Weak Polyelectrolyte Membranes with a Wide Ion-Exchange Capacity (IEC) Range and Limited Water Swelling in Clean Technologies for Sustainability

Abstract

Charged polymer membranes are of great interest in clean technologies for sustainability due to their tunable transport properties. Designing new innovative charged polymers for clean technologies is highly dependent on the mechanistic understanding of water and ion transport in these materials. In this context, we have designed a systematic library of weak polyelectrolyte membranes: cross-linked acrylic acid–poly(ethylene glycol) diacrylate (AA-PEGDA) networks, with a wide range of ion-exchange capacity (IEC = 0–4 mequiv/g) and limited water swelling (ϕ_W = 0.07–0.69). An acrylic acid (AA) monomer was chosen as a weakly charged group to control the charged group concentrations in the polymers, i.e., the maximum ion-exchange capacity (mIEC) at the varied external pH. Poly(ethylene glycol) diacrylates (PEGDAs) with different molecular weights were used as cross-linkers to control cross-linking densities in the networks. Specifically, in one fixed polymer composition, the charged (COO^–) group concentration can be systematically changed on the same chemical structure by controlling the external pH: the same polymer behaves like an uncharged neutral polymer (the degree of ionization, α = 0) at low pH, whereas at pH = pK_a, the same polymer is one-half charged (α = 0.5), and, at high pH (≫pK_a), is fully charged (α = 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 other polymers in the literature. In addition to changing the chemical structure of polymers, this AA-PEGDA series can provide extra freedom to modify transport properties via simply changing the pH and enable us to develop a mechanistic understanding of water and ion transport in charged polymer membranes.