Preparation Of PVDF Membrane With Modified-TIPS Method

Poly(vinylidene fluoride)(PVDF) membranes have been widely used inmembrane distallation, industrial and urban sewage treatment, orgnic/water separationand drinking water purity, due to its good thermal stability and excellent chemicalresistance. PVDF membranes can be prepared by different ways, such as thenon-solvent induced phase separation (NIPS), melt spinning-streching process and thethermally induced phase separation (TIPS) process. In this paper, the PVDF flatmembranes were prepared along with diluent mixtures via complex thermal inducedphase separation process．The polyvinylidine fluoride (PVDF) flat membrane wasprepared via modified thermally induced phase seperation (modified-TIPS) methodusing the mixture of triethyl phosphate (TEP)(good diluent for PVDF) and Diethyleneglycol monoethyl ether acetate (DCAC)(poor diluent for PVDF) as mixed solvent. Theeffects of TEP/DCAC ratio in diluent mixture and coagulation bath conditions onphase behavior have been investigated. The results of experiment showed that，thephase separation changed from Liquid-Liquid phase seperation to Solid-Liquid phaseseparation as the interaction between PVDF and diluent mixture increased．The crystallization kinetics of PVDF in diluent mixture was investigated throughdifferenial scanning calorimetry (DSC) and X-ray diffraction (XRD) measurement, themembrane morphologies were examined by scanning electron microscopy (SEM), Themembrane mechanical proprety, and membrane performance were measured. Theexperimental results indicated that the crystallinity increased by decreasing interactionbetween PVDF and diluent. Morphologies of the top of the membranes were found toheavily on the content of DCAC, the air-surface structure became porous, the glasssurface structure changed from a continuous morphology to spherical crystals, the pore size of the cross-section structure increased with the increase of DCAC. With theincrease of DCAC in the coagulation bath, the top surface evolved from a dense skin toa totally porous morphology, and the pore size of the cross-section of the membranedecreased. Furthermore, the results of the pure water flux and the tensile strength werefound to correlate with the morphologies of the membranes.The composition of coagulation bath have important influence to phase behaviorof the system, which would lead to different membrane structure and performance. Theamount of H2O in coagulation bath decrease the pure water flux and increase thetensile strength. The amount of solvent in the coagulation bath will increase thethickness of the membrane, increase the pure water flux and decrease the tensilestrength. The coagulation bath temperature have important influence on the phaseseparation mechanism of the system. When coagulation bath temperature is lower thancrystallization temperature of the system, the system is prone to S-L phase separation.When coagulation bath temperature is higher than crystallization temperature of thesystem, the system is prone to Liquid-Liquid phase separation.