Free Volumes in Polystyrene Probed by Positron Annihilation

Polystyrene specimens with different molecular weights were characterized by the positron annihilation technique. Lifetime spectra of positrons were measured in the temperature range between room temperature and ~150 °C. The observed lifetime spectra were resolved into three components, where the longest lifetime, τ3, was associated with pick-off annihilation of orthopositronium (o-Ps) trapped by free-volumes. Above the glass transition temperature (Tg), the temperature coefficient of τ3 was found to be about 4 times larger than that below Tg, wile no drastic change in the intensity corresponding to the annihilation of o-Ps was observed at Tg. These facts suggest that the glass transition mainly affects the expansion rate of the free volumes in polystyrene specimens