Effect of a tensile stress on the radiolytic degradation of poly(methyl methacrylate)

Abstract

Abstract Plaques (2 mm thick) of atactic, bulk polymerized poly(methyl methacrylate) (PMMA) were exposed to γ-rays (dose rate 2.5 kGy h −1 , doses ranging from 5 to 10 kGy), at ambient temperature in a nitrogen atmosphere. Tensile-loaded samples (stress: 47 MPa) were exposed together with unloaded ones. The molecular weight distribution was studied before and after irradiation by size exclusion chromatography calibrated by PMMA quasi homodisperse standards. The yield of chain scission G(S) was determined for six different doses and a total number of stressed/unstressed samples of 38. The results clearly reveal a small but significant effect of tensile loading on the rate of chain scission: G(S) stressed/ G(S) unstressed ≈ 1.2. Two possible mechanisms are discussed. The first involves a primary radiolysis of skeleton CC bonds. In this case, tensile loading would disfavour the cage recombination of radicals and, thus, increase G(S) . The second (and more frequent) mechanism results from a scission of the primary macroradical, a mechanism which could be stress-activated.