In Situ Nitroxide-Mediated Polymerized Poly (acrylic acid) as a Stabilizer/Compatibilizer Carbon ...

13 Pages·2014·4.5 MB·English

HindawiPublishingCorporation
JournalofNanomaterials
Volume2007,ArticleID74769,12pages
doi:10.1155/2007/74769
Research Article
In Situ Nitroxide-Mediated Polymerized
Poly(acrylic acid) as a Stabilizer/Compatibilizer
Carbon Nanotube/Polymer Composites
VitaliyDatsyuk,1LaurentBillon,1ChristelleGuerret-Pie´court,1SylvieDagre´ou,1
NicolasPassade-Boupatt,2SylvainBourrigaud,2OlivierGuerret,2andLaurenceCouvreur2
1EquipedePhysico-ChimiedesPolym`eres(EPCP),InstitutPluridisciplinairedeRecherchesurl’Environnementet
lesMat´eriaux(IPREM),UMR5254CNRS,Universit´edePauetPaysdel’Adour,H´elioparcPau-Pyr´en´ees,2AvenueAngot,
64053PauCedex09,France
2GroupementdeRecherchedeLacq,ARKEMA,64170LACQ,France
Received6April2007;Accepted31July2007
RecommendedbyDavidHui
Carbon nanotube (CNT) polymer composites were synthesized via in situ nitroxide-mediated diblock copolymerization.
Poly(acrylicacid)(PAA)waschosenasafirstblocktoobtainaprecompositeCNT-PAAwhichisreadilydispersibleinvarious
solventsincludingwater.Theimmobilizationofthestablepoly(acrylicacid)alkoxyaminefunctionalityonthenanotubesurface
occursduringthesynthesisofthefirstblockwithoutCNTpriortreatment.Thelivingcharacterofthisblockisestablishedby
spectroscopicmethodsandthenatureoftheCNT/PAAinteractionisdiscussed.Thislivingfirstblockofferstheopportunityto
reinitiatethepolymerizationofasecondblockthatcanbechosenamongawiderangeofmonomers.Thisversatilityisillustrated
withasecondblockcontainingmethylacrylate(MA)orstyrene(S).Scanningandtransmissionelectronmicroscopiesconfirm
goodCNTdispersioninthepolymernetwork,whiletransmissionelectronmicroscopyalsospotstheanchoragelocationsofPAA
on the CNT surface. Such nanotubes wrapped by diblock copolymers can be dispersed in various polymer matrices to create
CNT—polymercomposites.Conductivitymeasurementsshowthatthesecompositesobeyapercolation-likepowerlawwithalow
percolationthreshold(lessthan0.5vol%)andahighmaximumconductivity(upto1.5S/cmatroomtemperature).
Copyright©2007VitaliyDatsyuketal. This is an open access article distributed under the Creative Commons Attribution
License,whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperly
cited.
1. INTRODUCTION The highly anisotropic nature of the tube is critical to pro-
vide percolative behavior at low-volume fractions, result-
The recently discovered carbon nanotubes (CNT) have at- ing in graphite-like electrical and thermal properties at 1-
tracted great interest as fillers for novel polymer compos- 2vol% additions [5]. Overall, CNTs are generally consid-
ites[1–3].CNTpolymercompositesareoneansweramong ered in three categories: single-wall nanotubes (SWNTs),
others to the increasing worldwide demand for novel poly- double-wall nanotubes (DWNTs) and multiwall nanotubes
mermaterialswithimprovedphysicalandmechanicalprop- (MWNTs). In order to take into account the wide range
erties, for applications ranging from the aerospace domain of available carbon nanotubes, we have started our work
to high-tech automotive components. For instance, in or- with different types of CNT, such as double-walled car-
der to avoid electrostatic charging of an insulating poly- bonnanotubes(DWNTs)andmultiwalledcarbonnanotubes
mermatrixortoapplyelectrostaticpaintingprocesses,suf- (MWNTs).
ficient electrical conductivity of the polymer is needed [4]. CNTcompositesmustbetailoredinawaythatwillop-
By using carbon nanotubes as fillers in those materials, timize the benefits of CNT in terms of mechanical, electri-
one expects to introduce the required electrical conductiv- cal,thermal,andrheologicalproperties.Thiscanbeachieved
ity together with a potential increase in mechanical perfor- onlywhenthedispersioniswellmanagedandforthat,one
mance. Tube diameters can range from 1 to 100nm with mustfirstbeabletoexfoliateefficientlytheCNTwhichare
aspect ratios (length/diameter) greater than 100 or 1000. initially arranged in much entangled faggots. Second, one
2 JournalofNanomaterials
must be able to avoid the agglomeration of the CNTs once 2. EXPERIMENTAL
dispersed.Asforanyfillersorpigments[6–8],thestabilityof
thedispersionisobtainedbywrappingtheCNTswithpoly- 2.1. Carbonnanotubes
mers which will stabilize their dispersion—thanks to steric
The double-walled carbon nanotubes (DWNTs) were syn-
hindrance or electrostatic repulsion. However, it is always
difficult to exfoliate nanoassembled systems with polymers thesized by a catalytic chemical vapor deposition (CCVD)
method [27] under hydrogen atmosphere with 18%mol of
since the gyration radius of the polymer is of the same or-
CH at1000◦C,usingMoinadditiontoCoinanMgO-based
der of magnitude as the voids in the faggots. Several ap- 4
catalyst,whichwaseasilyremovedbyamildacidictreatment
proacheshavebeenreportedtoprepareCNTpolymercom-
(HCl).TheDWNTswerewashedwithdeionizedwateruntil
posites via exfoliation: solution processing [9], mesophase
neutrality,filtered,anddriedovernightat80◦Cinair.Statis-
mediated processing, melt processing, and chemical modi-
ticalstudiesofTEMimagesofindividualCNThaveshown
fication of the CNT surface. Among the latter techniques,
that most of them were double-walled and were 1-2nm in
Lou et al. [10] on one side and Liu et al. [11] on an-
diameterandmorethan100μminlength.
