Internal Friction in Irradiated Polymethylmethacrylate

Abstract
Some results of the influence of γ radiation on the mechanical properties of Polymethylmethacrylate are presented. Doses of 0, 50, 100, 150 and 200 kGy were employed. The internal friction (loss tangent) and storage shear modulus were measured at temperatures between 173 K and 300 K using a damped torsion pendulum. Compressive tests were made at room temperature with strain rates between 1.4x10^-3 s^-1 and 3.5x10^-2 s^-1. The materials studied were previously characterized by means of their molecular weight with the GPC technique. Loss tangent versus frequency curves are given, showing the α, α' and β relaxations. A linear relationship between the glass transition temperature, calculated at a fixed frequency, and the irradiation dose is obtained. An anomalous behaviour of the shear storage modulus of the material when it was irradiated at doses lower than 100 kGy was noticed. A similar response was observed in measurements of the yield point in the same material.