(499c) Effect of Mesh Confinement Versus Segmental Relaxation on the Diffusion of Penetrants in Polymer Networks

Passive membranes have been used for separations because they are less energy intensive compared to thermal separations. There remains a challenge to separate molecules with similar molecular weight and intramolecular interactions. Tight flexible polymer networks are proposed to improve separation efficiency because the sub-nm mesh sizes can discriminate differences in geometries of two molecules. Nevertheless, the mechanisms for how tight networks regulate transport for different molecules are not fully understood. We seek to provide a molecular-level understanding of tight network-based selectivity to design materials for separation applications with Molecular Dynamics simulation with the standard bead-spring model for semi-flexible polymers. The model is parametrized for poly-n-butyl-acrylate (PnBA) networks synthesized by experimental collaborators to model dilute spherical penetrant diffusion in the networks. We studied the effect of crosslink density on the alpha relaxation time and glass transition temperatures (T_g) for networks, and the results agree with experiments and theory. Diffusion coefficients for spherical penetrants with various diameters were calculated and we were able to determine the relative contribution of entropic mesh confinement and the activated segmental dynamics of the surrounding matrix to penetrant motion. We determined that crosslinks affect molecular transport though both mechanisms, but that the segmental motion of the polymer network was the dominant effect for systems considered by our collaborators. This is accompanied by the emergence of significant dynamic heterogeneity effects in penetrant hopping. Due to the importance of a distribution of network relaxation time scales, we extended our model to study penetrant diffusion in vitrimers, polymer networks with additional relaxation time scales via the presence of dynamic bonds. We show how these dynamic bonds affect molecular transport, and couple to crosslink density, penetrant size, and temperature.