Surface modification of electrospun polyester nanofibers with cyclodextrin polymer for the removal of phenanthrene from aqueous solution

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ContentslistsavailableatScienceDirect

Journal

of

Hazardous

Materials

j o ur na l h o me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / j h a z m a t

Surface

modiﬁcation

of

electrospun

polyester

nanoﬁbers

with

cyclodextrin

polymer

for

the

removal

of

phenanthrene

from

aqueous

solution

Fatma

Kayaci,

Zeynep

Aytac,

Tamer

Uyar

∗

UNAM-InstituteofMaterialsScience&Nanotechnology,BilkentUniversity,Ankara06800,Turkey

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•Electrospun PET nanoﬁbers were

surface modiﬁed with cyclodextrin

polymer(CDP).

•ThreedifferenttypesofnativeCD

(␣-CD, ␤-CD and ␥-CD)were used to

formCDP.

•Nanoﬁbrous structure of PET mats

was preserved after CDP surface

modiﬁcation.

•PET/CDP nanoﬁbers have shown

enhanced mechanical and thermal

properties.

•PET/CDP nanoﬁbers efﬁciently

remove PAH (e.g. phenanthrene)

fromaqueoussolution.

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Articlehistory: Received4March2013

Receivedinrevisedform5July2013 Accepted18July2013

Available online 25 July 2013 Keywords: Electrospinning Cyclodextrinpolymer Nanoﬁbers Polyester Phenathrene

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Surfacemodiﬁedelectrospunpolyester(PET)nanoﬁberswithcyclodextrinpolymer(CDP)wereproduced

(PET/CDP).CDPformationontoelectrospunPETnanoﬁberswasachievedbypolymerizationbetween

citricacid(CTR,crosslinkingagent)andcyclodextrin(CD).ThreedifferenttypesofnativeCD(␣-CD,

␤-CDand␥-CD)wereusedtoformCDP.Water-insolublecrosslinkedCDPcoatingwaspermanently

adheredontothePETnanoﬁbers.SEMimagingindicatedthatthenanoﬁbrousstructureofPETmatswas

preservedafterCDPsurfacemodiﬁcationprocess.PET/CDPnanoﬁbershaveshownrougher/irregular

surfaceandlargerﬁberdiameterwhencomparedtountreatedPETnanoﬁbers.Thesurfaceanalysesof

PET/CDPnanoﬁbersbyXPSelucidatedthatCDPwaspresentontheﬁbersurface.DMAanalysesrevealed

theenhancedmechanicalpropertiesforPET/CDPwherePET/CDPnanoﬁbershaveshownhigherstorage

modulusandhigherglasstransitiontemperaturecomparedtountreatedPETnanoﬁbers.Thesurface

areaofthePET/CDPnanoﬁbersinvestigatedbyBETmeasurementsshowedslightdecreaseduetothe

presenceofCDPcoatingcomparedtopristinePETnanoﬁbers.Yet,itwasobservedthatPET/CDPnanoﬁbers

weremoreefﬁcientfortheremovalofphenanthreneasamodelpolycyclicaromatichydrocarbon(PAH)

fromaqueoussolutionwhencomparedtopristinePETnanoﬁbers.OurﬁndingssuggestedthatPET/CDP

nanofiberscanbeaverygoodcandidateasafiltermaterialforwaterpurificationandwastetreatment

owingtotheirverylargesurfaceareaaswellasinclusioncomplexationcapabilityofsurfaceassociated

CDP.

∗ Correspondingauthor.Tel.:+903122903571;fax:+903122664365. E-mailaddresses:tamer@unam.bilkent.edu.tr,uyar@unam.bilkent.edu.tr,

tameruyar@gmail.com(T.Uyar).

1. Introduction

Electrospun nanofibers and their nanofibrous mats have demonstratedhugepotentialforfiltrationapplicationsduetotheir

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highsurface-to-volumeratioand nanoporousstructure[1–3].It hasbeenreported that electrospunnanofibrousmats are quite effective for particulate separation [2,3], liquid filtration [1–3], wastevaportreatment[4,5]aswellasdesalination[6]. Electro-spinninghasadvantageoverconventionalmembraneproduction techniques,sincevarietyoffunctionalnanofibrousmaterialscan beeasilyobtainedintheformofnonwovenmembraneswhichcan bereadilyusedasafilteringmaterial[1–3].Inaddition,thedesign flexibilityofelectrospunnanofibersforspecificsurface function-alitycanyieldbetteradsorptivecapacityandselectiveseparation performance[7,8].

