Elucidating the Role of Alkoxy Radicals in Polyethylene Radio-Oxidation Kinetics

Polyethylene is a widely used polymer and a prototypical
polyolefin.
Understanding degradation mechanisms at the atomic scale leading to
oxidation under the effect of temperature and/or irradiation is crucial
for many long-term applications. Here, we focus on a subset of reactions
belonging to the generally assumed radio-oxidation reaction scheme
and involving alkoxy radicals. We follow a theoretical approach based
on density functional theory, in order to assess the role of these
species, which are short lived and thus hard to detect experimentally.
For every considered reaction, we calculate the reaction enthalpy
and the energy barrier, and we evaluate the influence of the local
atomic environment, taking advantage of a model surface of a crystalline
lamella which mimics the interface between crystalline and amorphous
zones. Our results suggest that in certain conditions, the kinetic
pathway can bypass the formation of hydroperoxides. The concentrations
of alkoxy and peroxy radicals during radio-oxidation and their ratio
are important parameters controlling the predominance of chain scission
or crosslinking in the polymer. The data presented here constitute
part of a database that can be used to set up kinetic simulations
based on homogeneous chemical kinetics or Monte Carlo algorithms.