Fracture Toughness and Elastic Modulus of Epoxy-Based Nanocomposites with Dopamine ...

16 Pages·2017·17.59 MB·English

materials
Article
Fracture Toughness and Elastic Modulus of
Epoxy-Based Nanocomposites with
Dopamine-Modified Nano-Fillers
KwangLiangKoh1,XianbaiJi1,AravindDasari1,* ,XuehongLu1,SooKhimLau2and
ZhongChen1,*
1 SchoolofMaterialsScienceandEngineering,NanyangTechnologicalUniversity,50NanyangAvenue,
Singapore639798,Singapore;[email protected](K.L.K.);[email protected](X.J.);
[email protected](X.L.)
2 SingaporeInstituteofManufacturingTechnology,73NanyangDrive,Singapore638075,Singapore;
[email protected]
* Correspondence:[email protected](A.D.);[email protected](Z.C.);Tel.:[phone](A.D.);
[phone](Z.C.)
Received:24April2017;Accepted:7July2017;Published:10July2017
Abstract: This paper examines the effect of surface treatment and filler shape factor on the
fracture toughness and elastic modulus of epoxy-based nanocomposite. Two forms of nanofillers,
polydopamine-coated montmorillonite clay (D-clay) and polydopamine-coated carbon nanofibres
(D-CNF) were investigated. It was found that Young’s modulus increases with increasing D-clay
andD-CNFloading. However, thefracturetoughnessdecreaseswithincreasedD-clayloadingbut
increaseswithincreasedD-CNFloading.Explanationshavebeenprovidedwiththeaidoffractographic
analysisusingelectronmicroscopeobservationsofthecrack-fillerinteractions.Fractographicanalysis
suggeststhatalthoughpolydopamineprovidesastrongadhesionbetweenthefillersandthematrix,
leadingtoenhancedelasticstiffness,theenhancementprohibitsenergyreleaseviasecondarycracking,
resultinginadecreaseinfracturetoughness.Incontrast,1Dfibreiseffectiveinincreasingtheenergy
dissipationduringfracturethroughcrackdeflection,fibredebonding,fibrebreak,andpull-out.
Keywords: polydopamine; montmorillonite clay; carbon nanofibre; fracture toughness; elastic
modulus
1. Introduction
Epoxyhasbeenusedextensivelyforengineeringapplicationsduetoitscombinedadvantagesofhigh
stiffness,strength,creepresistance,andchemicalresistance.However,itsfractureresistanceisrelatively
lowbecauseoftherigidnetworkofcross-linkedpolymerchains.Significantefforthasbeenmadeinthe
pasttoimprovevariouspropertiesofepoxy, suchaschangingthebackbonechemistriesoftheepoxy
andcuringagent[1],modifyingthethermosetresinswiththermoplasticpolymer[2–4],andcreatinga
multiphasepolymericsystembymixingdispersedelastomericandthermoplasticphaseswiththeepoxy.
Nevertheless,theseexploredmethodshaveonlyprovidedlimitedimprovementtothefracturetoughness
ofthehighlycross-linked,highglasstransitiontemperatureepoxyandtheircomposites.Challengesremain
whenepoxymaterialsareintroducedtoapplicationsintheaerospaceorautomotiveindustries[5,6].
Towardstheendofthe20thcentury,theconceptofnanocompositesstartedtoemerge,offeringthe
industrywithpromisingresultsandauniquelevelofpropertyimprovement. Thismethodinvolves
theincorporationofnano-sizedorganicandinorganicnanoparticlestotheepoxymatrix. Researchers
from Toyota [7] increased the thermal and mechanical properties of polyamide-6 nanocomposite by
addingaverysmallamountofclaynanoparticles(~4wt%).Subsequentstudiesbyotherresearchersalso
Materials2017,10,776;doi:10.3390/ma10070776 www.mdpi.com/journal/materials
Materials2017,10,776 2of16
reportedsignificantimprovementsinstiffnessandstrengthbyaddingnano-sizedclayfillerscomparedto
micro-sizeinthesamepolymerclay,atlowloadingpercentagesofabout1–5%byweight[8–11].Other
formsofnanofillersincludingsphericalparticles,nanotubes,nanofibres,andother2Dmaterialshave
beeninvestigated,showinggreatimprovementinthemechanicalpropertiesofthematerials[12–15].
Generally,surfacetreatmentofnanofillersisrequiredforbetterinteractionwithhydrophobic
polymermatrices. Musseladhesiveproteins(MAPs)havereceivedagreatamountofattentionfor
their capability to adhere effectively to different types of surfaces. MAP contains a large quality
ofaparticularaminoacidknownasdopamine(DOPA)thatisresponsibleforthestrongadhesion
of mussels on different types of marine surfaces [16]. Podsiadlo et al. [17] have applied a small
amountofDOPAtoartificialnacretoachieveacompositefilm. Duetoitsstrongadhesion,DOPA
acts as an effective load transfer agent and, thus, improves the overall stiffness and toughness of
polymercomposites. Hanetal.[18]appliedDOPA-intercalatedclaynanosheetsfortheenhancement
of both adhesive strength and toughness of hydrogel for skin regeneration. Zhu et al. [19] added
DOPA-coatedcarbonnanotubesintopoly(vinylidenefluoride)matrix,theresultingcompositewas
abletoachieveabalancedhighpermittivityandlowdielectricloss. DOPA-coatedfillershavealso
beenreportedforenhancingthermalstability,mechanicalstrength,andtribologicalperformanceof
nanocomposites[20–22].
Despitetheseefforts,therehavebeenverylimitedstudiesonthefundamentalrootcausesthat
contribute to the cracking process, i.e., the micro-mechanisms of a fracture in the nanocomposite,
especiallythefillerandfiller-matrixinteraction. Inthisstudy,weinvestigatedhowDOPAcoating
treatmentaffectsthefracturetoughnessandelasticmodulusofepoxy-basednanocomposites. Two
typesofnanofillerswereused: montmorillonite(MMT)clayasa2Dfiller,andcarbonnanofibre(CNF)
asa1Dfiller. CompositeswithandwithoutDOPAcoatingonthefillersweretestedandanalysed. The
comparisonbetweenthe1Dand2Dfillersenablesustounderstandhowtheformfactoraffectsthe
fracturetoughnessandfractureprocess.
2. ResultsandDiscussion
2.1. VerificationsofDOPAPresenceonD-Clay
TheDOPA-coatedclay(D-clay)particleswereexaminedusingX-rayphotoelectronspectroscopy
(XPS)toverifythepresenceofDOPAontheMMTsurface. TheresultsfromXPSrevealedthatthe
sodiumatomswerenolongerdetectedandthesignalsforoxygenandsiliconwerelargelyreduced
ascomparedtotheuntreatedMMT(Figure1a). Atthesametime,ahigherconcentrationofcarbon
and nitrogen were detected on the D-clay surface similar to the observations reported on surface
functionalizednanofibresin[23]. Furthermore,usingFouriertransforminfraredspectroscopy(FTIR),
similarbandscanbefoundforbothpolymerizeddopamine(PDOPA)andDOPA(Figure1b). Thus,we
concludethatDOPAwaspolymerizedonthesurfaceoftheMMT.Itiswell-knownthattheintensive
bandinMMTat1015cm−1isduetoin-planestretchingofSi–O,andtheout-of-planestretchingwill
appearasatinyshoulderat1115cm−1. Additionally,bendingvibrationsofthehydroxylgroupsof
watergenerallyoccurat1641cm−1inMMT(stretchingwillbeat~3430cm−1). Therelativelybroad
bandbetween3200and3500cm−1 thatappearedinD-ClaycomparedtoMMTisacombinationof
stretchingvibrationsof–NHand–OH.Thebandat1618cm−1 inD-ClayassignedtoC=Cfurther
confirmsthepresenceofdopamine.
Materials2017,10,776 3of16
Materials 2017, 10, 776 3 of 16
O1s
Na-MMT
Na1s
Si2p
Si2s
O1s C1s
D-clay
N1s Si2p
Si2s
1200 1000 800 600 400 200 0
Binding energy(eV)

