The distribution of oxidation products in irradiated ultra-high molecular weight polyethylene

Ultra high molecular weight polyethylene (UHMWPE) has been irradiated in air and nitrogen to various received doses and stored in air for periods of time up to 84 months. The degree of oxidation has been measured using a microscope-FTIR technique, which allows assessment of the distribution of oxidation products throughout the thickness of the specimen. Maximum oxidation occurs close to the surface and is dose-dependent; however, the depth of penetration of the oxidised zone depends on the ageing period. Materials stored in vacuo for 35 days prior to irradiation under nitrogen show reduced levels of oxidation but react further with time, implying that the irradiated material still contains reactive species. Newly exposed surfaces of materials irradiated in air to a received dose of 50 Mrad and stored in air for 84 months also show evidence of renewed oxidation, but 'annealing' at 90°C for 24 h reduces the reactivity. Taken in conjunction with other observations, the results suggest that the dominant process during ageing of irradiated UHMWPE is oxidative chain scission. It seems probable that the active species are either radicals generated during the initial irradiation or secondary radical rearrangement products.