Physical ageing studies on amorphous poly(ethylene terephthalate). Part 1.?Enthalpic relaxation and electret properties

The physical ageing of amorphous poly(ethylene terephthalate) (PET) is studied by differential scanning calorimetry (DSC) and by thermally stimulated currents (TSC). The ageing of PET is studied for a range of ageing temperatures (T(a)) between 50 and 70-degrees-C at ageing times (t(a)) between 15 min and 25 h. The endothermic peak which appears in the glass-transition region is studied by the DSC technique and the maximum value of the enthalpy relaxation can be observed for T(a) between 60 and 65-degrees-C. At 70-degrees-C the structural relaxation to equilibrium is accomplished at 4 h. The physical ageing effect on the electret properties is studied by polarizing the PET sample before (P + A samples) or after the ageing (A + P samples) and further analysis by the TSC technique. In both cases, the effect of physical ageing on the position and charge of the alpha-dipolar peak is observed. The charge of the dipolar peak decreases with t(a), the decay in the P + A samples being sharper. The position of the alpha-peak proves to be more sensitive to the physical ageing than the temperature of the DSC endothermic peak. The TSC results are correlated with the calorimetric ones.