Preparation and characterization of PVDF-HFP copolymer hollow fiber membranes for membrane distillation

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) hollow fiber membranes were prepared by the dry/wet spinning technique at different copolymer concentrations from 17 to 24 wt%. All the spinning parameters were kept constant except the copolymer concentration. The temperature of both the internal and external coagulants was maintained at 40^oC. The effects of the copolymer concentration on the morphological properties of the hollow fibers were studied in terms of external and internal diameter and scanning electron microscopy (SEM). It was found that the thickness of all tested hollow fibers did not change significantly. An evolution of the cross-section structure with the increase of the copolymer concentration was detected. The cross-section of the hollow fiber prepared with the lowest copolymer concentration exhibited a finger-like structure in both the external and internal layers disappearing in the internal layer as the copolymer concentration increases. Finally, a sponge-like structure is formed through all cross-section of the hollow fiber prepared with the highest concentration. This may be explained based on the decrease of the coagulation rate with the increase of the copolymer concentration in the dope solution.