Pore structure control of PVDF membranes using a 2-stage coagulation bath phase inversion process for application in membrane distillation (MD)

Polyvinylidenefluoride (PVDF) membranes were fabricated using non-solvent assisted phase inversion technique (NIPS) to ultimately create a membrane distillation (MD) membrane. No pore forming additives were employed, in order to maintain the pore size distribution (PSD) as narrow as possible. Instead, a 2-stage coagulation bath system was used. The effects of the CB temperature, immersion time, polymer solution concentration, and thickness of the casted PVDF membranes were investigated. Prospective membranes with a uniform and open structure on the surface and an asymmetric interconnected pore structure all through the thickness of the membranes were obtained. A narrow PSD (0.14-0.2 µm) along with a high liquid entry pressure (LEP) range of 420-570 kPa and porosity values ranging from 57% to 79% were observed in the fabricated membranes, with a contact angle variance from 100° to 127°. The synergy of hydrophobicity, mechanical strength, high LEP and a very narrow PSD made the fabricated membranes suitable candidates for MD applications. Direct Contact (DCMD) and Vacuum MD (VMD) testing using these membranes were then carried out to analyze and understand their operational MD performance. The membranes were found to yield acceptable water vapor flux in MD and a narrower PSD than commercial PVDF membranes with similar nominal pore size. The salt rejection achieved was lower than expected, which was explained based on pore wetting.