Sonication-Induced Modification of Carbon Nanotubes

14 Pages·2017·2.33 MB·English

materials
Article
Sonication-Induced Modification of Carbon
Nanotubes: Effect on the Rheological and
Thermo-Oxidative Behaviour of
Polymer-Based Nanocomposites
RossellaArrigo1,2,* ID,RosaliaTeresi1,CristianGambarotti3,FilippoParisi4 ID,
GiuseppeLazzara4 ID andNadkaTzankovaDintcheva1
1 DipartimentodiIngegneriaCivile,Ambientale,Aerospaziale,deiMateriali,
UniversitàdegliStudidiPalermo,VialedelleScienze,Ed.6,90128Palermo,Italy;
[email protected](R.T.);[email protected](N.T.D.)
2 DipartimentodiScienzaApplicataeTecnologia,PolitecnicodiTorino,VialeT.Michel5,
15121Alessandria,Italy
3 DipartimentodiChimica,MaterialiedIngegneriaChimica“G.Natta”,PolitecnicodiMilano,
PiazzaL.daVinci32,20133Milano,Italy;[email protected]
4 DipartimentodiFisicaeChimica,UniversitàdegliStudidiPalermo,VialedelleScienze,Ed.17,
90128Palermo,Italy;fi[email protected](F.P.);[email protected](G.L.)
* Correspondence:[email protected];Tel.:[phone]
Received:25January2018;Accepted:1March2018;Published:5March2018
Abstract: The aim of this work is the investigation of the effect of ultrasound treatment on the
structuralcharacteristicsofcarbonnanotubes(CNTs)andtheconsequentinfluencethattheshortening
induced by sonication exerts on the morphology, rheological behaviour and thermo-oxidative
resistanceofultra-highmolecularweightpolyethylene(UHMWPE)-basednanocomposites. First,
CNTshavebeensubjectedtosonicationfordifferenttimeintervalsandtheperformedspectroscopic
and morphological analyses reveal that a dramatic decrease of the CNT’s original length occurs
with increased sonication time. The reduction of the initial length of CNTs strongly affects the
nanocompositerheologicalbehaviour, whichprogressivelychangesfromsolid-liketoliquid-like
as the CNT sonication time increases. The study of the thermo-oxidative behaviour of the
investigated nanocomposites reveals that the CNT sonication has a detrimental effect on the
thermo-oxidativestabilityofnanocomposites,especiallyforlongexposuretimes. Theworseningof
thethermo-oxidativeresistanceofsonicatedCNT-containingnanocompositescouldbeattributed
to the lower thermal conductivity of low-aspect-ratio CNTs, which causes the increase of the
local temperature at the polymer/nanofillers interphase, with the consequent acceleration of the
degradativephenomena.
Keywords: UHMWPE;CNTs;sonication;nanocomposites;rheology;thermo-oxidativestability
1. Introduction
The outstanding electrical, magnetic and mechanical properties of CNTs have motivated a
flurryofinterestinexploitingtheirpotentialasnanofillersinpolymer-basednanocomposites[1,2].
Indeed, several studies reported in the literature show that adding CNTs to a polymer matrix
causesareinforcementofmechanicalproperties[3]andanenhancementofthermalandelectrical
conductivity [4,5]. Furthermore, the introduction of CNTs in polymeric matrices can enlarge the
polymer application field, since this strategy allows the formulation of technologically relevant
Materials 2018,11,383;doi:10.3390/ma11030383 www.mdpi.com/journal/materials
Materials 2018,11,383 2of14
nanostructuredmaterialssuitableforadvancedapplications,suchasenergystorage[6],electronics[7],
catalysis[8]andsensors[9].
ThefinalpropertiesofCNT-containingnanocompositesstronglydependontheextentoftheCNT
dispersionwithinthepolymericmatrixandonthequalityofthepolymer/nanofillerinterfacialregion.
Duetotheirhighsurface-to-volumeratio,CNTstendtointeractthroughVanderWaalsforces,forming
agglomeratesandaggregatesthatcompromisetheobtainmentofahomogeneousdistributionofCNTs
thorough the host matrix and, hence, a proper transfer of the CNT properties to the polymer [10].