(a)

(b)
Figure 1. (a) XPS spectra of MMT clay and D-clay; and (b) FTIR spectra of MMT clay, D-clay, DOPA
Figure1.(a)XPSspectraofMMTclayandD-clay;and(b)FTIRspectraofMMTclay,D-clay,DOPA
and PDOPA.
andPDOPA.
Thermogravimetric analysis (TGA) was used to determine the amount of DOPA polymerized
Thermogravimetricanalysis(TGA)wasusedtodeterminetheamountofDOPApolymerized
on the surface of the MMT. From the TGA curve shown in Figure 2, the degradation of D-clay begins
onthesurfaceoftheMMT.FromtheTGAcurveshowninFigure2,thedegradationofD-claybegins
at ~230 °C and ends at about 700 °C. The weight loss between 600 °C and 800 °C is caused by
at ~230 ◦C and ends at about 700 ◦C. The weight loss between 600 ◦C and 800 ◦C is caused by
dehydroxylation of MMT clay which suggests the loss of structural –OH groups. From this result, we
dehydroxylationofMMTclaywhichsuggeststhelossofstructural–OHgroups. Fromthisresult,we
can conclude that the PDOPA has relatively low thermal stability, therefore, the amount of PDOPA
canconcludethatthePDOPAhasrelativelylowthermalstability,therefore,theamountofPDOPA
must be controlled to achieve the expected adhesive interaction while maintaining the thermal
mustbecontrolledtoachievetheexpectedadhesiveinteractionwhilemaintainingthethermalstability
stability required for the composite. A series of tests was conducted with DOPA loading from 0.1 to
requiredforthecomposite. AseriesoftestswasconductedwithDOPAloadingfrom0.1to2.0mg/mL,
2.0 mg/mL, and the thermal stability was determined by the starting temperature of degradation.
andthethermalstabilitywasdeterminedbythestartingtemperatureofdegradation. Itwasfound
It was found (results not shown here) that 1.5 mg/mL was able to provide the most stable DOPA
(resultsnotshownhere)that1.5mg/mLwasabletoprovidethemoststableDOPAcoating,aswellas
coating, as well as good dispersion of the fillers for the desirable mechanical properties. Higher
gooddispersionofthefillersforthedesirablemechanicalproperties. Higherconcentrationswouldnot
concentrations would not increase the thermal stability and coating thickness any further. Therefore,
increasethethermalstabilityandcoatingthicknessanyfurther. Therefore,unlessspecified,allthe
unless specified, all the results reported will be based on 1.5 mg/mL DOPA concentration.
resultsreportedwillbebasedon1.5mg/mLDOPAconcentration.
Materials2017,10,776 4of16
Materials 2017, 10, 776 4 of 16
Materials 2017, 10, 776 4 of 16
Pristine clay
100 Pristine clay
100
)
)%
%
(
(t 90
th90
h
g
gi
ie
WeW DD--ccllaayy
80
80
A
A
70
70
150 300 450 600 750
150 300 450 600 750
o
TTeemmppeerraa ttuurree((oCC))

