Reactions of a prototypical phenolic antioxidant with radicals in polyethylene: insights from density functional theory

Phenolic antioxidants are widely used to prevent oxidation, which is the main degradation process for many polymers, in particular polyolefins among which polyethylene is the most employed one. Although it is generally understood that one of the main mechanisms by which phenolic antioxidants prevent or slow down oxidation is by deactivating radicals and preventing the formation of alkyl radicals, detailed understanding at the atomic scale of the hierarchy of radical reactions is still lacking. Here, we investigate the interaction of a prototypical phenolic antioxidant, butylated hydroxytoluene (BHT), with radicals in a polyethylene model by means of static and dynamic simulations based on density functional theory. We focus on the H-transfer reactions between BHT and radical species by evaluating the associated energy barriers and analyses the conditions in which these relevant reactions occur by first principles molecular dynamics simulation. Our polyethylene model includes a realistic surface of a crystalline lamella, thus describing the local atomic environment in which the reactions mainly take place. Our results suggest that the H-transfer reaction of the BHT molecule with an alkoxy radical is spontaneous and the energy barrier is small (~0.1 eV) with a peroxy radical. Conversely, direct scavenging of alkyl radicals by BHT seems excluded. Our molecular dynamics simulations highlight the influence of steric hindrance and antioxidant diffusion within amorphous regions on antioxidant efficiency.