Nonisothermal and isothermal discharging currents in polyethylene terephthalate at elevated temperatures

The thermally stimulated discharge current and the isothermally final discharging current have been measured, in vacuum and in different ambient gases for “as-received” polyethylene terephthalate specimens, in order to understand the nature of the origin of the released current in the temperature range from glass–rubber transition temperature up to 220 °C. The behavior of the samples thermally treated in oxygen, in nitrogen and in ambient air was analyzed, the gases have been used for detecting the localized states in the material. The current spectrum is determined by the space-charge existing in the as-received sample, and by the adsorbed and/or absorbed gases and water vapors. The movement of the ions, resulting from the interaction of the adsorbed and absorbed gases with the parasitic space charge, in the field produced by the space charge, is responsible for observed change in polarity of the current during nonisothermal and/or isothermal measurements and for the appearance of the ρ or space-charge peak. This movement is considered to be thermally activated with a field-modified activation energy. The calculated activation energy, for the sample thermally treated in oxygen at different temperatures, was in the range (0.9–2.3)±0.1 eV. From the isothermal discharging current measurements, values for the exponent of time in the range from 0.04 to 0.7 were obtained suggesting a dispersive transport of the charge. The total charge density stored in the material is about 4×10−5 C and the corresponding trap density approximately 1023/m3. This charge is substantially larger than that determined by the pulsed electroacoustic method.