Use of Polyimide-graft-Bisphenol A Diglyceryl Acrylate as a Reactive Noncovalent Dispersant of ...

13 Pages·2010·4.91 MB·English

6542 Chem.Mater.2010,22,6542–6554
DOI:10.1021/cm101785t
Use of Polyimide-graft-Bisphenol A Diglyceryl Acrylate as a Reactive
Noncovalent Dispersant of Single-Walled Carbon Nanotubes
for Reinforcement of Cyanate Ester/Epoxy Composite
WeiYuan,†JunluoFeng,†ZaherJudeh,†JieDai,‡andMaryB.Chan-Park*,†
†SchoolofChemicalandBiomedicalEngineering,NanyangTechnologicalUniversity,
62NanyangDrive,Singapore637459,Singapore,and ‡DSONationalLaboratories,
20ScienceParkDrive,Singapore118230,Singapore
ReceivedJune26,2010.RevisedManuscriptReceivedNovember9,2010
Dispersion of single-walled carbon nanotubes (SWNTs) into individuals or small bundles and
strongnanotube/matrixinterfacialstrengthremainaschallengesinexploitingtheexcellentmecha-
nicalpropertiesofSWNTsinstructuralcomposites.NoncovalentfunctionalizationofSWNTsisan
attractiveoptionforcompatibilizationofnanotubeswiththematrixasitdoesnotdestroythenano-
tube graphene structure. We have designed and successfully synthesized a new reactive comb-like
polymer, polyimide-graft-bisphenol A diglyceryl acrylate (PI-BDA), which is shown to be highly
effectiveindispersingSWNTsintoindividualnanotubesorsmallbundles,asevidencedbytransmission
electron microscopy (TEM) and UV-vis-NIR absorption spectra. SWNTs dispersed with PI-BDA
were added to a thermosetting resin blend of cyanate ester and epoxy (CE-EP) (70/30, w/w) and
compositefiberswithSWNTloadingrangingfrom0to1.5wt%weresuccessfullyfabricated.The
strongπ-πinteractionbetweenSWNTsandPI-BDAbackbonewasevidencedbyRamanspectra,
and the covalent reaction between PI-BDA side-chain and matrix was confirmed by FT-IR and
1HNMR.Ourdesigneddispersantinteractsnoncovalentlywithnanotubesbutreactswiththethermo-
settingmatrix. With addition ofPI-BDA dispersant, only 1wt %SWNTsoptimally increasesthe
tensilemodulus,strength,andtoughnessby80%(to4.70(0.24GPa),70%(to142.3(6.9MPa),
and58%(4.1(0.4MJm-3)comparedtotheneatresinblend(withE=2.61(0.14GPa,σ=83.7(
3.3MPa,andT=2.6(0.2MJm-3).Improvementinthermalpropertieswasalsoobserved.
Introduction have generally been far inferior than theoretically pre-
dicted.6-9Forexample,Zhuetal.6reported30%increase
Carbon nanotubes (CNTs)are thoughtto be an ideal
in Young’s modulus (from 2.03 to 2.63 GPa) and 14%
materialforreinforcementinultrastronglightweightpolymer
increase in tensile strength (from 83.2 to 95.0 MPa) for
composites due to their low mass density, large aspect
epoxy composites reinforced with 1 wt % fluorinated
ratio (typically ca. 300-1000), and superior mechanical
single-walledcarbonnanotubes(SWNTs).Recently,our
properties.1,2Outstandingmechanicalproperties(specifi-
groupreported85%and17%increasesintensilestrength
cally,tensilestrengthof500-2000MPaandmodulusof
and modulus, respectively, for cyanate ester composites
15-169GPa)ofCNT/polymercompositeshaverecently
reinforcedwith1wt%SWNTs10and39%and18%,re-
been reported, though achieved with special nanotubes
spectively,forepoxycompositeswith0.5wt%SWNTs.11
and/ornonconventionalprocessing techniques.3-5 With
ThemodestimprovementswithCNTsarestillbeneficial
conventionalcompositeprocessingtechniquesandcommon
forreinforcingthermosettingresinsforadhesivesoreven
CNTs, the properties of resultant CNT-reinforced com- carbon fiber reinforced hierarchical composites12-14 in
posites,especiallyusingcommonthermosettingmatrices,
whichthesmallsizeofCNTsmakesthenanotube-reinforced
*Correspondingauthore-mail:[email protected].
(1) Coleman,J.N.;Khan,U.;Gun0ko,Y.K.Adv.Mater.2006,18, (8) Chen,Z.K.;Yang,J.P.;Ni,Q.Q.;Fu,S.Y.;Huang,Y.G.Polymer
689. 2009,50,4753.
(2) Moniruzzaman,M.;Winey,K.I.Macromolecules2006,39,5194. (9) Chen,D.;Liu,T.X.;Zhou,X.P.;Tjiu,W.C.;Hou,H.Q.J.Phys.
(3) Ma,W.J.;Liu,L.Q.;Zhang,Z.;Yang,R.;Liu,G.;Zhang,T.H.; Chem.B2009,113,9741.
An,X.F.;Yi,X.S.;Ren,Y.;Niu,Z.Q.;Li,J.Z.;Dong,H.B.; (10) Che,J.F.;Chan-Park,M.B.Adv.Funct.Mater.2008,18,888.
Zhou,W.Y.;Ajayan,P.M.;Xie,S.S.NanoLett.2009,9,2855. (11) Che,J.;Yuan,W.;Jiang,G.;Dai,J.;Lim,S.Y.;Chan-Park,M.B.
(4) Shim,B.S.;Zhu,J.;Jan,E.;Critchley,K.;Ho,S.S.;Podsiadlo,P.; Chem.Mater.2009,21,1471.
Sun,K.;Kotov,N.A.AcsNano2009,3,1711. (12) Qian,H.;Greenhalgh,E.S.;Shaffer,M.S.P.;Bismarck,A.J.Mater.
(5) Cheng,Q.F.;Bao,J.W.;Park,J.;Liang,Z.Y.;Zhang,C.;Wang, Chem.2010,20,4751.
B.Adv.Funct.Mater.2009,19,3219. (13) Qian,H.;Bismarck,A.;Greenhalgh,E.S.;Kalinka,G.;Shaffer,
(6) Zhu,J.;Kim,J.;Peng,H.;Margrave,J.L.;Khabashesku,V.N.; M.S.P.Chem.Mater.2008,20,1862.
