Aqueous Phase Separation of Responsive Copolymers for Sustainable and Mechanically Stable Membranes

Membranes are often used in environmentally friendly applications and as a sustainable alternative to conventional processes. Unfortunately, the vast majority of polymeric membranes are produced via an unsustainable and environmentally unfriendly process that requires large amounts of harsh reprotoxic chemicals such as N-methyl-2-pyrrolidinone and dimethylformamide. In this work, we investigate an aqueous phase separation (APS) system that uses weak polyelectrolytes, whose charge is dependent on the pH (weak polyelectrolytes), to produce membranes. Specifically the copolymer polystyrene-alt-maleic acid (PSaMA) is used. PSaMA contains responsive monomers, required for APS, and also unresponsive hydrophobic monomers that provide mechanical stability to the resultant membranes. This work demonstrates that by controlling the precipitation of PSaMA, it is possible to prepare a wide range of membranes; from microfiltration membranes capable of treating oily waste water to dense nanofiltration-type membranes with excellent micropollutant retentions and high mechanical stability. While similar materials in prior work could only withstand 4 bar, the membranes presented here demonstrate stable operation up to 20 bar. The only solvents used in this APS system are water and the green solvent acetic acid, thus making our APS process significantly more sustainable and environmentally friendly as compared to the conventional membrane fabrication methods.