亲水PVDF膜制备

Journal of Membrane Science 345 (2009) 331–339Contents lists available at ScienceDirectJournal of MembraneSciencej o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /m e m s ciPreparation of hydrophilic and fouling resistant poly(vinylidene fluoride)hollow fiber membranesFu Liu a ,b ,∗,You-Yi Xu a ,∗,Bao-Ku Zhu a ,Fan Zhang a ,Li-Ping Zhu aa Institute of Polymer Science,Key Laboratory of Macromolecule Synthesis and Functionalization,Ministry of Education,Zhejiang University,Hangzhou 310027,China bDepartment of Chemical Engineering &Chemical Technology,Imperial College,South Kensington Campus,London SW72AZ,UKa r t i c l e i n f o Article history:Received 9January 2009Received in revised form 4September 2009Accepted 8September 2009Available online 15 September 2009Keywords:PVDF hollow fiberAmphiphilic copolymer Fouling resistance Hydrogel layera b s t r a c tAmphiphilic brush-like copolymer P(MMA-r-PEGMA)was synthesized by a radical polymerization method.The copolymer was characterized by the nuclear magnetic resonance proton spectra (1H NMR)and gel permeation chromatography (GPC).Poly(vinylidene fluoride)(PVDF)hollow fiber membrane was then prepared by the phase inversion method using the copolymer as the macromolecular additive.The effect of P(MMA-r-PEGMA)on the phase inversion process was investigated using a light transmission experiment.The surface enrichment of amphiphilic copolymer in the PVDF hollow fiber was characterized by Fourier transform infrared attenuated total reflection spectroscopy (FTIR-ATR)and X-ray photoelec-tron spectroscopy (XPS).Morphological structures of both dry and wet membranes were observed by scanning electron microscopy (SEM).The hydrophilicity,water permeation flux,anti-fouling and carbon ink rejection performance of the hollow fiber were characterized respectively.All these results demon-strated that PVDF hollow fibers fabricated by the single-step method showed high permeation flux,good hydrophilicity and fouling resistance.© 2009 Elsevier B.V. All rights reserved.1.IntroductionPVDF membranes have been widely researched and applied in many areas,such as microfiltration [1–3],ultrafiltration,nanofil-tration [4–6],distillation [7]and reverse osmosis [8]due to the excellent chemical,thermal and mechanical stability originated from the PVDF semi-crystalline structure and strong C–F bond energy (530.5kJ/mol)[9].Despite these advantages,PVDF porous membranes still have some limitations,especially when treating aqueous solution like waste water containing organic molecules or proteins due to the hydrophobic nature or low surface energy,such as the low water flux,high energy-consumption,easy foul-ing by organic molecules or proteins.So it is necessary to improve the hydrophilicity of PVDF membrane through versatile methods,i.e.surface coating,surface grafting and blending.Surface coat-ing is usually unstable and might be washed away during the operation process.Surface grafting usually needs an extra step to modify the surface chemistry of the membrane,such as high energy electron beam [10,11],plasma [12,13],surface living/controlled grafting method [14,15],which makes surface grafting not suitable∗Corresponding authors at:Institute of Polymer Science,Key Laboratory of Macromolecule Synthesis and Functionalization,Ministry of Education,Zhejiang University,Hangzhou 310027,China.Tel.:+8657187953011;fax:+8657187953011.E-mail addresses:yoyodragon1980@ (F.Liu),opl-yyxu@ (Y.-Y.Xu).for an industrial scale production.The membrane with the desirable properties can be modified simultaneously using blending method during the membrane preparation process.Blending method can be considered as a single-step method for preparing a hydrophilic and anti-fouling membrane,indicating a potential application for large scale production.In previous studies,PVP or PEG was mostly used to blend with PVDF as pore-forming additives.However,these additives are not stable and could be easily washed away by water due to their lin-ear structure and the incompatibility with PVDF.The use of some amphiphilic copolymers might avoid these problems.Many such copolymers with different structures have been used to modify the membrane hydrophilicity or anti-fouling properties,such as comb-like PVDF-g-PEGMA [16],P(MMA-r-PEGMA)[17,18],PSf-g-PEG [19],PAN-g-PEO [20],block PEO-b-PSF [21],Pluronic