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Overview on the study of polymer ageing under ionizing radiations

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Involved people : Emmanuel Balanzat, Yvette Ngono-Ravache

Contracts : Labex EMC3 : TEPRI (2014-2015), RIBEG (2010-2012), COSTO (2010-2014)

The aim of these studies is to understand the processes involved in the radiation-induced ageing of synthetic polymers, either under oxygen or under vacuum, by analysing both macromolecule defects and gas emission. Instead of focussing on the settling of a database gathering radiation chemical yield of gas emission and oxygen consumption, at low doses, as in the previous period, we have considered the influence of most of the irradiation parameters (atmosphere, dose and dose-rate) and studied how the high LETs and the inhomogeneous energy deposition induced by Swift Heavy ions (SHI) impact the polymer oxydation. We have demonstrated that, at the contrary of beta and gamma, oxidation under SHI irradiation is scarcely dose-rate dependant. Moreover, at equivalent dose rates, oxydation is lower under SHI compared to beta irradiations. This is assigned to the increased recombination of alkyl radicals, instead of reacting with O2, due to the high ionisation densities induced by SHI. The influence of the ionization and excitation density on the creation of carbonyl groups, in a reduced set of polymers, was studied through the variation of LET. The influence of the excitation and ionisation density on carbonyl creation, under SHI, depends on the chemical structure of the polymer repeating unit. For polyolefins devoid of benzene rings, the carbonyl creation is almost LET independent. Conversely, when benzene rings are present in the polymer, the LET effect is tremoundeous. This behaviour is assigned to the increase in benzene ring breakage, with increasing LET, above a LET Threshold. Since the benzene ring possesses a huge radiation stabilisation effect, its breakage increases the sensitivity of the polymer towards irradiation and thus towards oxydation.

We have also focussed on the comprehension of the process at work at high doses. Namely, energy and species transfers towards radiation-induced defects. For this purpose, we used an inovative approach, using tailored polymers with pre-introduced well known concentrations of defects.

Evolution of G(H2) as a function of the trans-vinylene concentration [TV] either in PEC=C or radiation-induced in fully hydrogenated PE
ref : A. Ventura, T. Chenal, M. Bria, F. Bonnet, P. Zink, Y. Ngono-Ravache, E. Balanzat, M. Visseaux ; European Polymer Journal, 49(12), 4130-4140, 2013