功能高分子材料 (10)

功能高分子材料功能高分子材料 10 10 Highly PermeablePolymer MembranesContaining DirectedChannelsfor WaterPurif icationHongyang Ma Christian Burger Benjamin S Hsiao and BenjaminChu Department ofChemistry Stony BrookUniversity Stony Brook New York11794 3400 United StatesABSTRACT This viewpointdescribes theconcept ofusing nanopositebarrier layers containingdirected water channels to increase membranepermeabilityfor waterpurif ication In onepractical approach the channels formed atthe interface between theinterconnected nanof i brous scaf f old andthe polymer matrix were usedto guidethe transportof water molecules in adirected mannerand toalso excludecontaminant molecules This conceptwasdemonstrated byembedding overlappedoxidized multiwalled carbon nano tubes into thepoly vinyl alcohol PVA barrier layerfor ultraf i ltration UF We anticipatethat the same approachcan beextended bysubstituting oxidizedcarbon nanotubes withultraf i ne cellulose nanof i bers diameter about5nm which arederived fromwood pulpand areenvironmentally friendlyas wellasmore cost ef fective into highlycross linked polymerbarrier layers Theresulting thin f ilm nanof i brous posite TFNC membranes shouldexhibita permeationf l ux signif i cantlyhigher than those of conventional thin f ilm posite TFC membranes for nanof i ltration whilemaintainingthe samerejection capability The useof membranef i ltration topurify waterfrom diversesources such asseawater produced water industrialwastewater and otherpolluted water is acost ef fective waytoaddress theemerging problemof aclean watershortage whichhas beea majorchallenge forhumanity today 1Progress inmembranescience andtechnology duringthe lastfew decadeshasbeen slow and somemajor technicalbarriers toimprovethe energyef f i ciencyof f i ltration especially whendealing withtheremoval ofsmall molecules such assalt ionsin seawater desalination remain unresolved In conventionalnanof iltration NF and desalination e g reverse osmosis RO or forwardosmosis FO membranes the separationprinciple forremoving smallcontaminantmolecules e g sizes less than1nm from wateris mainlybased on the mechanism of size exclusionor solutiondif fusionof water molecules in the barrier layer 2 3When dealingwith thesolution dif fusion pathway the freevolume concept is oftenusedto explain the waterpassage in the amorphousmatrix 4However the freevolume in the barrier layer of theconventional membrane as conf i nedby highlycross linkedand restrainedpolymer chains is inevitablyorganized in atortuous manner Thus the permeablewater pathsareessentially randomor nondirected Recently the materialsmunity hasbeen pursuingtheconcept of incorporating directed water channels in thebarrier layer toimprove themembrane permeability 5Thisconcept isbased on themechanismofsizeexclusion inspiredby theNobel discoveryof hourglass shaped channels with apore size of0 28nm of Aquaporinproteins in the cellmembrane 6 7These protein channels canselectively passwatermolecules andexclude othermolecules thus regulatingthe f l owof water Recent studiesshowed that the waterpermeability inmembranescontaining suchprotein channelscan be5 1000times greaterthan those of conventionalmembranes 5 8 9Othernanoporous materials including carbon nanotubes 10zeolites 11and graphene oxide nanosheets 12that possessmolecularchannels in the nanometerrange have alsobeen introducedintoor used directly as the barrier layer For example thehollow coresof alignedcarbon nanotubes including single double and multiwallcarbon nanotubes with aninnerdiameter of lessthan1nm havebeen used as water channels where watermolecules couldpass throughthe corewith2 3orders ofmagnitude higherf l ux thanthoseofmercialmembranes 10Having Na A typezeolites withentrance poresof0 4nm incorporatedintothe polyamide barrier layer madebyinterfacial polymerization the resultingRO membraneshowedan80 increase inpermeability whenpared withmembraneswithout zeolites 11Recently the barrier layermade ofgeometrically optimizedgrapheneoxidenanosheetscontaining intercalatedwater layershas alsobeen foundtoexhibit almostunimpeded water peration capability