Study On The Fabrication Of Nanofiltration Membranes Modified By Interlayers Based On Diazotization-coupling Reaction

After the rapid innovation and development of technologies of membrane separation in recent decades,various membrane materials have been widely used in medicine,food,chemical industry,and water treatment.Among them,nanofiltration membranes is gradually being applied on a large scale due to its high permeability,good ion selectivity,and low operating pressure.At the same time,in fundamental research,the preparation of high-performance nanofiltration membrane materials has also become one of the hotspots in the field of membrane materials.In this study,the interlayer modification strategy was adopted to obtain a uniform and stable interlayer through an in-situ cross-linking reaction of organic macromolecules on the surface of the substrate,and then the nanofiltration membrane with high permeability and selectivity could be prepared through interfacial polymerization reaction.In addition,the influence from the surface properties of interlayer on the interfacial polymerization process was also discussed.The polysulfone（PSf）ultrafiltration substrate was modified by the aromatic dendrimer polyamidoamine macromolecules through self-coupling reaction,and a thin-film composite membrane was prepared by interfacial polymerization reaction based on the modified substrate.Specifically,at first,the fourth generation dendrimer（G4D）was synthesized via divergent method.Then,G4D molecules were covalently self-assembled in-situ on the surface of substrate through diazotization-coupling reaction to form a stable interlayer.The modified substrate has a higher pure water flux than the pristine PSf,due to the increased hydrophilicity and the inherent rigid intramolecular pores in G4D.The active layer prepared by interfacial polymerization on the substrate is thinner and has special nanostrip morphology.Thus,the reduced mass transfer resistance,the increased effective membrane area,and the substrate with increased pure water flux make the final prepared nanofiltration membrane possesses outstanding permeability(26.40±0.51 L·m^-2·h^-1·bar^-1).Meanwhile,the novel nanofiltration membrane also exhibits good monovalent/divalent ion selectivity and long-term operation stability.Taking into account the expensive cost and the relatively long-term synthesis process of the dendrimers.Therefore,a novel azo-based interlayer with abundant phenolic hydroxyl groups on the surface is designed to prepare high-performance nanofiltration membranes.Firstly,a dual diazonium salt with a distorted structure was prepared by diazotization reaction which serves as a crosslinking agent,and then a stable azo-based interlayer is formed through coupling reaction with tannic acid deposited on the surface of PSf.The active layer prepared on the modified PSf is also thinner and has special nanostrip morphology,compared with that prepared on the pristine PSf.Given the increase in the hydrophilicity of the substrate surface and the interaction between the hydroxyl groups on the surface of the interlayer and the amine monomer in the interfacial polymerization,a thinner active layer was prepared by further reducing the concentration of the amine monomer.The resulting nanofiltration membrane shows excellent permeability(17.40±1.09 L·m^-2·h^-1·bar^-1)and ensures a high rejection to sodium sulfate（98.2±0.5%）.