In-situ construction of superhydrophobic PVDF membrane via NaCl-H2O induced polymer incipient gelation for membrane distillation

In-situ construction of superhydrophobic polyvinylidene fluoride (PVDF) membrane was prepared through nonsolvent induced phase inversion process for direct contact membrane distillation (DCMD). Saturated sodium chloride aqueous solution (NaCl-H2O) was added to the casting solution as a green phase inversion inducing agent. The variation of polymer crystal and the formation mechanism of micro-nano structure on membrane surface were studied. The results showed that NaCl crystals could act as nucleus for the pre-gelation and crystallization of PVDF chains. The “harsh” nonsolvent, water, accelerated the gelation process. When the optimum amount of NaCl-H2O was added, superhydrophobic membrane with lotus leaf like micro-nano spherical structure and interconnected channels was obtained. The pure water contact angle reached 155.3° with DCMD flux of 54.5 L/m2∙h, which was about 1.6 times for the membrane without NaCl-H2O addition. The stability was also improved owing to the superhydrophobic structure.