Effects of mixed solvents on the structural morphology and membrane distillation performance of PVDF-HFP hollow fiber membranes

Abstract

Poly(vinylidene fluoride-hexafluoropropylene), PVDF-HFP, hollow fiber membranes were prepared by the dry/wet spinning technique, maintaining all the parameters the same except the solvent used to prepare the polymer solution. Different solvents namely, single N,N-dimethyl acetamide (DMAC) and different mixed solvents, DMAC and trimethyl phosphate (TMP) as well as N,N-dimethyl formamide (DMF) and TMP were employed. The relative affinity of the PVDF-HFP and the solvent(s) and the thermodynamic and kinetic aspects responsible for membrane formation were investigated and related with the structure of the prepared hollow fiber membranes. The structural and morphological properties of the hollow fiber membranes as well as the necessary parameters to be known for a membrane proposed for direct contact membrane distillation (DCMD) were studied by different characterization techniques. It was observed that an increase of TMP ratio in the solvents mixture resulted in a decrease of the finger-like structure of the external layer, an increase of the pore sizes of the hollow fibers and the DCMD permeate flux enhancement as consequence. When changing DMAC by DMF a thicker hollow fiber membrane was obtained and the DCMD was decreased due partly to the formation of macro-voids in the middle layer of the hollow fiber membrane.