Novel methodology for facile fabrication of nanofiltration membranes based on nucleophilic nature of polydopamine

Novel methodology for facilely fabricating nanofiltration (NF) composite membrane has been successfully developed by employing nucleophilic nature of polydopamine (PDA) chemistry. The self-polymerized PDA coating over polysulfone (PSf) substrate was utilized as a key intermediate layer for trimesoyl chloride (TMC) grafting followed by poly(ethyleneimine) (PEI) deposition to construct the hierarchically structured separation layer of NF membrane. In contrast to the electrophilic quinone moieties of PDA layer usually involved in the Michael addition and Schiff base reactions with polymeric amines for membrane preparation in previous reports, the phenolic hydroxyl groups of catechol moieties as well as amine groups at the PDA layer possess nucleophilic nature, which are capable of quickly coupling with the highly reactive acyl chlorides to form ester and amide bonds in the step of TMC grafting, resulting in TMC moieties covalently anchored at the PDA layer with free acyl chloride groups. Such created acyl chlorides are further coupled with the amine groups of branched PEI polymer in the PEI deposition procedure to form the stable amide bonds linking the PDA base layer and PEI upper layer in the resulting hierarchical separation layer. The properties of membranes prepared at different stages were characterized with respect to surface chemistry, pore properties, and separation performances to understand deeply the newly developed methodology for membrane preparation. Further studies focusing on NF properties and stability of the developed NF membrane revealed that such membrane shows high efficiencies in retention of divalent cations, small organic molecules as well as heavy metal ions, and exhibits desirable thermal stability and long-term performance stability.