other side reacted a living polystyrene chain obtained by
Themultiwalledcarbonnanotubes(MWNTs)werepro-
nitroxide mediated polymerization (NMP). This “grafting
to” technique is different from the in situ synthesis which vided by Arkema. They were also synthesized by a CCVD
methodandpurifiedbyacidictreatment.Statisticalstudyof
is based on the direct growth of polymer chains in the
high-resolution electron microscopy images of the MWNT
CNT network [12–15]. The in situ route is taking advan-
showedthat100%ofthemweremultiwalled,withdiameters
tage of the small dimensions of the starting monomers to
rangingfrom10to50nm.
swell the CNT network. Their polymerization provokes the
1,4-dioxane, toluene, styrene, acrylic acid, and methyl
exfoliation. This technique is similar to a “grafting from”
acrylatewerepurchasedfromAldrichandusedwithoutany
method but in fact chemical grafting is not always neces-
furtherpurification.
sary.Interestingly,nooneyettriedtocombineinsituCNT
composite synthesis and NMP polymerization before our
work. 2.2. Polymerizationofthefirstblock
Indeed, living polymerization is a very powerful tech-
nique to design polymer chains on the CNT surface. Re- Carbonnanotubes(DWNTsandMWNTs)(0.2g)weredis-
ports of CNT composites synthesis using surface-initiated persed in 15mL of 1,4-dioxane in a sonication bath for 1
ring-opening polymerization [16]orsurface-initiatedatom hour,and2gofacrylicacid(AA)werethenadded.Thereac-
transferradicalpolymerization(ATRP)[17–19]haveshown tionmixturewaskeptunderstirringwithamagneticstirring
that those in situ living polymerization techniques are very barrelfor1hour,andthensonicatedfor1additionalhour.
efficienttodisperseCNTinapolymermatrix.However,the An alkoxyamine NMP initiator (2-methyl-2-[N-tert butyl-
functionalizationoftheCNTsurfaceisalwaysacomplicated N-[diethoxy-phosphoryl-2, 2-di methyl propyl) aminoxy]
stepwhichpreventsanyindustrialdevelopment. propionicacid)(whichisdevelopedandcommercializedby
The aim of our work was thus to develop a simple ArkemaasBlocbuilder,andwillbereferredtoasIinthere-
method for the preparation of CNT-based polymer com- mainingpartofthepaper)wasthenaddedwithdifferentra-
posites,viainsitunitroxide-mediatedpolymerization.This tiosfrom[AA]/[I]=50to[AA]/[I]=300.Nitrogenbubbling
polymerization technique is industrially straightforward was used to deoxygenate the reaction mixture. Polymeriza-
[20] and offers the possibility to polymerize most of func- tion wasperformedat 115◦C. Finally,reaction mixture was
tionalvinylicmonomers(styrenic,acrylic,andmethacrylic) cooledtoroomtemperature.Theseconditionsweredirectly
[21,22]. derivedfromthoseelucidatedbyCouvreuretal.andusedto
Thus, using NMP to disperse nanotubes seems to be formpoly(acrylicacid)-basedblockcopolymersbyLaruelle
a new step towards the future development of nanotube- etal.[22,28].Thenetmaterialobtainedafterthatfirstpoly-
containingmaterials. merizationstepistermedasCNT-PAAprecompositeinthe
Theoriginalityofourroutecomesnotonlyfromthefact remainingpartofthepaper.
