Evolution of the Radicals in γ-Rays Irradiated Medical Grade Ultra-high Molecular Weight Polyethylene

Medical grade ultra-high molecular weight polyethylene (UHMWPE) was irradiated with gamma-rays. The types of irradiation-induced free radicals and their decay behavior under argon and different partial pressures of oxygen were studied by electron spin resonance spectroscopy (ESR). In argon, the alkyl radicals and allyl radicals are the main radical species with a total radiation chemical yield of 0.48/100 eV. Alkyl radicals have poor stability with a life time about 1 d at room temperature. In oxygen-containing atmosphere, free radicals are decayed mainly by oxidation reaction and the decay rate increases with the increase of oxygen partial pressure. The half-life of free radicals decreases from 224.0 h in 1x10(5) Pa argon to 1.8 h in 5x10(5) Pa O-2. It is estimated that the migration rate of free radicals trapped in the crystal region to the surface of the microcrystalline is very fast at room temperature, on the order of hours.