| Porous polymer membrane attracts continuous attention because of its promising applications in many fields such as separation and electrochemical energy storage.Fluoropolymers become excellent candidates for membrane preparation and application because of the high mechanical strength,thermal and chemical stability.Among them,the copolymer of poly(vinylidene fluoride-co-hexafluoro propylene)(PVDF-HFP)receives increasing attention,because the crystalline region of VDF would be helpful for strength maintaining as well as the amorphous HFP block might facilitate its processing.Therefore,this polymer has been fabricated into membranes with various microstructures for applications including membrane filtration,lithium ion batteries,and sensors.The phase inversion usually employs solvent with low volatility,and its removal is often realized by repeated washing with large amount of water.However,if volatile solvent was used for membrane preparation,this step of washing could be avoided,implying the advantage of VIPS.In this paper,PVDF-HFP was used as raw materials with additive,such as N,N-dimethylacetamide(DMAc)and Aliquat 336,to prepare a variety morphology of porous polymer membrane using one-step vapor induce phase separation.Meanwhile,the influencing factors on the morphology of the polymer membrane,such as the concentration of polymer solution,preparation temperature,selection of non-solvent atmosphere,additives and so on have been analyzed in detail.The applications of the prepared porous polymer membrane have been tested in this work.The research contents are summarized as follows:Tailoring morphology of PVDF-HFP membrane via one-step reactive vapor induced phase separationIn this work,we attempted to tailor the morphology of the PVDF-HFP membrane via one-step reactive vapor induced phase separation method.Namely,PVDF-HFP was dissolved in a volatile solvent and then was cast in an ammonia water vapor atmosphere to evaporate solvent.After complete evaporation of solvent,membranes with adjustable porous structure have been prepared.The process of the membrane preparation in ammonia water vapor was compared with those in other non-solvent vapors such as pure water and ethanol,and the effect of preparation temperature was also studied.Moreover,the microstructures of the membranes were analyzed by scanning electron microscopy,Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy and X-ray diffraction characterizations.Based on the results,a possible mechanism of dehydrofluorination induced cross-linking of PVDF-HFP chains has been suggested to understand the morphology formation.To our knowledge,this is the first report of using one-step reactive vapor induced phase separation strategy to tailor morphology of PVDF-HFP membrane.In addition,the membranes prepared in the ammonia water vapor exhibited elevated mechanical strength and display satisfactory separation efficiency for water-in-oil emulsions,suggesting promising potential in membrane separation field.Effect of high boiling point solvent on the preparation of PVDF-HFP membrane via reactive vapor induced phase separationThis part of the work further studies the influencing factors of reactive vapor induced phase separation,and explores the effect of solvent additives on the morphology and performance of polymer membranes.PVDF-HFP and the solvent additive N,Ndimethylacetamide(DMAc)were dissolved in acetone and then was cast in ammonia water vapor atmosphere to evaporate solvent.Scanning electron microscope(SEM)was used to observe the surface and cross-sectional morphology of the porous membrane.It was found that with increasing the amount of DMAc additive,the pore size of the membrane surface increased gradually,and the cross-section changed from bicontinuous structure to honeycomb porous structure.The contact angle measurement found that the contact angle of the membrane surface increased from 116.3 ± 3.2° to 131.8 ± 1.2° after the DMAc addition increased from 0%to 10%.Through the porosity and pure water flux tests,it was found that increasing the amount of DMAc additive can significantly increase the porosity and pure water flux of the porous membrane.The porosity and pure water flux of the membrane without DMAc additive were 55.3%and 53.6 ± 1.8 L/(m2·h),when the amount of DMAc additive up to 10%,porosity and pure water flux of the membrane are increased to 61.2%and 85.1 ± 3.7 L/(m2·h).Preparation of ionic liquid modified PVDF-HFP membrane by vapor induced phase separation and its applicationIn this work,Aliquat 336 modified PVDF-HFP membrane was prepared by one-step vapor induced phase separation.PVDF-HFP and Aliquat 336 were dissolved in tetrahydrofuran and then was cast in water vapor atmosphere to evaporate solvent.Using scanning electron microscope(SEM)to observe the surface and cross-sectional morphology of the membrane,it was found that with the increase the amount of Aliquat 336 from 0%up to 40%,the pore size of the membrane surface increased,and the crosssection changed from dense structure to cellular porous structure and then change to nodular structure.In addition,the energy spectrum analysis(EDS)was used to characterize the distribution of chlorine in the membrane cross section,and it was found that the enrichment of Aliquat 336 on the membrane surface is an important reason for the change of membrane morphology.The contact angle measurement found that increasing the amount of Aliquat 336 improves the hydrophilicity of the membrane.When the amount of Aliquat 336 increased from 0%to 40%,the surface contact angle decreased from 100.4±0.4° to 0°.The pure water flux test showed that the water flux increased first and then decreased with the increase of Aliquat336 content,and the pure water flux reached a maximum of 231.7±9.0 L/(m2-h)when the addition amount was 20%.The membrane also showed excellent copper ion selective adsorption capacity,indicating that the membrane preparation method has potential value in the adsorption and enrichment of metal ions. |
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