PROFILE OF OXIDATION IN IRRADIATED POLYETHYLENE

Following gamma irradiation in air which causes bond scission and yiel ds large concentrations of peroxy radicals, maximum oxidation and an i ncrease in crystallinity occurs on the surface of ultrahigh molecular weight polyethylene. Here, bimolecular reactions of peroxy radicals ge nerate carbonyls, mostly ketones. On the polymer surface, peroxy radic als continue to react over time periods of years to generate carbonyls and chain scission. Peroxy radicals in the interior of the polymer ab stract hydrogens and form hydroperoxides, inducing chain reactions and a slow but continue increase of ketone. Within the polymer sample, to a decreasing depth with increasing dose, a reduced concentration of o xygen is available to react with radiolytic radicals, so that more eff icient crosslinking and a low level of hydroperoxide chain reaction oc cur. After long periods of time a surface maximum in carbonyl concentr ation is produced. Heating polyethylene in high pressures of oxygen ac celerates the oxidative process. (C) 1998 John Wiley & Sons, Inc.