Barrera,E.V.NanoLett.2003,3,1107. (14) Bekyarova,E.;Thostenson,E.T.;Yu,A.;Kim,H.;Gao,J.;Tang,
(7) Sun,L.;Warren,G.L.;O’Reilly,J.Y.;Everett,W.N.;Lee,S.M.; J.;Hahn,H.T.;Chou,T.W.;Itkis,M.E.;Haddon,R.C.Langmuir
Davis,D.;Lagoudas,D.;Sue,H.J.Carbon2008,46,320. 2007,23,3970.
pubs.acs.org/cm PublishedonWeb12/02/2010 r2010AmericanChemicalSociety
Article Chem.Mater.,Vol.22,No.24,2010 6543
matrix(alsocalledthehybridmatrix)processablelikethe thiophene,25,26 polybenzimidazole,27 and polyimide,28,29
neatresin.Themechanicalpropertiesof“hybridmatrix” which interact with CNTs via strong π-π interaction,
areimprovedbecauseCNTshavestrength(10-63GPa)15,16 havebeenusedtodisperseCNTsandreinforcepolymer
far superior to most thermosetting matrices, and even matrixinpreviousstudies.Theπ-πinteractionhasbeen
carbonfibers(∼250MPa).17Further,thenanoscalesize suggestedtobethestrongestnoncovalentinteractionbya
ofCNTsenablesthemtobeappliedasreinforcementsin recenttheoreticalmodelingstudy.30However,thesestudies
low-dimensional(e.g.,2-D)structures,e.g.,polymerfibers, mainly focus on thermoplastic matrices and only a few
foams,andfilms,4,10,18whereotherconventionalmicroscale studiesusingnoncovalentlyfunctionalizedCNTstoreinforce
fillerswouldbetoolargeforinclusion.CNTscanalsobe thermosettingmatriceshavebeenreported.31
used to produce multifunctional structural composites Cyanateester(CE)resinsarehigh-performancestruc-
withuniquethermal,electrical,andopticalproperties.2 turalthermosetswidelyusedintheelectronicsandaerospace
Thegeneralpoorimprovementinthemechanicalpro- industries due to their outstanding adhesive, thermal,
pertiesofCNT/polymercompositecanbepartiallyattrib- mechanical, and electrical properties.32 A drawback of
uted to the poor nanotube dispersion and nanotube/ CEisitsbrittlenessresultingfromthehighlycross-linked
matrixstresstransfer.DuetostrongvanderWaalsforces structurewhichoftenrestrictsitsstructuralapplications.
betweenthenanotubes,CNTsareusuallybundledwhich CopolymerizingCEwithotherthermosettingresinssuch
wouldresultinintertubeslippagewithappliedstressand asepoxy(EP)hasbeenproventobeeffectiveintoughen-
poormechanicalpropertiesofCNTcomposites.Further, ing CE with enhanced processability and at low cost.33
thegraphenestructureofCNTsisatomicallysmoothand Thisthermostable,processable,andrelativelytoughcyanate
highlyhydrophobicsothatstresstransfertoatypicalpoly- ester/epoxyblendisafavorablematrixwidelyusedinelec-
mercompositematrix,whichisusuallyrelativelypolar,is tronicandaerospaceapplications.Additionofinorganic
poor.Toexploitthehighmechanicalpropertiesofnano- nanofillershasbeenconfirmedtobeaneffectiveapproach
tubesincomposites,thenanotubeshavetobewell-dispersed for toughening thermosets without compromising their
andthenanotube/matrixinterfacehastobestrong. thermalproperties.10,34
Inmostofthesolution-processedcompositestudies,the Thispaperreportsourdesignandsynthesisofareactive
CNTswerecovalentlyfunctionalized.6,19,20TheSWNTsused noncovalent polymeric dispersant for SWNTs: polyimide-
in both our two studies10,11 were covalently functionalized
graft-bisphenolAdiglycerylacrylate(PI-BDA)(Scheme1).
viagraftingoforganicmoleculesandseemtobebundled.
Thepolyimidebackbonecanstronglyinteractwiththenano-
It appears that covalent functionalization6,10,11,19,20 can
tube via noncovalent interaction, while the side chain
only moderately improve the mechanical properties of
facilitatesdispersionandthehydroxylgrouponthegraft
composites.Thehighdensityoffunctionalizationneeded
enables reaction with the matrix. The high efficacy of
for effective stress transfer disrupts the extended π con-
PI-BDAatdispersingSWNTsindimethylformamide(DMF)
jugation in nanotubes, leading to decreased mechanical
wasassessedwithtransmissionelectronmicroscopy(TEM)
properties.Moreover,covalentfunctionalizationistypi- and UV-vis-NIR absorption spectra. PI-BDA was
cally not effective for individually dispersing high con-
usedtodisperseSWNTsinathermosettingresinblendof
centration of CNTs. On the contrary, noncovalent
cyanateesterandepoxy(CE-EP)(70/30,w/w)(Scheme2).
functionalization using surfactant or polymer has been
TwoseriesofSWNTcomposites,onewithPI-BDAdispersed-
shown to be effective in individually dispersing nano-
SWNTsandonewithout,withnanotubecontentranging
tubes without compromising the nanotube graphene
from 0 to 1.5 wt % were fabricated. For convenience,
structure.21-23 Conjugated and aromatic condensation
SWNT/resin and SWNT/PI-BDA/resin composites are
polymers, such as poly(phenylene ethynylenes),24 poly-
hereafterdenotedasSWNTs/RandSWNTs/PI-BDA/R
respectively. The nanotube dispersion and nanotube/
(15) Yu,M.F.;Lourie,O.;Dyer,M.J.;Moloni,K.;Kelly,T.F.;Ruoff, matrixinteractionwereexaminedbyfield-emissionscan-
R.S.Science2000,287,637.
(16) Yu,M.F.;Files,B.S.;Arepalli,S.;Ruoff,R.S.Phys.Rev.Lett. ning electron microscopy (FE-SEM), Raman spectra,
2000,84,5552. FT-IR,and1HNMR.Thetensileproperties,glasstransi-
(17) Lubin,G.,Ed.HandbookofComposites;VanNostrandReinhold:
NewYork,1982;p234. tion temperature (Tg), and thermal decomposition tem-
(18) Jell,G.;Verdejo,R.;Safinia,L.;Shaffer,M.S.P.;Stevens,M.M.; perature(T )ofthecuredcompositesweremeasuredasa
Bismarck,A.J.Mater.Chem.2008,18,1865. d
(19) Zhou,C.J.;Wang,S.F.;Zhang,Y.;Zhuang,Q.X.;Han,Z.W. functionofSWNTcontent.
Polymer2008,49,2520.