FFiigguurree 22.. TTGGAA ccuurrvveess ooff pprriissttiinnee ccllaayy ((MMMMTT)) aanndd DD--ccllaayy ((11..55 mmgg//mmLL DDOOPPAA ccoonncceennttrraattiioonn)) iinn aaiirr..
Figure 2. TGA curves of pristine clay (MMT) and D-clay (1.5 mg/mL DOPA concentration) in air.
The D-spacing of the samples were measured using X-ray diffraction (XRD) and results are
TThhee DD--ssppaaccininggo offt htehsea smapmlepslews ewreemree amsuearesduruesdi nugsXin-gra yX-driafyfr adcitfifornac(tXioRnD )(XaRndDr)e asunldts raerseuslthso warne
shown in Figure 3. Typical d-spacing of the MMT, as-received, was found to be around 1.19 nm.
isnhoFwignu rien 3F.igTuyrpei c3a. lTdy-pspicaacli ndg-sopfacthinegM oMf tTh,ea Ms-rMecTe,i vaesd-r,ewceaivsefdo,u wndast ofobuenadr otou nbde 1a.r1o9unnmd .1.D19u rninmg.
During the polymerization of the DOPA in the presence of MMT, the d-spacing between different
tDhuerpinogly tmhee rpizoaltyimoneroifztahtieoDn OofP Athien DthOePpAre isne nthcee opfrMeseMnTc,et hofe Md-MspTa,c itnhge bde-tswpaeceinngd ibffeetrwenetencl adyiflfaeyreenrst
clay layers increases, suggesting the intercalation of DOPA chains. Different polymerization times
icnlacyre laasyeesr,ss uingcgreesatsinesg, tshuegignetsetricnagla tthioen inotfeDrcOalPaAtiocnh aoifn sD.ODPifAfe rcehnatinpso.l yDmifeferrizeantti opnoltyimmeesriwzaetrieonu steimdetos
owwbeetrareein uutssheeeddo ttpoot ioombbuttaamiinnt ittmhheee .ooApptftitimmeruupmmol yttiimmmeeer..i zAAafftttieeorrn ppfooorllyy1mmhee,rrtihizzeaatDtiioo-cnnl a ffyoorre x11h hhib,, ittthheeed DDa--pccellaaayky aeextxh2hθiibb=iittee6dd.0 2aa◦ pp,weeaahkki caahtt
2θ = 6.02°, which directly corresponds to a d-spacing of 1.47 nm using Bragg’s law (an increase of
d2θir e=c t6ly.0c2o°,r rweshpiochn ddsirtoecatldy- scporarceinspgoonfd1s.4 t7on am du-sspinagciBnrga gogf ’1s.l4a7w n(man uinsicnrgea Bseraogfg~’0s .2la8wn m(anfr oinmcrperaisseti noef
~0.28 nm from pristine MMT). Considering the polymerization times and the achieved d-spacing, a
M~0M.28T )n.mCo fnrosimde prirnisgtitnhee MpoMlyTm)e. rCizoantsioidnetriimnges thane dptohlyemacehriiezvaetidond -tsipmaecsin agn,da 2thhe paochlyimeveerdiz dat-isopnactiimnge, ias
2 h polymerization time is chosen as the optimum condition.
c2h ho speonlyamstehriezoaptitoinm tuimmec iosn cdhiotisoenn. as the optimum condition.

Figure 3. X-ray diffraction (XRD) curves of D-clay obtained at different polymerization times.
FFiigguurree 33.. XX--rraayy ddiiffffrraaccttiioonn ((XXRRDD)) ccuurrvveess ooff DD--ccllaayy oobbttaaiinneedd aatt ddiiffffeerreenntt ppoollyymmeerriizzaattiioonn ttiimmeess..
2.2. Filler Dispersion Characterisation of Nanocomposites
22..22.. FFiilllleerr DDiissppeerrssiioonn CChhaarraacctteerriissaattiioonn ooff NNaannooccoommppoossiitteess
Figure 4 shows the XRD scan of composites with different D-clay loading. A reference of uncured
FFiigguurree 44 sshhoowwss tthhee XXRRDD ssccaann ooff ccoommppoossiitteess wwiitthh ddiiffffeerreenntt DD--ccllaayy llooaaddiinngg.. AA rreeffeerreennccee ooff uunnccuurreedd
sample without the addition of the amine hardener (labelled as 5 wt % precursor) was also plotted.
ssaammppllee wwiitthhoouutt tthhee aaddddiittiioonn ooff tthhee aammiinnee hhaarrddeenneerr ((llaabbeelllleedd aass 55 wwtt %% pprreeccuurrssoorr)) wwaass aallssoo ppllootttteedd..
The d-spacing of the D-clay has expanded from the initial 1.53 nm to 2.35 nm after epoxy resin was
TThhee dd--ssppaacciinngg ooff tthhee DD--ccllaayy hhaass eexxppaannddeedd ffrroomm tthhee iinniittiiaall 11..5533 nnmm ttoo 22..3355 nnmm aafftteerr eeppooxxyy rreessiinn wwaass
added into the solution. This signifies that the epoxy resin has intercalated into the D-clay layers.
aaddddeedd iinnttoo tthhee ssoolluuttiioonn.. TThhiiss ssiiggnniififieess tthhaatt tthhee eeppooxxyy rreessiinn hhaass iinntteerrccaallaatteedd iinnttoo tthhee DD--ccllaayy llaayyeerrss..
Materials2017,10,776 5of16
Materials 2017, 10, 776 5 of 16
Materials 2017, 10, 776 5 of 16

