A comparison of the effects of α and γ irradiation on the crystallinity of polyethylene

Abstract

A comparison has been made of the effectiveness of Po-210 α-radiation and Co-60 γ-radiation in destroying crystallinity of “low-density” polyethylene, as measured by the infra-red absorption at 731 cm −1. At room temperature high doses of the order 10 3 Mrad were necessary to induce significant changes. In the absence of oxygen, α-radiation was found to be approximately twice as effective as γ-radiation in destroying crystallinity. This difference is ascribed to the greater density of radical radical interactions such as cross-linking within the alpha particle track. The possibility of local melting in the track due to the high rate of energy deposition is also discussed. In the presence of oxygen it was found that the effectiveness of α-radiation in destroying crystallinity was greatly increased and, since the crystalline regions of the polymer are relatively impermeable to oxygen, disordering of the close-packed chains is envisaged as taking place progressively from the crystallite surfaces as a result of the migration of radicals from the interior. The enhancing effect of oxygen is less marked in the case of α-irradiation and it is suggested that radical reactions within the track, by reducing the number of radicals free to migrate, are responsible for the diminished effect. At higher dose rates the effect of oxygen is negligible. Measurements over a range of alpha particle dose rates show that the loss of crystallinity increases with rate of energy input, but the effect, ascribed to gross heating of the specimen, is small. It is concluded that in spite of the very high rate of energy deposition by an alpha particle, the temperature within the track does not rise high enough, or long enough, to allow melting.