Macrovoids-Inhibited PVDF Hollow Fiber Membranes via Spinning Process Delay (SPD) for Direct Contact Membrane Distillation.

Polyvinylidene fluoride (PVDF) hollow fiber membrane was fabricated through the water-induced dope crystallization by allowing a facile spinning process delay (SPD) in the non-solvent induced phase separation (NIPS) process for direct contact membrane distillation (DCMD). The SPD was achieved by the addition of a small amount of water to the PVDF dope solution that was held in a closed container for a particular time. The crystalline property of the PVDF dope solution was investigated by differential scanning calorimetry (DCS). The obtained PVDF hollow fiber membranes were characterized with different techniques, including field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and the mechanical strength. Both the formation mechanism and properties were studied for the membranes with different SPD times. The results showed that macrovoids-inhibited PVDF membranes were obtained from 12 days of the SPD via the crystallization dominated membrane formation process. The obtained membrane 4D-12 exhibited desirable membrane structure and properties for DCMD, which includes improved liquid entry pressure of 2.25 bar, the surface water contact angle of 129°, the maximum pore size of 0.40 μm and the mean pore size of 0.34 μm. The membrane 4D-12 possessed a 2-fold increase in both energy efficiency and permeate water flux in DCMD, and the stable permeate water flux and salt rejection through 224 hours of continuous desalination operation. Compared to the commonly used approach by adding chemicals to the external coagulant, the SPD method provided a low-cost and environmentally friendly alternative to pursuing the macrovoids-free PVDF membranes for DCMD.