(640b) Nanoconfinement and Chemical Structure Effects on Permeation Selectivity of Self-Assembling Graft Copolymers

Permeation of small molecule solutes through thin films is typically described by the solution-diffusion model, but this model fails to capture the effects of nanostructure in the film. Other models focusing on diffusion through isolated nanopores indicate that confining permeation to channels slightly larger than the size of the solute can increase the influence of solute-pore  interactions on permeation rate. In this study, we investigate the mechanism of permeation through nanostructured polymers. We analyze how differences in polymer nanostructure affect the contributions of solute size and polymer-solute interactions on transport rate. For this purpose, we studied diffusion through two polymer thin films: A cross-linked, homogeneous film of poly(ethylene glycol phenyl ether acrylate) (PEGPEA), and a graft copolymer with PEGPEA side-chains, designed to self-assemble into bicontinuous ~1-3 nm PEGPEA domains through which transport occurs. We correlated the diffusion rates of several small molecules with their size, and their chemical affinity to PEGPEA as measured by the difference between their solubility parameters. Our results indicate that differences in chemical structure of the small molecules have a stronger influence on selectivity for the graft copolymer, whereas permeability through the homogeneous cross-linked polymer is dominated by size effects. Furthermore, permeation selectivity between aromatic and non-aromatic molecules was significantly higher for the nanostructured copolymer, likely enhanced by the nanoconfinement effects. These initial results indicate that permeation the formation of a self-assembled nanostructure in the copolymer leads to permeation rates controlled by the partitioning of the solutes into the nanochannels, potentially enabling the use of polymer self-assembly as membranes with chemical structure based selectivity.