FFiigguurree 44.. XXRRDD ccuurrvveess ooff DD--ccllaayy eeppooxxyy aatt ddiiffffeerreenntt wweeiigghhtt llooaaddiinnggss..
Figure 4. XRD curves of D-clay epoxy at different weight loadings.
AAfftteerr tthhee eeppooxxyy rreessiinn wwaass ccuurreedd wwiitthh tthhee aammiinnee hhaarrddeenneerr,, tthhee ppeeaakk aatt 22θθ == 33..88◦° wweeaakkeenneedd
After the epoxy resin was cured with the amine hardener, the peak at 2θ = 3.8° weakened
significantly. This result shows that during curing, intercalation of D-clay has taken place. This extent
significantly. Thisresultshowsthatduringcuring,intercalationofD-clayhastakenplace. Thisextent
significantly. This result shows that during curing, intercalation of D-clay has taken place. This extent
of intercalation could lead to great improvement in both mechanical and physical properties of the
ofintercalationcouldleadtogreatimprovementinbothmechanicalandphysicalpropertiesofthe
of intercalation could lead to great improvement in both mechanical and physical properties of the
polymer clay composite [24–27]. Transmission electron microscope (TEM) images taken from the
polymer clay composite [24–27]. Transmission electron microscope (TEM) images taken from the
polymer clay composite [24–27]. Transmission electron microscope (TEM) images taken from the
cured D-clay epoxy nanocomposites show that the D-clay are in the form of thin tactoids formed by
curedD-clayepoxynanocompositesshowthattheD-clayareintheformofthintactoidsformedbya
cured D-clay epoxy nanocomposites show that the D-clay are in the form of thin tactoids formed by
a few layers and randomly distributed throughout the epoxy matrix (Figure 5). The thickness of such
fewlayersandrandomlydistributedthroughouttheepoxymatrix(Figure5). Thethicknessofsuch
a few layers and randomly distributed throughout the epoxy matrix (Figure 5). The thickness of such
tactoids is about 10 nm and the length to thickness ratio in the range of around 50. At the same time,
tactoidsisabout10nmandthelengthtothicknessratiointherangeofaround50. Atthesametime,
tactoids is about 10 nm and the length to thickness ratio in the range of around 50. At the same time,
completely exfoliated single layers can also be observed in the area near the tactoids. It is, therefore,
completelyexfoliatedsinglelayerscanalsobeobservedintheareanearthetactoids. Itis,therefore,
completely exfoliated single layers can also be observed in the area near the tactoids. It is, therefore,
concluded that the PDOPA coating on the MMT greatly benefits the intercalation and exfoliation of
concludedthatthePDOPAcoatingontheMMTgreatlybenefitstheintercalationandexfoliationof
concluded that the PDOPA coating on the MMT greatly benefits the intercalation and exfoliation of
D-clay within the epoxy matrix because of the strong interaction between PDOPA and epoxy matrix
D-claywithintheepoxymatrixbecauseofthestronginteractionbetweenPDOPAandepoxymatrixto
D-clay within the epoxy matrix because of the strong interaction between PDOPA and epoxy matrix
to give a higher magnitude of intercalation and partial exfoliation of the D-clay within the epoxy matrix.
giveahighermagnitudeofintercalationandpartialexfoliationoftheD-claywithintheepoxymatrix.
to give a higher magnitude of intercalation and partial exfoliation of the D-clay within the epoxy matrix.

(a) (b) (c)
(a) (b) (c)
Figure 5. TEM micrographs of the D-clay epoxy at different weight loading. (a) 1 wt %; (b) 2 wt %;
Figure 5. TEM micrographs of the D-clay epoxy at different weight loading. (a) 1 wt %; (b) 2 wt %;
Fanigdu (rce) 55. wTtE %M. mThicer socgarlaep bhasr orefpthreeseDn-tcsl a5y00e pnomx yina atldl iffifgeurreenst. weightloading. (a)1wt%;(b)2wt%;
and (c) 5 wt %. The scale bar represents 500 nm in all figures.
and(c)5wt%.Thescalebarrepresents500nminallfigures.
On the other hand, TEM images taken from the cured DOPA-coated carbon nanofibre (D-CNF)
On the other hand, TEM images taken from the cured DOPA-coated carbon nanofibre (D-CNF)
epoxOy nnathneocootmheprohsaitneds ,sThEoMw tihmaatg tehse tDak-CenNfFro amret hinedciuvirdeduaDllOy PaAnd-c oraanteddomcalryb odnispnaernsoefidb trher(oDu-gChNouFt)
epoxy nanocomposites show that the D-CNF are individually and randomly dispersed throughout
ethpeo xeyponxayn omcoamtripxo (sFitiegsusrhe o6w). tPhDatOtPhAeD co-CaNtinFga oreni nthdei vCidNuFa lelynsaunrdesr agnodoodm dliyspdeisrpsieornse odf tDhr-oCuNgFh owuitththine
the epoxy matrix (Figure 6). PDOPA coating on the CNF ensures good dispersion of D-CNF within
ethpeo expyomxya tmriaxtr(iFxi,g aunrde t6h)i.sP isD aOttPrAibuctoeadt itnog thoen stthroenCgN inFteernascutiroens bgeotowdeednis PpDerOsiPoAn aonfdD t-hCeN epFowxyit hminattrhixe.
the epoxy matrix, and this is attributed to the strong interaction between PDOPA and the epoxy matrix.
epoxymatrix,andthisisattributedtothestronginteractionbetweenPDOPAandtheepoxymatrix.
Materials2017,10,776 6of16
Materials 2017, 10, 776 6 of 16
Materials 2017, 10, 776 6 of 16
(a) (b)
Figure 6. TEM microgr(aap)h s of the D-CNF epoxy nanocomposites with (a) low and(b (b)) high magnifications.
Figure 6. TEM micrographs of the D-CNF epoxy nanocomposites with (a) low and (b)
hFiigguhrme 6a.g TnEifiMc amtiiocnrosg.raphs of the D-CNF epoxy nanocomposites with (a) low and (b) high magnifications.
2.3. Storage Modulus
22..33.. SSAttoosrr aasggheeo MMwoondd uubllyuu sst he dynamic mechanical analysis in Figure 7, by adding D-clay into the epoxy
matrix, the storage modulus has an impressive improvement over epoxy without any filler addition.
AAss sshhoowwnn bbyy tthhee ddyynnaammiicc mmeecchhaanniiccaall aannaallyyssiiss iinn FFiigguurree 77,, bbyy aaddddiinngg DD--ccllaayy iinnttoo tthhee eeppooxxyy
The increase in modulus is close to 30% at 3 wt % clay loading. Among different loadings (0–3 wt %),
mmaattrriixx,, tthhee ssttoorraaggee mmoodduulluuss hhaass aann iimmpprreessssiivvee iimmpprroovveemmeenntt oovveerr eeppooxxyy wwiitthhoouutt aannyy ffiilllleerr aaddddiittiioonn..
the 1 wt % loading seems to have relatively better improvement compared to neat epoxy than the
TThhee iinnccrreeaassee iinn mmoodduulluuss iiss cclloossee ttoo 3300%% aatt 33 wwtt %% ccllaayy llooaaddiinngg.. AAmmoonngg ddiiffffeerreenntt llooaaddiinnggss ((00––33 wwtt %%)),,
subsequent increase of the loading. We deduce that at lower clay loading, the clay is able to disperse
tthhee 11 wwtt %% llooaaddiinngg sseeeemmss ttoo hhaavvee rreellaattiivveellyy bbeetttteerr iimmpprroovveemmeenntt ccoommppaarreedd ttoo nneeaatt eeppooxxyy tthhaann tthhee
and distribute better within the matrix.
ssuubbsseeqquueenntt iinnccrreeaassee ooff tthhee llooaaddiinngg.. WWee ddeedduuccee tthhaatt aatt lloowweerr ccllaayy llooaaddiinngg,, tthhee ccllaayy iiss aabbllee ttoo ddiissppeerrssee
aanndd ddiissttrriibbuuttee bbeetttteerr wwiitthhiinn tthhee mmaattrriixx..

