Mechanism of formation of hollow fiber membranes for membrane distillation: 2. Outer coagulation power effect on morphological characteristics

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) hollow fiber membranes with porous and skinless outer surfaces were prepared for desalination by direct contact membrane distillation (DCMD). N, N-dimethyl acetamide (DMAC)/distilled water mixtures extruded from the outermost channel of the spinneret were employed as external coagulants along the gap of the spinning process. During membrane formation, various types of interactions studied via Hansen solubility parameters were identified at the outer layer of the nascent fiber. Several characterization techniques and DCMD experiments were carried out in order to analyze the effects of the wet gap on the membrane morphology, properties and desalination performance. The skinlayer was avoided and permeate flux was enhanced when using the DMAC/water mixtures as the external coagulants. Increasing the concentration of DMAC in the nonsolvent mixture weakened the external coagulation power and the outer layer morphology of the fiber became more porous with larger surface pore sizes. Open-porous inner surface, skinless outer layer and spongy cross-section were observed for the hollow fiber membranes prepared with the highest amounts of DMAC in water, 50 and 60 wt%, used simultaneously as internal and external coagulants. The hollow fiber membrane prepared with the coagulant mixture 50 wt% DMAC in water exhibited good mechanical properties and excellent MD membrane characteristics, leading to the highest DCMD permeate flux with a good salt rejection factor.