F127[22],alternative poly(styrene-alt-maleic anhydride)[23],branched P123-b-PEGs [24]and hyperbranched-star [25],etc.In summary,amphiphilic copolymers have both hydrophobic and hydrophilic chain segments.For example,the hydrophobic main chain seg-ments (PMMA)have a good compatibility with PVDF and ensure that the copolymer will not be washed away from the membrane,while the hydrophilic side chain segments (PEGMA)will endow the membrane with the desirable hydrophilicity and fouling resistance through the surface segregation self-organization effect during the phase inversion process.The brush-like structure of P(MMA-r-PEGMA)guarantees its durable stability with PVDF and will not diffuse into the water,as does PEG or PVP.0376-7388/$–see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.memsci.2009.09.020332 F.Liu et al./Journal of Membrane Science345 (2009) 331–339Most previous research focused on the effect of amphiphilic polymer on the hydrophilicity,fouling resistance of PVDFflat sheet membranes.However,the preparation of PVDF hollowfiber mem-brane using amphiphilic copolymer P(MMA-r-PEGMA)appears not to have been studied.In particular,the effects of amphiphilic copolymer on the phase inversion process and the subsequent membrane structures and properties have not been investigated. In this paper,we aim to prepare the PVDF hollowfiber mem-brane with good hydrophilicity,high permeationflux,and fouling resistance.In addition,we will discuss the hydration behaviors of amphiphilic copolymer during the phase inversion process using the light transmission measurement,SEM observation of both dry and wet membranes.Most importantly,we have attempted to pro-vide an easy approach to prepare a hydrophilic and fouling resistant PVDF hollowfiber with highflux,which can be easily scaled up at industrial level,and open interesting perspectives for anti-fouling PVDF hollowfiber applications.Firstly,we synthesized P(MMA-r-PEGMA)by a radical polymer-ization method.1H NMR and GPC were used to characterize the structure and composition of the copolymer.A light transmission experiment was conducted to investigate the effect of the copoly-mer on the phase inversion process.Secondly,PVDF hollowfiber membrane blended with P(MMA-r-PEGMA)was fabricated by the phase inversion method.FTIR-ATR and XPS were used to study the surface enrichments of amphiphilic copolymer.Finally,the perfor-mance of the membrane including hydrophilicity,permeationflux, retention ratio,protein fouling resistance was studied.2.Experimental2.1.Materials and reagentsPVDF powders(FR-904,M n=380,000)obtained from Shang-Hai New Materials Co.were dried at80◦C for12h before use.Poly(ethylene glycol)methyl ether methacrylate(PEGMA, M n=1000,industrial grade)purchased from ShangHai TaiJie Chem. Co.wasfiltrated with Al2O3chromatography column to remove inhibitor and residual water.Methyl methacrylate(MMA)and N,N-dimethylacetamide(DMAc)were distilled before use.AIBN was recrystallized three times before use.Polyethylene glycol (PEG-600),polyvinylpyrrolidone(PVP)and tetrahydrofuran(THF) were purchased from ShangHai Chemical Co.Albumin,Bovine Serum(BSA,M n=67,000)was purchased from Sino-American Biotechnology Co.Phosphated buffered saline(PBS,0.01M)was prepared by dissolving pre-weighed quantities of potassium dihy-drogen phosphate(KH2PO4)and di-sodium hydrogen phosphate (Na2HPO4·12H2O)in deionized water.The ink solution was pre-pared with5mL carbon ink purchased from Shanghai oriental stationery and1L pure H2O.All other chemicals,unless other-wise stated,were obtained from commercial sources and used as received.2.2.Polymerization and characterizationIn a typical polymerization,purified PEGMA and MMA(feed weight ratio=10:7)were dissolved in DMAC at room temper-ature to make a homogenous solution.Then argon gas was bubbled through the mixed solution,and the reaction vessel was sealed with a rubber septum.A certain amount of AIBN ([AIBN]/[MMA+PEGMA]=1/400)was added and then the reaction solution was heated to65◦C.After10h,the polymer was precipi-tated in a mixture containing methanol and petroleum ether(1:9), and then dissolved in THF,concentrated by evaporation,and re-precipitated in similar petroleum ether/methanol mixtures.Finally, the copolymer P(MMA-r-PEGMA)was obtained after being dried for24h in vacuum at30◦C.Table1PVDF membranes with different