Cyclodextrins(CD)haveanoutstandingcapabilitytoform inclu-sioncomplexeswithvarietyofmoleculesthroughnon-covalent host–guestinteractionsduetotheirtoroid-shapedmolecular struc-ture[9].CD arequite applicablein pharmacy, cosmetics, food, textiles, since CD might enhance the solubility, stability, and bioavailabilityoftheguestmolecules[9–11].Inaddition,CDhave alsopotentialtobeusedasaﬁlteringmaterialduetotheirabilityto selectivelyforminclusioncomplexeswithorganicwastemolecules [12,13].CDarenontoxicandnaturalcyclicoligosaccharidesderived fromstarch.ThemostcommonCDtypesarenamedas␣-CD,␤-CD and_␥-CDhavingsix,sevenandeightglucopyranoseunits, respec-tively.TheseCDhavethesamecavitydepthwhichis∼7.8 ˚A,while thediameterofthecavityfor␣-CD,␤-CD,␥-CDare∼6,8,and10 ˚A, respectively[9].Hence,␣-CD,␤-CDand␥-CDshowdifferent capa-bilitiesfortheinclusioncomplexformationwiththesameguest molecule,becausetheformationofinclusion complexprimarily dependsonthesizematchandbindingforcesbetweenCDcavity andguestmolecule[14].

CDarewatersoluble,therefore,theycannotbeuseddirectly asafilteringmaterialfortheremovaloforganicpollutantsfrom water and wastewater. So, crosslinked and water-insoluble CD basedpolymersweresynthesizedforcapturingorganicpollutants fromthesurroundings[13].Alternatively,CDmoleculescouldbe permanentlyimmobilizedbychemicallygraftingontopolymeric fibers[15–18],orthesurfaceofthefiberscouldbemodifiedby crosslinkedCDpolymer[19–21]forfiltrationpurposesordelivery ofadditives.Moreover,inordertocombinethecomplex forma-tion capability of CD along with very highsurface areaof the electrospunnanofibrousmat,surfacefunctionalizationof electro-spunnanofiberswithCDwouldbequiteinterestingfordesigning efficientfilteringmaterials.Infact,inourrecentstudieswehave incorporatedCDintonanofibersbyelectrospinningofphysical mix-tureofpolymer/CDsolution[4,22,23].AlthoughmostoftheCD moleculeswereburiedinsidethefibermatrix,weobservedthat someCDmoleculeswerepresentonthefibersurface,andthese surfaceassociated CD moleculeswereeffectivefor theremoval oforganicmoleculesfromliquidmedia[22,23]andvaporphase [4].However,CDmoleculeswerephysicallyattachedtothefiber surface;so,theleaching ofCDmoleculesfromthefibersurface duringfiltrationespeciallyintheliquidmediawasinevitable. Con-sequently,permanentCDmodificationontoelectrospunnanofibers wouldbeidealfor designing novelfilteringmaterialsfor water purificationandwastewatertreatment.Eventhoughsurface mod-ificationsoffibersandnonwovenfabricsbyCDgrafting[15–17] orcoatingwithcrosslinkedCDpolymer[19–21,24]werereported, tothebestofourknowledge,thesurfacemodificationof electro-spunpolymericnanofiberswithcrosslinkedCDpolymerwasnot reportedpreviously.

In this study, we have achieved thesurface modification of theelectrospunpolyester(PET)nanofiberswithcyclodextrin poly-mer(CDP).Water-insolubleandcrosslinkedCDPcoatingontoPET nanofiberswasformedbythepolymerizationreaction between CDandcrosslinkingagent(citricacid).Foracomparativestudy, threetypesofCD(␣-CD,␤-CDand␥-CD)wereusedinorderto formCDPcoatingontoPETnanofibers.Themorphological,surface,

thermalandmechanicalpropertiesofsurfacemodifiedelectrospun PETnanofiberswithCDP(PET/CDP)wereexamined.Thefiltration performance ofthePET/CDPnanofibrousmatswasinvestigated byremovalofamodelpolycyclicaromatichydrocarbon (phenan-threne)fromaqueoussolution.

2. Materialsandmethods 2.1. Materials

Polyethyleneterephthalate(PET)chipsweregiftsfromKorteks (Bursa, Turkey). Dichloromethane (DCM, Sigma Aldrich, extra pure),trifluoroaceticacid(TFA,AlfaAesar,99%),acetonitrile chro-masol V (Sigma Aldrich, 99.9%), citric acid monohydrate-gritty puriss(CTR,SigmaAldrich,99.5–100.5%),sodiumhypophosphite hydrate(SHPI,SigmaAldrich),phenanthrene(SigmaAldrich,98%), andcyclodextrins(_␣-CD,_␤-CDand_␥-CD,WackerChemieAG)were purchasedandusedas-receivedwithoutanypurification.Distilled waterwasfromMilliporeMilli-Qultrapurewatersystem. 2.2. Preparationofpolymersolutionandelectrospinningof nanofibers

First,differentpolymerconcentrationswereusedforthe elec-trospinningofPETsolutioninordertoobtainuniformandbead-free PETnanofibers,and22.5%(w/v)polymerconcentrationwasfound tobe theoptimal. Therefore, 22.5% (w/v)PET wasdissolvedin TFA/DCM(50/50,v/v),andtheresultingsolutionwasloadedinto 5mLsyringefittedwithametallicneedlehavinganinnerdiameter of0.8mm.Then,thesyringewasplacedhorizontallyonthesyringe pump(KDScientific,KDS101).Thepolymersolutionwaspumped with1mL/hflowrateduringtheelectrospinning,andthedistance wassetto12cmbetweenneedletipandgroundedstationary cylin-dricalmetalcollector(height:15cm,diameter:9cm)coveredwith apieceofaluminumfoil.Avoltageof15kVwasappliedforthe electrospinningbyusinghighvoltagepowersupply(Matsusada, AUSeries).Theelectrospinningprocesswascarriedoutat24.5◦C and17%relativehumidityinanenclosedPlexiglasbox.

