Solvation and ion transport within nanoporous polymers derived from lyotropic mesophases

Investigations regarding the fundamental nature of ion motion within nanoporous membranes are critical to developing the next generation of selective materials for electrochemical processes. In this work, we fabricate nanoporous polymer films from a quaternary ammonium-based amphiphilic monomer, forming the Ia3d and H_I mesophases, controlled for pore dimension. We perform an in-depth investigation into the motion of mobile counter-ions within the nanoporous polymer via electrochemical impedance and 2DIR spectroscopy, validating the results using molecular dynamics. By varying the ion identity, we find that ions in lyotropically-derived polymers engage in structural diffusion, and that the strength of the interaction with the quaternary ammonium charge sites and highly mobile water drives conductive behavior. The nanopores enhance differences in diffusivity due to solvation shell depletion and condensed charge pair formation at the quaternary ammonium charge centers, irrespective of the broader morphological identity. Investigations based on pore size emphasize these conclusions for the H_I mesophase. By grasping the driving physics of these materials, we can develop rules for nanoporous polymer design in electrochemical separations.