(20) Gao, J. B.; Itkis, M. E.; Yu, A. P.; Bekyarova, E.; Zhao, B.;
Haddon,R.C.J.Am.Chem.Soc.2005,127,3847. (28) Delozier, D. M.; Watson, K. A.; Smith, J. G.; Clancy, T. C.;
(21) Richard,C.;Balavoine,F.;Schultz,P.;Ebbesen,T.W.;Mioskowski, Connell,J.W.Macromolecules2006,39,1731.
C.Science2003,300,775. (29) Li,N.W.;Zhang,F.;Wang,J.H.;Li,S.H.;Zhang,S.B.Polymer
(22) Yan,L.Y.;Poon,Y.F.;Chan-Park,M.B.;Chen,Y.;Zhang,Q. 2009,50,3600.
J.Phys.Chem.C2008,112,7579. (30) Yang,M.;Koutsos,V.;Zaiser,M.J.Phys.Chem.B2005,109,
(23) Zou,J.H.;Liu,L.W.;Chen,H.;Khondaker,S.I.;McCullough, 10009.
R.D.;Huo,Q.;Zhai,L.Adv.Mater.2008,20,2055. (31) Gong,X.Y.;Liu,J.;Baskaran,S.;Voise,R.D.;Young,J.S.Chem.
(24) Chen, J.; Ramasubramaniam, R.; Xue, C.; Liu, H. Adv. Funct. Mater.2000,12,1049.
Mater.2006,16,114. (32) Laskoski,M.;Dominguez,D.D.;Keller,T.M.J.Polym.Sci.,PartA:
(25) Kim,K.H.;Jo,W.H.Macromolecules2007,40,3708. Polym.Chem.2006,44,4559.
(26) Kim,K.H.;Jo,W.H.Carbon2009,47,1126. (33) Liang,G.Z.;Zhang,M.X.J.Appl.Polym.Sci.2002,85,2377.
(27) Okamoto, M.; Fujigaya,T.;Nakashima,N.Adv. Funct.Mater. (34) Pan, Y.; Xu, Y.; An, L.; Lu, H.; Yang, Y.; Chen, W.; Nutt, S.
2008,18,1776. Macromolecules2008,41,9245.
6544 Chem.Mater.,Vol.22,No.24,2010 Yuanetal.
Scheme 1. SynthesisofHydroxylPolyimide(PI)and waspurchasedfromWuxiResinFactory(China)withthetrade
Polyimide-graft-bisphenolADiglycerylAcrylate(PI-BDA) nameE20.
SWNTswerepurchasedfromChengduResearchInstituteof
OrganicChemistry(China);theywereproducedbythechemical
vapordeposition(CVD)methodandhavediametersof1-2nm,
lengthsof5-30μm,andpurityof∼90%.Theywerepurified
withthermaloxidation(350(cid:1)Cfor2hinair)followedbyacid
treatment(refluxingin6MHClsolutionfor12h)beforeuse.
SynthesisofHydroxylPolyimide(PI).Thereactionisshown
inSteps1-2inScheme1.Inatypicalexperiment,HAB(1.080g,
5.00mmol)wasdissolvedin40mLoffreshlydistilledDMAcin
around-bottomflaskunderargonprotection.Afterthesolution
wascooledat0(cid:1)Cfor15min,ODPA(1.550g,5.00mmol)was
addedtothesolutionwithvigorousstirring.Themixturewas
thenwarmedtoroomtemperatureandmagneticallystirredfor
24 h under argon atmosphere to form poly(amic acid) (PAA)
solution. Then dryxylene(40 mL) wasaddedto thissolution
andstirredat160(cid:1)Cfor3htoeliminatethewaterformedinthe
imidization reaction. After cooling to room temperature, the
reactionmixturewasaddeddropwiseintoalargeexcessofmethanol
toprecipitatethePI.Theprecipitatewasfilteredandrepeatedly
washedwithalargeamountofmethanolandthenwithTHF.
Theseparatedprecipitatewasdriedat80(cid:1)Cundervacuumfor
24h.Theyieldwas2.040g(83%yield).
SynthesisofPolyimide-graft-BisphenolADiglycerylAcrylate
(PI-BDA).ThereactionisshowninStep3inScheme1.DMAP
was added as the catalyst35 and BHT was added to prevent
homopolymerizationofacrylatedoublebondontheBDAside
chain.36Typically, PI(0.588g, 1.20mmol ofrepeatunit)was
dissolvedin40mLofdryDMSOat60(cid:1)Cinaround-bottom
flask with a water condenser under argon atmosphere. After
dissolutionofDMAP(0.293g,2.40mmol),asolutionofGBA
(1.188g,2.64mmol)togetherwithBHT(0.018g,0.08mmol)in
20mLofdryDMSOwasadded,andtheresultingmixturewas
stirredat100(cid:1)Cfor48h.AfterremovalofsomeDMSOwitha
rotary evaporator, the mixture was added dropwise into bulk
methanolwithvigorousstirring.Theprecipitatewasfilteredand
washed several times with 0.2 M HCl solution, then with 5%
NaHCO solution,andfinallywithwater.Theside-chaingrafted
3
polyimide(PI-BDA)productwasdriedundervacuumatroom
temperaturefor48h.Theyieldwas0.850g(51%yield).
PreparationofSWNTDispersions.ToprepareSWNT/PI-BDA
dispersion,10mgofpurifiedSWNTsand10mgofPI-BDAwere
firstaddedinto10mLofDMF.Thenthemixturewassonicated
withatipsonicatorfor10minfollowedbyfurthersonicationin
asonicatorbathfor30min.Theresultingstableandhomogene-
ous solution had a SWNT concentration of 1 mg/mL. Pristine
ExperimentalSection
SWNTsuspensioninDMFwaspreparedbythesameprocedure
Materials. 3,30-Dihydroxy-4,40-diaminobiphenyl (HAB, 97%) without the addition of PI-BDA. For Raman characterization,
waspurchasedfromTokyoChemicalIndustry.R-Glycidyltermi- theSWNT/PI-BDAsolutionwasfilteredandwashedthoroughly
withDMFtoremoveanyfreesurfactant,andtheresultantpowder
natedbisphenolAacrylate(GBA,Scheme2c) withamolecular
wasdried.
weightof450wassuppliedasEbecryl3605fromUCBchemicals
(Malaysia).Itwasfreeze-driedat-55(cid:1)Cfor2daysbeforeuse. EvaluationofSWNTDispersionStabilitybyVisualObservation.
