A quantum chemical and molecular dynamics simulation study on photo-oxidative aging of polyethylene: Mechanism and differences between crystalline and amorphous phases

Abstract

It is well known that polyethylene (PE) generates CO2 during photo-oxidative aging; however, there is still some ambiguity in the exact mechanistic steps involved. In this work, the photo-oxidative aging mechanism of PE is considered via quantum chemical (QC) calculations. This demonstrates that it is difficult for some previously proposed Norrish I mechanisms to occur because of the high energy required and forbidden transitions. A synergistic effect between oxygen and ultraviolet (UV) radiation is shown to promote the initiation of photo-oxidative aging, providing a more reasonable description of photo-oxidation than existing mechanisms. The charge differential density (CDD) indicates that UV radiation alters the electron distribution so that the electron transfer from the secondary C–H bond to the H–OO bond promotes the formation of HOO·. As is well established, after the initiation reaction, the formed radical is converted into hydroperoxide. UV radiation promotes the conversion of hydroperoxide into ketones via cleavage of the O–OH bonds and the formation of hydroxyl radicals. The hydroxyl radicals can further react with ketonic carbonyls, resulting in carbonic acid. Finally, the carbonic acid decomposes into CO2. The difference in photo-oxidative aging between the crystalline and amorphous phases of PE was studied by molecular dynamics (MD) simulation. The MD results show that the crystalline phase is more difficult to photo-oxidize than the amorphous phase because of the poorer permeation ability and solubility but not the diffusion ability of O2.