2.3. Formationofcyclodextrinpolymer(CDP)ontoPETnanofibers 10%(w/v)of␣-CD,␤-CDand␥-CDwasmixedindividuallyin 150mLaqueoussolutionat50◦C, andthen,10%(w/v)CTRasa crosslinkingagentand1.2%(w/v)SHPIasacatalystwereaddedto eachCDsolutionseparately,andstirredfor30minat50◦C.After allreactantsweredissolvedinaqueoussolution,threerectangular shaped(about12cm×11cm,0.4g)electrospunPETnanofibrous matswereimmersedintotheeachresultingsolutionandkeptfor 3hat50◦C.Thenthesenanofibrousmatsweredriedat105◦Cfor 10min,andthencuredat180◦Cfor7minfortheCDPformation ontoPETnanofibers.Finallytheresultingnanofibrousmatswere washedtwotimeswithwarmwater(40◦C)fortheremovalof unre-actedCDandCTRifanypresent,andthendriedat105◦Cfor7min. Inordertomakeclearidentification,CDPmodifiedPETnanofibers arenamedasPET/␣-CDP,PET/␤-CDPandPET/␥-CDPaccordingthe typeofCDused(␣-CD,␤-CDand␥-CD).

2.4. Characterizationsandmeasurements

The morphology and the diameter of the PET and PET/CDP nanofiberswereexaminedbyusingscanningelectronmicroscope (SEM,FEI-Quanta200FEG).Thenanofiberswerecoatedwith5nm Au/PdpriortoSEManalysis.Toreporttheaveragefiberdiameter (AFD)ofthenanofibers,around100fibersofeachsample were measured.

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The chemical surface analyses of the PET and PET/CDP nanofiberswerecarriedoutbymeansofhigh-performanceX-ray photoelectronspectroscopy(XPS,ThermoScientific).XPSdatawere takenbyafloodgunchargeneutralizersystemequippedwitha monochromatedAlK-␣X-raysource(hv=1486.6eV).Inorderto determinethesurfaceelementalcompositionswideenergysurvey scansofthenanofiberswereacquiredoverthe0–1360eVbinding energyrange,atpassenergyof150eVwithenergystepsizeof1eV from400_␮mdiametercircularspotinnanofibers.Thehigh resolu-tionspectrawererecordedforO1sregionatpassenergyof30eV andwithenergystepsof0.1eVinordertoanalyzethebonding states.

Thethermalanalysesofthesampleswereinvestigatedbyusing thermogravimetricanalyzer(TGA,TAQ500).TGAmeasurements werecarriedout underthenitrogenatmosphere, andthe sam-pleswereheatedfromroomtemperatureto600◦C(nanoﬁbers) or500◦C(CTRandCD)ataconstantheatingrateof20◦C/min.

Thedynamicthermo mechanical performanceof the nanofi-brousmatswasdeterminedusingadynamicmechanicalanalyzer (DMA, TA Q800) in tension film clamp at a constant fre-quencyof1Hz.Thesampleshavingsizeof10mm(gap)×∼3mm (width)_×_∼0.12mm(thickness)weremeasured.Theamplitudeof 20␮mwasapplied.Thestoragemodulusandlosstangent(tanı) ofthenanofibrousmatswererecordedintherangeof50–150◦Cat aheatingrateof3◦C/min.

Thesurfacearea,averageporediameter(mesopore)and cumu-lativeporevolumeoftheelectrospunPETandPET/CDPnanoﬁbers were examined using Brunauer–Emmett–Teller (BET) surface areaanalyzer(Quantachrome,IQ-Cmodel)withlow-temperature (77.35K) nitrogenadsorption isotherms measured over a wide rangeofrelativepressuresfrom0.00to1.00.Priortomeasurement, theeachsamplewasplacedina9mmcellanddegassedat323.15K for12hinthedegaspotoftheadsorptionanalyzer.Thesurfacearea ofthesampleswasdeterminedwithmultipointBETmethod.Onthe otherhand,densityfunctionaltheory(DFT)wasusedtodetermine cumulativeporevolume.

