Dual PVP roles for preparing PVDF hollow fiber membranes with bicontinuous structures via the complex thermally induced phase separation (c-TIPS)

Polyvinylpyrrolidone (PVP) is a widely used water-soluble polymer that can act as a pore-forming additive in ultrafiltration membrane preparation. Here, the PVDF hollow fiber membranes were prepared via the c-TIPS method by introducing PVP to the casting solution. The dual PVP roles in the membrane fabrication process enabled the production of PVDF hollow fiber membranes composed of bicontinuous structures. The PVP inhibited PVDF crystallization and then reduced the gelation temperature of the casting solution. They grew faster at coarsening, allowing greater growth of pores, and the formation of gap-connected pores and interlinked pores, which formed the bicontinuous structure. The PVP also acted as an effective pore-forming agent, giving porous outer surface. PVP can be removed almost completely from the membrane and did not lead to the wetting of PVDF membranes. Therefore, the synthesized PVDF membrane under optimal conditions could be used in the DCMD process with a maximum vapor flux of 18 ± 3 kg m−2 h−1, the pure water flux in ultrafiltration process reached 335.91 L m−2 h−1 bar−1, and the tensile stress was 5.8 MPa. This work presents a new route for preparing PVDF hollow fiber membranes using conventional spinning equipment with simple operation and moderately elevated temperature that is scalable to industry level.