Influence of Hydroxyl Groups on Water Transport in Highly Charged Ion Exchange Membranes

Ion exchange membranes (IEMs) are used in membrane-based separation processes, such as electrodialysis (ED), which is promising for brine concentration applications. One factor that limits the efficiency of brine concentration via ED is the net transport of water from the dilute stream to the concentrate stream through osmosis and electro-osmosis. Current commercial membranes have limited control over water transport properties. In order to address this deficiency, we need a deeper understanding of how polymer structure affects water transport through IEMs. We hypothesize that interactions between water and polar groups on the polymer backbone could be leveraged to synthesize membranes with decreased water permeability. To the best of our knowledge, there are no studies that have systematically investigated the effects of backbone-pendant functional groups with hydrogen bonding ability (e.g., hydroxyl groups) on water transport in IEMs. This project aims to fundamentally study hydroxyl-water interactions and the effect of these interactions on membrane water transport properties. To better understand the influence of hydroxyl groups on water transport properties, IEMs with a varying number of hydroxyl groups were synthesized. To isolate the effect of hydroxyl groups, other membrane properties (e.g., membrane water content and fixed charge concentration) were kept constant. To remove the effects of charge groups in the IEMs, homopolymers were also synthesized with a varying number of hydroxyl groups. Hydroxyl-water interactions in the IEMs and homopolymers have been studied using differential scanning calorimetry, osmotic permeability, and electro-osmosis. A better understanding of hydroxyl-water interactions will help rationalize the future development of IEMs with controlled water transport properties. This will provide the foundation to develop next-generation IEMs with decreased water permeability for use in ED applications such as brine concentration.