Oxidation of polyethylene under irradiation at low temperature and low dose rate. Part I. The case of “pure” radiochemical initiation

As part of a study on the kinetic modelling of polyethylene oxidation under irradiation at low temperature and low dose rate, this first part deals with the kinetic regime in which thermal initiation, linked to hydroperoxide decomposition, is negligible compared to radiochemical initiation due to polymer radiolysis. The kinetic analysis is based on results published 30 years ago by Decker, Mayo and Richardson. A small modification of their mechanistic scheme, consisting in the introduction of a non-terminating bimolecular combination of PO₂ radicals, leads to a more consistent set of radiochemical yield values. The most significant change is a decrease in the radiochemical yield of radicals G_i from 10 to 8. At 45 °C, termination of PO₂ radicals is not very efficient: 35-40% of the PO₂ + PO₂ encounters are non-terminating, 75% of the termination events lead to peroxide bridges, the rest is a disproportionation according to the Russell mechanism.