ESR Studies of Free Radical Decay in Irradiated Polyethylene

Abstract

A study has been made of the temperature and rates at which molecular hydrogen is evolved and free radical decay occurs when semicrystalline polyethylene, irradiated at liquid‐nitrogen temperature, is warmed to room temperature. Hydrogen evolution and free radical decay roughly parallel each other. The temperature of half reaction is the same as the temperature of the gamma‐maximum in the mechanical dispersion temperature curve. At constant temperature over the temperature range —71° to 41°C, the free radical decay is first order and the free radical concentration seems to decrease to a limiting value at each temperature. The first‐order decay constants are approximately constant with temperature. The data are interpreted in terms of free radicals decaying in spurs. If a constant activation energy of 15 kcal mole^—1 is assumed, the frequency factors decrease with rise of temperature. This decrease is interpreted on the basis of a decreasing probability of reaction for those free radicals that escape reaction at low temperatures, but react at higher and higher temperatures. An alternative model based upon the release of free radicals from traps as the temperature is raised is also discussed. The reaction rate constant for free radical decay calculated from D—H exchange experiments agrees with that deduced from the ESR measurements. It is also concluded that the persistent free radical in polyethylene irradiated to moderate doses is the allyl free radical.