Study On Preparation Of PVDF Porous Membrane By Two-step Method Of Surface Gel-submerged Precipitation Phase Inversion

With the scarcity of global freshwater resources,the reuse of freshwater resources is particularly important,among which membrane separation technology has made great contributions to the treatment of domestic sewage and industrial wastewater.Polyvinylidene fluoride（PVDF）membranes are widely used in microfiltration and ultrafiltration due to their excellent physical and chemical properties.PVDF membranes are mainly prepared by the immersion precipitation phase inversion method.Composition of the casting liquid,composition of the coagulation bath,temperature,and type of solvent have a great influence on the structure and properties of the PVDF porous membrane.In this paper,the surface gel-immersion Polyvinylidene fluoride（PVDF）porous membranes were prepared by a two-step precipitation phase inversion method.That is,spray solutions of different properties on the surface of the scraped solution film to make the surface of the solution film gel,and then immerse the surface gel solution film in a coagulation bath for phase inversion into a film.The microstructure and properties of PVDF are controlled by the composition of the spray solution.This paper mainly studies the following three aspects:（1）Polyvinylidene fluoride（PVDF）porous membrane was prepared by surface gel-immersion precipitation phase inversion two-step method,and ethanol（Et OH）/water solution and N,N-dimethylacetamide（DMAc）were sprayed on the surface of the solution membrane,respectively.)/water solution,the effects of different volume ratios of ethanol/water solution and different volume ratios of N,N-dimethylacetamide/water solution on the crystal structure,surface pore size and morphology of PVDF porous membranes were mainly investigated.The film-forming mechanism of the new film-forming method is also discussed.The structure and morphology were characterized by infrared spectrometer（FTIR-ATR）,wide-angle X-ray diffractometer（XRD）,differential scanning calorimeter（DSC）,field emission scanning electron microscope（FE-SEM）,etc.The instrument is used to characterize its separation performance.It was found that both the Et OH/H₂O solution and the DMAc/H₂O solution reduced the content ofβcrystals and increased the content ofαcrystals on the PVDF membrane.And the film is mainlyαcrystal,and with the increase of the volume ratio of Et OH and DMAc in the solution film spray solution,the crystallinity of the film shows an increasing trend.40.11%),the overall crystallinity of the sprayed DMAc/H₂O solution reached 46.1%（39.3%for the blank sample）.After spraying the mixed solution,the upper surface of the PVDF membrane changed from a dense skin layer to a porous structure,and the number of spherical particles increased.The cross section of PVDF membrane sprayed with DMAc/H₂O solution changed from finger-like macropore structure to sponge-like pore structure.The maximum porosity of the membrane sprayed with Et OH/H₂O solution is 95.70%（89.88%for the blank sample）,and the maximum average pore size is 0.92μm（about 1.68 times higher）;the maximum porosity of the membrane sprayed with DMAc/H₂O solution is 91%（90%for the blank sample）,the average pore size is up to 0.915μm（about 1.4 times higher）.Compared with the one-step PVDF membrane,the pure water flux of both is greatly improved.（2）Based on the influence of the novel membrane preparation method on the structure and morphology of PVDF porous membrane,it is considered to spray a solution containing hydrophilic groups on the surface of the solution membrane.The acetic acid solution of chitosan（CS）and polyvinyl alcohol（PVA）aqueous solution were sprayed on the surface of the solution membrane respectively,and the polyvinylidene fluoride（PVDF）porous membrane was prepared by the surface gel-immersion precipitation phase inversion two-step method.The effects of different mass fractions of chitosan（CS）acetic acid solution and different mass fractions of polyvinyl alcohol（PVA）solutions on the hydrophilic properties of PVDF porous membranes were mainly investigated.It was found that the addition of CS and PVA changed the structure and morphology of the membrane surface,increased the porosity and average pore size of the membrane,and transformed the PVDF membrane from a dense skin layer to a porous skin layer.Among them,the contact angle of the acetic acid solution sprayed with chitosan（CS）reached the minimum of 68.28°（82.57°for the blank sample）,which was reduced by about 1.21 times,thereby improving the water flux of the PVDF membrane,and the highest water flux reached 1226.26 L/m~2h,about 1.91 times higher than the blank sample（639.40 L/m~2h）;the minimum contact angle of the sprayed polyvinyl alcohol（PVA）solution reached 59.63°,and the maximum water flux reached 1085.2 L/m~2h,about 5.05 times that of pure PVDF membrane（214.9 L/m2h）.It was concluded that the addition of CS and PVA successfully improved the hydrophilic properties of the membrane.（3）In order to strengthen the attachment of chitosan to the surface of PVDF membrane,acetic acid solution and glutaraldehyde aqueous solution of chitosan are sprayed on the surface of the solution membrane.By changing the cross-linking time and the concentration of glutaraldehyde,the degree of cross-linking of the two on the membrane surface is controlled,and the effects of different cross-linking degrees on the hydrophilic properties and anti-fouling properties of PVDF porous membranes were studied.The results showed that chitosan and glutaraldehyde successfully formed a cross-linked network structure on the upper surface of the PVDF porous membrane,which strengthened the adhesion of chitosan on the upper surface of the membrane.It is a hydrophilic membrane（minimum reaches 47.8°）.The water flux is increased by about 3.31 times,and the acetic acid solution of simply spraying the polysaccharide increases the water flux by about 1.91 times,and the effect of cross-linking on the water flux is better.The highest BSA rejection rate reached 93.3%（68.3%for the blank sample）,the fouling degree of the membrane was reduced by about half,and the performance of the water washing recovery was also better than that of the blank sample.The anti-pollution performance of PVDF porous membrane has been greatly improved.