(168b) Transport Channels in Polymer-Grafted Nanocomposite Membranes for Vapor Separations
Inspired by the increasing need for polymeric membranes with tunable separation ability, here we focus on the mass transport of a small molecule penetrant in matrix-free thin films of PMA-grafted, nonporous silica nanoparticles. Preliminary permeability data measured with the quartz crystal microbalance technique are larger than the neat polymer and directly contrast conventional composite theory. We postulate that the self-assembly of the particles grafted with moderate to high grafting densities of moderate length polymer chains into ordered arrays creates interstitial spaces which the polymer chains can only fill by stretching. Solute particles are thus driven to diffuse through these channels as their placement in these regions serve to reduce the unfavorable entropy of the stretched chains. This, in turn, increases the available “free volume” within the film and creates channels, similar in spirit to transport channels in cell membranes, which facilitate penetrant diffusion. Importantly, we expect that these channels will disappear in the limit where the grafts become infinitely long: in this limit the medium should transport solute exactly as a normal polymeric membrane. The potential tunability of these transport channels could be a paradigm shifting discovery for this class of membrane materials.