Figure 7. Storage modulus of D-clay epoxy at different weight loading, me asured by dynamic
mechanical analysis (DMA).
FFiigguurree 77.. SSttoorraaggee mmoodduulluuss ooff DD--ccllaayy eeppooxxyy aatt ddiiffffeerreenntt wweeiigghhtt llooaaddiinngg,, mmeeaassuurreedd bbyy ddyynnaammiicc
mmeecchhaanniiccaall aannaallyyssiiss ((DDMMAA))..
Figure 8 shows that the storage modulus of D-clay composite is higher than that of untreated
virgin clay (V-clay) composite, while the epoxy without filler is even lower than the V-clay composite.
Figure 8 shows that the storage modulus of D-clay composite is higher than that of untreated
Figure8showsthatthestoragemodulusofD-claycompositeishigherthanthatofuntreated
The comparison indicates that the interfacial interactions, and thereby storage moduli, are indeed
virgin clay (V-clay) composite, while the epoxy without filler is even lower than the V-clay composite.
virginclay(V-clay)composite,whiletheepoxywithoutfillerisevenlowerthantheV-claycomposite.
enhanced by the DOPA treatment.
The comparison indicates that the interfacial interactions, and thereby storage moduli, are indeed
Thecomparison indicates thattheinterfacialinteractions, andtherebystoragemoduli, are indeed
enhanced by the DOPA treatment.
enhancedbytheDOPAtreatment.
Materials2017,10,776 7of16
MMataetreirailasl s2 2001177, ,1 100, ,7 77766 77 o of f1 166

