General consideration of the radiation chemistry of polymers

Conventional radiation chemistry of polymers is based on studies of irradiation with electron beams or gamma-rays, both of which are low ionization density (low LET) radiations. In such systems the radiation energy is deposited homogeneously and absorbed dose is expressed in terms of 'Grays' (Joules per kg). Heavy ion beams represent high ionization density (high LET) radiations that deposit their energy inside separate tracks, in the bulk of which the polymeric material is severely disintegrated into low molecular weight products. An 'average dose' can be defined in such systems, but the effective 'local dose' inside the tracks can only be estimated very roughly with a more or less arbitrary assumption of the effective volume of the track. In polymers subjected to low LET radiation the following chemical modifications are observed: gas evolution, formation of unsaturation, main-chain cleavage and/or cross-linking, depending on the chemical nature of the polymer. Oxygen, if present during irradiation, induces oxidative degradation processes, which may lead to significant main-chain scission, even in polymers of the cross-linking type. High LET radiations produce very severe damage to the polymeric material inside the tracks. The latter may be visualized by dissolving out the degraded material with aqueous solutions that do not affect the original polymer. This major chemical trasformation results from multiple bond scissons in single molecules, directly caused by the high density of energy deposition. Chemical effects of similar nature are produced under the action of glow discharges and of laser breams but the resulting products are not the same as those produced by ion beams in view of the large differences in irradiation conditions.