PVDF hollow fiber formation via modified NIPS method: Evolution elucidation of phase separation mechanism, structure and properties of membrane with coagulation strength varied

In this work, hollow fiber membrane was prepared from poly(vinylidene fluoride) (PVDF)/triethyl phosphate (TEP)/water systems by modified non-solvent induced phase separation technique (mNIPS), which was characterized in the process with relative high polymer content of 30 wt% in spinning dope. The ternary phase diagram and precipitation rate were determined to elucidate the various membrane formation mechanisms, and the membranes morphologies were observed by field emission scanning electronic microscopy (FESEM). It was indicated that the phase separation mechanism was found to heavily depend on the bath strength. Although the solid-liquid demixing process initiated the precipitation process, by changing the bath gradually from pure water to 40% TEP, the liquid-liquid demixing occurred earlier and earlier. Accordingly the top surface morphologies evolved from a dense skin (asymmetric membrane) to a totally porous morphology (symmetric membrane). In addition, the permeability, maximum stress at break, medium pore size and porosity of hollow fiber membranes were also investigated. In conclusion, the successful application of modified non-solvent induced phase separation (mNIPS) may provide an effective route to improve the membrane structure modulation.