4,40-Oxydiphthalicanhydride(ODPA,97%),4-(dimethylamino) PristineSWNTsandSWNT/PI-BDAsolutionswithSWNTcon-
centrationof1mg/mLwereprepared.Toclearlyobservewhether
pyridine(DMAP,99%),butylatedhydroxytoluene(BHT,99%),
sodium hydrogencarbonate(NaHCO ,99.5%), N,N0-dimethyl- thereareSWNTaggregatesinthesolution,partsofthesetwo
3
acetamide (DMAc), xylene, dimethyl sulfoxide (DMSO), N,N0- solutionswerealsodilutedby50timestoaffordsolutionswith
lownanotubeconcentration(0.02mg/mL).Allthesesolutions
dimethylformamide (DMF), and methanol were obtained from
Sigma-Aldrich. DMAc and DMSO were distilled over calcium (beforeandafterdilution)wereleftstandingfordifferenttimes
hydrideandxyleneoversodiumwirebeforeuse.Allotherchemicals
were used as received. Bisphenol A cyanate ester resin (CE, (35) vanDijk-Wolthuis,W.N.E.;Franssen,O.;Talsma,H.;vanSteenbergen,
Scheme2a)waspurchasedfromShanghaiHuifengTechnical& M.J.;Kettenes-vandenBosch,J.J.;Hennink,W.E.Macromolecules
1995,28,6317.
BusinessCo.,Ltd.(Shanghai,China)withthetradenameHF-1.
(36) Bahar,Y.;AylinZiylan,A.;NihanCelebi,O.;Duygu,A.J.Polym.
Diglycidyl ether bisphenol A epoxy resin (EP, Scheme 2b), Sci.,PartA:Polym.Chem.2008,46,2290.
Article Chem.Mater.,Vol.22,No.24,2010 6545
Scheme2. Structuresof(a)BisphenolACyanateEster(CE),(b)DiglycidylEtherBisphenolAEpoxy(EP),
and(c)r-GlycidylTerminatedBisphenolAAcrylate(GBA)
(immediately,7days,and6months)andthehomogeneitywas temperatures(T )ofneatCE-EPanditscompositesweredeter-
g
evaluatedbyvisualobservation. minedfromdifferentialscanningcalorimetry(DSC)performed
Fabrication of Fibers. SWNT/PI-BDA solution (1 mg onaMettlerToledoDSC822einstrumentunderN ataheating/
2
SWNTs/mL)wasfirstpreparedviathemethoddescribedabove. coolingrateof20(cid:1)C/minfrom50to230(cid:1)C.TheT wascalculated
g
CE-EPmatrixwaspreparedbymixingCE(70wt%)andEP fromthemidpointofthechangeinslopeonthesecondheating
(30wt%)at110(cid:1)Cfor30min.A2-gportionofCE-EPmatrix run.RamanspectrawereobtainedwithaRenishawinViaRaman
wasaddedtomeasuredquantitiesofSWNT/PI-BDAdispersion microscopewithHeNelaserexcitationwavelengthof633nm.
toproducesolutionsofSWNTs/PI-BDA/RwithdifferentSWNT UV-vis-NIRabsorptionspectraofthenanotubesolutions
loadings. After sonication in a sonicator bath for 10 min, the were recorded ona VarianCary 5000UV-vis-NIR spectro-
solution was cast onto glass substrates, which were placed on photometer. Optical microscopy characterization of SWNT-
ahotplateatabout50(cid:1)Cfor1htoslowlyremovemostofthe (0.2wt%)/RandSWNT(0.2wt%)/PI-BDA/Rcompositeswas
DMF.Theglasssubstrateswerethentransferredtoavacuum carriedoutonanOlympusSZX12microscopeatamagnifica-
ovenanddriedundervacuumat90(cid:1)Cfor1hand100(cid:1)Cfor2h. tion of 144(cid:2). Transmission electron microscopy (TEM) mea-
Subsequently,theblendwascollectedfromtheglasssubstrates surements were carried outona JEOL2100F high-resolution
andthebubbleswereremovedusingavacuumoven.Theblend scanningelectronmicroscopeoperatingat200kV.Pristineor
wasthenusedtofabricateSWNT/PI-BDA/Rcompositefibers PI-BDAfunctionalizedSWNTsdispersedinDMFwasdiluted
using our custom-made reactive spinning device.10,11 After anddropcastontoacarbon-coatedcoppergridfollowedbysolvent
spinning, the fibers were precured for 6 h under a UV lamp evaporation.ThedispersionofSWNTsincompositefiberswas
(intensity of30mW/cm2,withon/offcycleof15min/15 min) examined with a JEOL JSM-6700F field-emission scanning
whichwasfilteredwithadishofwater,andthenfurtherthermal electron microscope (FE-SEM). Uncured SWNT(1 wt %)/R
cured at 100 (cid:1)C/1 d under vacuum followed by 120 (cid:1)C/2 h, andSWNT(1wt%)/PI-BDA/Rspunfiberswereredispersedin
150(cid:1)C/2h,180(cid:1)C/2h,andpostcuringof200(cid:1)C/4hatatmospheric DMFwithonlymildshakingfollowedbyfiltrationthrougha
pressure. Neat CE-EP fibers and pristine SWNTs-reinforced 0.2-μmAl O membraneandwashingseveraltimeswithlarge
2 3
fibers(SWNTs/R)werealsopreparedbysimilarmethod.Detailsof quantityofDMFtoremoveanyfreepolymer.Thesesamples
fiber fabrication can be found in the Supporting Information werecoatedwithgoldandexaminedwithFE-SEM.Thefracture
Part1. surfacesofcompositesaftertensiletestingwerealsocoatedwith
Verifying the Reaction between PI-BDA and CE. To verify, goldforFE-SEMobservation.TensilepropertiesofneatCE-EP
with FT-IR, the addition reaction between -OH on PI-BDA andcompositefibersweredeterminedusinganInstronmodel
withtheCE,PI-BDA/CEblend(30/70,w/w)wasfirstdissolved 5543mechanicaltesteratroomtemperature.Thegaugelength
inDMF,thenseveraldropswerecastontoapotassiumbromide was25mmandthe diameterwasestimated byoptical micro-
(KBr)pellet.TheDMFwasremovedundervacuumat80(cid:1)Cfor scopy.A10Nloadcellandacrossheadspeedof2.54mm/min
2 h. Then the sample was characterized by FT-IR before and wereusedtodothetesting.Atleast10samplesweretestedand
afterheatingat120(cid:1)Cfor1h.Toverifythereactionusing1H theresultswereaveraged.
NMR,PI-BDA/CEblend(30/70,w/w)wasdissolvedindeuterated
dimethylsulfoxide (DMSO-d6), and 1H NMR spectra of the ResultsandDiscussion
samplewerecollectedbeforeandafterheatingat120(cid:1)Cfor1h.
