Ion and water transport in weak polyelectrolyte membranes at varying external pH for clean technologies in sustainability

Charged polymer membranes play a crucial role in clean technologies for sustainability such as energy-efficient separations, energy storage, and environmental remediation. Designing innovative charged polymer membranes with desirable structural and transport properties is required to advance these clean technologies. In order to design new innovative charged polymers, the effect of charged group contents on water and ion transport properties needs to be understood. However, two major challenges exist: (1) different polymer chemical structures used for comparison and (2) high water swelling hinder the mechanistic understanding of water and ion transport in charged polymer membranes. To overcome these limitations, we have designed a new library of weak polyelectrolyte membranes, i.e., acrylic acid – poly(ethylene glycol) diacrylate (AA-PEGDA) networks with a wide ion-exchange capacity range (IEC = 0 ~ 4 mequiv/g) and limited water swelling. An acrylic acid (AA) monomer was chosen as a weakly charged group to control the charged group concentrations in the polymers. Poly(ethylene glycol) diacrylates (PEGDAs) were used as crosslinkers to control crosslinking densities in the networks.

Specifically, in one fixed polymer composition, by controlling the external pH, the charged (COO^-) group concentration can be systematically changed on the same chemical structure: the same polymer behaves like an uncharged neutral polymer (the degree of ionization, a = 0) at low pH, whereas at pH = pKa, the same polymer is one half charged (a = 0.5), and, at high pH (pKa), is fully charged (a = 1), providing extra freedom to tune IECs and an opportunity to investigate water and ion transport using the same chemical structure, for the first time. The differences in polymer transport properties versus pH in the fixed chemical structure in this study can be achieved by substantially varying the chemical structure of the other polymers in literature. Our study will catalyze the design of new innovative charged polymer membranes for a broad range of applications in energy, environment, and health.