| Poly(vinylidene fluoride) (PVDF) membranes have been widely used in organic/water separation, membrane distillation, industrial and urban sewage treatment, drinking water purity and polymer electrolyte fabrication, due to its excellent chemical resistance and thermal stability. PVDF membranes are usually prepared by three methods: melt spinning-stretching (MS-S) process, non-solvent induced phase separation (NIPS) process and thermally induced phase separation (TIPS) process. In above methods, TIPS process can easily attain the objective of membrane structure control. In previous studies, only spherulitic structures of PVDF membranes prepared by PVDF/single diluent systems via solid-liquid phase separation were obtained. The PVDF membranes with spherulitic structures always present poor mechanical properties, dense skin layers and wide pore size distribution. In this paper, PVDF flat membranes and hollow fiber membranes were prepared along with diluent mixtures via TIPS process and polymer electrolytes based on PVDF flat membranes were prepared by phase inversion technique.The phase behavior of a ternary system of PVDF, Dibutyl phthalate (DBP) (good diluent for PVDF) and Di(2-ehylhexyl) phthalate (DEHP) (poor diluent for PVDF) was determined by drawing the phase diagram phase and observing the phase separated structure. The effects of DBP/DEHP ratio in diluent mixture, polymer concentration, polymer molecular weight and PVDF copolymer on phase behavior have been investigated. The experimental results showed that as the interaction between PVDF and diluent mixture increased, the phase separation changed from Liquid-Liquid phase separation to Solid-Liquid phase separation.The isothermal and non-isothermal crystallization kinetics of PVDF in diluent mixture via solid-liquid phase separation was investigated through differential scanning calorimetry (DSC) measurement. The effects of interaction between PVDF and diluent, cooling rate on the crystallization rate and crystallization mechanism were investigated. The information gained from this study will be useful to determine the condition of membrane fabrication via TIPS method. The experimental results indicated that the crystallization rate increased by increasing interaction between PVDF and diluent or increasing the cooling rate. Also, it was found that secondary crystallization existed in the process of non-isothermal crystallization and enhanced by increasing the DBP ratio in diluent mixture.PVDF flat membranes were prepared via TIPS process using diluent mixture of DBP and DEHP. Since this method has only one parameter, namely the DBP/DEHP ratio in diluent mixture, the membrane microstructure is easily and conveniently controlled. With the assistance of a pseudo-binary temperature-DBP ratio phase diagram of the PVDF-diluent mixture system, the membrane formation mechanism for different microstructures of membranes was proposed. Polymer electrolyte films based on PVDF flat membranes were prepared by phase inversion technique. The effects of microstructure and aggregation structure of membrane on the property of polymer electrolyte film have been investigated. It was found that PVDF flat membrane with uniform sponge-like microstructure possessed higher elastic modulus and stress than most membranes and the polymer electrolyte film prepared from that had an ambient temperature conductivity of 1.3×10-3S/cm. Also this polymer electrolyte film exhibited a good electrochemical stability up to 4.5 V versus Li+/Li.Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) flat membranes were prepared via the TIPS process using diluent mixture of DBP and DEHP. The effects of interaction between PVDF-HFP and diluent mixture on the microstructure and the aggregation structure of PVDF-HFP flat membrane has been investigated. Polymer electrolyte films based on PVDF-HFP flat membranes were also prepared by phase inversion technique. The properties of PVDF-HFP polymer electrolyte film were compared with PVDF polymer electrolyte film. It was found that PVDF-HFP polymer electrolyte film presented higher ambient temperature conductivity (4.07×10-3S/cm), higher holding liquid electrolyte property and wider electrochemical window (4.75 V versus Li+/Li), while the battery cycle properties of these two polymer electrolyte films were almost the same.Poly(vinylidene fluoride) (PVDF) hollow fiber membranes were prepared by TIPS using diluent mixtures of DBP and DEHP. Liquid paraffin (LP), which is a non-solvent for PVDF but a solvent for diluent mixtures, was used as bore liquid to make a lumen of the hollow fiber for purpose of exchange with diluent near the inner surface of hollow fiber. The membrane formation mechanism for different structures of PVDF hollow fiber membranes was proposed and the effect of membrane structure on the performance of hollow fiber membranes was investigated. It was found that PVDF hollow fiber membrane prepared via spinodal decomposition presented interconnected small pores in bulk-structure, smaller pores at outer surface and larger pores at inner surface. The formation of larger pores at inner surface was due to replacement of the rich diluent phase formed near the inner surface via Liquid-Liquid phase separation by LP. This hollow fiber membrane presented higher water permeability (541.5 L/m2.hr, 0.1 MPa), better elasticity and high solute rejection property (100% rejected, 0.16μm particle size of carbonic ink).
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