Study Of Microstructure And Properties Of PVDF Membrane With Complex Solvent System

Poly （vinylidene fluoride）（PVDF） is one of the widely used membrane materials,due to its excellent mechanical strength, thermal and chemical stability, radiationresistance. Membrane morphology was an important factor in determining themembrane performance，thus studies were focused on improving the performance of themembrane by changing the structure of the PVDF membrane at home and abroad now.In view of the disadvantage of the solution phase inversion that the control of themembrane structures is difficult, and membrane reproducibility is poor and membranemechanical strength is worse, this paper investigated the varying rules of the effects ofthe ratio of solvent and phase transformation condition on the structure and performanceof polyvinylidine fluoride （PVDF） membrane by dissolving PVDF in γ-BL/TEPcomplex solvent with EG and H₂O as coagulation bath respectively, aiming ataccumulating plenty of experiments for the preparation of high-performance PVDFmembrane. The main research conclusion are presented as follows:1. With the concentration of TEP in the complex solvent of γ-BL/TEP increased,the stability of the casting solution increased, and the speed of the precipitation andsolidification became slower. Thus a slightly delayed demixing behavior appearedinside of membrane producing polymer lean phase diffusion and rich coalescence,making the pore size of membrane between internal spherulite increased.When EG waschosen as the non solvent coagulation bath the speed of phase separation of castingsolution is slow after interphase demixing and the membrane structure is looser thanwater as a coagulation bath.When the concentration of TEP in mixed solvents is60%,PVDF membrane has a better comprehensive performance. 2. Adding solvent DMAc in coagulation bath, the chemical potential of diffusionmass transfer at the interphase of casting solution and coagulation bath reduced. Thedelayed liquid-liquid demixing occurred for the stability of the membrane-formingsystem enhanced. With the content of DMAc in the coagulation bath increased, the topsurface evolved from a dense skin to a totally porous morphology and the size of PVDFcrystal spherulite became smaller resulting in the porosity increased, water fluxincreased and the strength of membrane decreased. With water （referred as harshnon-solvent） as the coagulation bath, a dense structure could be formed for rapidlysolidification of PVDF after the phase separation of interface. The interaction ofalcohols（as a nonsolvent） and polymer was more stronger than water and polymer, sothe speed of phase separation of casting solution is slow after interphase demixing. It isin favor of the crystallization of PVDF in phase separation process, forming a thinnerskin layer and loose spherical particles membrane structure.When the content of DMAcin EG coagulation bath is30%and the content of DMAc in H₂O coagulation bath is60%, the PVDF membrane has a good permeability.3. The changing of the coagulation bath temperature affected the velocity of themutual diffusion of solvent and non solvent. If the temperature of the coagulation bathincreased, the diffusion of the non solvent is more rapid than the diffusion of the solvent.Consequently there is more non solvent diffusing into the sublayer, making the polymerlean phase diffusion and rich coalescence, thus resulted in the surface roughness andporosity increased forming a loose membrane structure with a high water flux and lowrejection rate. Using harsh non-solvent water as the coagulation bath, the speed of thephase separation increased as the temperature of the coagulation bath raised and thestructure of the PVDF membrane was denser than EG as the coagulation bath for thetime of the polymer lean phase diffusion and rich coalescence was shorter comparedwith EG coagulation bath. When the temperature of coagulation bath is30℃, theperformance of the PVDF membrane is good. At this time the pure water flux with EGas a coagulation bath is2100L·m^-2·h^-1and H₂O as a coagulation bath is290L·m^-2·h^-1.