Fabrication of hydrophobic bi-layer fiber-aligned PVDF/PVDF-PSF membranes using green solvent for membrane distillation

Green high-flux poly(vinylidene fluoride) (PVDF) membranes with hydrophobic properties are beneficial for membrane distillation (MD). In this study, PVDF and polysulfone (PSF) polymer powders were dissolved in PolarClean to form immiscible polymer solutions. After casting the PVDF-PSF membranes and inducing a phase inversion procedure in water, a high-porosity fiber aligned PVDF-PSF membrane was obtained. The PVDF-PSF membranes also exhibited high tensile strength and satisfactory vapor flux. However, the surface hydrophobic properties were relatively poor and the surface pore sizes were large. Therefore, a PVDF/PolarClean/propylene glycol (PG) solution was used to construct rough PVDF nanoparticles (NPs) on the PVDF-PSF membrane surface. The PVDF NPs decreased the membrane pore size and increased the surface roughness. The resulting membrane exhibited a highly hydrophobic membrane surface and a higher liquid entry pressure than the pristine PVDF-PSF membrane. In addition, the whole process did not contain any toxic solvents and the stability of the PVDF/PVDF-PSF membrane was satisfactory after ultrasonic treatment. In the desalination of saline water containing 3.5 wt% and 12 wt% NaCl solutions, the resulting membrane showed a high water flux, with a rejection rate higher than 99.99 % after 120 h. This study provides important guidance for polymer membranes for MD.