Toovercomethisdrawback,significanteffortshavebeendevotedtothemodificationoftheexternal
surfaceofCNTsinordertoenhancetheirabilitytodisperseinpolymersortoformstablesuspensions
inbothorganicandinorganicsolvents. ThefunctionalizationofCNTsurfacescanbeachievedby
exploitingseveralmethods,includingcovalentlinkage[11]andphysicalabsorption[12],and,besides
ensuringimproveddispersabilityoftheCNTsinpolymers,canprovidethemwithnewfunctionalities.
TheintrinsicpropertiesofCNTsthemselves—suchassurfacequality[13],wavinessand,above
all,theiraspectratio[14]—arealsoofparamountimportancetoobtainnanostructuredpolymer-based
materials with enhanced final properties compared to the corresponding neat matrices. Indeed,
thehighaspectratioofCNTsisassociatedwithahighsurface-to-volumeratio,thathelpspromote
theCNTsuperiorpropertiestothepolymermatrix[15,16]. Therefore,theimprovementofthefinal
propertiesofCNT-containingnanocompositesisinpartattributedtothenanoparticles’highaspect
ratio,andmanystudieshaveshownenhancedphysicalpropertiesofpolymernanocompositeswith
increasingnanofilleraspectratio[17,18].Kimetal.studiedtheeffectofCNTgeometryanddimensions
onthethermalconductivityofpolymer-basednanocomposites,showingthataneffectivecontrolof
thispropertycanbeachievedthroughtheadjustmentofthenanoparticlelength. Thedependenceof
thethermalconductivityofCNTsbasednanocompositesonthenanofilleraspectratiocanbeexplained
considering that a higher aspect ratio favors the formation of a percolation network, allowing the
conductionofphononsoveralongdistance,withoutanytransitionfromparticletoparticle[19].
TheaspectratioofCNTsalsoplaysafundamentalroleindeterminingthepercolationthreshold
in polymer-based nanocomposites, that is, the critical concentration of CNTs needed to form
interconnected networks of nanofillers within the polymer matrix. When the CNT concentration
reachesthepercolationthreshold,thenanocompositeelectricalconductivity(electricalpercolation
threshold) or storage modulus (rheological percolation threshold) suddenly increases, due to the
formationofasemi-3Dnetworkofnanofillerswithinthehostpolymermatrix[20,21]. Asreportedin
theliterature,anincreaseoftheCNTaspectratioreducesthecriticalconcentrationfortheformation
of a percolated network and nanoparticles with a very high aspect ratio can form interconnected
networksatverylowvolumefractions[22]. Ciprianoetal.[23]studiedtheeffectoftheCNTaspect
ratioontherheologicalbehaviorofpolystyrene-basednanocomposites,showingthatCNTswitha
higheraspectratiobringaboutamorepronouncedenhancementofboththestoragemodulusand
viscositycomparedtothatofnanocomposite-containingCNTswithaloweraspectratio.Similarresults
have been found by Potschke et al. in polycarbonate/CNT nanocomposites for which, at a given
CNTconcentration,thecomplexviscosityprogressivelyincreaseswithanincreaseintheCNTaspect
ratio[24].
UltrasonictreatmentiswidelyusedtodisperseCNTsinsolution,tofunctionalizeCNTsorto
formulatenanocompositesandnanomaterials[25,26]. TheeffectsofsonicationonCNTsderivefrom
theshearforcescomingfromcavitationphenomenathatareabletobreaktheCNTaggregatesformed
duetotheVanderWaalsinteractionsbetweentheCNTsthemselves[27]. However,thecavitation
phenomenacancausetheformationofstructuraldefectsontheCNT’ssurfaceand,eventually,their
breakage,withmodificationofinitialCNTlengthandaspectratio[28].
TheaimofthisworkistoevaluatetheeffectofultrasonictreatmentontheCNTaspectratioand
howthisissueaffectsthemorphologyandtherheologicalpropertiesofpolymer-basednanocomposites,
aswellastheirthermo-oxidativeresistance. Despitequalitativeandquantitativestudiesonreducing
CNTlengthbysonicationbeingpresentinthescientificliterature,inmanycasesthishasnotbeen
Materials 2018,11,383 3of14
accompaniedbyinvestigatinghowthealterationoftheCNT’sstructureandaspectratioinfluencesthe
rheologicalbehaviourand,aboveall,thethermo-oxidativestabilityofCNT-containingnanocomposites.