that we were the first to combine NMP “in situ” polymer-
ization and CNT but also because the block that we found 2.3. Copolymerizationofthesecondblock
to bring the most stable affinity with the CNT surface is a
poly(acrylic acid) block [23, 24]. Thus, it was often associ- Respectiveamountofthesecondmonomer(methylacrylate
ated with a styrenic block supposedly bringing the affinity (MA) or styrene (S)) was added to the reaction mixture of
fortheCNTsurface[18,25,26]. theCNT-PAAprecomposite.Themixturewasbubbledwith
The paper here reports a thorough investigation of the nitrogenandheatedto115◦C.Thenetmaterialobtainedaf-
differentstepsdrivingtothoseCNTdiblockcomposites,try- terthissecondstepisnamedCNT-PAA-PM2(nano-)com-
ing to elucidate the nature and the reason of such strong posite.Twodifferentcompositesarestudiedhere:CNT-PAA-
interactions. In addition, in order to validate the fact that PMA, with Poly (methyl acrylate) as a second block, and
this method is not affecting the intrinsic expected proper- CNT-PAA-PS,withpolystyreneasasecondblock.Notethat
ties of CNT composites, we studied the properties of such the PM2 second block is in fact a statistical block result-
diblock CNT composites when blended in a polymer ma- ingfromthecopolymerizationofM2monomerandtheAA
trix. monomerremainingafterthefirstblocksynthesis.However,
VitaliyDatsyuketal. 3
theamountofremainingAAislessthan10wt%compared Table1:CorrelationbetweenAAadsorptionandspecificsurface
toM2monomer. andoxygencontent,determinedbyXPS,fordifferenttypesofcar-
bonnanotubes.
2.4. Nuclearmagneticresonance CNT BET(m2∗cm−3) [O]at% AAadsorption(mg∗g−1)
DWNT 750 2.6 560
NMRspectraweredeterminedinsolutionofDMSO-d ina
6
MWNT 150 0.6 70
BruckeradvancedAM400MHzspectrometer(frequencyfor
31Pphosphorusis162MHzthechemicalshiftsaregivenin
ppmrelativelytoasolutionofH3PO4(85%)astheexternal
reference). It appears from Figure2 that the acrylic acid is rapidly
adsorbedontheCNTsurface.TheamountsofadsorbedAA
2.5. Scanningandtransmissionelectronmicroscopies on both DWNT and MWNT, in 1,4-dioxane, are in strong
correlation with the specific surface of nanotubes (deter-
ForSEM,anLEO1530VPmicroscopehasbeenused.Edges minedbyBET)andoxygencontentsontheirsurfaces(deter-
ofthesampleswereobservedafterbeingcutwithadiamond minedbyX-rayphotoelectronspectroscopy)(seeTable1).It
knifeandwereslightlyrecoveredwithafinecarbonlayerto isknownthatthispresenceofoxygenisduetohydroxylor
enhancelocalconductivity. carboxylic functions present at the surface of the CNT and
TEMobservationswerecarriedoutonaPhilipsCM200 usuallygeneratedbytheacidicpurificationofthenanotubes
microscope. Ultra thin sections were directly cut from the [29].Hence,onemustassumethatAAadsorbsonnanotubes
films formed after drying of the as-synthesized CNT poly- via interactions between the acid function of the monomer
mer composites, using an ultramicrotome and a diamond and −COOH and/or −OH groups present atthe CNT sur-
knifeatroomtemperature.Sectionswerepickeduponcop- face.ThishypothesisisalsosupportedbytheresultsofSon-
per grids. To enhance contrast between PAA and PMA, the nenbergetal.[30]whofoundoutthatCOOH−and/orOH−
sampleswerestainedbyimmersioninRuO4. functionalgroupsofsubstratespromotePAAadsorption.
Inordertoelucidateatwhatstageofthesynthesisprocess
2.6. X-rayphotoelectronspectroscopy the strong interaction between nanotubes and poly(acrylic
acid) takes place, we compared the thermal degradation
X-rayphotoelectronspectroscopy(XPS)wasperformedus-
of a precomposite produced according to in situ synthesis
ingaKratos165AxisUltra.
(named CNT-PAA precomposite) and that of the samples
ofequivalentcomposition,obtainedbymixinginsolutiona
2.7. Thermogravimetricalanalysis PAApreparedseparatelyandCNT(namedCNT-mixedPAA
inwhatfollows)(seeFigure3).
Thermogravimetrical studies were made on the TA Instru-
The thermogram of a pure PAA (see Figure3) shows
ments TGA2950TA under nitrogen at 10◦C/min with tem-
threedecompositionstageswhicharewellknown[31].The
peraturesrangingfrom20to600◦C.
first decomposition stage (I), in the range of 50–180◦C, is
attributedtothelossofabsorbedwater.Theseconddecom-
2.8. Electricalconductivitymeasurements positionstage(II),inanintervalof215–300◦C,corresponds
to the dehydratation (anhydrification of vicinal acid func-
The conductivity measurements were performed at room
tions)plusthedecarboxylationofthepolymer,whichresults
temperaturewithintheconventionalfour-wireprobeconfig-
in inter- and intramolecular bridging. The third decompo-
urationusingaKeithey6430ascurrentsourceandvoltmeter.
sition stage (III), in the range of 365–470◦C, is a result of
V(I)curvesweregenerallyohmiconseveraldecades.
