Nanofiltration membranes with hydrophobic microfiltration substrates for robust structure stability and high water permeation flux

Traditional nanofiltration membranes (NFMs) suffer from ultrafiltration substrates with low porosity, small pore size and relatively poor solvent stability. Herein, NFMs have been fabricated on a series of hydrophobic polymer microfiltration substrates to address these issues. Polyphenol-based coatings of tannic acid/diethylenetriamine (TA/DETA) were co-deposited on the hydrophobic substrates to improve their surface wettability and to make them appropriate for interfacial polymerization. The spreading behaviors of aqueous solutions, which are of significant importance to the formation of defect-free polyamide layers, were directly visualized by laser confocal microscopy. The influences of TA/DETA coatings on the interfacial polymerization were further demonstrated by both dynamic molecular simulation and nanofiltration performance evaluation. The as-prepared NFMs exhibit higher water permeation flux compared with traditional ones because of the large pore size and high porosity of the microfiltration substrates, as well as the relatively low cross-linking degree of polyamide layers. Internal stress during the nanofiltration process was calculated by the theory of thin plates and the results claim good pressure resistance for these NFMs. Therefore, the as-prepared NFMs can be steadily used under high operation pressures even up to 0.9 MPa, which are in accordance with the theoretical calculation. Furthermore, these NFMs also present good solvent resistance since the chemical stability of the no-polar hydrophobic substrates.