Themolecularfiltrationperformanceoftheresulting nanofi-brousmatsforwaterpurificationwastestedusingphenanthrene asamodelpolycyclicaromatichydrocarbon(PAH).First, phenan-threnewasdissolvedinacetonitrile,andthen10␮Lofthissolution was dropped in 50mL pure water in order to obtain 1.8ppm phenanthreneaqueoussolution.SquareshapedofPET,PET/␣-CDP, PET/␤-CDPand PET/␥-CDPnanofibrousmats(6cm×6cm)were immersedindividuallyinthe1.8ppmphenanthreneaqueous solu-tion(50mL).Wekeptthesizeofthematsidentical;however,the weightofeachPET/CDPmatwasabout0.38g,whilethatofPET nanofibrousmatwasabout0.63gduetodifferenceinthethickness ofthemats,sincethenanofiberswerecollectedindifferenttimes foreachsample.Itisquitedifficulttokeepthethicknessofthe electrospunmatsevenidenticaltime.Forfiltrationmeasurements, 0.5mLofeachsolutionwaswithdrawntomeasurephenanthrene concentrationinthesolutionandreplenishedwithsameamount ofwateratpre-determinedtimeintervals.Thephenanthrene fil-trationperformancefromaqueoussolutionbyPETandPET/CDP nanofibrousmats was investigated by highperformance liquid chromatography(HPLC,Agilient1200series)equippedwithVWD UVdetector.ThecolumnwasAgilientC18,150mm×4.6mm(5␮m pores) and thedetection was accomplished at 254nm. Mobile phase,flowrate,injectionvolumeandtotal runtimewere ace-tonitrile(100%), 0.6mL/min,10␮Land5min,respectively. Asa result,theamountofphenanthreneremaininginthesolutionwas determinedfromtheareaofphenanthrenepeakobservedinHPLC chromatograms.Thenthecalibrationcurvewaspreparedbyusing phenanthrene solutions (1.8ppm, 0.9ppm, 0.45ppm, 0.23ppm, 0.12ppm)and R2 _was_calculated_as_0.985._The_peak_area_under

curves wasconverted toconcentration (ppm)according tothe

calibrationcurve.Thisexperiment wasrepeatedthreetimesfor eachsample.Theresultswerereportedastheaverage±standard deviationofphenanthreneconcentrationremaininginthesolution. 3. Resultsanddiscussion

3.1. TheCDPformationontoelectrospunPETnanoﬁbers

In this study, polyester (PET) nanofibers were obtained by electrospinningof22.5% (w/v)PETsolutionin TFA/DCM(50/50, v/v), as it is schematicallygiven in Fig. 1a. The chemical reac-tioncannotoccurbetweencyclodextrin(CD)/citricacid(CTR)and PETnanofibers directly,since PET, a polymerbased on tereph-talic acid and ethylene glycol, does not contain free reactive groups.Therefore,wemodifiedthesurfaceoftheelectrospunPET nanofibersthroughthepolymerizationreactionbetweenCTRand CD[21,24,25].Water-insoluble cyclodextrinpolymer (CDP) net-workwasformedbythecrosslinkingreactionbetweenCD and CTR[26].ThreedifferenttypesofnativeCD(␣-CD,␤-CDand␥-CD) wereusedtoform␣-CDP,␤-CDPand␥-CDP.Initially,electrospun PETnanofibrousmatswereimpregnatedinasolutionofCD,CTR, andsodiumhypophosphite(SHPI, catalyst),and thendried, fol-lowedbycuringat180◦Cfor7min.CTRturnintoacyclicanhydride intermediatebythermaldehydrationatelevatedtemperature,and thenhydroxylgroupsofCDreactedwiththecarboxylgroupsof citricacid[25].ThemechanismoftheCDPformationis schemati-callydescribedinFig.1b.CDPwasformedasathree-dimensional networkstructureontoPETnanofibers.Duetocrosslinked struc-ture,theCDPisstableandwater-insoluble[27,28].Thereby,surface modificationof CDPonto PETnanofibersis permanent andcan resisttoleachingorwashingprocess[21,24,25,29].Theresulting CDPhavingtheessentialstructuralcharacteristicsofCDwasnot covalentlyfixedtothePETnanofibers,butitwasphysicallyadhered orwasentangledontoPETfibermatrix[21,24].CDPmodifiedPET nanofibersarecalledasPET/CDP.Therepresentativephotograph oftheeasilyhandledfree-standingPET/CDPnanofibrousmatand theschematicrepresentationofPET/CDPnanofibersaregivenin Fig.1c.

3.2. Morphologicalcharacterizationofthenanoﬁbers

Scanningelectronmicroscope(SEM)analysiswasperformedto investigateanymorphologicalchangesafterthesurface modifica-tionofPETnanofiberswithCDP.Fig.2showstherepresentative SEM images and average fiber diameter (AFD) of unmodified PET,PET/␣-CDP,PET/␤-CDPandPET/␥-CDPnanofibers.Asclearly seen from SEM images, the surface morphologies of all three PET/CDPnanofiberswereobviously differentfromthe unmodi-fiedPETnanofibers.ThesurfaceoftheunmodifiedPETnanofibers wassmooth anduniform, whereas thesurfacesofthePET/CDP nanofibersappearroughpossiblyduetoCDPlayerontonanofibers. The rough surface has also been reported for cotton fabrics graftedwithglycidylmethacrylate/␤-CD[17]and hydroxypropyl-CD grafted woven PET vascular prosthesis [20,21]. Moreover, surface irregularities at certain points were also observed in the SEM images of PET/CDP nanofibers. Similar morphological observations were also reported for cotton fabricgrafted with monochlorotriazinyl-␤-CD/butylacrylate [30].Inbrief,therough andirregularsurfaceofmodifiedPETnanofiberssuggestedthe suc-cessfulattachmentofCDPontoPETnanofibers.Moreimportantly, CDPsurfacemodificationprocessdidnotdeformthefibrous struc-tureofPETasclearlyseenfromtheSEMimages.Theunmodified PETnanofibershave870_±260nmofAFD,whiletheAFDofPET/ ␣-CDP, PET/␤-CDP and PET/␥-CDP weremeasured as 1200±350, 1290±490and950±270nm,respectively.TheincreaseintheAFD

