Morphological properties of poly(vinylidene fluoride-cotetrafluoroethylene membrane): effect of solvents and polymer concentrations

Poly(vinylidene fluoride-co-tetrafluoroethylene) (PVDF-co-PTFE) flat sheet membranes were prepared via phase inversion process by means of immersion precipitation. The effects on the microstructure of prepared membranes by using different solvents and the effect of polymer solution composition (polymer concentration) on the membrane morphologies and the membrane porosity were studied. Three different solvents were employed, which were, N,N-dimethylacetamide (DMAc), Nmethyl-2-pyrrolidone (NMP) and N,N-dimethylformamide (DMF). Different polymer concentrations were used20 wt.%, 25 wt.% and 30 wt.%. Brookfield viscometer was used to measure the viscosity for each polymer solution before the membrane was casting. The morphological study for the cross-sectional area of PVDF-co-PTFE membrane was carried out by using scanning electron microscope (SEM). The membrane porosity was determined by using the weight difference and density resulted from the immersing of dry membrane into octanol for 15 seconds. The results showed that the polymer concentration was a particularly important parameter. At different polymer concentration, the precipitant plays an important role and finally influence the prepared membrane microstructure. At higher concentration of PVDF-co-PTFE polymer, the prepared membrane possessed a sponge-like membrane structures, whereas at lower concentration, a finger-like structure was obtained. It was also found that, the membrane porosity decreases with the increasing of polymer concentration. The effect of different solvent reflected to different morphologies of the prepared flat sheet membranes. Prepared membrane with more sponge-like structure indicated a weak dissolution solvent was used. Hence, a small amount of water needed to induce precipitation and reduced the precipitation rate