A co-casting route enables the formation of skinless, hydrophobic poly(vinylidene fluoride) membranes for DCMD

Abstract

Unlike the dual-layer membranes in which dual-layers attached tightly after co-casting, the attainment of independent poly (vinylidene fluoride) (PVDF) membranes with uniformly rough surface structures necessitates the delamination of top-layer membranes from the underlying PVDF membranes. The coverage of top-layer membranes via co-casting is expected to reduce the solvent/nonsolvent exchange rate for the underlying PVDF membranes during a phase inversion process. In this context, a polyethersulfone (PES) membrane was co-cast onto a PVDF membrane followed by phase inversion in a water bath to fabricate PVDF membranes for membrane distillation (MD). The spontaneous delamination of PES from PVDF membranes occurs once the membrane solidifies. Due to the combined effect of the additional PES layer and the solvent retained at the PES/PVDF interface, delayed liquid-liquid (L-L) demixing and crystallization of the PVDF solution take place during the phase separation process, resulting in the formation of a porous surface with symmetric cross-section. Results show that the PVDF concentration significantly affects the membrane morphology. Increasing the PVDF content from 16 wt% to 20 wt% leads to a marked morphological change for membrane cross-section from cellular pores to PVDF particles, which indicates that the phase inversion mechanism varies from delayed L-L demixing to solid-liquid (S-L) demixing; this is mainly affected by the solution viscosity. Due to the roughness increment by the particulate surfaces, the hydrophobicity of co-cast PVDF membranes (PVDF-co) is distinctly enhanced compared to the single-layer PVDF membranes. Furthermore, the pore sizes of the PVDF-co membranes are enlarged, and the thicknesses are reduced. Therefore, the PVDF-co membranes have much higher fluxes than the single-layer PVDF membranes, especially for PVDF-co-20 membranes composed of large PVDF particles which show superior pore interconnectivity than the cellular pores. A long-term stability test is conducted with the PVDF-20-21 membrane for 48 h using a 3.5 wt% NaCl solution at the feed and permeate temperatures of 60 °C and 20 °C, respectively. Fairly steady fluxes are observed over the testing course, as well as a continuous high salt rejection of above 99.95%