thefinaldegradationofthepolymer.Significantdifferences
appearindecompositiontestsofbothCNT-containingsam-
3. RESULTSANDDISCUSSION
ples at similar PAA amounts. CNT-mixed PAA showed be-
havior typical for the pure PAA with decomposition start-
3.1. CharacterizationoftheCNT-PAAprecomposite
inginthe50–180◦Crange.Inthisphase,thecurvesofpure
Inoursyntheticroute(asdescribedin[23,24]andrecalled PAA and CNT-mixed PAA are overlapping. In the case of
inFigure1),weusedaninsitunitroxide-mediatedpolymer- theinsitupolymerizedCNT-PAAprecomposite,wecanob-
izationwithanalkoxyamineNMPinitiator(I)thatallowsthe servethatthepolymerdegradesathighertemperatures(on-
synthesisofwell-definedblockorgradientcopolymerswith set at 250◦C), with a smooth threshold at 300◦C and final
agoodcontrolofthepolymerpolydispersity[20]. decompositionoccurringat570◦C.Thiscanbeexplainedby
Carbonnanotubesweredispersedin1,4-dioxanewhich stronginteractionsbetweenCNTandpolymerchains,which
isagoodsolventleadingtostableCNTdispersions[23,24] increases the thermal stability of the poly(acrylic acid). In
andalsoallowingforthecontrolofthepolymerizationofthe their paper, Lou et al. reported a similar effect of MWCNT
acrylicacid[28]. onpolystyrenethermaldegradationonsetwhenpolystyrene
In order to understand the mechanism of the forma- is chemically bonded to the tubes [10]. This also confirms
tionofthepoly(acrylicacid)chainsonthenanotubesurface, whatwealreadyobservedbyXPSanalysis[23,24].
monomeradsorption-desorptionkineticswerestudied.Re- To understand deeper such a strong grafting, we com-
sultsarepresentedinFigure2. pared the stability of a dispersion in water of a CNT-PAA
4 JournalofNanomaterials
PrecompositeCNT-PAA CompositeCNT-PAA-PM2
1ststep 2ndstep
MonomerAA Nitroxideradical MonomerM2
CNT
Figure1:Schematicillustrationofthecompositepreparation.
45 7
40
6
35
5
30
%) %) 4
ht( 25 ht(
g g 3
Wei 20 Wei
2
15
1
10
5 0
0 −1
0 20 40 60 80 100 120 0 20 40 60 80 100 120
Time(mn) Time(mn)
Adsorption Adsorption
Desorption Desorption
(a) (b)
Figure2:Adsorption-desorptionkineticsoftheacrylicacidonnanotubesurfaceindioxane(CNTconcentration:1wt%;AAconcentration:
10wt%):(a)DWNT;(b)MWNT.
precompositeandaCNT-mixedPAAsampleatpH=9(see The interesting feature of the initiator (I) that was used in
Figure4).Theformerleadstotheformationofalong-time thisworkconsistsinthefactthatitbearsaphosphorusatom.
stable dispersion in water, while after 1 day, the latter sedi- Thus, it was possible to study chain ends by 31P NMR (see
ments completely. This difference of behavior stresses once Figure5). The chemical shift of starting (I) is a single pick
again the permanence of the CNT-PAA interactions which at26.6ppmthisis;acharacteristicofanalkoxyaminewhere
are generated during the in situ process. The reason why thenitroxideisattachedtoatertiarynonchiralcarbonatom
a simple mixing of PAA and nanotubes does not lead to ((cid:2)).Whenthisalkoxyamineisagrowingpoly(acrylicacid)
similar interactions can be attributed to the fact that poly- chain, the nitroxide is now linked to an enantiomeric sec-
mer chains are not able to penetrate the aggregates of nan- ondary carbon atom (•), since the nitroxide moiety is also
otubes. Steric hindrance prevents any intereaction between chiral (∗) (see Figure5(b)). The resulting alkoxyamine is
polymer chains and chemical functionalities at the surface a mixture of two diastereoisomers with their own chemi-
ofthenanotubes,orcreatesaninsufficientamountofinter- cal shift. In Figure5(b), we reproduced the 31P NMR spec-
actionsbetweenmacromolecularchainsandtheouterCNT. trumoftheCNT-PAAprecomposite,andwesawnolonger
In the case of CNT-PAA precomposite, the chains immo- a single pick in the region characteristic for alkoxyamines
bilized at the surface of the CNT are generated in situ and buttwobroadpickstypicalofthediastereoisomericformsof
form an electrostatic and steric barrier against agglomera- the polyacrylic alkoxyamines. This NMR study on polymer
tion. chainsofCNT-PAAprecompositeisfullysupportedbypre-
InordertofullycharacterizetheCNT-PAAprecompos- viousworkonnitroxide-mediatedpolymerizationofacrylic
ites,weinvestigatedthechainendofthepoly(acrylicacid). acidreportedbyLefayetal.[32].Notethatinthispaper,the
VitaliyDatsyuketal. 5
synthesized CNT-PAA precomposites was performed. TEM
100 microgramsofbothCNT-PAAprecompositesarepresented
inFigure7.