Characterization.1HNMRspectraofPI,PI-BDA,andPI-BDA/ SynthesisandCharacterizationofPIandPI-BDA.The
CE(30/70,w/w)blendwererecordedonaBruker(300MHz)NMR structures of the synthesized PI and PI-BDA were con-
instrumentusingDMSO-d assolvent.Fouriertransforminfrared firmed by 1H NMR spectroscopy. The complete assign-
6
(FT-IR)spectraofPI-BDA/CE(30/70,w/w)blendbeforeandafter mentoftheprotonsignalsisshowninFigure1.Inthe1H
heatingat120(cid:1)Cfor1hwererecordedonaNicolet5700FT-IR NMRspectrumofPI(Figure1A),characteristicpeaksof
instrument.ThemolecularweightofPAAandPI-BDAwasana- the phenolic OH and aromatic protons are shown at δ
lyzed by gel permeation chromatography (GPC) carried out on 10.1(a) and 7.1-8.2 (b-g) ppm, respectively. In the 1H
aShimadzuLC-20ASeriesGPCsystemequippedwithapump,BC-
NMRspectrumofPI-BDA(Figure1B),thephenolicOH
PLgelmixedcolumns,andarefractiveindexdetector(RID-10A).
signalscompletelydisappearedwhilethearomaticprotons
DMFwith0.02MLiBrwasusedastheelutingsolventandpoly-
of the PI backbone appeared at δ 7.0-8.2 ppm (b-g).
styrene standards were used as the reference. Thermogravimetric
analysis(TGA)ofneatCE-EPanditscompositeswasperformedon Newpeaksatδ6.7ppm(k)andδ7.0ppm(l)areattrib-
aNetzschSTA409PG/PCinstrumentunderanitrogenatmosphere, uted to the aromatic protons of the grafted BDA side
withaheatingrateof10(cid:1)C/minfrom50to800(cid:1)C.Glass-transition chain.Thepeaksatδ5.8(s,CH ),6.1(q,CH),6.2ppm
2
6546 Chem.Mater.,Vol.22,No.24,2010 Yuanetal.
Figure1. 1HNMRspectraof(A)PIand(B)PI-BDA.
(r,CH )areattributedtothe-OCOCHdCH terminal dilutedSWNT/PI-BDAsolutionappearsfreefromSWNT
2 2
group.Thepeakatδ1.5ppm(m)isattributedto-CH aggregates(Figure2A,viald).After7days,pristineSWNTs
3
groups,whilethepeaksatδ3.0-4.5ppmareassignedto completelysettledatthebottomofthevials(Figure2B,
thealiphaticprotonsof-OCH(iando)and-OCH (h,j, vials a and b). The SWNT/PI-BDA solutions were very
2
nandp)inBDAsidechain.Thecompletedisappearance stableanddidnotprecipitateafter7days(Figure2B,vials
ofthepeakatδ10.1ppm(a)andtheappearanceofnew c and d), or even after six months of standing at room
resonancepeaks(h-s)confirmthatBDAis100%grafted temperature. These phenomena indicate that PI-BDA
oneverypendant-OHofPI-BDA.Themolecularweights significantlyimprovesthesolubilityofSWNTsinDMF
(M )ofpolyamicacid(PAA)andPI-BDAweremeasured andprovideslong-termdispersionstability.
n
by GPC. Compared to PAA (M =2.7(cid:2)104 g/mol), The dispersion of pristine SWNTs and PI-BDA func-
n
PI-BDA has expectedly higher molecular weight (M = tionalizedSWNTsinDMFwasalsoexaminedbyTEM
n
3.9(cid:2)104g/mol).ThepolydispersityindicesofPAAand (Figure3).AsshowninFigure3A1andA2,pristineSWNTs
PI-BDAwere1.38and1.35,respectively. arehighlyentangledwitheachotherandthebundlesize
SolubilityofPI-BDAFunctionalizedSWNTs.Thedis- (about30nmorlarger)isnotuniform.Figure3A3reveals
persionstabilityofSWNTsolutionspreparedinDMFwith thattheSWNTsurfaceisveryclean.Afterfunctionaliza-
andwithoutPI-BDAwasevaluatedbyvisualobservation tionwithPI-BDA,thebundlesizebecomesmuchsmaller
at different standing times after sonication (Figure 2). (Figure3B1),andmostoftheSWNTsappeartobedispersed
Immediatelyaftersonication,highSWNTconcentration as individual tubes or very small bundles (Figure 3B2).
solutions(1mg/mL)ofbothpristineSWNTsandSWNTs/ AhigherresolutionTEMimage(Figure3B3)revealsclearly
PI-BDAareblack(Figure2Avialsaandc),andwecannot thattheSWNTsarecoveredwithalayerofcoating,whichis
tellwhethertherearenanotubeaggregatesinthesesolu- believedtobePI-BDAwrappedontheSWNTsurface.It
tionsduetothedarkness.Afterdilutionto0.02mg/mL, shouldbenotedthatthethicknessofthePI-BDAlayeris
suspended nanotube aggregates can be clearly seen in notuniform.AlowermagnificationTEMimage(Supporting
pristineSWNTsolution(Figure2A,vialb),whereasthe Information, Figure S5) shows that the SWNT length
Article Chem.Mater.,Vol.22,No.24,2010 6547
dispersions have suspended SWNT concentrations of
about0.02mg/mLforeaseofcomparisonamongvarious
PI-BDA to SWNT ratios. After standing for 3 days or
centrifugationat6000rpmfor1h,thereisacleardepen-
denceofthePI-BDAtoSWNTsmassratioontheamount
ofdispersedSWNTs.TheSWNTconcentrationincreases
firstbutlevelsoffwhenthePI-BDAtoSWNTsmassratio
reaches1:1,suggestingthat1:1istheoptimalmassratio
for preparing SWNT/PI-BDA dispersion. The SWNT
concentration of SWNT/PI-BDA (mass ratio 1:1) after
centrifugation is about 0.0158 mg/mL, which is higher
than that of polyvinylpyrrolidone (PVP, M =29000,
w
Sigma-Aldrich)dispersedSWNTspreparedbythesame
method (0.0105 mg/mL), indicating higher efficacy of
PI-BDA atdispersing SWNTsthan the commercial dis-
persantPVP.39
SWNTDispersionandInterfacialBondinginComposites.