compositions.No.PVDF/P/PEG/PVP/DMAc No.PVDF/P/PEG/PVP/DMAc#112/0/0/0/88#516/0/3.2/0.8/100#210/2/0/0/88#616/1.6/3.2/0.8/100#38/4/0/0/88#716/3.2/3.2/0.8/100#416/0/0/0/100#816/4.8/3.2/0.8/100Gel permeation chromatography of the copolymer was con-ducted at30◦C in anhydrous THF based on polystyrene standards in WATERS1525(USA).1H NMR was performed in deuterated chloro-form using Avance DMX-500spectrometer(Germany).The PEGMA content in the copolymer was also determined in this way.2.3.Membrane preparation and characterizationTo prepare doping solutions for spinning,different ratios of PVDF,PVP,PEG,and P(MMA-r-PEGMA)were dissolved in DMAc at60◦C for24h(see Table1).Prior to spinning,the doping solu-tion was degassed under vacuum at room temperature overnight to remove the air bubbles.To study the PVDF membrane kinetics precipitation process during the phase inversion,light transmission experiments were carried out(#1,#2,#3).The effects of the content of amphiphilic copolymer in the casting solution were studied.The standard pro-cedure is as follows:the solution was cast onto a glass plate and put into a chamber with a light,and then water coagulation submerged the membrane.The intensity change of light transmission caused by the precipitation process was recorded by a computer.Asymmetric PVDF hollowfiber membranes were fabricated by dry–wet spinning process(#4,#5,#6,#7,#8).Specifically,the dope was pressurized to2kg through a spinneret by N2and went through a certain air gap of15cm,and then was immersed into the water bath at50◦C.Both internal and external coagulation bath were composed of water at50◦C.Finally,the nascent hollowfiber was taken up by a roller at10–15m/min.The hollowfiber mem-branes were then immersed in fresh water for3days to remove the residual solvent,the PVP or PEG additives were washed away com-pletely by changing water frequently.Finally,the prepared hollow fibers were dried in the air and then sealed in a U-like module for thefiltration test.2.4.Membrane structure and performance characterizationFTIR-ATR spectroscopy was carried out on a Bruker Vector22 FTIR spectrophotometer.The spectra were measured in the wave number range of4000–500cm−1.The spectra were collected by cumulating32scans at a resolution of2cm−1.The X-ray photoelectron spectroscopy(XPS,PerkinElmer Instru-ments,USA)was used to further study the surface chemical change of PVDF membranes.It was equipped with Al K␣at1486.6eV and 300W power at the anode.A survey scan spectrum was taken and the surface elemental composition was calculated from the peak area with a correction for atomic sensitivity.The error in all the binding energy(BE)values reported is±0.1eV.Scanning electron microscopy(SEM,Sirion-100FEI)was used to characterize the surface and cross-section morphologies of the membranes.The cross-section was fractured in liquid nitrogen.A thin layer of gold was coated on the membrane surface and cross-section before measurement.To study the hydrogel effect of amphiphilic copolymers,a piece offiber was lightly wetted before being coated.The water drop adsorption on the membrane was measured at 25◦C using a sessile-drop technique with a video-based high speed contact angle measuring device(OCA20,Germany Dataphysics)toF.Liu et al./Journal of Membrane Science 345 (2009) 331–339333characterize the hydrophilicity of porous membranes more reli-ably.The change of the contact angles was recorded as a function of the drop age to determine the time required for the adsorption of the drop.To test the protein static adsorption of the membranes,PVDF hollow fibers were cut into 5.0cm pieces,and 5pieces of 5.0cm fibers were put into a glass vial containing 10mL of 1.0g/L BSA solu-tion (pH =7.0in PBS).The vials were vibrated in a shaking bath at constant temperature of 25◦C for 24h to reach the BSA adsorption equilibrium.The concentrations of BSA in the solution before and after adsorption were measured with a UV spectrophotometer (UV-1601,Shimadzu,Japan).The amount of BSA static adsorption on the PVDF hollow fibers was calculated based on the UV adsorption [26,27].Water flux experiments were performed in a U-like membrane module filtration apparatus.Ten hollow fibers with the efficient length 30cm were assembled into a U-like membrane module with the inside-out configuration.The permeation flux was measured using the cross-flow mode.Pure water was