TEMimagesfullyconfirmourhypothesisaboutthedif-
80
ferenceinPAAorganizationontotheCNTsurface.Thepre-
%) I composite synthesized with a high initiator concentration
oss( 60 ([AA]/[I]=50)hasastructurewherepoly(acrylicacid)en-
l capsulates the carbon nanotubes. It is in good agreement
ht II
g with what wasobserved on the C1s XPS spectra,wherethe
Wei 40
signal corresponding to nongraphitic carbon atoms is pre-
dominant. This signal can indeed be attributed to the car-
20 III bon atoms of polymer chains that cover the CNT surface.
Ontheotherhand,PAAformsspotsontotheCNTsurface
0 whenpolymerizedwithalowinitiatorconcentration.Inthis
0 100 200 300 400 500 600 case, the extreme surface of the precomposite is still dom-
Temperature(◦C) inated by the presence of graphitic carbon atoms from the
CNTwhichexplainstheaspectoftheXPSspectrumofthis
PurePAA
sample.
CNT-mixedPAA
InsitupolymerizedCNT-PAA
3.3. Mechanismofgrafting
Figure3:Thermogravimetricanalysis(TGA)tracesundernitrogen
ofpurePAA,MWNT-PAAprecomposite,andMWNT-mixedPAA. TheNMRinvestigationrevealedthatmostofthechainswere
terminated by an alkoxyamine. We initiated the polymer-
ization of a second block PM2, which is built from a mix-
ture of the remaining acrylic acid of the first block and an-
authorsevaluatedtheamountofchainsbearinganitroxide
other vinylic monomer (M2). Growing of the second poly-
tobeabove85%.Hence,wehavespectrometricevidencethat
merblockPM2thusledtogradualchangeofthehydrophilic
ourprecompositeissuitabletorestartasecondblockpoly-
feature of the final composite depending on the second
merization.
monomer M2, promoting the good dispersion of the CNT
in different polymer matrices. Study of the stability of the
3.2. Effectoftheinitiatorconcentration dispersionshelpedusinunderstandingthenatureoftheat-
tachmentofPAAtotheCNTsurface.
Syntheses of CNT-PAA precomposites were performed In this paper, we will discuss the cases where M2 is
by using two different initiator concentrations so that styrene (S) or methyl acrylate (MA). By varying the poly-
the monomer-to-initiator ratios were [AA]/[I] = 300 and merization time of the second block, we obtained CNT-
[AA]/[I] = 50. Changing the initiator concentration is ex- PAA-PM2compositeswithvarioussecondblocklengthsand
pected to vary the grafting density, the length of polymer different CNT amounts (from 0.9wt% to 5.9wt%, accord-
chains, and the respective amounts of CNT and polymer ing to thermogravimetric studies). We tried to determine
chainsintheCNT-PAAprecomposites.Samplesweretested the molecular weight of polymer chains by gel permeation
byXPS(seeFigure6). chromatography (GPC) and we collected the supernatant
Figure6 shows the deconvolution of the C1s peak into of centrifuged CNT-PAA-PM2 composites diluted in ace-
fivemaincontributions.Wefocusourattentiononthetwo tone. Despite several washings in acetone, the resulting su-
mostsignificantpeaks:oneat284.2eVwhichisattributedto pernatantappearedslightlygrayish,indicating thepresence
the graphitic carbon atoms of the nanotubes, and a second of grafted nanotubes and preventing the direct measure-
peak at 285.0eV, which can be related to structural defects mentsofthemolecularweightofthediblockcopolymerby
at the surface of the graphitic sheet. The low initiator con- GPC [17]. For the moment, molecular weights of the dif-
centration sample ([AA]/[I] = 300) shows a strong peak at ferent diblocks are supposed to be comparable to the one
284.2eV, corresponding to the graphitization of the carbon obtainedinabsenceofCNT.Thepossibilitytocleaveselec-
nanotubes,andalowpartofdefectcarbonatoms,ascanbe tivelythelinkagebetweenCNTanddiblockisstillunderin-
deducedfromthelowpeakat285.0eV.Ontheotherhand, vestigation,inordertocharacterizepreciselythegrafteddi-
one can see from the spectrum on the left that the sample blocks.
with[AA]/[I]=50showstheoppositetrend,withasharpde- The stability of dispersions of various CNT polymer
creaseoftheamountofgraphiticcarbonatomsonthesam- composites is summarized in Table2. This study allowed
plesurface,andanextremeincreaseofthecarbonatomsin us to determine the most probable mechanism of interac-
anotherstateratherthanthetypicalgraphiticpuresp2 one. tion between PAA chains generated in our process and the
These results must be interpreted by keeping in mind that tubes.
the XPS is a surface analysis method and may be explained The first possibility is the radical grafting of an
byadifferenceofthepolymerchainsconformationontothe alkoxyamineasreportedbothbyJe´roˆmeandAdronov(route
CNTsurface.Toconfirmthishypothesis,aTEMstudyofthe IinFigure8)[10,11].Ifthatwastheonlyreason,thenthere
6 JournalofNanomaterials
1 2 3 1 2 3
(a) (b)
Figure4:StabilityofCNTwaterdispersions:(a)1hourafterdispersion;(b)1dayafterdispersion.1:insitusynthesizedprecomposite;2:
CNT-mixedPAA;3:pureCNT.
