relaxations present in amorphous and partially crystallized poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) and poly(ethylene terephthalate) (PET) has been carried out by thermally stimulated depolarization currents (TSDC), differential scanning calorimetry (DSC), and X ray diffraction. Amorphous films have been crystallized thermically at temperatures between 170°C and 200°C (PEN); 100°C and 150°C (PET) by the thermal stimulation by steps method (TSS). DSC measurements of these samples show the appearance of a small endothermic prefusion peak once the crystallization of the sample is completed. This peak increases and shifts towards higher temperatures as the sample is further thermally treated. Associated with the appearance of this endothermic peak, the ρ relaxation passes through a maximum with a sharp decrease with further heat treatment. X-ray diffraction measurements of these samples show that the decrease in the ρ peak is associated to the improvement of the amorphous-crystal interphases. This can explain as well the endothermic peak that appears by DSC