Volatile evolution induced by energetic He++ ions in a polyurethane and the effects of previous gamma irradiation

Irradiation of polymer samples using an accelerated beam of He++ ions passed through a 10 μm thick window of havar foil, has been performed. Such an irradiation simulates the effects of large α radiation doses, on a vastly reduced time-scale. The experimental set up was designed to allow analysis of volatiles evolved from the irradiated samples by means of a residual gas analyser (RGA). This was located in close proximity to the sample chamber. A radiation study on a poly(urethane) materials using an RGA to analyse volatiles indicated the dominant degradation products to be H2, CO and CO2. A series of polyurethane samples previously conditioned by γ irradiation to between 1 and 5 MGy were irradiated in the ion beam. Identification of differences in trends in the rates of volatile evolution between these samples indicated the precise vacuum conditions at the time of irradiation had a major influence. There was also an indication that the surface of the sample had a small effect on rates of volatile evolution. Comparative plots of CO and CO2 evolution for series of 5 × 1 MGy irradiations indicated variations in behaviour between samples with different γ doses. Evolution during the first 1 MGy was inhibited for the unirradiated sample, the extent of inhibition diminished with increasing γ dose and was no longer evident in a sample with 1.5 MGy γ dose. H2 does not show an equivalent inhibition. Evidence for a low dose crosslinking reaction is put forward as the reason for the inhibition. Chemical reaction mechanisms are postulated and used to explain the differences in behaviour observed between CO/CO2 and H2.