forced to permeate from the inside to the outside of the hollow fiber.The filtration procedure of the hollow fiber membrane is described as follows:the membrane was pre-compacted at 0.15MPa for 20min,and then the pure water flux (J 0)was mea-sured at 0.1MPa every 5min.For the fouling resistance test,the pure water was exchanged with a 1.0g/L BSA solution (pH =7.0in PBS).BSA solution flux (J B )was measured at 0.1MPa every 5min.Afterwards,the protein fouled hollow fiber was surface-washed by the circulation of pure water through the membrane module for 1h,and then the pure water flux after surface washing (J 1)was mea-sured.For the retention experiment,the pure water was exchanged with 5.0mg/L ink solution.The ink particle distribution of the per-meate solution was analyzed by the dynamic light scattering device (90Plus,USA).3.Results and discussion3.1.Characterization of amphiphilic copolymer P(MMA-r-PEGMA)The hydrophilic PEG side chains are distributed randomly on the hydrophobic PMMA main chains to form amphiphilic brush-like copolymer P(MMA-r-PEGMA).Fig.1shows the 1H NMR spectrum of P(MMA-r-PEGMA)with the feed monomerratioFig.1.500MHz 1H NMR spectrum for P(MMA-r-PEGMA)b with the initial monomer ratio MMA:PEGMA =10:7.Table 2Number of O–Hx and C–Hx protons in MMA and PEGMA.Number of protons per repeat unitMMA (A)PEGMA (B)O–CHx 385C–CHx55PEGMA:MMA =10:7.From the 1H NMR spectrum,we can see that the peak appearances of the chemical shift at 3.3ppm are assigned to –OCH 3,3.5ppm are assigned to –OCH 2,4.1–4.2ppm are assigned to COOCH 2–,and 0.7–2.3ppm are assigned to H–in the copolymer main chains.The mole fraction of PEGMA (X PEGMA )in copolymer was calculated as 14.3%from the NMR data by X PEGMA =A A +Bwhere A and B are the solutions to the simultaneous system of equa-tions,5A +5B =I C–CHx ,3A +85B =I O–CHx ,and where I C–CHx and I O–CHx are the total intensities of the resonances for each type of bond-ing environment.The number of protons per repeat unit MMA and PEGMA is in Table 2.The weight content of PEGMA (W PEGMA )in copolymer was calculated as 62.5wt%by W PEGMA =X PEGMA M 0PEGMAX PEGMA M 0PEGMA +(1−X PEGMA )M 0MMAwhere M 0PEGMA =1000,M 0MMA =100,which is comparable with the feed weight fraction 58.8%,indicating that the whole copoly-merization can be controlled to some extent.GPC results show that the number-average molecular weight and the weight-average molecular weight of the copolymer is 27,600(M n )and 85,311(M w ),respectively.Therefore the synthesized copolymer with the suitable molecular weight could be used to modify the PVDF mem-brane as a macromolecular additive.All these results show that the amphiphilic brush-like copolymer P(MMA-r-PEGMA)was success-fully synthesized by the radical polymerization method.3.2.Kinetic analysis of PVDF/P(MMA-r-PEGMA)in phase inversion processPVDF/P(MMA-r-PEGMA)mixture can form a thermodynamic compatible solution for fabricating stable membranes as the PMMA backbone chains have a well-known good miscibility with PVDF [28].The kinetic behavior of PVDF/P(MMA-r-PEGMA)during the phase inversion process,especially the influence of amphiphilic copolymers on the precipitation process was investigated by light transmission experiments.From Fig.2,it can be seen that:(a)there is a delayed onset of demixing in the first phase,and the delayed demixing time (t 3>t 2>t 1)increased with the content of P(MMA-r-PEGMA)(#3>#2>#1);(b)in the second phase,the demixing curve decreases less with the increasing content of P(MMA-r-PEGMA).Increasing the content of P(MMA-r-PEGMA)in the casting solution will change the phase inversion process of PVDF/P(MMA-r-PEGMA)membrane from instantaneous demixing to delayed onset of demixing.The amphiphilic copolymer with the hydrophilic PEG side chains possesses lower interface free energy with water than PVDF chains [29,30].The hydrophilic chains were driven to segregate onto the interface between water and membrane,and then combine with the surrounding water molecules,which made the nascent mem-brane more transparent due to the hydration effect as suggested in Fig.3.The hydration effect would make the amphiphilic copolymers self-organize both on the membrane surface and inside the pore,which would reconstruct an ultrathin dense hydrogel layer.The surface segregation movement of amphiphilic copolymer P(MMA-334 F.Liu et al./Journal of Membrane