The free positron lifetime in amorphous polymers at low temperature

A series of amorphous polymers was studied by positron annihilation lifetime (PALS) spectroscopy at 20 K each experiment lasted more than 1 week. The lifetime spectra were deconvoluted into three exponential components. In isothermal conditions, an increase of the free positron lifetime is observed while the parapositronium (pPs) and orthopositronium (oPs) lifetimes do not change with the measuring time. This upward trend was correlated to the oPs intensity change. To explain the time dependence of the free positron lifetime, a model based on heterogeneities in the electronic density and accumulation of shallowly trapped electrons (STEs) caused by the 22 Na source irradiation is proposed. The fittings give two parameters the intrinsic free positron lifetime τ 0 2 and the Onsager radius R PALS 0 .