STRUCTURE DEPENDENCE OF GAMMA-RAY IRRADIATION EFFECTS ON POLYETHYLENES STUDIED BY POSITRON ANNIHILATION

摘要

The radiation effects on five kinds of polyethylenes (PEs) were studied from the viewpoints of mechanical properties, free radicals and positron annihilation. The degradation of the mechanical strength of samples irradiated by ~(60)Co γ-rays was evaluated based on the elongation at break and the tensile strength. Fully stretched PE with a crystallinity larger than 99% was greatly affected by γ-irradiation. In low-density PEs free radicals were detectable only above 70 kGy, and sharply increased above 100 kGy. A detectable number of free radicals was obtained even at 0.1 kGy in PEs having high crystallinity. At around 100 kGy, the increase in the intensity (I_3) of ortho-positronium (o-Ps), seems to be associated with an increase in the mechanical strength. Contrary to the expectation that the o-Ps seeks intermolecular space holes only in amorphous regions, the I_3 obtained in PEs with nearly 100% crystallinity was almost half that of low-density PEs, and τ_3, had similar values. This suggests that even in crystalline regions the o-Ps seeks intermolecular space holes comparable to those in the amorphous regions.