The Study Of Polyvinylidene Fluoride Membrane Via Thermally Induced Phase Separation

Polyvinylidene Fluoride(PVDF) is a white powder crystal polymer, It has excellent physicaland chemical properties,because its high mechanical strength、resistance to chemical corrosion、high temperature resistance、resistance to oxidation and radiation, etc, It has became a goodmembrane material been widely used in water purification, sewage treatment, machine/waterseparation, etc.PVDF microporous membrane are usually pruduced via the Immersion precipitation(NIPS)and Thermal induced phase separation(TIPS) method. NIPS method limit the scope of the choiceof solvent because of the NIPS being conducted at low temperature. The principle of TIPS methodis a "high temperature dissolving, low temperature phase"that expanded the choice of polymers anddiluent and opened up new ways of preparation of microporous membrane via phase separation.Recently, TIPS method is used for further study by many scholars,a complex phase separationprocess composed of thermal induced phase separation and the solvent exchange of phaseseparation happened at the same time Through Regulating polymer, thinner, and kinds of bathgel.It was called complex thermally induced phase separation method.In the complex thermallyinduced phase separation method method, a mixture composed of a water soluble latent solventand polymer as membrane components, and the polymer can be dissolved under the melting pointbelow.When the homogeneous solution into liquid cooling fluid, because of a complex phaseseparation process composed of thermal induced phase separation and the solvent exchange ofphase separation happened at the same time,and the main phase separation is TIPS.Membranestructure is composed of thin cortex and thick spongy structure.In this paper, PVDF microporous membrane were prepared based on the TIPS and complexthermally induced phase separation method method.First, PVDF porous membranes were prepared by the TIPS method of PVDF blended with asolid water-soluble Caprolactam（CPL）.CPL is a water-soluble diluent at room temperature, Itcan be directly extracted with water, so the membrane process requirement can reduce,and it isgreen environmental protection to the environment.The crystallization properties of the binarysystem were analyzed by DSC,The effects of polymer concentration on the micro-structure ofPVDF membrane were observed and studied by SEM. The effect of polymer concentration on rate water flux, BSA retention, tensile strength, were studied.This study show that PVDF porousmembranes prepared by the TIPS method of PVDF blended with a solid water-soluble CPL tend toform spongy structure.The results showed that water flux of membrane is1100L/m~2.h and BSAretention rate is20%with mass fraction of20%PVDF.Second, under the condition of determined above membrane components, we studied theeffect of method of complex thermally induced phase separation on membrane structure andperformance. In the experiment we selected the water or ethanol as the coagulation bath, study theeffect of polymer concentration and cooling temperature on membrane structure and theperformance.Results show that water as coagulation bath, the membrane structure is asymmetric,composed of thin cortex and thick spongy structure,water flux of membrane is180L/m~2.h and BSAretention rate is60%with mass fraction of20%PVDF, because of the mass transfer rate of ethanolis less than water, the phase separation is thermal induced phase separation when Ethanol ascoagulation bath, membrane was MF.and water flux of membrane is3000L/m~2.h.Finally,Study the effect of composite diluent on PVDF membrane structure andperformance.In the experiment,we make the CPL blended with diacetone alcohol or propylenecarbonate as compound diluent. PVDF porous membranes were prepared by the complexthermally induced phase separation method of PVDF blended with different proportion ofcompound diluent.The results show that whether diacetone alcohol or propylene carbonate makemembrane pore size smaller.when the mixture composed of CPL and diacetone alcohol as acomplex diluent,PVDF microporous membrane structure is Spherulite because of Solid-liquidseparation.when the mixture composed of CPL and propylene carbonate as a complexdiluent,PVDF microporous membrane structure is spherulite because of liquid-liquid separationand tensile strengt more stronger.Composite diluent on membrane is water flux decreases, butintercept rate increased obviously compared with single diluent.