Structural Design And Performance Optimization Of PVDF Hollow Fiber Membranes By Melt-spinning-heat-stretching Method

Membrane filtration has become an important separation technology due to theobvious advantages of high separation efficiency,low energy cost,and environmental friendly.Poly(vinylidene fluoride)(PVDF)is one of the most important membrane materials due to its good processability,high mechanical strength,excellent thermal and chemical stability.PVDF hollow fiber membranes have been widely used in many separation fields,which play an important role for solving the scarcity of clean water.Generally,PVDF hollow fiber membranes are prepared by non-solvent induced phase separation(NIPS)and thermally induced phase separation(TIPS)processes.These two preparation processes usually need a large amount of organic solvent and low-molecular-weight diluent respectively,which would produce a series of complex toxic wastewater.Specifically,treating the produced wastewater is difficult and expensive due to their low molecular weight and high toxicity.In the past decade,most researches mainly focused on treating wastewater using membrane technology,but ignoring the wastewater production itself during membrane preparation process.According to the report,the wastewater contributed to more than 95%of the total waste produced during the membrane fabrication process.In order to achieve completely green and sustainable development of membrane technology,the researches of green preparation process should be paid more attention.The goal of this work is to achieve green preparation of PVDF hollow fiber membranes.PVDF and polyethylene oxide(PEO)were selected as matrix phase and pore forming agent,respectively.Meanwhile,the different structure materials were used as dispersed phase,such as poly(tetrafluoroethylene)(PTFE),polysulfone(PSF),and graphene(GE).Based on the new principles and methods of polymer processing and the interfacial pore theory,PVDF hollow fiber membranes were fabricated by melt-spinning and hot stretching(MSHS)method through optimization the spinning and stretching conditions.In particular,there were not any organic solvent and low-molecular-weight diluent used during the preparation process.In addition,the pore structure of PVDF hollow fiber membranes was decorated by vacuum filtration or chemical vapor deposition method based on a solvent-free route.Specifically,the PEO wastewater could be treated by membrane filtration to recycle,which could achieve zero discharge of wastewater.PVDF,PTFE and PEO were selected as matrix phase,dispersed phase,and poreforming agent,respectively.PVDF/PTFE hollow fiber membranes were fabricated by MSHS process.The membrane preparation process and pore formation mechanism were discussed and investigated in detail.Meanwhile,the effect of stretching ratios on the membrane structure and property was studied.The prepared membranes with different stretching ratios exhibited excellent tensile strength in the range from 23.0 to62.6 MPa.The biggest pore size of the prepared membranes with stretching 100%(M-100)was about 0.317?m.In particular,the prepared PVDF/PTFE hollow fiber membranes could switch the pore size with different pressures based on inner pressure operation,showing obvious pressure-responsive property.In order to improve the separation accuracy of PVDF/PTFE hollow fiber membranes,Graphene oxide(GO)was used to cover the interface pores of the pristine PVDF/PTFE hollow fiber membranes via vacuum filtration,and polypyrrole(PPy)was polymerized on the surface to further decorate the membrane structure.Interestingly,the modified membranes exhibited a multilayer structure due to synergistic effect of GO and PPy.The structure and property of the prepared PVDF/GO/PPy hollow fiber loose nanofiltration(NF)membranes were investigated in detail.The hydrophilicity of the modified membranes improved obviously.Moreover,polyethylene glycol(PEG)molecular weight cut off was about 3580 Da,and the smallest pore size of skin layer decreased to 2.5-4 nm.Furthermore,the prepared PVDF/GO/PPy hollow fiber loose NF membranes presented a high dye rejection(?98.5%)for AO 10,whereas a low salt rejection for Na Cl(about 4%),showing a great potential for separating dye/salt.PSF molecular chain contained the rigid aromatic rings,which was introduced into the PVDF melt for improving the interface layer between the matrix phase(PVDF)and the dispersed phase(PSF).PVDF/PSF hollow fiber membranes were fabricated by MSHS process.Due to the poor compatibility of the two phases,the interface pore structure could occur and be controlled precisely through the further stretching.Interestingly,the pore size of the prepared membranes could be controlled from 0.172to 0.550?m by changing the stretching ratios.Accordingly,N₂ flux increased from 14.1to 383.2 m³ m^-2 h^-1,and pure water flux increased from 128.0 to1463.7 L m^-2 h^-1.In order to improve the separation accuracy of PVDF/PSF hollow fiber membranes,ZIF-8,HNTs and PVA were used to modify the membrane pore structure.The synthesis of ZIF-8 and the preparation of the casting solution were both completed in a water system.The structure and property of the prepared PVDF/ZIF-8 hollow fiber loose NF membranes were investigated in detail.The research results showed that the synthesized ZIF-8 suspension had a good dispersion effect,and the obtained ZIF-8particles possessed high specific surface area with 1450.6 m² g^-1.After modification,the pore size of the PVDF/PSF membranes decreased from more than 300 nanometers to several nanometers.PEG molecular weight cut off was about 5344 Da.Furthermore,the water contact angle reduced from 91.1°to 54.2°.In particular,the prepared PVDF/ZIF-8 hollow fiber loose NF membranes presented a high dye rejection(?99.0%)for CR,whereas a low salt rejection for Na Cl(about 2%),showing a great potential for separating dye/salt accurately.In addition,the flux could reach 21.6 L m^-2 h^-1 after filtration 360 min.In order to further improve the permeability of PVDF/PSF hollow fiber membranes,GE was introduced into the melt,PVDF/PSF/GE hollow fiber membranes were prepared by MSHS method.The effect of GE content on forming membrane performance was studied,and the effect of dry/wet dispersion GE process on membrane performance was explored.Meanwhile,the generated PEO wastewater was treated,and its reusable performance was studied.The research results showed that the wet process could make GE uniformly dispersed.With the addition of GE,PVDF/PSF/GE hollow fiber membranes obtained multiple interface pores.The membrane pore size and permeability were significantly higher than those without GE.In addition,the pore size of the prepared membranes could be controlled from 0.258 to 1.066?m by changing the stretching ratios.Accordingly,N₂ flux increased from 41.5 to 768.4 m³ m^-2 h^-1,and pure water flux increased from 174.5 to 2782.9 L m^-2 h^-1.In particular,the concentrated PEO aqueous solution could be widely used in the pharmaceutical industry,oil and mining,building materials industry,light and textile industry,which could achieve zero discharge of wastewater during membrane preparation process.