Enhanced permeability and stability of PVDF hollow fiber membrane in DCMD via heat-stretching treatment

The schematic of PVDF membranes structure evolution during heat-stretching treatment under different conditions. Shrinkage of PVDF hollow fiber and polymer chain in length and diameter (a) without stretching and (b) with stretching when heated at 120 °C in air or water. [Display omitted] •Heat-stretching treatment yielded high permeate flux and membrane stability.•Heat-stretching treatment significantly affected the membrane's inner surface.•High temperature enhanced the permeate flux and anti-wetting ability.•Pore filling and stretching could inhibit membrane shrinkage. Membrane distillation (MD) demonstrates excellent desalination application potential for high salt concentration wastewater. However, keeping the membrane flux and rejection high during long-term operation remains one of the critical challenges. In this work, a heat-stretching treatment method was proposed to improve the structure of polyvinylidene fluoride (PVDF) hollow fiber membrane and enhance its stability in the MD process. The membrane's pore size decreased after a heat-stretching treatment in water at 120 °C for 1 h. However, the liquid entry pressure of water (LEPw) value and membrane strength increased significantly, making it less prone to pore deformation during the drying or MD process and thus improving the membrane flux and stability in the MD process. The heat-stretching treatment made the PVDF chain arrangement more compact and oriented, avoiding shrinkage in the length direction. Besides stretching, the shrinkage of hollow fiber can be constricted by the combinedeffects of pore filling and stretching due to water or glycerol filling in the membrane pores, thus eliminating the capillary stress caused by liquid evaporation during the air-drying. Consequently, the heat-stretching treatment broke the trade-off between high permeate flux and high stability in the MD process.