The effects of fluorocarbon special surfactant (FS-30) additive on the phase inversion, morphology and separation performance of poly(vinylidene fluoride) (PVDF) membranes

The effects of additive, fluorocarbon special surfactant (FS-30), on the mechanism of phase inversion, morphology and separation performance of the PVDF membranes formed by non-solvent induced phase inversion (NIPS) method from the PVDF/LiCl/FS-30/DMAc system were investigated. The addition of FS-30 significantly improved membrane performance and achieved a simultaneous increase in the pure water flux and BSA rejection of the membrane, which was of great value for membrane application. With the addition of FS-30, the pure water flux increased from 114.68 LMH of the original membrane M0 to 393.76 LMH of the blend membrane M7. At the same time, the BSA rejection rate of the membrane was also increased continuously when the FS-30 content was less than 5 wt% and reached 99.48% at M5. The change in membrane properties can be attributed to the phase inversion process as well as the morphological structural features of the membrane. The addition of FS-30 increased the porosity and improved the hydrophilicity of the membrane by 28.06%. However, the average pore diameter of the membrane was reduced from 0.339 μm to 0.241 μm and a special “stage pore” structure was produced due to the unique properties of FS-30. Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) imaging showed that FS-30 was lost during the coagulation bath and did not remain on the membrane, but affected the crystal composition of the α-phase and β-phase in PVDF membrane as well as promoted the phase inversion process to instantaneous phase separation. Meanwhile, the effect of FS-30 on the morphology of the membrane was characterized by scanning electron microscopy (SEM), and the morphological changes of the membrane was analyzed from the aspects of thermodynamics and kinetics of the casting solution in combination with the phase inversion process of the membrane. It was found that the influence of FS-30 on the kinetics of the casting solution did not balance the effect on the thermodynamic instability, and the thermodynamic effect still dominated, making the FS-30 blend membrane have a more obvious asymmetric structure. Moreover, the effect of the blend membrane on the rejection of algae in surface water and bacteria in sewage was also characterized with the help of chlorophyll fluorescence technique and bacterial inoculation culture experiments respectively, and the result confirmed that the FS-30/PVDF membranes had excellent properties.