Effect of the physical ageing on electrical conductivity and dielectric measurements of amorphous poly(ethylene terephthalate)

Electrical conductivity and dielectric relaxation measurements have been carried out during the physical ageing of amorphous poly(ethylene terephthalate) in the range of ageing temperatures T _a between 50 and 70°C and ageing times t _a between 10 min and 72 h. Both the intensity of the current and the imaginary part of the complex dielectric permittivity ∈" (or dissipation factor tan δ) decrease with t_a, at T_a constant, as a consequence of the structural relaxation of the material, reaching a constant value in a time which depends on the temperature. As T _a increases, the decay of the intensity and ∈" (or tan δ) increases as a consequence of the greater extension of the relaxation which shows a maximum in the interval 60-65°C for t _a⩾6 h