Tuning the permeability and selectivity in polymer-grafted nanoparticle membranes for gas separations

Polymer-grafted nanoparticle (PGNP) based membranes have emerged as a strong candidate for key gas separations due to their free volume dependent permeability, selectivity, aging resistance, and compatibility with commercial applications. In this work, we elucidate the mechanisms of gas separations in PGNP membranes by demonstrating the role of polymer interpenetration on gas permeability and selectivity (CO₂, CH₄, H₂, O₂, N₂, etc.). We tune the polymer interpenetration by controlling the PGNP film formation. We observe that PGNP films cast using rapid solvent evaporation with spin casting trap the PGNPs in a “hard-sphere” like state with more free volume due to the collapse of polymer chains. This in turn results in higher gas permeability and lower selectivity. Furthermore, the slow evaporation of solvent using a room-temperature solution casting process allows the polymer chains more mobility during the solvent evaporation, resulting in the interpenetration of polymer chains, and thus arresting the system in a low-free volume state. This slow evaporation strategy shows lower gas permeabilities but higher selectivities. We believe this strategy of tuning gas permeability and selectivity is expected to be versatile for a variety of polymer-based gas separation membranes.