Diffusion-limited free radical mechanisms in peroxide-initiated crosslinking of low-density polyethylene

There have been many attempts to describe the kinetics of the peroxide-initiated crosslinking of low-density polyethylene (LDPE). However, these kinetic models have not comprehensively captured the free radical behavior during LDPE crosslinking. In this study, the evolution of the reduced reaction rate (R_r), derived from the mechanistic kinetic model, is employed to gain detailed insight into free radical behaviors in LDPE during crosslinking. The curve of R_r over time, which follows a bell shape, demonstrates that the complexity of the crosslinking chemistry incorporates diffusional limitation, leading to ineffective free radical terminations. When combined with structural characterization of functional trans-vinylene groups and hydroxyl groups of antioxidants, our analysis highlights a diffusion-limited mechanism associated with a continuous decrease in the effective consumption of peroxide and a sequential increase in radical scavenging. These observations are a result of the intramolecular disproportionation induced by the cage effect in peroxides and the unimolecular termination of LDPE radicals by antioxidants during network formation, respectively. Consequently, a refined kinetic model is proposed, which accounts for the effect of diffusional limitation on free radical terminations, thereby providing an accurate description of the LDPE crosslinking process. This comprehensive understanding of free radical crosslinking mechanisms in LDPE will facilitate the precise control and optimization of the properties of this reaction.