Weak polyanion and strong polycation complex based membranes: Linking aqueous phase separation to traditional membrane fabrication

In recent work, Aqueous Phase Separation (APS) based on pH change induced polyelectrolyte complexation has shown great potential for the preparation of sustainable polymeric membranes with tunable structures. Unfortunately, thus far this has only been possible with a single polyelectrolyte combination. In this work, we demonstrate that this APS approach extends beyond a single system by preparing sustainable membranes from polyelectrolyte complexes (PECs) of the weak polyanion poly(acrylic acid) (PAA) and the strong polycation poly(diallyldimethylammonium chloride) (PDADMAC). PE solutions are mixed in an acidic medium where PAA is uncharged, and then this mixture is cast and immersed in a coagulation bath at a pH where PAA becomes charged and able to form a PEC with the oppositely charged PDADMAC. Since this process includes both phase separation and PE complexation, it is expected that membrane structure and performance is influenced by a combination of many factors. Casting solution pH, PAA molecular weight, and coagulation bath pH all directly affect the phase separation behavior of PAA/PDADMAC complexes in ways similar to conventional nonsolvent induced phase separation (NIPS). In addition, coagulation bath salinity and PE mixing ratio influence the complexation behavior. Through tuning of all these parameters it is possible to create a wide variety of membrane structures, ranging from nodular symmetrically porous membranes, to asymmetric membranes with cellular pores and in some cases dense top layers. The nodular membranes show good performance as microfiltration membranes with excellent oil retention (>95% for 3–4 µm droplets) and good water permeances. However for the cellular membranes, filtration led to collapse of the porous structure, emphasizing the importance of PE selection for membrane applications.