Passive membranes have been used for separations because of lower energy demand 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. Similar to experimental observations, diffusion coefficients for spherical penetrants with various diameters were calculated and we obtained a stronger exponential dependence of diffusion coefficient and the ratio of system temperature to glass transition temperatures T<sub>g</sub> of networks at different crosslink densities. These results highlight the importance of mesh confinement and segmental motion in determining penetrant diffusivity in crosslinked networks the success of our simple model to capture key physics of molecule transport in dense crosslinked networks at temperature close to T<sub>g</sub>. We also extend the model to study penetrant diffusion in vitrimers, polymer networks with associative dynamic covalent bonds. We aim to understand how the rearrangement of network topology and the rate of bond exchange impact molecular transport.

Coarse-grained model for penetrant diffusion in permanent and dynamic crosslinked networks

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