(58t) Development of a Novel Nanofiltration Membrane Based on the Polydopamine Chemistry

This work reports on the fabrication of new nanofiltration (NF) membrane with a robust and permeable 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.

The resulting PEI deposited membrane M-PEI shows NF characteristic with mean pore size of 0.23 nm and MWCO of 292 Da, while possesses high rejection efficiency towards divalent cation Mg2+, small organic solutes, and heavy metal ions because of its positively charged surface feature and small pore size. Surface chemistry studies reveal that M-PEI has very smooth and hydrophilic layer surface owing to the existence of uniform PEI coverage with plenty of hydrophilic free amine groups. Furthermore, the M-PEI NF membrane is proved to be thermally stable in the range of feed temperature tested, owing to the strong adhesion of PDA base layer on the PSf substrate, as well as the existence of covalent bonds linking the middle TMC grafting layer with both PDA coating and PEI upper layer. This work demonstrates that the composite membrane with high separation efficiency could be effectively fabricated on the basis of nucleophilic nature of polydopamine. Further study focusing on the functionalities such as chlorine resistance and anti-fouling of the desalination membrane fabricated on basis of this newly developed methodology is ongoing in our lab.