Activated Gas Transport in Polymer-Grafted Nanoparticle Membranes

Abstract

Carbon capture and related gas separation processes are critical tools in our efforts to combat climate change. While polymer membranes are seen as a central construct to achieve these goals, their performance needs further improvement to meet current sustainability goals. It is in this context that membranes composed of polymer-grafted nanoparticles (GNPs) become highly germane. Previous work has shown that gas transport in pure GNP membranes can be strongly enhanced relative to that in the corresponding neat polymer. Additionally, we found that larger gases display greater degrees of permeability enhancement than smaller ones. There is currently no clear understanding of these two underpinning facts, and to address this issue, we have characterized the activation energy driving the permeability of multiple gases in several GNP membranes. Our results show that gases smaller than a critical size display the same activation energy as in the native polymer─any enhancement therefore is associated with changes in local gas/chain dynamics, e.g., the speeding up of polymer dynamics due to the local stretching in a brush conformation. Our results are consistent with the notion that small gases are present in the whole polymer layer. Conversely, larger gases show substantially lower activation energies and, in contrast to the neat polymer, exhibit essentially no dependence on penetrant size. Sorption measurements illustrate that this is a result of the enhanced solubility of larger gases in GNPs, relative to that in the pure polymer, which is reflected in their significantly more favorable enthalpies of solvation. Larger gases thus preferentially sorb into the interstitial spaces in the GNP assembly, thereby resulting in even higher permeability enhancements in the GNP layers relative to smaller gases. GNPs, therefore, provide us with multiple handles to control gas transport, a fact that offers critical insights into their utility for gas separations.