Relating Geometric Nanoconfinement and Local Molecular Environment to Diffusion in Ionic Polymer Membranes

Understanding structure–diffusion relationships in ionic polymer membranes not only illuminates fundamental mechanisms for membrane transport but also provides information for further membrane development. In this study, we employ NMR diffusometry and molecular dynamics (MD) simulations to investigate diffusion coefficients and activation energies of diffusion ( E a ) for both a lithium-form perfluorosulfonic acid membrane and aqueous solutions of lithium triflate. NMR shows that, at high water/ion molar ratio (λ), E a for water diffusion is slightly lower in the membrane than in solutions. Conversely, membrane transport exhibits much higher E a at low hydration as compared to solutions. MD simulations of a model system consisting of carbon nanotubes with varying diameter reveal that E a of diffusion clearly relates to both the geometric nanoconfinement of the hydrophilic pathways in the membrane and the local molecular environment. These results demonstrate that E a of diffusion can serve as a revelatory tool for the study of molecular transport processes and the coupling of morphology with transport at the nanoscale. This study thus provides new insights and new experimental and computational models for understanding transport in a wide range of polymer membranes, such as those used in molecular separation applications.