Activated Gas Transport in Polymer-Grafted Nanoparticle Membranes

Carboncapture and related gas separation processes are criticaltools in our efforts to combat climate change. While polymer membranesare seen as a central construct to achieve these goals, their performanceneeds further improvement to meet current sustainability goals. Itis in this context that membranes composed of polymer-grafted nanoparticles(GNPs) become highly germane. Previous work has shown that gas transportin pure GNP membranes can be strongly enhanced relative to that inthe corresponding neat polymer. Additionally, we found that largergases display greater degrees of permeability enhancement than smallerones. There is currently no clear understanding of these two underpinningfacts, and to address this issue, we have characterized the activationenergy driving the permeability of multiple gases in several GNP membranes.Our results show that gases smaller than a critical size display thesame activation energy as in the native polymer-any enhancementtherefore is associated with changes in local gas/chain dynamics,e.g., the speeding up of polymer dynamics due to the local stretchingin a brush conformation. Our results are consistent with the notionthat small gases are present in the whole polymer layer. Conversely,larger gases show substantially lower activation energies and, incontrast to the neat polymer, exhibit essentially no dependence onpenetrant size. Sorption measurements illustrate that this is a resultof the enhanced solubility of larger gases in GNPs, relative to thatin the pure polymer, which is reflected in their significantly morefavorable enthalpies of solvation. Larger gases thus preferentiallysorb into the interstitial spaces in the GNP assembly, thereby resultingin even higher permeability enhancements in the GNP layers relativeto smaller gases. GNPs, therefore, provide us with multiple handlesto control gas transport, a fact that offers critical insights intotheir utility for gas separations.