A comparison of the effects of ultra-violet and gamma radiation in polymethylmethacrylate

Abstract

The exposure of polymethylmethacrylate (PMM) and many other macromolecules to high-energy $\gamma$- or electron radiation produces a degradation of the molecular chains. This may result from either ionization or excitation. Ultra-violet light results in excitation only. This paper compares the effects of these forms of radiation. The degradation of PMM has been studied in thick or thin films, and in solutions in benzene. It is found that at room temperature degradation (random main chain fracture) occurs with ultra-violet radiation, whereas at higher temperatures the reaction is one of chain depolymerization. The energy absorbed per main chain fracture is about 550 eV with ultraviolet radiation, so that less than 1% of the quanta absorbed are effective; this compares with about 65 eV of $\gamma$-energy absorbed per fracture. Apart from this difference in efficiency, the two reactions are very similar. In both cases the number of fractures is proportional to dose and the radicals formed (as seen by e.s.r.) are identical in character. Somewhat similar changes also appear in the optical spectrum, although these may be modified by surface oxygen. These results show no significant contribution from any ionization produced by high-energy radiation.