whilerejecting mostother smallmolecules such as helium 12Self assembly orphase transformation of molecules such asliquidcrystals13or block copolymers 14are alternativeapproachesto incorporatedirected water channels in thebarrier layer For example membranes madeby postcross linking oflyotropic liquidcrystal moleculesin aspecial QIcubicphase with aPn3m structure were foundto containcontinuouswater channelshaving diametersof about0 75nm Such membranesexhibited arejection ratiohigher thanReceived April5 xxAepted May17 xxPublished May24 xxView pointpubs acs macroletters xxAmerican ChemicalSociety723dx doi 10 1021 mz300163h ACS MacroLett xx 1 723 72699 5 against sodiumchloride andat a parable fl uxas themercialmembranes 13A recentsmall angle neutronscattering SANS study ofmembranes madeof sulfonatedpenta block copolymers Kraton Inc also indicatedthepresence ofcontinuous ionicwater channels in the barrierlayer which ledto improved permeability 14All of the above mentioned methods however face dif f erentchallengesfor practicalapplications in termsof processability durability and cost performance ef f ectiveness For example theAquaporin proteinchannels haveto beimbedded ina polymermatrix to obtainsuf f i cientmechanical stability while thispositeformat representing asubstantial reductionon thesurfacenumber densityof Aquaporinproteinchannels greatlydecreases the permeability Aligned carbon nanotubes needtobe connectedand sealedby a water impermeablebinder wheresuch afabrication processdrastically increasesthe membranecost For zeolite based membranes the watertransport rateinhydrophilic zeoliteinner poresseems to be signif i cantlylowerthan thatin hydrophobihannels of carbonnanotubesorAquaporin proteins rendering alow permeabilitygain Inparticular the grapheneoxide based membraneseems tobeonly applicable to permeatewatermoleculesin thegaseousphase insteadof theliquid phase The conditionsfor formingandsecuring directed water channelsfrom theformation of aliquid crystalphase followedby polymerizationare trickyanddif fi cultto scale up Finally the water channels formedin tri penta blockcopolymersare tortuousand noteasily tunable while thepermeability advantageover othermercialmembranes isnot apparent In our laboratory we havebeen pursuinga diff erentpathwayto incorporatedirected water channelsin the barrierlayer Ourapproach takesadvantage of the naturalformationof theinterface betweenan interconnected nanof i brous scaf f old and abarrierpolymer matrix where the interface can be usedto guidewatertransport andto excludepredesigned contaminantmolecules Our fi rstpublication insupport of this approachinvolvedthe fabricationof high flux ultraf iltrationthin filmnanof i brousposite TFNC membranes basedonananoposite barrierlayer containingoxidized carbonnanotubesembedded incross linked poly vinyl alcohol PVA for oiland waterseparation 15In thatwork wehypothesized thatthe increasein waterpassage isdue totheexistence ofboth internalnanochannels i e the inner cores ofnanotubes and externalnanochannels i e the interfacialgapsbetween oxidizedcarbonnanotubesand thePVA polymermatrix We nowbelieve thatthe latterplays thedominant rolebecause 1 the internalnanochannels arenot interconnectedbutthe externalnanochannels are since theconcentration ofnanotubesinthe polymer matrixwas muchhigher thantheoverlap concentration and 2 the operatingpressure as lowas30psi might betoo lowto inducewatermoleculesenteringinto thehydrophobic coreofcarbonnanotubes 16Based onthe abovefi ndings we nowpropose toexpand ourviewpointonthe directedwater channels which arecontrolledby thegap between the relativelysmooth surface of theoxidizedcarbonnanotubesand the surface of the barrierpolymer matrix 17The gapdistance determinesthe ef f ectivesievesize While itis quiteeasy tofabricate arelatively largegapsize forUF membranes it beesmore challengingwithdecreasing gapsize To resolvethe scaleup transformation wesubstitute themodif iedcarbonnanotubes withenvironmentallyfriendly andmore cost ef fective ultraf i ne cellulose nanof i bersinthe barrierlayer toincrease themembrane permeabilityfornanof iltration