Fluoropolymer membranes are applied in membrane operations such as membrane distillation and membrane crystallization where hydrophobic porous membranes act as a physical barrier separating two phases. Due to their hydrophobic nature, only gaseous molecules are allowed to pass through the membrane and are collected on the permeate side, while the aqueous solution cannot penetrate. However, these two processes suffer problems such as membrane wetting, fouling or scaling. Membrane wetting is a common and undesired phenomenon, which is caused by the loss of hydrophobicity of the porous membrane employed. This greatly affects the mass transfer efficiency and separation efficiency. Simultaneously, membrane fouling occurs, along with membrane wetting and scaling, which greatly reduces the lifespan of the membranes. Therefore, strategies to improve the hydrophobicity of membranes have been widely investigated by researchers. In this direction, hydrophobic fluoropolymer membrane materials are employed more and more for membrane distillation and membrane crystallization thanks to their high chemical and thermal resistance. This paper summarizes different preparation methods of these fluoropolymer membrane, such as non-solvent-induced phase separation (NIPS), thermally-induced phase separation (TIPS), vapor-induced phase separation (VIPS), etc. Hydrophobic modification methods, including surface coating, surface grafting and blending, etc., are also introduced. Moreover, the research advances on the application of less toxic solvents for preparing these membranes are herein reviewed. This review aims to provide guidance to researchers for their future membrane development in membrane distillation and membrane crystallization, using fluoropolymer materials.

Poly(vinylidene fluoride) (PVDF) ultrafiltration (UF) membrane was prepared by vapor and non-solvent-induced phase separation (VNIPS) process, and the relationships between preparation conditions, phase separation behaviors and membrane structures were discussed. The phase diagram was generated by cloud point titration, the effects of environmental relative humidity, air exposure time and casting solution temperature on the structure and performance of the resulted membrane were investigated. The addition of polyvinylpyrrolidone (PVP) additives to the casting solution made the system thermodynamically unstable and led the turbidity curve shift toward the solvent/non-solvent axis. The experimental results showed that both average pore size and porosity increased with higher relative humidity. When the relative humidity increased from 35 to 85%, the mean pore size increased from 35 to 70 nm. The effect of exposure time on membrane structure was related to the air environment. It was found that under low temperature and low humidity conditions, exposure time had little effect on membrane structure. When the relative humidity was 75%, the mean pore size and distribution increased with the exposure time extension from 1.0 to 30 s. The mean pore size of the membrane could be reduced by reducing the relative humidity. When the temperature of the casting solution increased, the membrane pore size reached maximum at 80 °C, and the spherulite on the surface of the membrane decreased. The spherulite morphology almost disappeared when the temperature of the casting solution reached 120 °C. In addition, with the increase in the casting solution temperature, large pores appeared in the sub-layer of the membrane, which gradually widened the pore distribution, leading to the decrease in the tensile strength. The preparation condition was optimized as low temperature of casting solution, low humidity and low temperature of the environment. The fabrication process showed the potential for scaling-up production of the PVDF UF membrane by the VNIPS technique.