Preparation of high-performance PVDF hollow fiber microporous membranes via the c-TIPS process with water-insoluble solvent as the second solvent

Triethyl phosphate (TEP) is a promising solvent for the c-TIPS process given its high boiling point and water miscibility. However, casting solutions containing high TEP concentration generally trigger gelation, giving a dense outer surface in PVDF membrane. In this work, we fabricated PVDF hollow fiber membranes with a porous outer surface and excellent mechanical strength via a complex thermally induced phase separation (c-TIPS) method and applied them in the DCMD process. A water-insoluble solvent, dibasic ester solvents (DBEs), was used as the second solvent to modulate the phase separation process of the PVDF/TEP system. The mass transfer between the coagulation bath and the DBEs solvent was analyzed. It could decrease the polymer-solvent interactions and the rate of solvent efflux in the coagulation bath, contributing to the development of a porous membrane structure. When the weight ratio of TEP/DMA was 1, the optimized PVDF hollow fiber membrane exhibited a tensile strength of 8.34 MPa, an average pore diameter of 0.14 μm, giving the permeate flux of 18.56 ± 2.5 kg m−2 h−1 in DCMD and excellent operational stability when subjected to high brine concentration. Thus, the fabrication method shows a promising potential for tailoring the surface porosity and bulk structure. The PVDF membranes developed in this study demonstrate potential for use in membrane distillation and related applications. Schematic diagram of mass transfer and PVDF membrane morphology formed with TEP and mixed solvents (TEP/DBEs) during c-TIPS process. [Display omitted] •DBEs are water insoluble solvents.•DBEs can act as pore-forming agent for PVDF hollow fiber synthesis via c-TIPS.•DMA content in solvent mixture determines porosity on membrane surface.•Resultant PVDF has porous surface, high DCMD flux, and good strength.