Specifically,inthiswork,CNTsweresubjectedtoasonicationtreatmentfordifferenttimeintervals
andthethusmodifiednanofillerswereusedtoformulateultra-highmolecularweightpolyethylene
(UHMWPE)-basednanocomposites. Therheologicalbehaviorandthethermo-oxidativeresistanceof
formulatednanocompositeshavebeendeeplyevaluatedanddiscussed,consideringtheeffectofthe
sonicationtimeontheinitialdimensionsofCNTs.
2. MaterialsandMethods
2.1. Materials
UHMWPEisacommercialgradeproductpurchasedbySigma-Aldrich(SaintLouis,MO,USA)
intheformofawhitepowder. Itsmainpropertiesare: averagemolecularweightM =3÷6MDa,
w
softeningpointT=136◦C(Vicat,ASTMD1525B),meltingpointTm=138◦C,degreeofcrystallinity
about48.1%(estimatedthroughDifferentialScanningCalorimetrymeasurementconsideringaheat
of fusion for perfectly crystalline polyethylene ∆Hf = 288.84 J/g), and density (cid:36) = 0.94 g/mL at
25◦C.Theaveragemolecularweightwasdetectedbymeansofintrinsicviscosity[η]measurements
accordingtoASTMD4020-05usingaLaudaPro-linePV15viscometer. TheUHMWPEwasdissolved
indecahydronaphtalene(reagentgradeSigma-Aldrich)at150◦Candmaintainedundermagnetic
stirringforonehour. Theviscosimetricaveragemolecularweight,estimatedfromthreeindependent
measuresof[η]usingtheMargoliesequation,Mv=5.37×104[η]1.37,wasMv∼=4.9×106g/mol.
Multiwalledcarbonnanotubes,CNTs,werepreparedbythetypicalchemicalvapordeposition
(CVD)protocol,usingethyleneasthecarbonsource[29]. Thepurificationwasperformedwith50%
aqueous sulfuric acid, obtaining carbon nanotubes with outer diameters ranging between 14 and
20nm,innerdiametersintherangeof2to5nm,length1÷10µm,andpurity>98wt%. Afterthe
purificationprocess,thecontentofcarboxylicgroupswasestimatedtobeabout0.5%bymeansofXPS
analysisandacid/basetitration.
Allchemicalsandreagentswereusedasreceived,withoutfurtherpurification.
2.2. CarbonNanotubes(CNTs)Sonication
Sixidenticalsamplesof0.2gofCNTswereplacedin50mLone-neckroundbottomflasks,then
20mLofdistilledwaterwereaddedtoeachone. Theflaskswereplacedinanultrasonicbath(water
volume2.5L,power260W)takingcaretohavethewaterlevelhigherthanthelevelofthesuspension.
Thesixreactionmixturesweresonicatedatroomtemperaturefordifferenttimeintervals,particularly,
15, 30, 60, 90 and 120 min. The ultrasound treated CNTs were labeled as CNT-X, where X is the
sonicationtime.
2.3. NanocompositePreparation
TheUHMWPEpowderand1wt%ofuntreatedCNTsandsonicatedCNTsweremanuallymixed
atroomtemperatureandtheresultingpowderwashomogenizedbythoroughgrindinginaporcelain
mortartoavisuallyhomogeneousstate. Afterwards,thecompositepowderwaskeptundermagnetic
stirringforabout12h,untiltheachievementofahomogeneousblackpowder.
Theblendswerethenhotcompactedat210◦Cfor5minandunderapressureof1500psitoget
thinfilms(thicknesslessthan100µm)forthesubsequentanalyses.
TheneatUHMWPEwassubjectedtothesameprocedureforcomparison.
2.4. Characterization
To assess the surface quality of CNTs, micro-Raman spectroscopy was performed at room
temperature through a Renishaw (Gloucestershire, UK) Invia Raman Microscope equipped with
Materials 2018,11,383 4of14
a532nmNd:YAGlaserexcitationand100mWpower. Non-confocalmeasurementswerecarriedout
intherange3200–100cm−1withaspectralresolutionbetween0.5and1cm−1.
