MECHANISM OF AGING IN IRRADIATED POLYMERS

Samples of ultra-high molecular weight polyethylene (UHMWPE) and nylon 66, of grades suitable for use in medical prostheses, were subjected to irradiation using a **6**0Co source. The effects of a 10 Mrad dose on percentage crystallinity, tensile behavior and dynamic mechanical response were established 3 days after irradiation, and irradiated samples were then retested at intervals over a one year period. Crystallinity in UHMWPE was found to increase progressively during this period with consequent changes in mechanical behavior. It is proposed that the observed ageing process arises from the breakdown of metastable groups in the polymer, which are formed from the breakdown of metastable groups in the polymer, which are formed on irradiation with consequent release of long inter-lamella tie chains. This removes an inhibition to crystal perfection processes, which can occur at room temperature in polyethylene. It is proposed that the metastable species introduced on irradiation is peroxy or hydroperoxy and that this decomposes on ageing to a carbonyl group, which concurrent chain scission.