The stress cracking behaviour of poly(methyl methacrylate) after exposure to gamma radiation

Two major causes of polymer failure are discussed in this paper: degradation by gamma radiation and the environmental stress cracking. Both phenomena are reasonably well established in the literature with many papers dealing with them separately. However, the combined action of gamma radiation and stress cracking is a novel topic described in this work. Injection moulded bars of poly(methyl methacrylate) (PMMA) were exposed to gamma radiation to different doses and tested for mechanical properties and molecular weight. The exposed bars were then put in contact with ethanol, a stress cracking agent of PMMA. The stress cracking effects were evaluated by solvent absorption, stress relaxation and mechanical testing during solvent exposure. The results confirmed the high deterioration of PMMA by both gamma radiation and the active fluid. When degraded PMMA was exposed to ethanol the stress cracking effects tended to be higher in comparison to the undegraded polymer. This was possibly a result of the lower molecular weight and the higher affinity to ethanol. A study of fracture surface topography using scanning electron microscopy revealed surprising dendritic features in gamma degraded PMMA exposed to ethanol. These features were more evident when lower crosshead speeds were used during stress-strain experiments.