Molecular simulation study of penetrant diffusion in vitrimer networks.

The diffusivity of penetrants in polymer networks can be tailored by network topology, which is relevant to applications such as chemical separation membranes or the design of barrier coatings. Recent studies on permanent polymer networks have revealed how cross-linking affects both segmental relaxation and the entropic mesh confinement, and consequently, both physical phenomena affect penetrant diffusive dynamics. We build on this finding and investigate how penetrant diffusion occurs in vitrimers, materials with the same network topology as permanent networks but capable of topological rearrangement via bond exchange reactions. The structural relaxation of vitrimer networks is influenced by the kinetics of these bond exchange reactions. We employ molecular dynamics simulations to investigate how dynamic network rearrangement affects penetrant diffusion for various penetrant sizes, temperatures, vitrimer cross-link densities, and bond exchange rates. Our studies show that the diffusivity of small penetrants is largely unaffected by varying bond exchange rates; however, as the penetrant size increases, faster bond exchange rearrangements lead to enhanced diffusivity. Notably, this enhancement cannot be fully accounted for by changes in penetrant alpha hopping times, suggesting that bond exchange kinetics primarily affect the mesh confinement of penetrant diffusion in vitrimers.