Molecular Simulation Study of Penetrant Diffusion in Vitrimer Network

The diffusivity of penetrants in polymer networks can be tailored by the network topology, enabling applications ranging from efficient chemical separation as a membrane to the inhibition of penetrant diffusion as a barrier coating. Recent studies on permanent polymer networks have revealed that how the segmental relaxation of the network structure and the entropic mesh confinement effect govern penetrant diffusive dynamics. Building on this finding, we investigate vitrimers, materials with the same functional network topology as permanent networks but capable of topological rearrangement via bond exchange reactions, potentially facilitating penetrant diffusion. The structural relaxation of vitrimer networks is influenced by the rate of these reactions. We employ molecular dynamics (MD) 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 reflected in changes in penetrant alpha hopping times, suggesting that bond exchange rearrangements are also coupled with the mesh confinement effect in the polymer matrix, which affects the diffusion mechanism in vitrimers.