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Electron-irradiated specimens of linear polyethylenes, aged at ambient conditions for periods between 79 and 112 months, were examined by electron spin resonance spectroscopy (ESR). Free radicals were found in all irradiated specimens, spin concentrations ranging from 0.5 × 1016 spins/g to 3.5 × 1016 spins/g and being nearly proportional to the original irradiation dose. For a given polyethylene, the radical concentration rises monotonically with crystallinity, suggesting that the free radicals are trapped in the crystalline phase. This would account for their longevity. At a given irradiation dose and crystallinity, the radical concentration is lower in normal molecular weight polymer than in ultrahigh molecular weight material. This difference may be explicable in terms of tie-chain concentration.

The effects of high-energy irradiation on the structure, properties, and processing of polymers are of considerable scientific and commercial significance and have been reported in several books and review articles [101]. In this massive literature, two of the major radiation-induced changes reported are crosslinking and degradation, sometimes leading to a loss of crystallinity. In fact, in this literature polymers are generally classified as either "crosslinking type" or "chain-scission type." In the present review we shall not be concerned with radiation-induced crosslinking, although its existence in many polymers is, of course, acknowledged. Nor will our review cover radiation-induced polymerization or grafting. All these topics have already been adequately covered in the extensive radiation literature, such as the references cited above.