Science345 (2009) 331–339Fig.2.Light transmission curves of PVDF/P(MMA-r-PEGMA)membranes with dif-ferent compositions during the phase inversion (#1,#2and #3,respectively).Fig.3.The schematic diagram of the hydration of P(MMA-r-PEGMA)and the sur-rounding H 2O molecules.r-PEGMA)is suggested in Fig.4.The hydrogel layers were also observed in SEM through comparing dry membrane with a wet one.The advantage of ultrathin dense hydrogel layer is that it helps to improve the water flux and selectively at the same time,which will be further discussed in later experiments.The light transmission results revealed that the amphiphilic copolymer depressed the pre-cipitation process of PVDF/P(MMA-r-PEGMA)membrane and the hydration effect enhanced the surface segregation of amphiphilic copolymer at the same time.3.3.Membrane surface chemical compositionFig.5shows the FTIR-ATR spectra for qualitative analysis of PVDF hollow fiber outer surface with differentcomposi-Fig.5.ATR-FTIR spectra of PVDF/P(MMA-r-PEGMA)hollow fiber membranes (#4,#5,#6,#7and #8membrane,respectively).paring #4membrane (pure PVDF)with #5membrane (PVDF/PEG/PVP =16/3.2/0.8),it is found that they show exactly the same curves.There are no C O and –OH peaks at around 1730cm −1and 3000cm −1in #5membrane,which means that PEG and PVP as pore-forming additives have been completely washed away by water during the phase inversion or immers-ing process.Consequently,the hydrophilicity of PVDF membranes will disappear due to the loss of PEG or PVP.The role of the PEG and PVP in the later experiments is to cause pores and increase the water flux of the membrane.We therefore added the amphiphilic copolymer to enhance the hydrophilicity and fouling resistance of the PVDF membranes.The typical band at 1730cm −1assigned to C O increases with the content of copolymer in #6,#7and #8membranes.Increasing the content of copolymer in the membrane will subsequently increase the surface segregation of amphiphilic copolymers,which could potentially increase the sur-face hydrophilicity and protein fouling resistance.#7membrane with the suitable composition (3.2%copolymer in dope)showed good morphology without any shrinkage,it was chosen for the later experiment.The outer surface chemical compositions of #7hollow fiber were determined by XPS quantificationally.Fig.6shows the appearance of O1s (binding energy,535.0eV)indicating the pres-ence of P(MMA-r-PEGMA)in the PVDF hollow fiber clearly.Fig.7shows the C1s core level scan spectra of PVDF/P(MMA-r-PEGMA)membrane.C1s can be resolved into seven peaks corresponding to CF 2(295.9eV)and CH 2(291.84eV)assigned to PVDF,CH (290eV),C–COO (290.72eV),C–O (291.45eV PEG),COO–C (291.79eV)and COO (294.03eV)assigned to P(MMA-r-PEGMA)according totheFig.4.The schematic diagram of the surface segregation and hydration of P(MMA-r-PEGMA)during the PVDF membrane phase inversion.F.Liu et al./Journal of Membrane Science345 (2009) 331–339335Fig.6.XPS survey scan spectra of outer surface of PVDF hollowfiber membrane#7Fig.7.The C1s core level scan spectra of PVDF hollowfiber membrane#7(right). (left).Fig.8.SEM micrographs of PVDF hollowfiber membrane#6and#7(A:#6;B:#7;A1and B1:outer surface;A2and B2:inner surface;A3and B3:cross-section).336 F.Liu et al./Journal of Membrane Science345 (2009) 331–339 previous paper[17,18,31].The binding energy of each peak couldbe offset about5.0eV due to the different measuring environments.The surface mole fraction of P(MMA-r-PEGMA)was0.16calculatedasϕP=A COOA COO+A CF2where A COO and A CF2are areas of the COO and CF2peak components.The surface weight content66.4%of P(MMA-r-PEGMA)cal-culated as W P=((ϕPϕ1M1+ϕPϕ2M2)/(ϕPϕ1M1+ϕPϕ2M2+(1−ϕP)×M0))×100%(ϕ1andϕ2are the weight fraction of PEGMA and MMA,respectively from NMR results,M1,M2,M0are1000, 100,64,respectively)is much higher than the bulk weight con-tent16.7%(calculated according to the membrane composition listed in Table1,as PVP and PEG has been washed away from the membrane),which further confirm that amphiphilic copoly-mers prefer to segregate and enrich on the membrane surface rather than disperse uniformly in the membrane bulk.This sig-nificant surface segregation of the amphiphilic copolymer in PVDF membrane during the immersion precipitation phase inver-sion process is caused by the hydration effect and the relatively low interfacial energy between the