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Fig.1. Schematicrepresentationsof(a)electrospinningofPETnanofibers,(b)formationmechanismofCDPand(c)therepresentativephotographofPET/CDPnanofibrous matanditsSEMimageandschematicrepresentationofPET/CDPnanofibers.

of PET/CDPnanofiberscompared to unmodified PETnanofibers could be due to the coating of the CDP onto PET nanofibers. Additionally,slightswellingofnanofibersduringthemodification processmightalsohaveresultedinfiberdiameterincrease. 3.3. Surfacechemicalcharacterizationofthenanofibers

Thesurface chemicalcharacterization ofPET/CDPnanofibers wasperformedbyusingX-rayphotoelectronspectroscopy(XPS) in order to further demonstrate the coating of CDP onto PET nanofibers. Table 1 shows elementary compositions based on wideenergysurveyspectraoftheunmodifiedPETnanofibersand PET/CDPnanofibers.Thesurveyspectracomprisingtwopeaks:C

Table1

AtomicconcentrationsgeneratedfromXPSwideenergysurveyscans.

Samples C(%) O(%)

PET 72.21 27.79

PET/␣-CDP 64.92 35.08 PET/␤-CDP 61.29 38.71 PET/␥-CDP 67.89 34.31

1sandO1sareconsistentwiththemolecularstructureofPETand

CDP.TheXPSdatashowedthattheunmodiﬁedPETnanoﬁbershave

C1s:O1s=72.21:27.79(%)whichisinfullagreementwiththe

lit-erature[31].Oxygencontentonthesurfaceofthesampleswas

increasedwiththemodificationofCDPontoPETnanofibers.Thus, theappearanceofhigheroxygencontentprovidesanevidenceof thepresenceofCDPonthePETfibersurfaces.

High-energyresolutionO1sXPSspectrawerealsorecordedto getmoredetailedchemicalstateinformationaboutsurface chem-istryofthePET/CDPnanofibers.Fig.3showsthenormalizedO1s spectraofPETandPET/␥-CDPnanofibers.Theassigneddifferent componentswithinthesespectraandtheirindividualizedfitting parameters(peakbindingenergyand%arearatio)arealsogiven inTable2.SincetheO1sspectraofallPET/CDPnanofibers(PET/ ␣-CDP,PET/␤-CDPandPET/␥-CDP)aresimilartoeachother,those XPSdataacquiredforPET/␣-CDPandPET/␤-CDPnanofiberswere notgiven.TheO1sspectrumofunmodifiedPETnanofibersclearly representthetwotypesofoxygenatomswithintheestergroups; ␲-bondedoxygen(C O*₎_and_␴-bonded_oxygen_(C _O*₎_at_binding

energiesof531.54and533.12eV,respectively[32–35].Theratioof thesepeaksis56.2:42.1,whichisinreasonableagreementwiththe theoreticalratioof50:50[36].InadditiontotheseexpectedO1s

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Fig.2. RepresentativeSEMimagesandAFDof(a)PET,(b)PET/␣-CDP,(c)PET/␤-CDPand(d)PET/␥-CDPnanoﬁbers.Theinsetsshowhighermagniﬁcationimages.

peaks,PETnanofibershaveaverysmallpeaksituatedat534.52eV assignedtoadsorbedwater[32].AftertheCDPmodificationonthe surfaceofPETnanofibers,thecontributionofadditionalO1s fit-tingpeakat532.35relatedtoaliphaticC O*_H_came_into_view._The

appearanceofC O*_H_component_belongs_to_hydroxyl_groups_and

carboxylgroupsofCDPelucidatedthesuccessfulsurface modiﬁca-tionofPETnanoﬁberswithCDP.Moreover,asitwasexpected,CDP

modificationontoPETnanofibersresultedinsignificantincreaseof relativeXPSsignalintensityintheO1speaksituatedat533.06eV assigned to␴-bonded oxygen(C O*₎_compared _with_␲-bonded

oxygen(C O*₎_located_at_532.35_eV._In_brief,_there_are_three

differ-entcomponents(C O*_,_C _O*_and_C _O*_H)_for_O_1s_{high-resolution}

spectraofthePET/CDPnanofibers.Theincreaseinoxygencontent ofPET/CDPnanofiberscomparedtounmodifiedPETnanofiberswas

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Fig.4.(a)TGAthermogramsofCTRandthreeCDtypes(␣-CD,␤-CDand␥-CD),(b)TGAandderivativeTGA(inset)thermogramsofnanoﬁbers.

