Tuning the structure and performance of polyelectrolyte complexation based aqueous phase separation membranes

Aqueous Phase Separation (APS) provides a new and sustainable platform to fabricate polymeric membranes entirely in water. Still, little is known on how the casting solution and coagulation bath compositions can be used to tune membrane structure and performance. This work comprises a detailed investigation on the tuning parameters avaliable to tailor the morphology, pore size distribution, and water permeability of polyelectrolyte complex membranes prepared from poly(sodium 4-styrenesulfonate) (PSS) and polyallylamine hydrochloride (PAH). To avoid complexation of PAH and PSS in the casting solution, an optimum amount of base (NaOH) must be added to deprotonate PAH. In addition, the monomer mixing ratio of PSS to PAH significantly influences membrane morphology by modulating the interactions between the two polyelectrolytes. Coagulation bath pH can be used to control the driving force for complexation. Decreasing bath pH facilitates the formation of denser membranes, allowing ~97% protein retentions, whereas increasing bath pH leads to more open membrane structures. Changing the concentration of crosslinker in the coagulation bath allows tuning of membrane pore size from ~2 nm to ~46 nm, while simultaneously influencing membrane mechanical properties. Overall, this work highlights several key parameters to tune APS membrane morphology, demonstrating the versatility of APS to prepare optimized sustainable membranes for specific applications.