FFiFgigiugurureer8e 8. 8.D .D DMMMAAA cc ucururvvreveses soo fof nfn eneaeatat ete peppooxoxyxy,y,e ,e pepopoxoxyxyyw w wiitthihth5 5 5 ww wtt %t% % DD D--cc-lclaalyay,y,5 ,5 5 ww wtt %t% % VV V--cc-lclaalyay,y,a ,an andndd5 5 5w w wtt %t% % oo rorggragananonococlclaalyay.y. .
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111w w wtt% t% %a a daddddidtiitioitoinonno o fofD fD -Dc-lc-aclaylay.y.A .A tAt2 t2 w2 w twt% t% ,%,t ,ht hteheie ni ncinrcecrareesaaesseie si si~s ~ 2~2525%5%%.. F.F rFroromomm pp rprerevevivoioiuoususs XX RXRDRDD rr eresesusulutltslsts aa nandnddT T ETEMEMM
oobobsbseserervrvavataitoitoinonsns,s,D ,D D-c-c-laclalyaysysas ar arereie ni nitnetetrercrcacalaalaltaetetdeddw w witihtihtihninint ht hteheee ep epopoxoxyxyym m maatartritxrixixw w witihtihths so somommeeoe o foft fth hteheec c lclaalyayyl la alyayeyerercr c ocomommpplpleelteteetlelyyly
eexexfxfoofloliialaitateetdedd ini nin tth hteheee e pepopoxoxyxyym m maatartrtixrixi.x. T.T Thhihissis ii lllilululsuststrrtaratateetsess tt hhtahatat ttt hhtihissis ss tstaattateete oo fof dfd idsisipspeperersrsisiooinonn hh ehelelpplspss ini nin imi mimpprprorovovivninigngg tht htehee
sstsitfitffinffnfenesessssos f oDof f- cDDla-c-yclaelaypy o exepyponoxaxyny o ncnaoanmnoopccooosmmitpepsoo.ssIitnietecsso. .n ItnIrn a cscto,ontnhtrteraaYssto,t u, tnhtghe’e s YmYoooudununglg’us’ss momfoeodpduoulxulyussV o-ocfl fa eyeppcoooxmxyy p VoVs-ci-tclealsayy
dcrcooompmpppeoodssitbietyess ad bdrorooupptpp2ee4dd% b bywy ai atbhbooauudt td2 2i4t4%i%o nw woitfiht2h a waddtdd%itiitoiVon-nc o loaf fy2 .2 w wt t% % V V-c-clalayy. .
SSiSmimimilialialrarlyrlyl,y,a ,as asss sh shohowowwnnni ni ninF F iFgigiugururere9e 9 b9b,b,t ,th hteheeY Y oYouounungng’gs’s’ms m moododuduluululsusos o fofD fD D--CC-NCNNFFFc co comommppopososistietietei ni nicncrcrereaeasasesesesws w witithihthfi f liflliellelrerr
lololaoadadidninigng.g.T .T hThehererereei is sisa a nanni in nicncrcrereaeasaseseeo o fof~ f~ 1~1818%8%% iin nin YY oYouounungng’gs’s’ms m moododuduluululsusos o foft fth hteheec c ocomommppopososistiteietsesws w witithihth1 1 1w w wtt% t% %a a daddddidtitiioitoinonno o fof f
DDD--CC-CNNNFF.F.A .A Att2 t2 2w w wtt% t% %,,t ,th hteheei in nicncrcrereaeasaseseei is sis~ ~ 2~ 24 24%4%%..T .T hTheheeY Y oYouounungng’gs’s’ms m moododudululuulsusso o foft fth htehee uu unntntrrtereaeatateetdedd vv ivirrigrgiginnin CC CNNNFFF (( VV(V--CC-CNNNFF)F) )
ccocomommppoposositsietietsesds d rdroropoppppepededdb b ybyya ab abobououtut1 t19 19%9%%w w witihtihtht ht hteheae ad addddidtiitioitoinonno of of2 f2 w2 w wtt% t% %V V -VC-C-NCNFN.FF. .

Figure9.Young’smodulusofcompositeswithdifferentfillerloading(a)MMTclay(b)CNF.
FFigiguurere 9 9. .Y Yoouunngg’s’ sm moodduululus so of fc coommppoosistietes sw witihth d dififfefererennt tf iflilleler rl oloaaddiningg ( a(a) )M MMMTT c clalayy ( b(b) )C CNNFF. .
Materials2017,10,776 8of16
Materials 2017, 10, 776 8 of 16
ThTehec ocnotnrtarsatsbt ebtewtweeenenD DOOPPAA-c-ocoaatetdedfi flillelersrsa anndd vviirrggiinn fifilllleerrss cclleeaarrllyy iinnddiiccaatteess tthhee iimmppoorrttaannccee of
ofininteterrfafacciaial laaddhheessioionn bbeettwweeeenn tthhee mmaattrriixx aanndd tthhee fifilllleerrss.. TThhee DD--ccllaayy aannddD D-C-CNNFF,,h ahvaivnignga ast rsotrnogng
