Effects of PVDF-HFP concentration on membrane distillation performance and structural morphology of hollow fiber membranes

Poly(vinylidene fluoride-hexafluoropropylene), PVDF-HFP, hollow fiber membranes were prepared by the dry/wet spinning technique using different copolymer concentrations in the dope solutions ranging from 17 to 24 wt.%. All the spinning parameters were maintained constant except the copolymer concentration. The morphological properties of the hollow fiber membranes were studied in terms of scanning electron microscopy (SEM), atomic force microscopy (AFM) and void volume fraction. The effects of PVDF-HFP content in the spinning solutions were also studied by measuring the water entry pressure and direct contact membrane distillation (DCMD) permeate flux of the hollow fiber membranes. An increase in the copolymer concentration of the spinning solution resulted in a decrease in the precipitation rate and a transition of the cross-section structure from a finger-type structure to a sponge-type structure. Pore size, nodule size and roughness parameters of both the internal and external hollow fiber surfaces were determined by AFM. It was observed that the pore size decreased in both the internal and external surfaces of the hollow fiber membranes with increasing the copolymer concentration and reached a minimum value at the outer surface for PVDF-HFP concentrations greater than 20 wt.%. Water entry pressure values were decreased whereas both the void volume fraction and the DCMD permeate flux increased with decreasing the copolymer concentration