Identifying mechanisms of penetrant diffusion in highly crosslinked polymer networks*

Controlling the diffusive transport of molecular penetrants in polymers is essential for advancing the engineering design of materials for membrane separation. We seek to understand how highly crosslinked polymer networks near their glass transition temperatures (T_g) may affect penetrant motion and hence play a significant role in separation efficiency. Crosslinking results in an increase in T_g of networks and a decrease in averaged mesh size, possibly leading to higher selectivity of different molecular penetrants. Based on parameters of poly-n-butyl-acrylate networks synthesized by our experiment collaborators, we have designed a Molecular Dynamics simulation model to address the influence of crosslinking degree on the dilute spherical penetrant diffusion and relaxation. We characterized the diffusion coefficients and alpha relaxation times of penetrants to investigate how penetrant motion couples to both the segmental relaxation and the mesh confinement of networks over a wide range of temperatures and crosslink densities. The results reveal the primary significance of the segmental relaxation effect with mesh confinement effect being of secondary importance for the present system. Moreover, we have extended the model to non-spherical penetrants and investigated how penetrant chemistry/geometry affects the above two mechanisms and their effect on penetrant diffusion, which suggests important new strategies for enhancing selective polymer membrane design.