Figure5showstheFE-SEMimagesofSWNTsinuncured
SWNT(1wt%)/RandSWNT(1wt%)/PI-BDA/Rspun
fibers after removal of free polymer on a 0.2-μm Al O
Figure2. Visualobservationof(aandb)pristineSWNTand(candd) 2 3
SWNTs/PI-BDAinDMFatdifferentstandingtimesaftersonication: membrane. Pristine SWNTs form large aggregates
(A)immediately;(B)7days.TheSWNTconcentrationofaandcis1mg/mL (Figure 5A1) while PI-BDA functionalized SWNTs are
andofbanddis0.02mg/mL.
homogeneouslydispersed(Figure5B1).Highermagnifi-
cation images reveal that SWNTs in SWNT/PI-BDA
(at least 1-2 μm) is not significantly reduced after non-
were debundled very well and the nanotube bundle size
covalentfunctionalizationwithPI-BDA,whichisdifferent
(Figure5B2)ismuchsmallerthanthatofpristineSWNTs
from the observation of chopped CNTs (length usually
(Figure5A2).Theseobservationsareconsistentwiththe
lessthan1μm)aftercovalentmodification.37,38Thehigh
dispersion state of pristine and PI-BDA functionalized
efficacyofPI-BDAatdispersingSWNTsintoindividual
SWNTs in the fully cured composite fibers that will be
nanotubesorsmallbundleswasalsoverifiedbyUV-vis-
discussedbelow.
NIR (Supporting Information, Figure S6). We postu-
Figure6showsopticalmicrographsoffullycuredSWNT/R
late that the high efficiency of PI-BDA at dispersing
andSWNT/PI-BDA/RcompositefiberswithSWNTload-
SWNTs is attributed to its comb-like structure, where
ingof0.2wt%.(CompositeswithhigherSWNTloadings
thePIbackbonehasstrongπ-πinteractionwithSWNTs areopaque.)Manyblackspotswithsizesupto10μmcanbe
wallandtheBDAgraftimpartsstrongrepulsiveforcesto
clearlyseenintheSWNT(0.2wt%)/Rfiber(Figure6A),
SWNTsviasterichindrance.
indicating nonuniform dispersion of SWNTs. In contrast,
The efficacy of PI-BDA at dispersing SWNTs was
SWNT (0.2 wt %)/PI-BDA/R shows homogeneous dis-
quantitatively evaluated through absorption spectra mea-
persionofSWNTsthroughoutthematrix,andnoobvious
surementandBeer-Lambertlaw(A=εlc),whereAisthe
aggregateswereobserved(Figure6B).
absorbance at a particular wavelength (here we chose
To further evaluate the dispersion and morphology
500nm),εistheextinctioncoefficient,listhepathlength
ofSWNTsincompositefibers,crosssectionsofSWNT-
(1cmforourcell),andcistheconcentration.Todeter-
(1 wt %)/R and SWNT(1 wt %)/PI-BDA/R composite
minetheextinctioncoefficient(ε),theabsorbanceofvery
fibers after tensile testing were examined with FE-SEM
diluteSWNT/PI-BDA(massratio1:1)solutionatdiffer-
(Figure7).InSWNT(1wt%)/Rcompositefibers,aggre-
ent concentrations was measured. Figure 4A shows a gateswithsizesofabout1μmcanbeclearlyseen,indicating
representativecurveandtheabsorbanceat500nmplotted
nonuniformdispersion(Figure7A1,circled).ManySWNTs
against SWNT concentrations (inset in Figure 4A). The
arepulled out, leaving holes on the surface (Figure7A2
linear-least-squaresfittothedatagaveaslopeof37.60,
and A3, arrows), indicating weak interfacial adhesion
whichwasusedtocalculatetheextinctioncoefficient(i.e.,
betweentheSWNTsandtheCE-EPmatrix.Incontrast,
ε=37.60 mL mg-1 cm-1) for determination of SWNT
PI-BDAfunctionalizedSWNTsarehomogeneouslydis-
concentration. Figure 4B shows the extracted SWNT
tributedintheCE-EPmatrixwithoutanylargeaggregates
concentrations of SWNT/PI-BDA dispersions prepared
(Figure7B1).Somenanotubesseemtohavepartiallypulled
usingdifferentPI-BDAtoSWNTmassratiofrom0.125:1to
outfromthesurfacebutthepull-outlengthoftheSWNTs
2:1indifferentconditions:(1)immediatelyaftersonication;
issignificantlyreduced(indicatedbysquaresinFigure7B2
(2)afterstandingfor3days,and(3)aftercentrifugation
and B3) compared with the nanotubes in SWNTs/R;
at6000rpmfor1h.Immediatelyaftersonication,allthe
othernanotubesarebrokenonthesurfaceandtheends
(37) Forrest,G.A.;Alexander,A.J.J.Phys.Chem.C2007,111,10792.
(38) Yuen,S.M.;Ma,C.C.M.;Chiang,C.L.;Lin,Y.Y.;Teng,C.C. (39) Hasan,T.;Scardaci,V.;Tan,P.H.;Rozhin,A.G.;Milne,W.I.;
J.Polym.Sci.,PartA:Polym.Chem.2007,45,3349. Ferrari,A.C.J.Phys.Chem.C2007,111,12594.
6548 Chem.Mater.,Vol.22,No.24,2010 Yuanetal.
Figure3. TEMimagesof(A1-A3)pristineSWNTsand(B1-B3)PI-BDAfunctionalizedSWNTs.
Figure4. (A)AbsorptionspectrumofSWNTs/PI-BDA(massratio1:1)ataconcentrationof0.0125mg/mLinDMF.Insetshowsabsorbanceat500nm
oftheSWNTs/PI-BDA(1:1)inDMFatdifferentconcentrations.Thestraightlineisalinear-least-squaresfittothedata.(B)SWNTconcentrationsof
solutionspreparedusingdifferentPI-BDAtoSWNTmassratioindifferentconditions:immediatelyaftersonication;afterstandingfor3days,andafter
centrifugationat6000rpmfor1h.
aretightlyembeddedinthematrix(indicatedbycirclesin inthepeakintensityofD-bandwasobservedinSWNTs/
Figure7B2andB3).TheseFE-SEMimagessuggestthat PI-BDA(b)andSWNT(1wt%)/PI-BDA/R(d),indicat-
the nanotube/matrix interface between PI-BDA function- ing that the nanotube graphene structure was well pre-
alizedSWNTsandtheCE-EPmatrixisstrongerthanthat servedafternoncovalentfunctionalizationwithPI-BDA.
