Preparing Of Polyvinylidene Fluoride Membrane Via Blended Phase Separation

Poly(vinylidene fluoride)(PVDF), a semicrystalline polymer material, has been the hotspot of the membrane materials research because of its good chemical and thermally stability, as well as its high chemical resistance to solvent and radiation. The researches of the PVDF membranes now mainly focus on the structure control. PVDF membranes were prepared via blending the thermally induced phase separation and non-solvent induced phase separation, and factors that affect the membrane structure and performance were studied.Poly(vinylidene fluoride)(PVDF) was used as matrices; N, N-dimethylacetamide(DMAc) was used as solvent; propiophenone, isophorone, triethyl phosphate and gamma-butyrolactone were used as diluents respectively. Casting solution was prepared at a certain temperature. Membranes were obtained by adding the casting solution into the non-solvent (water). Diluents were removed by extraction. Membrane structures were studied by scanning electron microscope (SEM). Separation performances of the membranes were evaluated by the pure water flux and the BSA rejection, and the tensile strength of the membranes was also studied.Results showed that membrane presented bi-continuous structure when the solvent/diluents ratio was 1:1 and the system temperature was below 100℃. When the temperature was above 100℃, sphero-crystal structure was obtained. While the membrane structure changed from sphero-crystal to bi-continuous, the pure water flux increased from 18Lm-2h-1 to 48Lm-2h-1. While the pure water flux increased, BSA rejection remained at a relatively high level. When the membrane structure was sphero-crystal, the BSA rejection was above 80%; when the membrane structure changed to bi-continuous, the BSA rejection was also above 70%. When the system temperature was at a high level(such as 120℃), the sphero-crystal structure membrane had a thick cortex, so the membrane showed a relatively high tensile strength(7.41MPa). When the system temperature decreased, the membrane structure changed to bi-continuous and the tensile strength decreased to 4.21MPa. Under the system temperature of 80℃and the solvent/diluents ratio of 1:1, bi-continuous structure was obtained when the polymer concentration was between 12% and 16%.