CH3 CH(cid:2)3 (cid:3)
RorS/R
HO C C O N HO C C CH2 CH CH2 CH O N
∗ RorS n ∗ RorS/S
O CH3 CH O CH3 C C CH
P HO O HO O P
O (OCH2CH3)2 O (OCH2CH3)
ppm 30 25 20 15 ppm 30 25 20
(a) (b)
Figure5:(a)31PRMNspectraofinitiator(I)and(b)ofCNT-PAAprecomposite,showingthepresenceofanalkoxyaminechainend.
should be only a minor difference between the grafting of ThestabilityofdispersionsofCNT-PAA-PScompositein
a polystyrene (composite C) and a polyacrylic alkoxyamine tolueneindicatestwofeatures.First,bycomparingwiththe
(composite B). We see in Table2 that poly(acrylic acid) is CNT/PScompositewhichdoesnotprovideastabledisper-
boundtotheCNT,whereasthepolystyrenegeneratedinthe sion in toluene, we understand that the PAA first block in-
sameconditionsoftemperatureanddilutionpresentsanon- ducesstrongattachmentofthePSblockontheCNT.Second,
permanentinteraction. sincethesecondblockiswellanchored,itsgrowthhastaken
Asecondpossibility(routeIIinFigure8)correspondsto placefromthesurfaceofthetubes.Thismeansthatthestart-
the grafting of acid functions of the polymer on the CNT ingPAAblockgraftedontheCNThadkeptitslivingness,in-
surface. That is possible through esterification of the acid dicatingthatthemechanismofgraftingcannotbetheradical
withalkoxypendantfunctionsontheCNTorthroughanhy- couplingbetweentheCNTandthelivingalkoxyamine.This
drification with pendant acid functions. Such reactions are alsoimpliesthatarelevantamountoflivingPAAwasgrafted.
knowntotakeplaceatelevatedtemperature,asreportedby We cannot exclude the fact that route I occurs, but we
Baskaranetal.[33].TheNMProutethatwechosepresents estimatefromtheseobservationsthatrouteIIiscertainlythe
theadvantageofbeingprocessedattemperatureabove110◦C dominant mechanism ofanchorageofthegrowingPAAon
allowingsuchreactions. theCNT.
VitaliyDatsyuketal. 7
×102 ×102
C1s C1s
120
S)
P
C
( 80
y
nsit
nte 40
I
300 296 292 288 284 280 276 300 296 292 288 284 280 276
Bindingenergy(eV)
Name: Energy FWHM Area(%) Name: Energy FWHM Area(%)
C1s-1: 284.625 0.700 C1s-1: 284.540
C1s-2: 285.715 1.311 C1s-2: 285.300
C1s-3: 286.900 2.340 C1s-3: 286.561
C1s-4: 289.926 1.216 C1s-4: 289.492
C1s-5: 291.197 1.617 C1s-5: 291.215
(a) (b)
Figure 6: Deconvolution of the C1s peak of the CNT-PAA precomposites, synthesized with two different initiator concentrations:
[AA]/[I]=50and300(lefttoright,resp.).
6 0
5 6
8 8
C C
20nm540K 20nm390K
(a) (b)
Figure7:TEMmicrographsoftheCNT-PAAprecomposites,synthesizedwithdifferentinitiatorconcentration:[AA]/[I]=50ontheleft,
and[AA]/[I]=300ontheright(scalebare=20nm).
3.4. Electronmicroscopyof aredisperseduniformlyinthepolymermatricesbuttheyare
theCNT-PAA-PM2composite difficulttoimageduetotheirsmallsize.Suchstructuresof
CNTdispersedinpolymerhavebeenobservedinthelitera-
Whilethefirstblockofpoly(acrylicacid)affordedagooddis- ture[34].Fieldemissiongunscanningelectronmicroscopy
persionabilityoftheCNTinwater,bybeingabletorestarta (EFG-SEM) is in progress to characterize the samples con-
secondpolymerizationfromthealkoxyaminechainend,we tainingDWNT,becauseofthebetterresolutionnecessaryto
wereabletovarytheaffinityofthenanotubeswithallkinds imageDWNT.
ofhydrophobicmedia.Atthispoint,wechosetofocusour Toinvestigatethedispersionatananometerscaleandto
studyonthedriedmaterialdirectlyinordertocharacterize characterizetheanchorageofthepolymeronthenanotube
the composites. We led microscopy investigations and then surface,TEMwasused.First,inFigure10,weconfirmthat
electricalmeasurementsoncastfilmsobtainedbyslowevap- the carbon nanotubes, initially forming aggregates, are iso-
orationofadioxanesolutionofthecomposite. latedafterthecopolymersynthesis.Figures10(a)and10(b)
The CNT-PAA-PM2 composite films were observed revealthenanostructuringofthePAA-PMAdiblock(respec-
at different scales. Figure9 shows SEM micrographs of tive contents in the diblock are about 10wt% and 90wt%)
MWNT-PAA-PMA and MWNT-PAA-PS composites, re- in the presence of carbon nanotubes. Thanks to Ru0 , PAA
4
spectively, containing 3.9wt% and 5.9wt% of carbon nan- chains are indeed stained preferably and appear to be con-
otubes. On these micrographs, MWNTs appear as shining centratedinnanospheres(blackareas)whosesizesareinthe
points.Onbothsamples,onecanobservesomeareasofrel- order of magnitude of ten nanometers. Note that the mi-
ativelyhighconcentrationsofcarbonnanotubesinthecom- crographs (see Figure10(c)) of pure PAA-PMA synthesized
posites. However, apart from these clusters, the nanotubes in the same way but in the absence of CNT they show a
8 JournalofNanomaterials
Table2:StabilityofCNT-basedcompositesinappropriatesolvents.