adahdehseiosniown iwthitthh ethmea mtriaxt,raixr,e aarbel eabtolee ntoa belneaebfflee cetifvfeeclytivtrealnys tfrearnsstfreers ssftrroesms tfhroemep tohxey empoatxryix mtoatthriex fitoll etrhe
(wfhililcehr (hwashiachh ihgahse ra shtiifgfhneers ss)t.ifAfnseassr)e.s Aulst ,at hreesouvlte, rtahlel eolvasetriacllc oenlassttainc tcionncsrteaanste sinbcerceaauseses btheecafiullseer tihsea bfilleler
toisu nadbleer ttaok uensdomeretaokfet hsoemster eosfs .thUen sttrreeastse.d Ufinltlerer,aotendt hfielloerth, oernh tahned o,tdhoere shnaontdh, advoeesth neostt hroanvge itnhtee rsftarcoeng
wiitnhtetrhfeacme awtriitxh. tThhee mreaftorriex,. tThheesrterfeosrsei,s thlaer gsterleyssu nisd learrtgaekleyn ubnydethrteamkeant rbixy othnely m. TahtreixY oonulnyg. ’Tshme oYdouulnugs’s
demcroedauselussb deeccaruesaeseosf bloecsasuosfev oof lluomsse otfr ivcopluemrceetnrtica gpeerocfenthtaegleo aodf thbee aloriandg bpeharaisnego pfhtahsee eopf othxey empoaxtryi xm.atrix.
2.52..5F. rFarcatuctruerTeo Tuoguhgnhenssess
ThTehfer afrcatuctruerteo tuoguhgnhensessos botbatianiendedb ybyth tehsei nsignlge-leed-egdegne ontochtcehded3- 3p-opionitnbt ebnednidnigng(S (ESNEN-3-P3BP)Bt)e tsetsits is
shsohwownnin inF igFiugruerse1s0 1a0n adn1d1 1.1T. hTehKe IKc aIc nadndG IGcIvc avlauleuseso fotfh tehDe -Dcl-aclyayep eopxoyxyn annaoncoocmompopsoitseitseas raerleo lwowerer
thtahnatnh tahtaotf otfh tehne enaetaetp eopxoyx.yT. hTehaed addidtiiotnioonf o2f w2 wt%t %cl calyaylo laodaidnigngin inth teheD D-c-lcalyayn annaoncoocmompopsoitseitseesx ehxihbiibtsits
atao utoguhgnhensessosf o0f. 508.5M8 PMaP·ma∙1m/21/2a nadnd8 88.87.474J/ Jm/m22f oforrK KIcIca annddG GIcI,c,r eressppeecctitviveelyly..T Thhisisr erpeprerseesnentstsa ad rdorpopo fof
111.81%.8%fo rfoKr IKcaIc nadnd3 63.69.%9%fo froGr GIcIac sasc ocommpparaerdedto ton neaetaet pepoxoyx.yH. Howoweveevre,rt,h tehaed addidtiiotinonof o2f w2 wt%t %u nutnretraetaetded
V-Vcl-acylaeyx hexibhiitbsiatst ao utoguhgnhesnseossf 0o.f8 304.8M34P Ma·Pma1∙/m21a/2n adn2d9 209.10.J1/ mJ/m2f2 ofrorK IKcIac nadndG GIc,Ic,r ersepspecetcitvievleyl.yT. hTihsiiss ias nan
inicnrecraesaesoef o2f3 2.13%.1%fo froKr IKcaIc nadnd1 0140%4%fo froGr IGcIac sasc ocmompparaerdedto ton enaetaet peopxoyx.y.
WWithitCh NCFN,FK,I cKaIcn adndG IGcIvc avlauleuseso fobf obtohthD D-C-CNNFFa nadndV -VC-NCNFFep eopxoyxyn annaoncoocmompopsoitseitseasr aerhe ihgihgehretrh tahnan
thtahtaotf otfh tehne enaetaetp eopxoyx.yT. hTehaed addidtiiotnioonf o2f w2 wt%t %cl aclyayin inth tehDe D-C-NCNFFn annaoncoocmompopsoitseiteex ehxihbiibtsitas tao tuoguhgnhensesss
ofo0f. 807.8878M MPaP·am∙m1/12/2 aanndd 220022..55 JJ//mm22 ffoorr KKIcI canandd GGIc,I cr,ersepsepcetcivtievlyel.y T.hTihs iiss iasna inncinrecaresae soef o3f33.03%.0 %forf oKrIcK aIncd
an4d3.403%.0 %forf oGrIGc aIcs acsocmompapraerde dtot onenaeta tepepooxyxy. .TThhee aaddddiittiioonn ooff 22 wwtt %% VV-C-CNNFFe xexhhibibitistsa ato tuoguhgnhensessos fof
0.803.863M6 MPaP·am∙m1/12/2a anndd 228800..44 JJ//mm22 ffoorr KKIcI caanndd GGIc,I cr,ersepspecetcitvievleyl.y .TThhisi srerpeprerseesnentst sanan inincrceraesaes eofo f262.67.%7% fofro rKIc
KIacnadn d989.80.%0% fofro GrGIc Iacsa csocmompapraerde dtot onenaeta etpeopxoyx.y .

