Relaxation behavior of polymers probed by positron annihilation lifetime spectroscopy

Abstract

Relaxation behaviors of polyethylene, polypropylene, and polycarbonate have been studied by positron annihilation lifetime spectroscopy (PALS). In PALS positron sources made of radioisotopes are used to inject positrons into polymer as a micro probe. The injected positron probes can induce radiation effect, which plays an important role in detecting the polymer relaxation behavior through electrons trapped in shallow potentials at low temperature. Monitoring the intensity (I3) of orthopositronium (o-Ps), transitions of γ and δ relaxation can be measured by PALS as a secondary effect. In this experiment, the change of I3 below Tg is connected with the number of the trapped electrons, which can be excited from the shallow potential by the thermal motion of polymer structures and visible light irradiation. In the PALS measurements of non-irradiated PP samples, relaxation of methyl groups was observed as low as 50 K, which can be assigned as the δ relaxation. Relaxations of β and γ were also observed for the non-irradiated PP samples between 100–370 K. However for the 3 MGy Y-ray irradiated PP samples, only β relaxation was observed because the large radiation dose caused a large number of scissions of -CH3 groups from main chains and the characteristics changed. For the irradiated samples, radiation hardening was observed.