Rapid in-situ covalent crosslinking to construct a novel azo-based interlayer for high-performance nanofiltration membrane

The strategy of introducing a hydrophilic interlayer (IL) on the substrate has been widely used in the preparation of nanofiltration (NF) membranes. In this paper, we perform a rapid in-situ coupling reaction of tannic acid (TA) and a dual diazonium salt (DDS) to construct a novel stable azo-based interlayer on the polysulfone (PSf) ultrafiltration membrane. Through exploring the concentration and the deposition time of TA, the protocol of fabricating the interlayer with the optimal hydrophilicity is determined. Relevant characterizations indicate that the thickness of the interlayer is quite thin, and the defects at the surface of the pristine substrates disappear after modification. Interestingly, the color of the substrate with interlayer changes dramatically after pouring piperazine (PIP) solution thereon, implying that there is considerable interaction between the phenolic hydroxyl groups at the interlayer and the PIP molecules in aqueous phase. By further reducing the PIP concentration, an active layer of which thickness is only about 35 nm was obtained. The separation performance evaluation suggests that the optimized novel NF membrane exhibits a 2.71-fold increase in water permeance (17.40 ± 1.09 L m−2h−1 bar−1) compared with the conventional one. Moreover, the discussion about the role of the interlayer on interfacial polymerization (IP) is also provided with advanced significance.