Polymer membranes are critical to many sustainability applications that require the size-based separation of gas mixtures. Despite their ubiquity, there is a continuing need to selectively affect the transport of different mixture components while enhancing mechanical strength and hindering aging. Polymer-grafted nanoparticles (GNPs) have recently been explored in the context of gas separations. Membranes made from pure GNPs have higher gas permeability and lower selectivity relative to the neat polymer because they have increased mean free volume. Going beyond this ability to manipulate the mean free volume by grafting chains to a nanoparticle, the conceptual advance of the present work is our finding that GNPs are spatially heterogeneous transport media, with this free volume distribution being easily manipulated by the addition of free polymer. In particular, adding a small amount of appropriately chosen free polymer can increase the membrane gas selectivity by up to two orders of magnitude while only moderately reducing small gas permeability. Added short free chains, which are homogeneously distributed in the polymer layer of the GNP, reduce the permeability of all gases but yield no dramatic increases in selectivity. In contrast, free chains with length comparable to the grafts, which populate the interstitial pockets between GNPs, preferentially hinder the transport of the larger gas and thus result in large selectivity increases. This work thus establishes that we can favorably manipulate the selective gas transport properties of GNP membranes through the entropic effects associated with the addition of free chains.

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We measure the activation energy for the local segmental dynamics of polymer chains densely grafted to nanoparticles (NPs) using quasielastic neutron scattering. We aim to understand the underpinning physics of the experimentally measured enhanced gas transport in polymer grafted nanoparticle-based membranes relative to the neat polymer (without NPs), especially the permeability maximum, which occurs at intermediate chain lengths. We find that the activation energy goes through a minimum as a function of chain length, while the elementary jump size goes through a maximum around the same chain length. These results, likely, are the dynamic consequence of a structural transition of the grafted polymer brush from “extended” to “interpenetrated” with increasing chain length at fixed grafting density. Evidently, the regimes of different graft chain lengths near this structural transition are associated with lower activation energy, likely due to fluctuation effects, which also lead to enhanced gas transport.