13C-NMR study of crosslinking and long-chain branching in linear polyethylene induced by 60Co gamma ray irradiation at different temperatures

H-type crosslinking and Y-type long-chain branching have been examined for linear polyethylene samples irradiate at different temperatures with ⁶⁰Co gamma rays in vacuum by solution-state ¹³C-NMR spectroscopy. In this study, relatively low molecular weight samples were irradiated with doses less than a gel dose in both solid and molten states. Resonance lines associated with H-links and Y-branches have clearly been observed for each irradiated sample and their radiation yields significantly depend on the irradiation temperature and the morphology of the samples. In particular, the G-value for the production of Y-branches is higher than that for H-links at lower temperatures, while the latter is superior to the former at higher temperatures. This may be due to the decrease in the concentration of the primary radicals, which are formed by main-chain scissions and associated with the production of Y-branches, by the recombination with small fragments at higher temperatures. The radiation yields of methyl branches and double bonds are also briefly discussed.