Mechanism of formation of hollow fiber membranes for membrane distillation: 1. Inner coagulation power effect on morphological characteristics

Hollow fiber membrane morphology and inner surface structure were analyzed based on the thermodynamic and kinetics of the phase inversion process. Different nonsolvent mixtures formed by N, N-dimethyl acetamide (DMAC) and distilled water were considered. By means of Hansen solubility parameters, lower interaction between the nonsolvent and the mixed solvent (DMAC/trimethyl phosphate, TMP) was observed when greater amount of DMAC was added in the nonsolvent. The spinning solution became thermodynamically more stable and kinetically showed slower coagulation rates, being both related to the resultant membrane formation. In this first study poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) hollow fiber membranes were prepared by the dry/wet spinning technique employing the above cited nonsolvent mixtures as an internal coagulant. Their effect on the internal structure of the membrane evolved towards a more open-porous inner surface when increasing the solvent content in the bore liquid. Various characterization techniques were used in order to analyze the adequacy of these membranes for desalination by direct contact membrane distillation (DCMD). It was found that all the properties together with the permeability of the hollow fibers improved when the coagulation power of the nonsolvent was reduced.