Particle Dispersion Controls the Gas-Separation Properties of Polymer-Grafted Nanoparticle Membranes

Membranes based on polymer-grafted nanoparticles (PGNPs) have emerged as strong candidates for key gas separations due to their tunable permeability, selectivity, mechanical properties, and aging resistance. Here, we show the importance of keeping nanoparticles spatially well-dispersed through the whole process of grafting chains onto their surfaces─this is illustrated by measuring the gas-separation properties of PGNP membranes. Specifically, while we start with well-dispersed bare NPs in solution, this dispersion state is affected when we functionalize their surface with the polymer initiating agent, which then causes local NP agglomeration (i.e., effectively leading to a larger NP core, to which chains are grafted) and poor gas-separation performance relative to our previous results on PGNP membranes. Instead, when we cap the NPs with a protective layer that prevents NP agglomeration during surface functionalization, followed by grafted polymer synthesis, we obtain significantly higher gas permeabilities. While these results can be rationalized by the fact that the gas-permeation properties of smaller NPs grafted with polymer chains are better than those of larger NPs at the same effective grafting density and graft chain length, these results emphasize the important role of particle dispersion during all steps of the formation of PGNPs.