Temperature and time dependence of orthopositronium formation in polystyrene

Three measurement protocols were performed to investigate the temperature d ependence of positron lifetime spectra for polystyrene over the temperature range -70 to 180 degrees C. The polystyrene samples were first subjected t o a heating schedule in sequential 10 degrees C increments. This was follow ed by a cooling schedule, again in 10 degrees C increments. The third sched ule involved rejuvenation of the specimen above T-g between each measuremen t. The measured lifetime spectra were resolved into three components. We ob served a minimum in the temperature dependence of the o-Ps intensity I-3 at similar to 20 degrees C for polystyrene specimens subjected to the heating and cooling schedules, but not for the rejuvenated samples. The influence of the duration of e(+)-irradiation on the lifetime and intensity of o-Ps a nnihilation was investigated under isothermal conditions. A decrease in o-P s intensity I-3 with duration of exposure to e(+)-irradiation was found who se rate was maximal at 23 degrees C. The results suggest that the time depe ndence of the o-Ps intensity in the glass can be interpreted within the fra mework of the spur-model. Specifically, the variation of I-3 is ascribed to the formation at low temperature of free radicals with small electron affi nities. After correcting for the e(+)-irradiation effect, no significant ag ing effect in the glassy state was detected in the o-Ps intensity and lifet ime. (C) 1999 Elsevier Science Ltd. All rights reserved.