Irradiated polyethylene I—Free radical formation

Abstract

Several aspects of alkyl and allyl free radical formation in Marlex 50 polyethylene are examined. The free radicals were generated with 60Co γ-rays and examined qualitatively and semiquantitatively by electron spin resonance techniques. Irradiations were performed at −196°C. The following conclusions are derived from this study: (1) the alkyl radical is relatively stable in the glassy amorphous region of polyethylene and relatively unstable in the viscouselastic region. Alkyl radical decay takes place by a diffusion process; (2) in the decay process, few of the alkyl radicals, if any, are transformed to allyl radicals, and (3) preliminary results suggest that allyl radical formation at room temperature involves an intermediate other than the alkyl radicals. Conclusion (1) is derived from the following: (a) isochronal measurements of the relative alkyl to allyl radical concentrations, as a function of temperature, decrease markedly as the temperatures go through the glass transition range; (b) a 12 kcal/mole activation energy value for alkyl radical decay, obtained from data above the glass transition temperature, T g, is the same as that obtained from diffusion studies in polyethylene at higher temperatures; and (c) temperatures below T g provide a linear relationship between the specific heat, C p, for amorphous polyethylene and the decay rate. This relationship indicates that the molecular motions which determine the value for C p also govern diffusion. Conclusion (2) is derived from a comparison of the residual allyl radical concentration at room temperature, following alkyl radical decay, with the initial alkyl radical concentration for radicals formed at −196°C. The comparative concentrations show that a very small fraction of allyl radicals remains following alkyl radical decay. Conclusion (3) is suggested from a comparison of the larger G values for allyl radical formation at room temperature irradiations with the smaller G values for alkyl and/or allyl radical formation at −196°C irradiations.