in SWNT/R composite. The strong interfacial bonding ComparingtheGbandofSWNTs/PI-BDA(b),SWNTs-
canbeascribedtotwoeffects:(i)strongπ-πinteraction (1 wt %)/R (c), and SWNTs(1 wt %)/PI-BDA/R (d) to
between nanotube and the backbone of PI-BDA and thepristineSWNTssample(a),upshiftsofabout5,3,and
(ii) the compatibility and covalent reaction between the 7cm-1,respectively,wereobserved.The3cm-1upshiftin
BDAsidechainandthematrix. SWNTs(1wt%)/R(c)comparedtopristineSWNTs(a),
Theπ-πinteractionbetweenSWNTsandthebackbone and the 2 cm-1 higher upshift in SWNTs(1 wt %)/
of PI-BDA was verified by Raman spectra. Figure 8A PI-BDA/R(d)comparedtoSWNTs/PI-BDA(b)canbe
showsRamanspectraof(a)pristineSWNTs,(b)SWNTs/ attributedtotheπ-stackingofCE-EPresinmoleculeson
PI-BDA, (c) SWNT(1 wt %)/R fibers, and (d) SWNT- thenanotubes.41The4-5cm-1Ramanupshiftsduetothe
(1wt%)/PI-BDA/Rfibers.Itisknownthatcovalentfunc- PI-BDA (comparing d to c, and b to a) indicate that the
tionalizationcanintroducedefectsintoCNTs,leadingto electronic environment of the SWNT surface has changed
increased intensity of D-band (at about 1330 cm-1).6,40 after functionalization with PI-BDA, with or without CE-
Unlikecovalentfunctionalization,nosignificantincrease EP matrix. This is believed to be due to the strong π-π
(40) Peng,H.Q.;Alemany,L.B.;Margrave,J.L.;Khabashesku,V.N. (41) Tournus,F.;Latil,S.;Heggie,M.I.;Charlier,J.C.Phys.Rev.B
J.Am.Chem.Soc.2003,125,15174. 2005,72,075431.
Article Chem.Mater.,Vol.22,No.24,2010 6549
Figure5. FE-SEMimagesofSWNTsinuncured(A1andA2)SWNT(1wt%)/Rand(B1andB2)SWNT(1wt%)/PI-BDA/Rspunfibersafterremovalof
freepolymerona0.2-μmAlO membrane.
2 3
attributed to the formation of new -OC(O-)dNH
bands,43,44ispresentbutsmallasthesamplewasheated
to80(cid:1)C(forDMFremovalaftercasting).Afterfurther
heating at 120 (cid:1)C for 1 h (spectrum 8B(d)), the relative
intensityof-OC(O-)dNHband(1678cm-1)increased,
suggestingthereactionofmore-OHgroupswith-OCN.
Theincreasedintensitiesofthebandsat1369and1567cm-1
implytheformationofsometriazinestructures.34Acontrol
experimentwasalsoperformedinwhichpureCEwithout
Figure6. Opticalmicrographsof(A)SWNT(0.2wt%)/Rand(B)SWNT
PI-BDAwasused,andtherewasnodifferenceintheFT-IR
(0.2wt%)/PI-BDA/Rcompositefibersafterpostcuring.
spectraofthepureCEtreatedat80(cid:1)C/2hand120(cid:1)C/1h
(Datanotshown).
interactionbetweenhighlyconjugatedSWNTwallandPI-
The reaction between PI-BDA and CE was also con-
BDAbackbonewithhighcontentofaromaticrings.23,25
firmedby1HNMRspectra.Figure8Cshowsthe1HNMR
TheBDAsidechainsalsocontributetothegoodinter-
spectraofPI-BDA/CE(30/70,w/w)mixtureinDMSO-d
facialadhesionbetweenSWNTs/PI-BDAandCE-EPmatrix. 6
beforeandafterheatingat120(cid:1)Cfor1h.Afterheatingat
TheBDAsidechaincontainsbisphenolAmoietylikethose
120(cid:1)Cfor1h(spectrum8C(b)),theintensityofthepeaks
inthecyanateesterandepoxyresins,whichincreasesthe
at around 9.2 ppm resulting from the NH protons of
compatibility and miscibility between SWNTs/PI-BDA
-OC(O-)dNHgroups43significantlyincreased.Additional
andCE-EPmatrix.Moreover,the-OHgroupsonBDA
evidenceofreactionisthatthe-CH peaks(atδaround
side chains can react with -OCN group of CE to form 3
1.6ppm)and aromatic peaks (at δ 6.7-7.2 ppm) of CE
iminocarbonate(-OC(O-)dNH)bonds(Scheme3).42-44
(overlapped with the peaks due to -CH and aromatic
The formation of iminocarbonate was confirmed with 3
protons in PI-BDA, respectively) split into two or more
FT-IRand1HNMR.
peaks, which can be attributed to unconsumed CE, and
To increase detectability of the reaction between PI-
adductsbetweenPI-BDAandCE(ortriazineoligomer)
BDA and CE, a blend containing larger proportions of
(Scheme3).43Alltheseconfirmthat-OCNgroupsonCE
PI-BDA to CE (30/70, w/w) was heated and analyzed
matrix covalently react with -OH groups on PI-BDA
with FT-IR and 1H NMR. Figure 8B shows the FT-IR
dispersanttoform-OC(O-)dNHbonds.
spectraof(a)neatCEbeforecuring,(b)PI-BDA,andPI-
Mechanical Properties. The tensile properties for neat
BDA/CE(30/70,w/w)(c)beforeand(d)afterheatingat
CE-EP,SWNT/R,andSWNT/PI-BDA/Rcompositeswith
120(cid:1)Cfor1h.Inspectrum8B(c),thebandat1678cm-1,
different nanotube loadings are summarized in Table 1
and Figure 9. Representative tensile stress versus strain
(42) Grigat,E.;Putter,R.Angew.Chem.,Int.Ed.1967,6,206. curvesareshowninFigure9A.WithoutaddingPI-BDA
(43) Lin,C.H.Polymer2004,45,7911. dispersant,SWNTs/Rcompositesshowlimitedincreases
(44) Liang,K.W.;Li,G.Z.;Toghiani,H.;Koo,J.H.;Pittman,C.U.
Chem.Mater.2006,18,301. intensileproperties.Thehighestincreaseintensilemodulus
6550 Chem.Mater.,Vol.22,No.24,2010 Yuanetal.
Figure7. FE-SEMimagesofcross-sectionalfracturesurfacesof(A1-A3)SWNT(1wt%)/Rand(B1-B3)SWNT(1wt%)/PI-BDA/Rcompositesafter
tensiletesting.