Composite CNT+PAA(A) CNT-PAA(B) CNT-PS(C) CNT-PAA-PS(D)
Process Blend Insitu Insitu Insitu
Stability∗ No Yes(water) No(toluene) Yes
∗
ofthedispersioninappropriatesolvent.
CH3 CH(cid:2)3 (cid:3)
HO C C O N Acrylicacid HO C C CH2 CH CH2 CH O N
120◦C n
O CH3 CH O CH3 C C CH
P HO O HO O P
O (OCH2CH3)2 O (OCH2CH3)
(I) R1
MWCNT N O
R2 COOH
(II)
−H2O
Figure8:PossiblemechanismsofgraftingofNMPgrowingpolyacrylicacid(I)byradicalreactionofthemacroalkoxyamineontheCNT
conjugatedsurfaceor(II)byesterificationoranhydrificationofacidfunctions(red)withpendantalcoholoracidfunctionalities(green
circles).
1μm 200nm
(a) (b)
2μm 100nm
(c) (d)
Figure9:ScanningelectronmicroscopyimagesoftheMWNT-PAA-PMAandMWNT-PAA-PScomposites.
VitaliyDatsyuketal. 9
verysimilarnanostructuringofthediblockcopolymer.Such percolationlawbecausethepolymermatricesaretoodiffer-
nodular organization is expected for the range of composi- entdependingonthepolymerizationtimeusedforthesec-
tionthatwehaveexplored(from4%to10%ofPAAinthedi- ond block. Indeed, for the DWNT-PAA-PMA (30minutes)
block).ThepresenceofCNTduringthepolymerizationdoes sample, the ratio of PAA/PMA is around 10 units/90 units
notchangethediblockorganizationinthiswindowofblock determined by NMR 1H, whereas for DWNT-PAA-PMA
ratio. (160minutes)theratiois10units/280units,sothatwecan
Figure10(b) shows a very exciting feature since at this hardlyconsiderthatthematrixisstillthesame.
level of magnification, we can easily point out domains of
PAA which are regularly spotted on the surface of a nan- 3.6. DilutionofCNT-PAA-PM2compositesin
otube. Those domains indicate the sites of interaction be- apolymermedium
tween the nanotube and the polymer chains. Contrary to
whatisgenerallyassumedintheliterature,weseeherethat The last step that we can achieve to prepare CNT com-
thegraftingisdiscreteandthatitdoesnotfitwithamodelof posites consists in diluting CNT-PAA-PM2 composite in a
continuouswrappingofthesurface.Wearecurrentlywork- polymer for which PM2 is a compatible block. We chose
ingonmeasuringthespacingofthosedomainswhichshould to blend CNT-PAA-PMA with an emulsion of poly(methyl
becorrelatedtotheaveragedistancebetweenoxygenatoms acrylate) (EPMA), while CNT-PAA-PS was solvent-mixed
presentatthesurfaceofthenanotube. (1,4-dioxane) with high molecular weight polystyrene PS.
EPMA was synthesized in laboratory by typical emulsion
3.5. ConductivityoftheCNT-PAA-PM2composites polymerization methods, using a sodium persulfate as an
initiator and nonionic surfactant TRITON X 400. This ex-
The electrical conductivities of the composites CNT-PAA- amplewaschosentoillustratethepossibilityofintroducing
PM2weremeasuredfordifferentlengthsofthePM2second CNT into aqueous acrylic paints. The composites obtained
block. In all cases, whatever the second block, PMA, or PS through these blends were then dried by slow evaporation
is,andwhatevertheCNT,DWNTorMWNTs,weobserved tocastafilm.Thefilmsthatweobtainedwillbereferenced
asharpincreaseoftheelectricalconductivitywhenincreas- asCNT-PAA-PMA/EPMAandCNT-PAA-PS/PS.Theyboth
ingtheCNTamountandasaturationbehavioratlargeCNT showedagoodCNTdistribution(checkedbyTEMandSEM
amounts.Thisisthesignatureofapercolationbehavior. imageswerenotshown).
As an example, from the same precomposite DWNT- Inourexperimentalsetup,theconductivityrangeislim-
PAA (whose CNT content was 32wt%), a second block itedtovaluesabove10−8S/cm.Onceagain,theconductivity
of PMA was polymerized for three different polymeriza- curves,showninFigure11,revealpercolationbehaviors.The
tion times (respectively, 30, 120, and 160minutes) lead- percolation threshold is in both cases estimated at around
ing to three different final amounts of CNT (respectively, 0.5vol%, and the maximum conductivity is above 1S/cm.
