Fabrication Of Poly(Vinylidene Fluoride) Porous Membrane Via Thermally Induced Phase Separation

Poly(vinylidene fluoride) (PVDF) membrane was most recently fabricated via immerse precipitation method with its outstanding physical and chemical properties. However, hollow fiber (no support material) by this method often forms finger-pore microstructure and effect the membrane strength. Therefore, it is disadvantage to extend its applied field. In the paper, PVDF porous membrane was prepared via thermally induced phase separation (TIPS). The effects of thermodynamic and dynamic parameters on membrane structure and property were studied. These lay a theoretical and practical foundation for a new technology of preparing PVDF porous membrane with high properties.Firstly, three diluents were selected including diethylene glycol monobuthyl ether, dimethyl phthalate and dibutyl phthalate by miscibility experiment with PVDF. The study results showed that all the casting systems exhibited solid-liquid phase separation when a single diluent was used and the membrane was apt to a spherulite structure which brought disadvantage to the membrane properties.Secondly, two or more diluents with different solubility parameters were mixed as combined diluent. The results showed that the solubility between polymer and combined diluent and system phase seperation behaviors were controlled by changing the composition of diluents, solid-liquid phase separation changed into liquid-liquid phase separation, and the liquid-liquid phase separation region broadened as the increasing content of poor solvent in combined diluent. Then, spherulite congeries structure in membrane was restrained due to the occurrence of liquid-liquid phase separation. Additionally, the addition of nucleating agent increased the nucleation density in some sense and improved the membrane structure correspondingly.On the basis of above-mentioned study, PVDF porous flat membrane was casted using the combined diluent. The study results showed that the membrane pore diameter, porosity and pure water flux decreased as the increase of polymer concentration, while the membrane strength and elongation at break increased correspondingly. With the increasing of the coagulation bath temperature, the spherulite congeries in the membrane structure exhibited more obviously; the membrane porosity decreased little while the membrane surface pore diameter andpure water flux increased gradually.Thirdly, the effect of CaCO₃ addition on membrane structure was studied. It is found that CaCO₃ particles affected the crystal dynamic of PVDF. Hence, it limited the spherulite congeries structure and a symmetrical bi-continuous morphology dominated all the membrane body. The decreasing size of CaCO₃ was propitious to this trend. Furthermore, the porosity of blend membrane was improved when membrane was treated by acid. In all cases, membrane structure can be perfected by melt-blending PVDF/CaCO₃/combined dilute pseudo-ternary system.At last, PVDF crystalline behavior in different conditions was investigated by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray scattering (WAXD). The results showed that PVDF presented a crystalline phase in all the studied systems; furthermore, it is irrespective to cooling condition and combined diluent. However, cooling condition influenced the PVDF crystallinity obviously.