Free radicals trapped in polyethylene crystals

Abstract

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.