A slow-fast phase separation (SFPS) process to fabricate dual-layer hollow fiber substrates for thin-film composite (TFC) organic solvent nanofiltration (OSN) membranes

A novel slow-fast phase separation (SFPS) process is proposed to fabricate the desirable ultrafiltration dual-layer hollow fiber substrate for effective interfacial polymerization. By controlling the ratios of non-solvents to volatile co-solvents in both the outer- and inner-layer dopes, the outer and inner layers undergo slow and fast coagulation, respectively, in the dry-jet wet-spinning co-extrusion process. The inner layer is highly porous for high solvent flux permeation, while the outer layer possesses a surface with nano-sized pores that have a sharp size distribution for effective thin film polymerization. The addition of polyvinylpyrrolidone in the outer dope further reduces the pore size, and enhances surface hydrophilicity as well as mechanical strength. The substrate was cross-linked and subjected to interfacial polymerization for fabricating thin-film composite (TFC) membranes. The effects of the substrate properties of various hollow fiber membranes from the SFPS process on the TFC membrane were systematically investigated in terms of morphology and organic solvent nanofiltration (OSN) performance. The membrane shows a 99.3% rejection of ramazol brilliant blue (MW: 626.54gmol^-1) and a methanol flux of 14.4lm^-2h^-1 at 16bar. •Novel slow-fast phase separation (SFPS) process to fabricate the dual-layer hollow fiber membrane.•SFPS process leads to desirable surface and nano-sized pores with a sharp size distribution.•The substrate from SFPS is favorable for effective interfacial polymerization.•The TFC hollow fiber membrane possesses 99.3% rejection at 16bar.