Ion Transport in Self-Assembled Membranes with 1-nm Scale Pores

Polymers with precisely-sized pores on the order of 0.5 – 1.5 nm offer the possibility of tailoring species selectivity by deconvoluting size-exclusion and affinity-based mechanisms for controlling transport. They represent a qualitatively different approach relative to dense polymer membranes. However, there is a lack of fundamental understanding regarding transport of both ionic and non-ionic species under such strongly confined situations. Here, we examine transport of ions in polymer membranes with well-defined 1-nm scale pores. The membranes are charged hydrated nanoporous polymers produced by crosslinking self-assembled lyotropic mesophases with direct hexagonal and gyroid nanostructures. They are mechanically and chemically resilient materials that exhibit limited swelling (< 2.5%) on exposure to excess water and support relatively fast transport of a variety of anions. We use a combination of electrochemical impedance spectroscopy, pulsed-field gradient NMR and molecular simulations to elucidate anion and water transport. Our work addresses the role of hydration, pore geometry and anion identity on transport using a combination of electrochemical and NMR measurements, and molecular simulation. Our results highlight the role of hydration separate of morphology, the role of pore size separate of hydration, and the role of local heterogeneity on ion transport, and strong correlations of ion polarizability with conductivity. These results provide new insight that can be exploited to develop mechanisms for tailoring ion selectivity in nanoporous polymers and shed new light on transport under nanoscale confinement in the presence of charge.