Leveraging the unusual polymer physics of grafted nanoparticles to design improved gas separation membranes

Polymers are typically grafted to nanoparticles (NPs) to prevent their undesired agglomeration in organic, chain-based matrices. Surprisingly, we find that these grafted nanoparticles show unusually high gas permeability properties. We have found that grafting chains to surfaces at high grafting densities causes the chain to stretch in a direction normal to the surfaces (“brush”) – this accelerates chain dynamics, presumably leading to faster gas diffusion. This idea parallels thinking in the membrane community which has increasing moved towards stiffer chains, with frustrated local packing and hence higher free volume, as a means of increasing gas permeability. Our physics-based approach, however is significantly different in focus than previous chemistry inspired ideas, and has the additional advantageous. Significantly, these NPs form a long lived colloidal glassy state. This state is found to be accompanied by hardly any aging effects, a significant problem for systems based on ungrafted stiffer chains. Also, we can tune the gas separation properties of these materials by varying casting protocols, a standard technique used to manipulate the properties of polymer glasses.