Effect of Permanent Crosslinks and Temperatures on Network Relaxation and Penetrant Diffusion in Tight Polymer Networks*

Selective separation of mixtures of organic molecules is particularly difficult when the components have similar molecular weight and intermolecular interactions. Tight flexible polymer networks are promising candidates for selective transport because of their strong selectivity to molecular size and shape and lower energy demand. However, there remains a need for a molecular-level understanding of network-based selectivity to design materials for separation applications. We performed Molecular Dynamics simulation of dilute penetrant diffusion in dense polymer networks with varying crosslink density. We found that glass transition temperatures (T_g) for networks shows a nearly linear relation with different crosslink densities, which compared well with measurements from experimental collaborators. To understand how segmental dynamics changes with crosslink densities and temperatures, we calculated the network relaxation time and found that the relaxation time dramatically increases as temperature gets close to T_g. We calculated the diffusion coefficient of penetrants in networks with varying crosslink density and temperature. The dependence between diffusion coefficient and temperature or size ratio of penetrant to network mesh size qualitatively agree with experimentally determined diffusivity of fluorescent dyes.