Fabrication of hydrophobic flat sheet and hollow fiber membranes from PVDF and PVDF-CTFE for membrane distillation

• PVDF-CTFE membrane had optimal membrane morphology, structure and hydrophobicity.
• The PVDF-CTFE membrane possessed a unique structure with narrow pore size distribution.
• Rougher membrane surface was observed for PVDF-CTFE membrane.
• PVDF-CTFE membrane had higher flux and salt rejection over 24 h MD operations.

Hydrophobic polyvinylidene fluoride (PVDF) membranes have been used in membrane distillation, however its performance in terms of permeation flux and salt rejection still needs to be improved. In this study, polyvinylidenedifluoride-co-chlorotrifluoroethylene (PVDF-CTFE), a PVDF based copolymer, was employed to fabricate both flat sheet and hollow fiber membranes. Two pore forming additives (PEG and LiCl) were used in order to examine their effects on the membrane micro-structure and consequently the distillation performance. The crystalline property of the polymers was studied via X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) to reveal its role in the membrane formation. The membrane morphology (SEM), surface property (AFM) and membrane properties (contact angle, pore size, porosity and pore size distribution) were systematically examined and compared among PVDF, PVDF-CTFE membranes as well as their counterparts with pore forming additives. The results revealed that PVDF-CTFE membrane exhibited optimal membrane morphology, pore structure and hydrophobicity, thus delivering better distillation performance (permeation flux up to 62.09 kg/m2 h, and distillate conductivity as low as 5 μS/cm) in comparison with their PVDF membrane counterparts.