Transmissionelectronmicroscopy(TEM)analyseswereperformedonahomogeneousdispersion
oftheCNTsin2%aqueoussodiumdodecylsulfate(SDS,suppliedbySigma-Aldrich)usingaPhilips
(Amsterdam,TheNetherlands)CM200fieldemissiongunmicroscopeoperatingatanaccelerating
voltageof200kV,withtheaimofinspectingthemorphologyoftreatedCNTs. AGatan(Pleasanton,
CA,USA)US1000CCDcamerawasusedand2048×2048pixelsimageswith256greylevelswere
recorded.Forthespecimenpreparation,afewdropsofthewatersolutionsweredepositedon200mesh
laceycarbon-coatedcoppergridandair-driedforseveralhoursbeforeanalysis.
Inordertoevaluatethedimensionsoftheultrasound-treatedCNTs,dynamiclightscattering
measurementsonthehighdilutedCNTdispersions(10−3w/w%)werecarriedoutusingaZetasizer
NANO-ZS(MalvernInstruments,Malvern,UK).A5mMsodiumdodecylsulphate(SDS)aqueous
solutionwasusedtoimprovetheCNTs’colloidalstability. Thefield-timeautocorrelationfunctions
wereanalysedbyinverseLaplacetransformation(ILT)thatprovidethedistributionofthedecayrates
(Γ)forthediffusivemodes. Forthetranslationalmotion,thecollectivediffusioncoefficient(D)given
byD=G/q2,whereqisthescatteringvector(4πnλ−1sin(θ/2);withnbeingthewaterrefractive
index,λthewavelength(632.8nm),andθthescatteringangle(173◦)). Thedistributionoftheapparent
hydrodynamicdiameters(d )wascalculatedbyusingtheStokes−Einsteinequation.
H
Therheologicalbehaviorofformulatednanocompositeswasstudiedthroughrheologicaltests
conducted by means of a strain-controlled rotational rheometer (ARES G2 by TA Instruments,
NewCastle,DE,USA)inparallelplategeometry(platediameter25mm). Thecomplexviscosity(η*)
wasmeasuredperformingfrequencysweeptestsatT=210◦Cfrom10−2to102rad/satamaximum
strainof2%. Asprovedbypreliminarystrainsweepexperiments,suchanamplitudeislowenoughto
beinthelinearviscoelasticregime. Besides,linearstressrelaxationmeasurementswerecarriedout,
submittingthesamplestoasinglestepstrainγ =1%,andtheshearstressevolutionduringtimeσ(t)
0
wasmeasuredtoobtaintherelaxationmodulusG(t)=σ(t)/γ . Asregardsthereproducibilityofthe
0
obtainedresults,itwassatisfactory(±5%).
Thenanocompositemorphologywasinspectedusingopticalmicroscopy(Leica(Torino,Italy)
Microscope)inreflectionmodeatamagnificationof20×. Imageswereacquiredonthesurfaceofthe
nanocompositefilms.
AFouriertransforminfraredspectrometer(FTIR)(SpectrumTwoFTIRspectrometer,PerkinElmer,
Waltham,MA,USA)wasusedtorecordtheinfraredspectra. FT-IRanalyseswerecarriedouttoinfer
theadvanceofthedegradationphenomenaonnanocompositefilms. Specifically,thesampleswere
firsttreatedinanairovenatT=120◦C,whichisatemperaturelowerthantheT ofthepolymerbut
m
highenoughtoacceleratethedegradationprocesses. Then,FT-IRspectrawerecollectedperforming
16scansbetween4000and500cm−1onsamplessubjectedtothermo-oxidationfordifferentexposure
times. The carbonyl index (CI) was calculated as the ratio between the carbonyl absorption area
(1850–1600cm−1)andtheareaofareferencepeakatabout2019cm−1.
