(20b) Self-Assembling Random Zwitterionic Copolymers As Charge-Selective Nanofiltration Membranes
The purification of water is vital for many chemical separation processes. However, feedstocks containing foulants and/or chlorine represent harsh conditions in which many membranes are unable to operate. Accordingly, the development of membranes that can function effectively in such conditions is an important research endeavor. A zwitterion is defined as a molecule that contains an equal number of positive and negative charges. Random zwitterionic copolymers (RaZCs) are linear copolymers comprised of randomly distributed zwitterionic repeat units and non-zwitterionic repeat units. RaZCs self-organize to form a network of zwitterionic nanochannels that are surrounded by the non-zwitterionic repeat unit of the RaZC. The zwitterionic channels act as the effective 1 nm pores. Previous work shows that thin film composite membranes featuring these RaZCs as their selective layer exhibit high flux, exceptional fouling resistance, and size-based small molecule selectivity along with low salt rejection. However, for many applications, improved charged solute and divalent salt rejection would be beneficial. This can be achieved by incorporating charged groups into these polymers that segregate into the zwitterionic domains, and exclude ions by hindering their rate of transport through the membrane. In this work, we show that RaZCs that combine zwitterionic, charged, and hydrophobic repeat units are versatile materials for the fabrication of robust membranes that exhibit high flux, divalent ion rejection, fouling resistance, and remain stable upon exposure to chlorine. Well-designed charged RaZCs lead to membranes that exhibit salt rejections comparable to commercial nanofiltration (NF) membranes (e.g. >95% Na2SO4 rejection). Charged groups segregated into these nano-scale channels also impart exceptional charge-based selectivity between organic small molecules to the membrane. Furthermore, the pore chemistry of these membranes can be tuned by a simple approach, thus rationally tuning membrane selectivity for desired applications. This approach represents a unique pathway to fabricating industrially relevant membranes with tunable separation properties.