Morphological effects on formation and behavior of radicals in γ‐irradiated polyethylene single crystals

Abstract

Polyethylene single crystals differing in lamellar thickness, both as‐grown and annealed with different lamellar thickness, were irradiated by γ‐rays to a dose of about 10⁷ rad at liquid nitrogen temperature in vacuo, and then ESR measurements were made. It was found for the as‐grown crystals that alkyl radicals were concentrated at the crystal surface. For the annealed crystals it was found that the radical concentration was greater than in the original crystals because of an increase in disorder with annealing. By assuming that the crystals form blocks upon annealing and that the surface and the interior of the blocks have the same trapping capacities for radicals as in the original crystals, the dependence of the size of the blocks upon variation in annealing temperature and the original lamellar thickness was estimated. This estimate is supported by the theory of the thickening process of single crystals. Two types of radical reactions with different reaction rates were found to occur simultaneously at room temperature. The rapid process was independent of lamellar thickness and was related to the reaction of radicals mainly in the surface region and the defects within the crystals. The slow process was strongly dependent on the lamellar thickness (i.e., the reaction rate was much depressed as the lamellar thickness was increased) and was inferred to be closely related to molecular motions manifested in viscoelastic measurements by the crystalline dispersion α_c.