especiallyduetotheappearanceofC O*_H_for_the_samples._The

presenceofCDPonthefibersurfaceisquiteimportantintermsof thefiltrationapplicationofPET/CDPnanofibrousmats[4,22,23]. 3.4. Thermalcharacterizationofthenanofibers

ThethermalcharacteristicsofthePET/CDPsampleswere inves-tigatedbyusingthermogravimetricanalyzer(TGA).InFig.4,the TGAthermogramsofCTRandCD(␣-CD,␤-CDand␥-CD)(Fig.4a), andunmodifiedPETandPET/CDPnanofibers(Fig.4b)aregiven. Moreover,thederivativeTGAthermogramsofnanofibersarealso shownasinsetinFig.4b.TheweightlossforCTRstartedataround 130◦C,andCTRcompletelydegradedbefore250◦C.TGA thermo-gramsofCD(␣-CD,␤-CDand␥-CD)presentedaninitialweight lossbelow100◦Candamajorweightlossbetween300and350◦C whichcorrespondtothewaterlossandmaindegradationofCD, respectively[37].ThemaindegradationofPETnanofibersoccurred between375and475◦C. ForthePET/CDPnanofiberstwomajor weightlosseswererecordedbetween200–350◦Cand375–475◦C whichcorrespondtomainthermaldegradationofCDPandPET, respectively. The% weightloss between200and 350◦C corre-spondingtoCDPinthePET/CDPnanofiberswas23%,44%and32% forPET/␣-CDP,PET/␤-CDPandPET/␥-CDPnanofibers,respectively, suggestingthattheamountofCDPcoatingontoPETnanofiberswas ontheorderof␤-CDP>␥-CDP>␣-CDP.Whenthederivativeweight %losswasanalyzed(Fig.4b),itwasobservedthatthepeakpoint fortheunmodifiedPET(∼437◦_C)_shifted_slightly_to_higher

temper-ature(∼445◦_C)_for_the_PET/CDP_nanoﬁbers._This_indicated_that_the

modificationofPETnanofiberswithCDPresultedinslightlyhigher thermalstabilityduetomoreenergyrequirementfor decomposi-tionofthesesampleshavingcrosslinkedstructure.Theincreased thermalstabilityhasbeenalsoobservedforCDgraftedpolyamide 6fabrics[38].Moreover,thecharyieldwashigherforPET/CDP nanofiberswhencomparedtounmodifiedPETnanofiberspossibly owingtothecrosslinkedCDPstructure providinghighercarbon residueuponburning.

Table2

FittingparametersoftheO1sXPSspectraofPETandPET/␥-CDPnanoﬁbers. Samples Fittingpeaks Bonds Peakbinding

energy Arearatio (%) PET O1s#1 C O* _533.12 _56.2 O1s#2 C O* _531.54 _42.1 O1s#3 AdsorbedH2O 534.52 1.7 PET/␥-CDP O1s#1 C O* _533.06 _42.5 O1s#2 C O* _531.5 _27.7 O1s#3 C O*_H _532.35 _29.8

3.5. Mechanicalcharacterizationofthenanoﬁbers

Dynamicmechanicalanalyzer(DMA)wasusedtoinvestigate

theeffectofCDPmodiﬁcationonthethermomechanical

proper-tiesofthePETnanoﬁbers.Thestoragemodulusandlosstangent

(tan_ı)oftheunmodiﬁedPETandPET/CDPnanoﬁbrousmatswere

recordedupto150◦C(Fig.5).Thestoragemodulusofthe

sam-plesdecreasedwithincreasingtemperatureduetothetransition fromglassystatetorubbery state.Itwasobservedthatstorage modulusofthePET/CDPnanofibrousmatswasmuchhigherthan theunmodified PET nanofiberspossibly dueto stiffeningeffect of crosslinkedCDP coating. Sincethe transferredstress forPET nanofiberswas sharedby CDP coating,the storagemodulus of PET nanofibersenhanced withCDP modification. Moreover,for CDPmodifiednanofibers,tanıpeakshiftedtothehigher tempera-tureregionindicatingthattheglasstransitiontemperature(Tg)for

thesenanofiberswashigherwhencomparedtounmodifiedPET nanofibers.TheTgvalueofPETnanofiberswas92◦C,whiletheTg

valuesofPET/CDPnanofiberswererecordedas109,112and113◦C forPET/␣-CDP,PET/␤-CDPandPET/␥-CDPnanofibers,respectively. ThisresultsuggestedthatthemobilizationofPETmacromolecular chainswereaffectedandthesegmentalmotionofPETchainswere hinderedbyCDPmodification.Furthermore,broadertan␦peaks observedforPET/CDPnanofiberswhichcanbeoriginatedfromtwo TgvaluescorrespondtonotonlyPET,butalsoCDP[39].

