Micro-structure of gamma-ray irradiated polyethylenes studied by positron annihilation

Effects of gamma-ray irradiation on high density (HDPE) and low density (LDPE) polyethylenes were measured by positron annihilation at temperatures between 100 and 420 K. The effect of the irradiation on the intensity I3 and the lifetime τ3 of the longest lived component was significant below the glass transition temperature Tg, while they were little affected above Tg. For LDPE a marked minimum was observed in the I3 vs T curve. The lower edge of the minimum, corresponding to glass II and glass I transitions, was found to shift to lower temperatures by the irradiation. In HDPE both I3 and τ3 decreased on irradiation mainly due to radiation induced free radicals. The gel fraction of HDPE was small when irradiated below Tm, while it rose sharply on irradiation near Tm. Positron lifetime parameters of this highly crosslinked HDPE showed a distinct difference compared to HDPE irradiated below Tm. Usefulness of the positron annihilation method is discussed in conjunction with studying micro-structure of polymers.