Porous PVDF asymmetric hollow fiber membranes prepared with the use of small molecular additives

Preparation of polyvinylidene fluoride (PVDF) asymmetric hollow fiber membranes was studied by introducing small molecular additives, which include nonsolvents (water, ethanol and 1-propanol) and inorganic salt (LiCl). Dimethylacetamide (DMAC) was used as a solvent. Water was used as an external coagulant, while water, ethanol or a mixture of water and ethanol was used as an internal coagulant. The prepared PVDF hollow fiber membranes were characterized in terms of water flux, and molecular weight cut-off for the wet membranes. Average pore size and effective surface porosity were determined using the gas permeation method for the dried membranes. The cross-sectional structure of the hollow fibers was examined by scanning electron microscopy. The collapsing pressure and wetting pressure of the dried membranes were also tested. The effect of polymer concentration, non-solvents, the mixture of non-solvent and LiCl, internal coagulant and post-treatment was studied in details. The PVDF hollow fibers prepared from the small molecular additives exhibit good mechanical strength and excellent hydophobicity. The PVDF hollow fiber spun from the non-solvent alone exhibits a quite low permeability, while good PVDF porous hollow fiber membranes were prepared using a mixture of the water/LiCl, or 1-propanol/LiCl as the additive.