Polymer-grafted Nanoparticle Membranes with Exceptional Gas Separation Performance

There is considerable interest in developing novel gas separation membranes that improve on current, pure polymer-based technologies. Pure polymer grafted nanoparticle based membranes offer the exciting opportunity to tunable small gas permeabilities but are still limited compared to current exotic polymers. Here, we discuss gas transport in GNPs incorporating small amounts of ungrafted polymer chains with judiciously chosen chain lengths. We find that the addition of homopolymer allows us to almost independently increase membrane gas selectivity, and thus exceed the Upper Bound representing the best currently available membrane materials. X-Ray and neutron scattering suggest that gas transport in GNPs is spatially inhomogeneous with solutes bigger than a critical size being moved primarily through the interstitial spaces between the NPs, while small solutes are carried more homogeneously through the whole layer. High molecular weight free chains segregate into these interstitial regions and preferentially hinder large solute motion, thus greatly improving selectivity. Controllably manipulating transport in these inhomogeneous materials thus offers the opportunity to construct gas-separation membranes with exceptional performance.