3.6. Surfaceareaofthenanoﬁbers

Thesurfacearea,averageporediameterandcumulativepore volumeofthePETandPET/CDPnanoﬁberswereinvestigatedby BET measurementsandthedatais summarized inTable3.The resultsindicatedthatthemultipointBETsurfaceareaof electro-spunPETnanoﬁbersis6.03m2_/g._The_surface_area_decreased _to

1.56,0.57and0.72m2_/g_for_PET/_␣-CDP,_PET/_␤-CDP_and_PET/_␥-CDP

nanofibers,respectively.AsmentionedinSEMcharacterization,the surfacemodificationofthePETnanofiberswithCDPresultedinthe irregularitiesonthefibersurfacesandAFDforthesesampleswere

Table3

Surfacearea,averageporediameterandcumulative porevolumedataofthe nanoﬁbers.

Samples MultipointBET surfacearea(m2_/g) Averagepore diameter(nm) DFTcumulative porevolume(cc/g) PET 6.03 15.3 1.03×10−2 PET/␣-CDP 1.56 12.4 3.22×10−3 PET/␤-CDP 0.57 13.6 1.28×10−3 PET/␥-CDP 0.72 14.0 1.05×10−3

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Fig.5.DMAthermogramsofnanoﬁbrousmats(a)storagemodulusand(b)losstangent(tanı).

increasedaswell,andtherefore,thesurfaceareaofthePET/CDP

nanoﬁbersweredecreased.Thesurfaceirregularitiesofnanoﬁbers

suchascrosslinkedCDP coating areclearly observed especially

intheSEMimagesof thePET/␤-CDPandPET/␥-CDPnanoﬁbers

(Fig.2candd).Hence,thesurfaceareaofPET/_␤-CDPandPET/ ␥-CDPnanofiberswaslessthanthatofPET/␣-CDP.Moreover,since PET/␤-CDPhasthelargestAFDamongthesamples(Fig.2c),the sur-faceareaofPET/␤-CDPnanofiberswasslightlylessthanPET/␥-CDP nanofibers.ItiswellknownthattheAFDhavegreateffectonthe surfaceareaoffibers[40].Wehavealsocalculatedthemesopore structure(averageporediameterandcumulativeporevolume)of thePETandPET/CDPnanofibers.Itwasobservedthat,theaverage porediameterandcumulativeporevolumedeterminedbydensity functionaltheory(DFT)alsodecreasedaftersurfacemodificationof thePETnanofiberspossiblyduetothecrosslinkedCDPcoatingonto nanofibersurfacewhichresultedinsurfaceirregularities.Inshort, thesurfaceareaofthePET/CDPnanofiberswasdecreasedduetothe presenceofCDPcoatingcomparedtopristinePETnanofibers, nev-ertheless,asdiscussedinthefollowingsection,PET/CDPnanofibers weremore efficientfor the removalof the phenanthrenefrom aqueoussolutionwhencomparedtopristinePETnanofibers. 3.7. Molecularfiltrationperformanceofthenanofibersforwater purification

ThemolecularﬁltrationcapabilityofPETandPET/CDP nanoﬁ-brous mats has been tested using a phenanthrene as a model polycyclicaromatichydrocarbon(PAH).Phenanthreneisacommon

pollutantandcanforminclusioncomplexeswithCD[12,41,42]. Fig.6summarizesthecumulative%decreaseofphenanthrene con-centrationovertimewhenPETandPET/CDPmatshavebeenkeptin aqueoussolutionofphenanthrene.AsseeninFig.6,the concentra-tionofphenanthreneintheaqueoussolutiondecreasedwithinthe contacttime.TheadsorptionofphenanthrenebyPETnanofibersfor thefirst2hwasobserved,andthentheconcentrationof phenan-threneslightlydecreasedovertime.Ontheotherhand,thedecrease ofphenanthreneconcentrationforPET/CDPmatswasmore sig-nificant.AlthoughlessamountofPET/CDPnanofiberswereused comparedtoPETnanofibersfor filtrationtest, theremoval effi-ciencyofthephenanthrenefromitsaqueoussolutionwasbetter whenPET/CDPnanofiberswereused.Water-insolubleCDPcanbe veryeffectiveinremovalofmanyorganicpollutantsfromaqueous media,sinceCDcavityiscapableofforminginclusioncomplexes withawidevarietyoforganicmolecules[9–11,13,18,22,28]. There-fore,thesurfacemodificationofelectrospunPETnanofiberswith CDPincreasedtheefficiencyoffiltrationbyfacilitatingcomplex formationwithphenanthrenecompounds.Here,allthreePET/CDP nanofibersdemonstratedtheabilitytofunctionasamolecularfilter forwaterpurificationthroughcomplexationofthephenanthrene withCDP.Asit mentioned in theprevioussection itis notable thatthesurfacearea,averageporediameterandcumulativepore volumeofnanofibersweredecreasedafterCDPmodification. How-ever,themolecularfiltrationefficiencywasstillfurtherimproved forPET/CDPnanofiberscomparedtopristinePETnanofibersdue totheCDPstructure ontonanofibers,whichplaysa crucialrole inmolecularcapturingofphenanthrene.WhentheCDtypeswere

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Fig.7.RepresentativeSEMimagesof(a)PET,(b)PET/␣-CDP,(c)PET/␤-CDPand(d)PET/␥-CDPnanofibersafterthefiltrationtest.Theinsetsshowhighermagnification images.

