Dynamic mechanical studies of irradiated polyethylene

Abstract

The dynamic mechanical properties of high density polyethylene irradiated in a 60Co source at dosages of 108–109 r.e.p. have been investigated from 80°K to 450°K at audiofrequencies using a resonance technique. With increasing radiation dose the slope of the modulus v. temperature relation above the melting temperature, 410°K, increases, indicating an increasing degree of crosslinking of the network structure. The damping peak in the 390°K region is also observed to decrease in magnitude. At gamma-ray doses of 6 × 108–109 r.e.p. the crosslinking efficiency is found to be somewhat greater for the high density polyethylene than for a low density polyethylene irradiated at room temperature under similar conditions. The effect of a subsequent heat treating and annealing operation has been studied for both low and high density polyethylene and for both 60Co irradiation and pile irradiation. For doses up to 1018 n.v.t. or so, the primary effect of heat treatment on the mechanical properties of the pile-irradiated high density samples is to decrease the values of dynamic modulus and increase the values of the mechanical loss over the entire temperature range from 100 K to 400 K. This is attributed to a reduction in crystallinity and is confirmed by room temperature measurements of crystallinity and density. It is also observed that, especially after subsequent heat treatment above the original melting temperatures, samples of high density polyethylene crosslinked to a similar degree in either a 60Co source or a nuclear reactor exhibit comparable dynamic mechanical behaviour. For samples of high density polyethylene irradiated in a pile to a dose of 1.5 × 1018 n.v.t. the influence of oxygen on the dynamic mechanical properties of laboratory size samples has been found to be negligible.