Study on the post-irradiation oxidation of polyethylenes using EPR and FTIR technique

•Crystallinity and lamellae size played important role on post-irradiation oxidation.•Trans-vinylene groups were highly active to participate in the oxidation reaction.•OIR and carbonyl were not directly produced by the oxidation of initial radicals.•Intermediate formed firstly and then induced oxidation with fresh radical formation. To investigate the post-irradiation oxidation process, different polyethylenes (UHWPE, HDPE, LDPE) were irradiated by gamma rays in Ar and then exposed to air. The effect of crystalline structure on the post-irradiation oxidation process was evaluated combined by FTIR with EPR technique. Crystallinity and thickness of lamellae play a major role on post-irradiation oxidation, largely affecting the decay rate of radicals and the degree of oxidation. New radical species (named OIR) and carbonyl groups were mainly oxidation products detected. On the contrary, trans-vinylene double bonds were highly involved in the oxidation reaction. Of particular interest, it was found that the oxidation products were not directly from the oxidation of initial radicals. So, an intermediate species was expected to be formed firstly which induced continuous oxidation of polymers. Finally, a new mechanism for post-irradiation oxidation of PE was then proposed. This work would provide a new sight for study on the post-irradiation oxidation of polymers as well as oxidation mechanism.