Control over Charge Density by Tuning the Polyelectrolyte Type and Monomer Ratio in Saloplastic-Based Ion-Exchange Membranes

Membranes based on polyelectrolyte complexes (PECs) can now be prepared through several sustainable, organic solvent-free approaches. A recently developed approach allows PECs made by stoichiometric mixing of polyelectrolytes to be hot-pressed into dense saloplastics, which then function as ion-exchange membranes. An important advantage of PECs is that tuning their properties can provide significant control over the properties of the fabricated materials, and thus over their separation properties. This work studies the effects of two key parameters─(a) ratio of mixing and (b) choice of polyelectrolytes─on the mechanical, material, and separation properties of their corresponding hot-pressed saloplastic-based ion-exchange membranes. By varying these two main parameters, charge density─the key property of any IEM─was found to be controllable. While studying several systems, including strong/strong, strong/weak, and weak/weak combinations of polyelectrolytes, it was observed that not all systems could be processed into saloplastic membranes. For the processable systems, expected trends were observed where a higher excess of one polyelectrolyte would lead to a more charged system, resulting in higher water uptake and better permselectivities. An anomaly was the polystyrenesulfonate-polyvinylamine system, which showed an opposite trend with a higher polycation ratio, leading to a more negative charge. Overall, we have found that it is possible to successfully fabricate saloplastic-based anion- and cation-exchange membranes with tunable charge densities through careful choice of polyelectrolyte combination and ratio of mixing.