(E) is 33% (from 2.61 ( 0.14 to 3.47 ( 0.18 GPa) at ofSWNTs.1,45Thistheoreticalestimateisconsistentwith
SWNTloadingof1wt%,andthehighestincreaseintensile our observation. The elongation at break of SWNT/
strength(σ)is28%(from83.7(3.3to107.0(11.0MPa) PI-BDA/R composite increases initially at SWNT loading
atSWNTloadingof0.5wt%.FurtherincreaseinSWNT of0.2wt%(from5.0(0.4to6.2(0.7%)andthende-
content impairs tensile properties. The elongation at creasesgraduallyto3.7(0.4%forthe1.5wt%composite.
break(ε)decreasescontinuouslyfrom5.0(0.4to2.5( AlltheSWNT/PI-BDA/RcompositeswithSWNTload-
0.4%asSWNTloadingincreasesfrom0to1.5wt%,and ingrangingfrom0.2to1.5wt%showhighertoughness
thetoughness(T)hasnosignificantimprovement.With than neat CE-EP resin. Increases of 100% and 58% in
SWNTsdispersedbyPI-BDA,increasingnanotubecon- toughnessareachievedatSWNTloadingof0.2wt%and
tent from 0 to 1 wt % leads to a continuous increase in
1 wt %, respectively. As manifested in the FE-SEM
bothtensilemodulusandstrength.ForSWNT(1wt%)/
fractographoftheSWNT(1wt%)/PI-BDA/Rcomposite
PI-BDA/R composite, the tensile modulus and strength
(Figure7B1),well-dispersedSWNTsandstrongSWNT-
increased by 80% and 70%, respectively, from 2.61 (
matrixbondingeffectivelyresistthepropagationofcracks
0.14to4.70(0.24GPaand83.7(3.3to142.3(6.9MPa duringdeformation,34thusincreasingthefracturetough-
(relativetoneatCE-EPresin).1.5wt%SWNTs/PI-BDA
ness. The poor reinforcement effect of SWNTs without
resultedindecreasedtensilemodulusandstrength,which
dispersantispossiblyduetotheaggregationofnanotubes
can be attributed to the poor wetting of SWNTs as
in the matrix and weak interfacial bonding as discussed
reflected from more pulled-out SWNTs on the fracture
previously.Thisiscorroboratedbygreaterimprovements
surface of the SWNT(1.5 wt %)/PI-BDA/R composite
of the tensile modulus, strength, and toughness of the
(SupportingInformation,FigureS7).Othershavecalcu-
compositeswiththePI-BDAdispersant.
latedthat1vol%(∼1wt%)ofSWNTsissufficientto
Usingonly1wt%PI-BDAfunctionalizedSWNTs,the
ensure that all of the polymer molecules are within
absolutetensilepropertiesofSWNT(1wt%)/PI-BDA/R
one radius of gyration (5 nm) of a nanotube, implying
composites(E=4.70(0.24GPaandσ=142.3(6.9MPa)
difficultiesincompletewettingofhighloading(>1wt%)
are higher than those of most other CNT reinforced
(45) Shaffer,M.;Kinloch,I.A.Compos.Sci.Technol.2004,64,2281. thermoplasticsandthermosets,includingCNT/CE10and
Article Chem.Mater.,Vol.22,No.24,2010 6551
Figure8. (A)Ramanspectraof(a)pristineSWNTs,(b)SWNTs/PI-BDA,(c)SWNT(1wt%)/Rcomposites,and(d) SWNT(1wt%)/PI-BDA/R
composites.(B)FT-IRspectraof(a)neatCEbeforecuring,(b)PI-BDA,(candd)PI-BDA/CE(30/70,w/w)(c)beforeand(d)afterheatingat120(cid:1)Cfor1h.
(C)1HNMRspectraofPI-BDA/CE(30/70,w/w)inDMSO-d (a)beforeand(b)afterheating120(cid:1)Cfor1h.
6
CNT/epoxy6,7,11,46-48 thermosetting composites. Because forbothtensilestrengthandmodulusachievedwith1wt%
noworkappearsaboutCNTsreinforcedCE-EPcomposite, ofSWNTs/PI-BDA(E=4.70(0.24GPa(80%increase)
herewecompareourresultswiththatofsolution-processed andσ=142.3(6.9MPa(70%increase))arehigherthan
CNTsreinforced epoxy(atypicalthermosettingmatrix) thetabulatedincreasesforEandσwhichareusuallyless
composites. The detailed comparison is summarized in than 30-40%7,11,47 The tensile reinforcement efficacy
TableS1inSupportingInformation.Itshouldbenoted can also be quantitatively evaluated by calculating the
thatthecomparisonsareonlyapproximateasthenano- Young’smodulusandtensilestrengthperunitweightfrac-
tubesused,matrices,processingtechnique(e.g.,solution tion(dE/dW anddσ/dW ).11,49Inthisstudy,dE/dW
NT NT NT
mixing versus direct mixing), etc., which are listed in anddσ/dWNTreachto252GPaand8120MPa,respectively,
Table S1 differ. Nanotubes used could be single-walled, at0.5wt%ofSWNTsandto209GPaand5860MPaat
double-walled, or multiwalled.6,8,48 From Table S1, we 1 wt % ofSWNTs. These values are also superior com-
canseethatmosttensileproperties arereportedforlow pared to the results of CNT/epoxy composites reported
CNTcontents(about1wt%)asthemechanicalproper- recently (Supporting Information, Table S1).6,7,11,47,48
ties usually deteriorate with higher loadings. While the The significant mechanical enhancements achieved here
epoxy resins used differ chemically, most have strength can be attributed to the (i) high aspect ratio of SWNTs
(σ)andmodulus(E)ofabout60-90MPaand2-3GPa, with well-preserved graphene structure surface due to
respectively.Thereportednanotubedispersionmethods noncovalentfunctionalization;(ii)homogeneousdisper-
sionofSWNTs/PI-BDAinCE-EPmatrix;and(iii)strong
for the tabulated composites involve covalent function-
lizations.6,11 Our absolute values and percent increases π-π interaction between SWNTs and the backbone of
PI-BDA,andcovalentreactionbetweenPI-BDAdisper-
santandtheCE-EPmatrix.
(46) Spitalsky,Z.;Tasis,D.;Papagelis,K.;Galiotis,C.Prog.Polym.
Sci.2010,35,357. The tensile strength of CE-EP composites reinforced
(47) Zhu, J.; Peng, H. Q.; Rodriguez-Macias, F.; Margrave, J. L.;
with SWNTs can be predicted by a standard model for
Khabashesku, V.N.;Imam,A.M.; Lozano,K.;Barrera,E.V.
Adv.Funct.Mater.2004,14,643.
(48) Gojny, F. H.; Wichmann, M. H. G.; Fiedler, B.; Schulte, K. (49) Yan,Y.H.;Zhao,S.A.;Cui,J.;Yang,S.B.J.Polym.Sci.,PartA:
Compos.Sci.Technol.2005,65,2300. Polym.Chem.2009,47,6135.