4.7wt%, 1.9wt%, and 1.5wt%) in the DWNT-PAA-PMA Because of the few number of points, no determination of
composites. Their respective conductivities were 0.59S/cm, the parameters σ0, xc, and t, using a least square method,
0.22S/cm,and10−5S/cm,showingthatthecompositesreach was possible. However, the gray tendency curves shown in
veryhighconductivitiescomparedtotheclassicalconductiv- Figure11areusedasthebestfitoftheexperimentalvalues.
ity value of polymers (below 10−14S/cm for PMA and PS). They correspond, respectively, to the following equations:
NotethattheconductivityofpressedpelletsofpureDWNT σ =1.14(x−0.5)2 andσ =0.1(x−0.27)2.Inbothcase,the
wasfoundaround25S/cmatroomtemperature,andtheone critical index t = 2 is close to t = 1.94, that is, the value
ofDWNT-PAAprecompositewasaroundoneorderofmag- predictedbythepercolationtheoryfor3Dcomposites[36].
nitudeless.Theseresultsshowthatabove1.9wt%,thecom- Notealsothelowvaluesofthethreshold,0.5and0.27,that
posites conductivity was dominated by the DWNT-PAA or confirmtheinterestinthesecompositesasconductivefillers
bytheDWNTmatconductivities. in different polymer matrices. In the case of MWNT-PAA-
Themeasuredconductivityshouldfollowapercolation- PSblendedwithPS,evenforlowcontent(10%ofMWNT-
likepowerlawnearthethresholdσ = σ(x−x )t,wherexis PAA-PS for 90% of industrial PS), the conductivity of the
c
thevolumefractionoftheconductivefiller,x istheperco- resulting composite is very important (around 10−3S/cm)
c
lationthreshold,andt istheconductivitycriticalexponent. [24].
Inthecomposites,themassfractionisrelatedtothevolume
fractionthroughtheratioofthedensityofthepolymerma- 3.7. ApplicationoftheMWNT-PAAprecompositesto
trixoverthedensityofCNT.ThedensityofMWNTisusu- “insitu”emulsionpolymerization
allyestimatedto2g/cm3[34,35],andthedensitiesofPSand
PMAarecloseto1g/cm3,sothatwhatfollows,weconsider Wefinallydiscusstheexampleoftheuseofthepoly(acrylic
that the volume fraction is around twice less than the mass acid)-modified carbon nanotubes in in situ emulsion poly-
fraction. Note that this assumption is probably not a cor- merization.SeriesoftheMWNTpoly(styrene-co-butylacry-
rectestimationinthecaseofDWNTsthatareaggregatedin late) composites were synthesized by emulsion polymeriza-
bundles for which density is smaller (around 1.3g/cm3 for tion,inthepresenceofMWNT-PAAprecomposites.Wevar-
SWNTbundles). iedtheamountofMWNTinthefinalMWNTpoly(styrene-
InthecaseofCNT-PAA-PM2composites,withdifferent co-butyl acrylate) composites from 0.7 to 4wt%. The con-
lengthsofthesecondblockPM2,itisdifficulttoconsidera centration of the polymers in emulsion ranged from 24 to
10 JournalofNanomaterials
(a) (b) (c)
m m m
n n n
200 100 100
(a) (b) (c)
Figure10:TransmissionelectronmicroscopyimagesoftheMWNT-PAA-PMAcomposite.Blackareascorrespondtoregionsstainedby
Ru0 ,wherePAAisdominant.
4
10 MWNT-PAA-PMAmixedwithEPMA 10 MWNT-PAA-PSmixedwithPS
1 1
0.1 0.1
0.01 0.01
m) 1E−3 m) 1E−3 100%MWNT-PAA-PS
c c
(S/ 1E−4 (S/ 1E−4 0%PS
y CNT-PAA-PMA/EPMA y 50%MWNT-PAA-PS
vit 1E−5 vit 1E−5 50%PS
ucti 1E−6 ucti 1E−6 10%MWNT-PAA-PS
ond 1E−7 ond 1E−7 90%PS CNT-PAA-PS/PS
C C
1E−8 1E−8
1E−9 1E−9
1E−10 1E−10
1E−11 1E−11
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0 0.5 1 1.5 2 2.5 3
CNT(%vol) CNT(%vol)
(a) (b)
Figure11:Four-wireconductivitymeasurementsofthecompositesCNT-PAA-PMA/EPMAandCNT-PAA-PS/PS.Dashedareascorrespond
toconductivityregionsthatarenotaccessiblewithourapparatus.
(a) (b)
Figure12:Poly(styrene-co-butylacrylate)emulsionsynthesizedwithout(1)andinpresence(2)ofMWNT-PAAprecomposite.
45%wt. Monomers were used under the ratio [S]:[BA] = The studies of MWNT polymer composites synthe-
1:1. The nonionic surfactant Igepal CO 890 was used as an sized by in situ emulsion polymerization are underway but
emulsifier, and the potassium persulfate was the initiator. thesynthesizedcarbonnanotubes-filledpolymercomposites
Polymerizationwasperformedat70◦C.Figure12reportsthe showagoodstabilityinemulsionandcanbeusedasabasis
photosoftheobtainedlatexes. forthepreparationofconductivepaintingsandcoatings.