The degradation of solid polymethylmethacrylate by ionizing radiation

Abstract

When polymethylmethacrylate in the solid form is irradiated in the atomic pile, or with γ-radiation, two reactions predominate: breakdown of the main chain, and decomposition of the side chain with evolution of gases. The former was followed by changes in viscosity, the molecular weight of the irradiated polymer being inversely proportional to the radiation dose (plus a small quantity of R 0 which depends on the initial molecular weight). The degradation is thought to proceed by random rupture of main-chain C— C bonds by rearrangement of the excited polymer, and 61 eV are absorbed per fractured bond. For each main-chain rupture approximately one ester side chain is also decomposed. The cage effect is thought to prevent direct dissociation, and reaction occurs by rearrangement of the molecule to give relatively stable entities. Added substances reduce the amount of degradation, possibly by transfer of energy from the excited polymer molecule. This is only possible if the excited molecules have an appreciable life before decomposition. Viscosity measurements on irradiated polymethylmethacrylate in the solid form offer a possible means of measuring high-energy radiation doses in the range of about 1 million röntgens and upwards.