ANALYSIS OF THE THERMALLY STIMULATED DISCHARGE CURRENT AROUND GLASS-RUBBER TRANSITION-TEMPERATURE IN POLYETHYLENE TEREPHTHALATE

The nature of the thermally stimulated discharge current (TSDC) for po lyethylene terephthalate samples in the temperature range from room te mperature to above glass-rubber transition temperature of the amorphou s phase is analyzed. The well conditioning of the sample is strictly n ecessary In order to have a good reproducibility and accuracy of resul ts. A main peak was observed whose maximum temperature moves towards a lower value with the decreasing of the amount of charge that flows th rough the sample during polarization. The peak position changes as wel l, if the sample is polarized in air or in oxygen and the nature of ch ange is more important in the case of oxygen, The shape of the peak is complex and at least four shoulders have been identified around 85, 9 0, 105, and 125 degrees C using the cleaning technique. The activation energy tends to increase with repetition of the TSDC runs, in the gla ss-rubber transition temperature range, in the case when the cleaning technique is used for the peaks separation. For the conditioned sample s, there is a good agreement between the experimental results and the analytical expression of the current, particularly in the region where it reaches a maximum, and so relevant values for the characteristic p arameters of the peak are determined. The time interval or the short c ircuiting of the sample, at roam temperature, before the TSDC measurem ent, strongly influences the initial rise of the current and consequen tly the parameters of the peak. A possible redistribution of the inter nal field arising from the injected charge, the heterocharge, and the existing charge in the sample as received, has been put forward to acc ount for the experimental evidences. The conclusion is that the curren t is mainly determined by the space-charge released from the traps tha t are likely continuously distributed in energy. For the stated polari zation conditions, the charge is released from the shallow traps with an activation energy in the range 0.23-0.32 eV and a concentration of similar to 10(18)/m(3). The dipolar charge is of little importance. (C ) 1997 American Institute of Physics.