compared,allthreePET/CDPsamplesshowedapproximatelysame filtrationefficiencyfortheremovalofphenanthreneattheendof filtrationtest.AlthoughTGAsuggestedthattheamountofCDP coat-ingontoPETnanofiberswasontheorderof␤-CDP>␥-CDP>␣-CDP, itislikelythatnotalltheCDmoleculesareavailablefor complexa-tion.So,thethreePET/CDPsamplesmayhavecomparableamount ofCDcavityavailableforcomplexation.Evenso,theaverage per-centageremovalofphenanthrenewithrespecttoinitialtimewas slightlybetterforPET/␣-CDPand thisispossiblybecauseofthe highersurfaceareaofPET/␣-CDPnanowebcomparedtoPET/␤-CDP and PET/␥-CDP nanowebs. We have alsoinspected the dimen-sionstabilityofthePETandPET/CDPnanofibers,andweobserved thatthematskepttheirnanofibrousstructureafterthefiltration test(Fig.7).Inshort,thesurfacemodificationofelectrospunPET nanofiberswithCDPenhancedtheefficiencyofitsfiltration perfor-mancebyfacilitatingcomplexformationwithorganiccompounds suchasphenanthrene.

4. Conclusion

In this study, we have achieved thesurface modification of electrospunPETnanofiberswithCDP.First,PETnanofiberswere obtained via electrospinning, then, water-insoluble crosslinked CDPcoatingwasformedontoPETnanofibersbypolymerization reaction betweenCD and crosslinking agent (citric acid).For a

comparativestudy,threedifferenttypesofCD:_␣-CD,_␤-CDand ␥-CDwereusedtoformCDPontoelectrospunPETnanofibers.The imaging analysisbySEMrevealedthat nanofibrousstructureof thePETnanofiberswaspreservedaftersurfacemodificationwith CDP.Yet,thesurfaceofthePET/CDPnanofiberswasrough/irregular, whereasthatofunmodifiedPETnanofiberswassmooth.Moreover thediameterofthePETnanofibersincreasedafterCDPmodification possiblyduetothepresenceofCDPlayerontonanofibersand/or swellingofthenanofibersduringmodificationprocess.BET mea-surementsindicatedthatthesurfaceareaofthePET/CDPnanofibers wasdecreasedduetothepresenceofCDPcoatingcomparedto pris-tinePETnanofibers.ThepresenceofCDPcoatingonthesurfaceof PETnanofiberswassupportedbyXPSanalyses.Thethermal anal-ysisofPET/CDPnanofiberscarriedoutbyTGAshowedtwomain thermaldegradationstepscorrespondingtoCDPandPET degra-dation.ThemodificationofPETnanofiberswithCDPresultedin slightlyhigherthermalstability,andthecharyieldwashigherfor PET/CDPnanofiberscomparedtounmodifiedPETnanofibers.The TGAdataalsoindicatedthattheamountofCDPcoatingontoPET nanofiberswasontheorderof␤-CDP>␥-CDP>␣-CDP.DMAresults elucidatedtheimprovementofmechanicalpropertiesforPET/CDP nanofibers,thatis,PET/CDPnanofibershaveshownhigher stor-agemodulusand higherglasstransitiontemperaturecompared tounmodifiedPETnanofibers.Thefiltrationperformance ofthe CDPsurfacemodifiedPETnanofiberswastestedbyremovalofthe

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polycyclicaromatichydrocarbonwastemolecule(phenanthrene) fromitsaqueoussolution.AlthoughthesurfaceareaofthePET/CDP wereless,weobservedthatPET/CDPnanofibershaveshownbetter filtrationefficiencywhencomparedtothepristinePETnanofibers due to the inclusion complexation capability of CDP onto PET nanofibers.Initially,theaveragepercentageremovalof phenan-threnewithrespecttotimewasslightlybetterforPET/␣-CDP,but attheendoffiltrationtesttheallPET/CDPsamplesshowedmoreor lesssamefiltrationefficiencyfortheremovalofphenanthrenefrom theaqueoussolution.ItwasalsoobservedthatPET/CDPmatshave kepttheirnanofibrousstructureafterthefiltrationtest.Inbrief,our resultsindicatedthatPET/CDPnanofibershaveshownthe poten-tialstobeusedasafilter/membraneforwaterpurificationowing toveryhighsurfaceareaofelectrospunnanofibersand surface associatedCDP,sinceCDmoleculeshaveinclusioncomplexation capabilitywithpolycyclicaromatichydrocarbonsandothertypes oforganicwastemolecules.

Acknowledgements

StatePlanningOrganization(DPT)ofTurkeyisacknowledged forthesupportofUNAM-InstituteofMaterialsScience& Nano-technology.Dr.T.UyaracknowledgesTUBITAK-TheScientiﬁcand Technological Research Council of Turkey for funding project #110M612andEUFP7-PEOPLE-2009-RGMarieCurie-IRGfor fund-ingNANOWEB(PIRG06-GA-2009-256428).F.Kayaciacknowledges TUBITAK-BIDEBforthenationalPh.D.studyscholarship.

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