3. ResultsandDiscussion
3.1. AssessmentoftheEffectofUltrasonicTreatmentontheCNTDimensions
First of all, to assess the effect of sonication on the morphology and dimensions of CNTs,
preliminaryanalysesofuntreatedCNTsandCNTssubjectedtothesonicationtreatmentfordifferent
timeintervalswerecarriedout. Morespecifically,toevaluatetheeffectofthetreatmentontheCNTs’
defectcontent,Ramananalyseswereperformed. Figure1areportsthespectraobtainedforuntreated
CNTsandCNT-120. IntheRamanspectra,twoprominentpeaksweredetected: theD-bandlocatedat
1340cm−1andtheG-bandcenteredatabout1580cm−1. Thefirstisknownasdefect-induceddisorder
mode,andreferstothecarbonatomsontheCNTsurfacehavingsp3hybridization;thesecondisthe
so-calledtangentialmodeandisrelatedtothegraphiticstructureoftheCNTsurface[30]. InFigure1a,
Materials 2018,11,383 5of14
eachspectrumhasbeennormalizedsuchthattheG-bandintensityisconstant. Itisclearlyvisiblethat
Mthaeteriinaltse 2n01s8it, y11o, xf t heD-bandismorepronouncedfortheCNTssubjectedtothemaximumsonic5a otif o1n4
timethanfortheuntreatedCNTs,indicatingdamagetotheCNTstructureandtheformationoflattice
the formation of lattice defects due to the sonication treatment. The unusual high intensity of the D-
defectsduetothesonicationtreatment. TheunusualhighintensityoftheD-bandfortheuntreated
band for the untreated CNTs can be explained considering that the purification process can cause the
CNTscanbeexplainedconsideringthatthepurificationprocesscancausethemodificationofthe
modification of the CNT surface, with the anchoring of oxygen-containing groups. To qualitatively
CNTsurface,withtheanchoringofoxygen-containinggroups. Toqualitativelyevaluatethecontentof
evaluate the content of structural defects, the ratio ID/IG between the intensities of the two Raman
structuraldefects,theratioI /I betweentheintensitiesofthetwoRamanbandshasbeencalculated
D G
bands has been calculated for all CNTs and the obtained values are reported in the inset in Figure 1a.
forallCNTsandtheobtainedvaluesarereportedintheinsetinFigure1a. Indeed,itisknownfrom
Indeed, it is known from the literature that this ratio accounts for the concentration of defect site content
theliteraturethatthisratioaccountsfortheconcentrationofdefectsitecontentonCNTstructures[31];
on CNT structures [31]; specifically, a higher value of the ratio means that more defects are formed.
specifically,ahighervalueoftheratiomeansthatmoredefectsareformed.
1600
sonication ID/IG
D band time
[min]
0 0.96
Gband
1200 15 1.05
30 1.1
60 1.12
ps 90 1.13
y, c 800 120 1.2
nsit
nte
I
400
CNTs-0
CNTs-120
0
1200 1300 1400 1500 1600 1700 1800
Raman shift, cm-1
(a) (b)
Figure 1. (a) Raman spectra of untreated CNTs and CNTs subjected to sonication for 120 min (in the
Figure1.(a)RamanspectraofuntreatedCNTsandCNTssubjectedtosonicationfor120min(inthe
inset the values of ID/IG ratio of all CNTs samples are listed); (b) representative TEM micrographs of
insetthevaluesofID/IGratioofallCNTssamplesarelisted);(b)representativeTEMmicrographsof
investigated CNTs.
investigatedCNTs.
The ID/IG ratio is 0.96 for the untreated CNTs progressively increased with the increasing
The I /I ratio is 0.96 for the untreated CNTs progressively increased with the increasing
sonication DtimeG, until it reaches a value of 1.20 for the CNT-120. This feature suggests that, due to the
sonication time, until it reaches a value of 1.20 for the CNT-120. This feature suggests that, due
sonication treatment, progressive damage to the CNTs’ graphitic structure occurs, with the formation
to the sonication treatment, progressive damage to the CNTs’ graphitic structure occurs, with the
of structural defects on the CNT surface, the concentration of which rises with the increasing
formationofstructuraldefectsontheCNTsurface,theconcentrationofwhichriseswiththeincreasing
sonication time.
sonicationtime.
