Sustainable Membrane Production through Polyelectrolyte Complexation Induced Aqueous Phase Separation

Nonsolvent induced phase separation (NIPS) is the most common approach to produce polymeric membranes. Unfortunately, NIPS relies heavily on aprotic organic solvents like N-methyl-pyrrolidone. These solvents are unsustainable, repro-toxic for humans and are therefore becoming increasingly restricted within the European Union. A new and sustainable method, aqueous phase separation (APS), is reported that eliminates the use of organic solvents. A homogeneous solution of two polyelectrolytes, the strong polyanion poly(sodium 4-styrenesulfonate) (PSS) and the weak polycation poly(allylamine hydrochloride) (PAH), is prepared at high pH, where PAH is uncharged. Immersing a film of this solution in a low pH bath charges the PAH and results in a controlled precipitation, forming a porous water-insoluble polyelectrolyte complex, a membrane. Pore sizes can be tuned from micrometers to just a few nanometers, and even to dense films, simply by tuning the polyelectrolyte concentrations, molecular weights, and by changing the salinity of the bath. This leads to excellent examples of microfiltration, ultrafiltration, and nanofiltration membranes. Polyelectrolyte complexation induced APS is a viable and sustainable approach to membrane production that provides excellent control over membrane properties and even allows new types of separations.