andbeyond Before wedescribe theexpandedapproach we fi rstbrief ly reviewour recentresults ofusingnaturally ourringcellulose nanof i bersfor waterpurif ica tion 18 21It hasbeen well demonstrated thatultraf i nepolysaharide e g cellulose andchitin nanof i bers ornanowhiskerscan bederived fromplant orcrustacean materialsusinga binationof chemicaland mechanicaltreatments 22These nanof i bers can beconsidered asnascent crystalsoraggregates ofnascent crystalsinthebiomass where thediameterof thesenanof i bers rangesfrom5to20nm and thelength rangesfrom afew hundrednanometers tomicrometers Variations of these dimensionsdepend onthe sourcesandprocessing methodschosen For example thesurfaceofcellulose nanof i bersderived fromwood pulpis naturallycoveredwith hydroxyl groups which arepartially convertedintocarboxylate and aldehyde groupsby theTEMPO oxidationmethod 23In thiscase carboxylate groupsprovide negativechargesin water andaldehydegroups can be thermallycross linked withhydroxylgroupsor chemicallybonded withotherfunctional groups 18Since asmall fi ber diameter leadsto veryhigh surface to volume ratio 600m2 g the ultraf i necellulose nanof i bers can be usedas asuperadsorbent withahigh adsorptionef fi ciencyto removecharged molecules e g positively chargedmetal ions 20or negatively charged virusesifthe carboxylategroups areplexed withpositively chargedmolecules such aspolyethyleneimine PEI 24 Another uniquecharacteristicof thesenanof i bers isthe highcrystallinityFigure1 Left Schematic hierarchicalstructure ofthin film nanof i berposite membranecontaining threelayers of randomly deposited fi berswith diff erentdiameters Right Cross sectional SEMviews ofthe barrierlayer andnanof i brous scaf f oldinatypical TFNCmembrane 18ACS MacroLetters Viewpointdx doi 10 1021 mz300163h ACS MacroLett xx 1 723 726724 70 which signifi cantlyincreases theirchemical stabilityandbacteria resistance As aresult ultrafi ne cellulose nanofi berscan be useddirectlyasthebarrierlayer where alayer of100nm thicknesswould resultinamean poresize ofabout20nm suitable forultrafiltration UF 18Figure1illustrates thehierarchicalstructure ofTFNC membranescontaining threelayersofrandomlydepositedfiberswith diff erentdiameters thetop barrierlayer basedon ultrafinecellulose nanofibers diameter about5nm the midlayerelectrospun nanofi brousscaf f old fiber diameterabout150nm and thebottomnonwoven fi broussupport fiberdiameterabout20 m Asthe porositiesof eachlayer areall higherthan65 the resultingfluxin thismembrane is3 10times higherthanthoseofmercial UFmembranes but withaparablerejectionratio 18The hierarchicalstructure inthe TFNCmembrane Figure1 is suitableonly forhigh fluxUF applicationsbecause themeanporesize about20nm inthebarrierlayeris confi nedby the diameterof ultrafinecellulosenanofibers about5nm involved 21The toplayer structure however can beusedas ascaf fold forfurther fabricationof NF RO or FOmembranes To bespecif ic the poresofthe top cellulosenanofiber layercanbe fi lledby adense polymer matrixtodecrease the effectivepore sizeofthebarrierlayerusing thefollowing twoapproachesbased onrecent findings In oneillustration awater soluble cross linkable monomer e g polyethylene glycoldiacrylate mixed withultrafinecellulosenanofibers couldbe polymerizedasa barrierlayer In thesecond illustration interfacialpolymerization couldbe carriedout inthetopcellulosenanof iber layerofaTFNC UFmembrane as shownin Figure1 to producea nanoposite barrierlayer containing theinterconnectedcellulosenanofi brousscaf fold andthe highlycross linked polyamidematrix There shouldbe anumber ofdiff erentapproaches for the introductionof functionalizedultrathincellulosenanofibersas directedwater channels Thekey pointis thatwe believethatthesurfaceofultrathinnanof iberscanbeusedasapractical andcost effective waytosubstantially increasethepermeabilityof filtrationmembranes Although theimprovedpermeabilitycanbeattributed totheformation of water channelsintheinterface betweencellulosenanof ibers andthepolyamidematrix the nature oftheinterfacein theabove twocases maybe diff erent Further studiesare inprogress A