| In this paper the polyvinylidene difluoride (PVDF) membrane formation mechanism during dry-wet phase separation process was studied, and the effect of techanical parameters on membrane morphological structure and property was discussed.Some new concepts such as 'dynamic phase separation', 'sodification line of polymer-rich phase', 'pre-gelation morphological structure' were proposed. The membrane formation mechanism stands on these concepts. It was considered that the mass transfer process and the phase separation process went along simultaneously in any differential area of casting solution and changed its composition and mechanical property. The morphological structure of polymer-rich and polymer-lean phase produced by this dynamic phase separation was an unsteady pre-gelation structure. The polymer-rich and polymer-lean phase would grow, aggregate or deform on the effect of interfacial tension or swell stress of polymer-lean phase. Only when the composition of polymer-rich phase crossed over the solidification line the pre-gelation sructure changed no more. The membrane morphological structure was then formed.The mechanical and phase separation property of PVDF solution were reserached. It was crystalline solid-liquid phase separation gelation, not vitrification, caused the polymer-rich phase solidification. Temperature decreasing, non-solvent or polymer concentration increasing are the three factors to trigger off gelation. The crystalline solid-liquid phase separation happened under nucleation and growth mechanism. The dispersed polymer crystalline nuclei grown and connected with each other to form the supported network and the polymer solution transformed to gelation and solidified. At the gelation point the transformed process from fluid to solid arised in about 3 to 4 minute. The gelation point of polymer solution is influenced by its amount. The greater the polymer solution amount, the lower the temperature and the higher the nonsolvent and polymer concentration at gelation point.The influence of additive on phase separation was studied. The additive was PVP,PEG or LiCl respectively. The phase diagram for polymer solution of PVDF/DMAc/Water/Additive system was measured using cloud-point method. The phase separation phenomena during membrane formation process were recorded using light transmission experiment. It was found that both gelation and liquid-liquid demixing existed during phase-inversion process of casting solution with different additive. Gelation appeared early at lower nonsolvent concentration than liquid-liquiddemixing in the range of 30~60℃. Increasing temperature would increasing thenonsolvent concentration at gelation point. LiCl as additive leads to a strong action for PVDF solution to form gel than PEG or PVP. The gelation period prolonged along with the sequence of PVP, PEG or LiCl as additive during membrane formation process. The polymer concentration of nascent membrane at the onset of liquid-liquid demixing increased and restricted the development of macrovoids. The strong gelation tendency of polymer solution intensified the solidification of nascent membrane and restricted the macrovoids development further. Thus the macrovoids structure extended to the bottom and stopped with the sponge-like structure when PVP was used as additive. When the additive was PEG or LiCl, the macrovoids developed insufficiently and terminated in the middle of membrane section with the sponge-like structure and then passed to nodular structure in bottom. The effective porosity and flux decreased and crystallinity increased along with the sequence of PVP, PEG and LiCl as additive.The hydrophobicity of membrane with LiCl as additive was barely changed, but the water-resisting pressure of membrane with PVP or PEG as additive decreased about 0.2bar. the hydrophobicity of membrane reverse side exceled than that of obverse.. For the PVDF-DMAc-PVP-H2O casting solution system, low temperature of casting solution and coagulation bath tend to increase the effective porosity of membra...
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