Weak Polyelectrolyte\nMembranes with a Wide Ion-Exchange\nCapacity (IEC) Range and Limited Water Swelling in Clean Technologies\nfor Sustainability

Charged polymer membranes are of great interest in clean\ntechnologies\nfor sustainability due to their tunable transport properties. Designing\nnew innovative charged polymers for clean technologies is highly dependent\non the mechanistic understanding of water and ion transport in these\nmaterials. In this context, we have designed a systematic library\nof weak polyelectrolyte membranes: cross-linked acrylic acid–poly(ethylene\nglycol) diacrylate (AA-PEGDA) networks, with a wide range of ion-exchange\ncapacity (IEC = 0–4 mequiv/g) and limited water swelling (ϕ<sub>W</sub> = 0.07–0.69). An acrylic acid (AA) monomer was chosen\nas a weakly charged group to control the charged group concentrations\nin the polymers, i.e., the maximum ion-exchange capacity (mIEC) at\nthe varied external pH. Poly(ethylene glycol) diacrylates (PEGDAs)\nwith different molecular weights were used as cross-linkers to control\ncross-linking densities in the networks. Specifically, in one fixed\npolymer composition, the charged (COO<sup>–</sup>) group concentration\ncan be systematically changed on the same chemical structure by controlling\nthe external pH: the same polymer behaves like an uncharged neutral\npolymer (the degree of ionization, α = 0) at low pH, whereas\nat pH = p<i>K</i><sub>a</sub>, the same polymer is one-half\ncharged (α = 0.5), and, at high pH (≫p<i>K</i><sub>a</sub>), is fully charged (α = 1), providing extra freedom\nto tune IECs and an opportunity to investigate water and ion transport\nusing the same chemical structure for the first time. The differences\nin polymer transport properties versus pH in the fixed chemical structure\nin this study can be achieved by substantially varying the chemical\nstructure of other polymers in the literature. In addition to changing\nthe chemical structure of polymers, this AA-PEGDA series can provide\nextra freedom to modify transport properties via simply changing the\npH and enable us to develop a mechanistic understanding of water and\nion transport in charged polymer membranes.