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.