schematicdiagram of directed andnondirected waterchannelsinthe nanopositebarrierlayercontaining thenanof i brousscaf fold andthe polymermatrixis shownin Figure2 The directedwater channelsare formedthrough theformationof interfacebetween the cross linked nanofi bersandthe polymermatrix while thegap thicknessmay beregulatedby physicalinteractions orchemical bondingbetweenthe two This thicknesscan directlyaffect theselectivity ofmoleculestoberemoved In addition the natureof thenanofiber surface that is neutral versuscharged positivelycharged versusnegativelycharged or hydrophilicversushydrophobic should alsoplay acrucial rolein fine tuning theselectivityorthecapability toreject thecharged molecules suchasmetal ions It isclear thattheeff ectsof theseparameterson thepermeability orthe rejectionratio incorrespondingmembranes arelargely unknown which havebee anactiveand worthyresearch topic worthy offurther investigations Although onecan expectthat watermolecules travelasignif i cantly shorterdistance throughthe interconnectedinterfacethan throughthe tortuouspath viamolecular cavitiesinthe polymermatrix the watertransport ratesalong thetwopaths willbe verydifferent Based onrecent experimentalandputational resultsofwatertransport inthe coresof carbonnanotubes10and proteinchannels 8 9we arguethat thehydrophobianofibersurfaceshould leadto greaterpermeabilityimprovement thanthe hydrophilianofibersurface This hypothesisis beinginvestigated in ourlaboratory In summary we havedemonstrated apractical approachtoincorporate directedwater channelsinthebarrierlayerthat cansignificantlyimprove themembrane permeabilityfornano filtration anddesalination The formationofwaterchannels isthroughthe naturalourrence ofinterfacebetween theembedded nanofibrousscaffoldandthe polymermatrix insteadof theinnercoresof nanoporousmaterials There areseveraladvantages ofadopting directedwaterchannelsinthebarrierlayer throughthis approachover theconventional barrierlayerwith nondirected waterchannels 1 Directed waterchannelscan becreated attheinterfacebetween twodomains ofanybicontinuous phase e g organic andinorganic crystalline andamorphous hydrophobic andhydrophilic inthebarrierlayer not confi nedonly bythenanopositecontainingthenanofibrousscaffoldandthe polymermatrix 2 The passagedistancethrough directedwaterchannelscanbesignificantlyshorter thanthat throughnondirectedwaterchannels 3 Thewidth ofthedirectedwaterchannelsis adjustable dependingon theinteractions physical or and chemical betweenthetwobicontinuous phases 4 The selectivityofthesedirected waterchannelscanbefine tuned bysurface functionalizationof onephase e g hydrophilic hydrophobic charged or chelategroups 5 The densityofdirectedwaterchannelscan becontrolledbythesurface to volume ratiooftheembeddedphase e g interconnectednanofibrousscaff olds withsmallf iber diameterscan providea particularlyhighsurface to volume ratiotoincreasethe throughputfor waterpurif ication Finally we notethat although wehave usedthe uniqueFigure2 Schematic representationofthenatureofwaterchannelsinthe nanopositebarrierlayer A skeletonof overlappedcellulosenanof ibers yellow guides acontinuously connectedsystem ofdirectedwaterchannels blue formed bythe connectedhollowcylindrical gapsbetweenthenanofibers andthepolymermatrix pink The cut out inthe redcircle sketchesthecross linked natureof thenanofiberinterconnects The nanofiberskeleton isanchored atoasionaldirect contactswith thepolymermatrix not shown Nondirected molecularcavities inthepolymermatrix alsocontributeto theoverall waterflow throughthebarrierlayer ACS MacroLetters Viewpointdx doi 10 1021 mz300163h ACS MacroLett xx 1 723 726725hierarchical fibrousstructure asa supportinourNF examples thesameconceptisapplicabletothe useofconventionalUFsupport AUTHOR INFORMATIONCorrespondingAuthor E mail bhsiao notes sunysb B S H bchu notes sunysb B C NotesThe authorsdeclare nopeting fi nancialinterest ACKNOWLEDGMENTSFinancial supportofthiswork wasprovided byan EAGERgrantfrom theNational ScienceFoundation DMR 1019370 REFERENCES 1 Shannon M A Bohn P W Elimelech M Georgiadis J G Mari nas B J Mayes A M Natu