amphiphilic copolymer and water.3.4.Membrane morphologies and structuresThe membrane SEM morphologies are showed in Fig.8.The cross-section shows the typical asymmetric structure.The pore size of the outer surface(A1,B1)is much smaller than the inner surface (A2,B2).The outer surface would go through a15cm air gap and form a much denser skin layer due to the delayed onset of demix-ing.While the inner surface would contact borefluidfirstly and form more porous layer due to instantaneous demixing.Moreover, it can be observed that higher content of P(MMA-r-PEGMA)(i.e.B, #7membrane)will cause larger pore size and higher porosity.Comparing the dry and wet state membrane SEM pictures with both5000×and1000×magnifications in Fig.9,we can see that the dry membrane exhibits a clear porous distribution while the wet membrane exhibits a uniform nonporous layer over a porous bining the kinetics analysis and surface chemical composition,we can conclude that nonporous hydrogel layers were formed through the surface segregation and hydration effectofFig.9.SEM micrographs of PVDF hollowfiber membrane#7(C:dry membrane;D:wet membrane;C1and D1:inner surface,5000×;C2and D2:inner surface,1000×;C3 and D3:outer surface,100,000×).F.Liu et al./Journal of Membrane Science 345 (2009) 331–339337amphiphilic copolymer P(MMA-r-PEGMA)combining with sur-rounding water molecules in the aqueous state.This hydrogel layer structure can also be supported by some recent research on membrane surface grafting [32–34].Most of their work focused on immobilizing some polymer chains on the membrane surface to form the thin layer with hydrogel structure.However,in our work,this kind of hydrogel layer could be formed by the segrega-tion of amphiphilic copolymer during the single-step fabrication of the hollow fibers by the immersion precipitation phase inversion method.3.5.Hydrophilicity and fouling resistanceThe relative hydrophilicity of the porous membrane was deter-mined reliably by the droplet dynamic adsorption process.As shown in Fig.10,the contact angle of the inner surface drops dra-matically from 82◦to 0◦in 15s,while that of outer surface drops from 82◦to 45◦in 102s.Obviously,the inner surface has a bet-ter hydrophilicity than the outer surface.This difference could be explained that the inner surface has a more porous structure (see-ing SEM Figs.8and 9).Besides,the inner surface contacted the bore fluid (water)simultaneously during the spinning process and the surface segregation of the amphiphilic copolymer happened first,while the outer surface of the membrane went through an air gap before immersing into the water coagulation,and thus depressed the surface segregation of the amphiphilic copolymer to some extent.Consequently,the inner surface has a higher distri-bution of amphiphilic copolymer than the outer surface,indicating a better hydrophilicity.The static protein adsorption in Fig.11shows that increasing the content of P(MMA-r-PEGMA)resulted in less BSA adsorption on the PVDF membrane.This is due to the well-known protein-resistance of PEG side chains,arising from the hydrophilicity,large excluded volume,and unique coordination with surrounding water molecules in aqueous solution [35,36].To evaluate the protein-resistance of PVDF hollow fiber membrane adequately,a dynamic fouling resistance experiment was also performed.From Fig.12,we can see that permeation flux increases tremendously with increas-ing the content of P(MMA-r-PEGMA)in the membrane.The BSA solution permeation flux decreases to some extent due to the non-specific adsorption of BSA on the membrane.However,the pure water flux could recover remarkably after simple water washing.For example,the pure water flux of #7membrane with 16.7%content of P(MMA-r-PEGMA)is 693.0L/m 2h,however thefluxFig.10.Changes of contact angles on both inner and outer surfaces with drop age for PVDF hollow fiber membrane#7.Fig.11.BSA static adsorption of different PVDF/P(MMA-r-PEGMA)hollow fiber membranes (#5,#6,#7,#8).deceases to 381.9L/m 2h due to the BSA adsorptive fouling,and the pure water flux recovers to 600.3L/m 2h after the water wash-ing.While the water flux of #5membrane is only 106.3L/m 2h,and it decreases dramatically to 16.5L/m 2h due to the BSA fouling.The water flux of #5membrane would not recover even after water washing.Fig.13further illustrates the good fouling resistance more