Multiscale understanding of ion and water transport in weak polyelectrolyte membranes for sustainable technologies

Polyelectrolyte membranes play critical roles in advancing membrane-based sustainable technologies. Mechanistic understanding of ion and water transport in polyelectrolyte membranes can enable us to design of next-generation membranes to address grand challenges of our time. Toward this goal, we designed a new library of weak polyelectrolyte membranes based on acrylic acid–poly(ethylene glycol) diacrylate (AA-PEGDA) network with a wide ion-exchange capacity range (IEC = 0 – 4 mequiv/g) and limited water swelling. In this model system, on the same chemical structure, the charged group concentration can be systematically varied (degree of ionization = 0 – 1) by controlling the external pH, without changing the chemical structure of the polymer. Using this unique feature, we systematically studied sorption and diffusion of ion and water in the polymers via multiscale characterization, with the goal of establishing the design principles of polyelectrolyte membranes. For sorption, we reported the solubilities of counter-ions and co-ions, with the minimum effects of water swelling. The sorption behavior was then discussed using the framework of Donnan and Flory-Rehner models, to determine the governing factors of electrostatics, polymer-ion interactions, and dielectric effects on the sorption. For diffusion, microscopic diffusivities of ion and water were reported using PFG-NMR and dielectric spectroscopy and compared with macroscopic diffusivity data. The diffusion phenomena was interpreted using the multiscale frameworks including the Mackie-Meares and Yasuda models as well as non-percolating model for low-hydration regime. Our multiscale studies in polyelectrolyte membranes can help us to develop innovative membranes for sustainability.