Nonisothermal and isothermal discharging currents in polyethylene terephthalate at elevated temperatures

The thermally stimulated discharge current and the isothermally final disch arging current have been measured, in vacuum and in different ambient gases for "as-received" polyethylene terephthalate specimens, in order to unders tand the nature of the origin of the released current in the temperature ra nge from glass-rubber transition temperature up to 220 degrees C. The behav ior of the samples thermally treated in oxygen, in nitrogen and in ambient air was analyzed, the gases have been used for detecting the localized stat es 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 gas es and water vapors. The movement of the ions, resulting from the interacti on of the adsorbed and absorbed gases with the parasitic space charge, in t he field produced by the space charge, is responsible for observed change i n polarity of the current during nonisothermal and/or isothermal measuremen ts and for the appearance of the rho or space-charge peak. This movement is considered to be thermally activated with a field-modified activation ener gy. The calculated activation energy, for the sample thermally treated in o xygen at different temperatures, was in the range (0.9-2.3) +/- 0.1 eV. Fro m 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 i s about 4 x 10(-5) C and the corresponding trap density approximately 10(23 )/m(3). This charge is substantially larger than that determined by the pul sed electroacoustic method. (C) 1999 American Institute of Physics. [S0021- 8979(99)04803- 3].