clearly.The fouling extent defined as (1−J B /J 0)×100%decreases with increasing the content of P(MMA-r-PEGMA).The flux recov-ery extent after water washing defined as (J 1−J B )/(J 0−J B )×100%increases substantially.Therefore the addition of copolymer into PVDF membrane improved the water flux and as well as the protein fouling resistance.The BSA adsorption belongs to reversible fouling and the flux can be easily recovered by simple water washing.It is speculated that the hydrogel layers formed by the high hydration effect of amphiphilic copolymers both on the surface and inside the pores played an important role in depressing the protein adsorptive fouling.3.6.Filtration performance#7membrane with 16.7wt%copolymer has a high water flux (693.0L/m 2h)and also showed good morphology without any shrinkage,so it was chosen to test the carbon ink rejectionper-Fig.12.Effect of P(MMA-r-PEGMA)content on the permeation flux (J 0:pure water flux,J B :BSA solution flux,J 1:pure water flux after water washing).338 F.Liu et al./Journal of Membrane Science345 (2009) 331–339Fig.13.Effect of P(MMA-r-PEGMA)content on the flux changes during filtration and after waterwashing.Fig.14.The particle distribution of both feed and permeation solutions for mem-brane #7.formance.As shown in Fig.14,the feed carbon ink solution demonstrated a normal particle size distribution with a minimum size 128nm,while the particles in the permeation solution cannot be detected at all.This result shows that carbon ink particles above 128nm can be completely rejected by the PVDF hollow fiber mem-brane.From the insert picture,it can be seen that the black solution became completely transparent after filtration.This is a very signif-icant result,especially with the pure water flux up to 700L/m 2h.The amphiphilic copolymer would segregate to the water interface and be hydrated with adjacent H 2O molecules.The hydration effect would promote the formation of nonporous hydrogel layers,which could behave as the water channel and particles retention barrier at the same time.4.ConclusionAmphiphilic brush-like copolymer P(MMA-r-PEGMA)has been synthesized by the radical polymerization method.NMR result shows the content of PEGMA in P(MMA-r-PEGMA)is 62.5wt%,which can be controlled by the feed monomer ratio to some extent.GPC could further verify that the copolymer has the suitable molecular weight (M w =85,311)to be used as a macromolecu-lar additive for PVDF hollow fiber membrane.Light transmission experiment demonstrates that amphiphilic copolymer P(MMA-r-PEGMA)results in a delayed onset of demixing and depress the precipitation process of PVDF/P(MMA-r-PEGMA)membrane.The amphiphilic copolymer would segregate onto the membrane/H 2O interface to reconstruct a hydrogel layer due to the hydration effect with surrounding H 2O molecules.FTIR and XPS proves that P(MMA-r-PEGMA)prefers to enrich on the membrane sur-face.SEM confirms the existence of a nonporous hydrogel layer in the aqueous state.The contact angle of the inner surface dropped from 82◦to 0◦in 15s,indicating that the PVDF hol-low fiber has a very good hydrophilicity.The pure water flux can reach 693.0L/m 2h and carbon ink particles above 128nm were rejected by the membrane completely.The membrane showed a good protein fouling resistance and also the flux could be recovered easily by simple water washing.All these results demon-strated that PVDF hollow fiber membranes prepared by the surface segregation of P(MMA-r-PEGMA)during the phase inversion pro-cess showed high permeation flux and good fouling resistance.This single-step method for fabricating PVDF hollow fibers with good performance has the potential to be scaled up to industrial level.AcknowledgementsFinancial support from the National 973Foundation of China (No.2009CB623402)and National Nature Science Foundation of China (No.20804034)is gratefully appreciated and acknowledged.References[1]A.K.Fritzsche,A.R.Arevalo,M.D.Moore,V.B.Elings,K.Kjoller,C.M.Wu,Thesurface structure and morphology of polyvinylidene fluoride microfiltration membranes by atomic force microscopy,J.Membr.Sci.68(1992)65–78.[2]Y.Chang,Y.-J.Shih,R.-C.Ruaan,A.Higuchi,W.-Y.Chen,i,Preparationof poly(vinylidene fluoride)microfiltration membrane with uniform surface-copolymerized poly(ethylene glycol)methacrylate and 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