FigFuigruer1e0 1.0C. rCitriictaiclaslt rsetrsessisn itnentesnitsyitfya cfatoctroorf onfa nnaoncoocmompopsoitseistews withitdh idffiefrfeenretnfitl fleilrlelro alodaidngin(ga )(aM) MMMTcTl acylay
(b()bC) NCFN.F.
Materials2017,10,776 9of16
Materials 2017, 10, 776 9 of 16

FigureF1i1g.uCrer i1t1ic. aClrsittircaailn steranienr egnyerrgeyle raesleearsaet eraotef onfa nnaoncoocmomppoossititeess wwiitthh ddiiffffeerreennt tfifilllelre lrolaodaidngin (ga)( Ma)MMTM T
clay(b)clCayN (Fb.) CNF.
2.6. Fra2c.t6o. gFrraapcthoigcrAapnhaicl yAsnisaloyfsiDs -oCf Dla-yCNlaya nNoacnoomcopmospiotseistes
A comparison of neat epoxy, V-clay, and D-clay epoxy composites fracture surfaces are shown in
Acomparisonofneatepoxy,V-clay,andD-clayepoxycompositesfracturesurfacesareshownin
Figure 12. Both composites have rougher surface features compared with neat epoxy, indicating that
Figure12.Bothcompositeshaveroughersurfacefeaturescomparedwithneatepoxy,indicatingthatthe
the crack propagation has occurred by a longer path. The V-clay epoxy nanocomposites have larger
crackpropagationhasoccurredbyalongerpath. TheV-clayepoxynanocompositeshavelargerfacets
facets than D-clay. The larger fractured facets on V-clay composites is probably due to the difference in
thanD-clay. ThelargerfracturedfacetsonV-claycompositesisprobablyduetothedifferenceinclay
clay dispersion: a better dispersed D-clay composite (smaller clay unit) is able to provide higher number
dispersoifo cnl:aya pbaertttiecrleds ipsepre urnseitd voDlu-cmlaey. Icmopmoprtoansittley,( tshmerael laerre clalargyeu nnuimt)biesras bolfe setocopnrdoavryid meihcriogchrearcknsu inm tbheer of
claypafrrtaicctluersedp eVr-culanyi tcovmolpuomsiete.sI (mFipguorret a1n3ctl,dy,).t Mheicrreoa-crreaclakringge rneulemasbese trhseo cfrascekc otinpd starreyssmes iacnrodc driascsikpsatiens the
fractureadddVit-icolnaayl caommoupnots oitfe esn(eFriggyu. rAes1 a3 rce,dsu).ltM, thicer Vo--cclraayc kcoinmgproeslieteass sehsotwhe acnr eanckhatnipcesdt rferascsteusrea nredsidstiasnsicpe.a tes
additioVn-acllaaym doouesn ntootf aednheerrgey t.oA thsea epreosxuyl tm,athtreixV a-sc lsatryoncoglmy paso siti tdeosessh ino wD-aclnaye,n whhainchce ids efvraidcetuncreedr ebsyi sthtaen ce.
decrease in Young’s modulus of V-clay composite. Therefore, it is understandable why the secondary
V-claydoesnotadheretotheepoxymatrixasstronglyasitdoesinD-clay,whichisevidencedbythe
cracks can be easily formed during the fracture of V-clay composites.
decreaseinYoung’smodulusofV-claycomposite. Therefore,itisunderstandablewhythesecondary
TEM images were taken at sub-critical points of the crack initiation site of D-clay epoxy
crackscanbeeasilyformedduringthefractureofV-claycomposites.
nanocomposites (Figure 13a). Several discontinuous cavities are identified in the D-clay layers. Some
TEM images were taken at sub-critical points of the crack initiation site of D-clay epoxy
long microcavities in between clay layers can also be seen. Most of such cavities are formed inside
nanocotmhep Dos-ciltaeys (laFyigerusr ein1d3icaa)t.inSge vtehraat lddeilsacmoinntaitniouno uofs cclaavyi tlaieyserasr ehaidvee nintidfieeedd itnaktehne pDla-cclea. yUlnadyeerr sh.igSho me
longmmicargonciafivciattiieosn i(nFibgeutrwe 1e3ebn),c tlhaeysel amyiecrrosccraanckas lasroe bcoensfeiremne.dM too sbte oinfistiuatcehd cwaivthitiine sthaer geaflolerrmy oefd cilanys ide
theD-clalayyerlsa, yneort sati nthdei ceaptoinxgy Dth-acltady einlatemrfiancaet. iTohne oofbcslearvyaltaioyne rcsonhfairvmesi nthdaet ewdeatakklye nbopnldaecde .clUayn ldaeyrerhs igh
magnifidcelaatmioinnat(eFdi gduurreing1 3frba)c,tutrhee. sTehims oibcsreorcvraaticokns thaarte thceo cnlfiayr mwietdh wtoeabke initneri-tliaayteerds swtreitnhgitnh dtheleamgianlaleterdy of
clay ladyuerrisn,gn fortacatutrteh pereopviodxeys aD s-tcrloanyg isnutpeprfoarct eto. tThhe eeaorbliesre revxaptliaonnaticoonn ffiorrm thse tfhraacttuwree atokulyghbnoenssd derdopc lay
based on fractographic analysis.
layersdelaminatedduringfracture. Thisobservationthattheclaywithweakinter-layersstrength
In contrast, in V-clay epoxy composites as the degree of intercalation is poor (Figure 13c) and
delaminated during fracture provides a strong support to the earlier explanation for the fracture
with little interaction to the matrix, the microcavities (the fracture path) are dominated by the
toughnessdropbasedonfractographicanalysis.
microcracks initiated along the epoxy—V-clay interface (Figure 13d). This is the reason for the
Incontrast,inV-clayepoxycompositesasthedegreeofintercalationispoor(Figure13c)and
observed microcrack deflection and formation of the secondary cracking. These observations again
with lirtetliteerianttee trhaec itmiopnorttoantchee ofm suartfraicxe, mthoedimficiactrioonc aovf ictliaeys la(tyheers firna scttruenregthpeantihn)g athree idntoemrfaincea bteedtwbeeyn the
microctrhaec kclsayi naintdia ttheed epalooxnyg mtahtreixe. p oxy—V-clay interface (Figure 13d). This is the reason for the
observedmicrocrackdeflectionandformationofthesecondarycracking. Theseobservationsagain
reiterat etheimportanceofsurfacemodificationofclaylayersinstrengtheningtheinterfacebetween
theclayandtheepoxymatrix.
Materials2017,10,776 10of16
Materials 2017, 10, 776 10 of 16
Materials 2017, 10, 776 10 of 16
(a) (b)
(a) (b)
(c) (d)
(c) (d)
(e) (f)
Figure1F2ig.uSrec a1n2.n Sincagnneilnegc te(reloe)c ntrmoni cmroicsrcooscpoypy( S(SEEMM)) mmiiccrrooggrarpahp hof odfifdfeirfefnetr efrnatc(tffu)rraec tsuurrfeacseus.r f(aa)c ecrsa.ck(a )crack
initiation site of neat epoxy; (b) fast fracture site of neat epoxy; (c) crack initiation site 2 wt % V-clay
initiatioeFnpigosuxirtyee; (1od2f). nfSacesaatn tfnreainpcgtou xerelye ;csit(trbeo )no ffm a2si cwtrfotr s%acco Vtpu-ycr l(eaSyEs iMetpe)o omxyfi;cn r(eoe)ag rtcareappchko oxifny idt;iia(ffctei)orecnnr sta ictferka ociftn u2ir tewi astt u%irof naDc-escsilt.a e(ya 2)e pcwroaxtcy%k; V-clay
epoxy;(aidnni)dtif a(aftsi)o tfnaf srstai tfcerta uocfrt uenreseai stt eietepo oofxf2 y2; w w(btt) %% fa DsVt- -fccrlalaacyyt uepereop xsoiytx.e y o;f( en)eactr eapcokxiyn; i(tci)a ctrioacnk sinitietiaotfio2n wsitte% 2 wDt- c%la Vy-celpayo xy;and
(f)fastferpaocxtuy;r (eds) iftaesto ffra2cwturte% sitDe -ocfl a2 ywet p%o xVy-c.lay epoxy; (e) crack initiation site of 2 wt % D-clay epoxy;
and (f) fast fracture site of 2 wt % D-clay epoxy.
(a) (b)

(a) (b)

Figure13.Cont.