Cold crystallization effects in free charge relaxation in PET and PEN

A comparative study of free charge relaxation in amorphous and partially cr ystallized poly(ethylene-2,6-naphthalene dicarboxylate) (PEN) and poly(ethy lene terephthalate) (PET) has been carried out by thermally stimulated depo larization currents (TSDC), differential scanning calorimetry (DSC), and X- ray diffraction. Amorphous films have been crystallized thermally at temper atures between 170 and 200 degrees C (PEN); 100 and 150 degrees C (PET) by the thermal stimulation by steps method. The windowing polarization (WP) te chnique has been applied to form PET and PEN thermoelectrets. TSDC of these electrets polarized at 86 degrees C (PET) and 130 degrees C (PEN) show onl y one peak which is attributed to space charge relaxation (p peak). The evo lution of this peak has been fitted to the general kinetic order model. DSC measurements of these samples show the appearance of a small endothermic p refusion peak once the crystallization of the sample is completed. This pea k increases and shifts towards higher temperatures as the sample is further thermally treated. Associated with the appearance of this endothermic peak , the p relaxation passes through a maximum with a sharp decrease with furt her heat temperature. The X-ray diffraction measurements of these samples s how that the decrease in the p peak is associated with the improvement of t he amorphous-crystal interphases. (C) 2000 Elsevier Science Ltd. All rights reserved.