To qualitatively evaluate the morphological modifications of CNT structures associated with the
ToqualitativelyevaluatethemorphologicalmodificationsofCNTstructuresassociatedwiththe
defect formation on sonication treatment, a visual inspection using TEM was carried out. In Figure
defectformationonsonicationtreatment,avisualinspectionusingTEMwascarriedout. InFigure1b,
1b, representative TEM micrographs of untreated CNTs and CNTs subjected to sonication for
representativeTEMmicrographsofuntreatedCNTsandCNTssubjectedtosonicationfordifferenttime
different time intervals are reported. For the sonicated CNTs, alterations of the nanoparticles’
intervalsarereported. ForthesonicatedCNTs,alterationsofthenanoparticles’structuralintegritycan
structural integrity can be noticed, with the appearance of waviness and presence of amorphous
benoticed,withtheappearanceofwavinessandpresenceofamorphouscarbononthenanoparticle
carbon on the nanoparticle surface, while untreated CNTs show a well-ordered graphitic structure.
surface,whileuntreatedCNTsshowawell-orderedgraphiticstructure.
Defect formation on CNTs induced by the sonication treatment can lead to nanoparticle scission.
DefectformationonCNTsinducedbythesonicationtreatmentcanleadtonanoparticlescission.
To verify the effect of the sonication on the CNTs’ length, the distribution of the hydrodynamic
To verify the effect of the sonication on the CNTs’ length, the distribution of the hydrodynamic
diameter of the untreated and sonicated CNTs was evaluated by dynamic light scattering (DLS)
diameter of the untreated and sonicated CNTs was evaluated by dynamic light scattering (DLS)
measurements. The average apparent hydrodynamic diameters are related to the characteristic
measurements. Theaverageapparenthydrodynamicdiametersarerelatedtothecharacteristiclengths
lengths of the anisotropic particles, as well as to the shell hydration and the aggregation phenomena.
oftheanisotropicparticles,aswellastotheshellhydrationandtheaggregationphenomena. Itis
It is known from the literature that the apparent hydrodynamic diameter for anisotropic particles is
knownfromtheliteraturethattheapparenthydrodynamicdiameterforanisotropicparticlesisadirect
a direct measure of the translational diffusion behaviour of the scattering objects and it can be
measureofthetranslationaldiffusionbehaviourofthescatteringobjectsanditcanbecorrelatedtothe
correlated to the CNTs’ actual characteristic sizes [32–34]. In particular, for a cylinder, the value of
the hydrodynamic diameter can be calculated by considering that [35]
2L
Dh =2s(cid:3398)0.19(cid:3398)8.24/s(cid:3397)12/s2 (1)
where L is the cylinder length, and s = ln(L/r) where r is the radius of the cylinder.
Materials 2018,11,383 6of14
CNTs’actualcharacteristicsizes[32–34]. Inparticular,foracylinder,thevalueofthehydrodynamic
diametercanbecalculatedbyconsideringthat[35]
2L
D = (1)
h 2s−0.19−8.24/s+12/s2
Materials 2018, 11, x 6 of 14
whereListhecylinderlength,ands=ln(L/r)whereristheradiusofthecylinder.
In Figure 2a, the obtained distribution functions of the average hydrodynamic diameters are
In Figure 2a, the obtained distribution functions of the average hydrodynamic diameters are
reported, for untreated CNTs and for CNTs subjected to sonication treatment for different time
reported,foruntreatedCNTsandforCNTssubjectedtosonicationtreatmentfordifferenttimeintervals.
intervals. The depicted curves clearly show that the average hydrodynamic diameter of the CNTs
ThedepictedcurvesclearlyshowthattheaveragehydrodynamicdiameteroftheCNTsprogressively
progressively decreases with increasing sonication time. Figure 2b reports the average hydrodynamic
decreaseswithincreasingsonicationtime. Figure2breportstheaveragehydrodynamicdiameterasa
diameter as a function of sonication time.
functionofsonicationtime.
30
150
CNTs-0
CNTs-15
m120
n
20 CNTs-30 er,
% CNTs-60 amet 90
uency, CNTs-90 mic di
q CNTs-120 a
Fre din 60
10 dro
y
H
30
0
0
10 100 1000 0 15 30 60 90 120
Hydrodinamic diameter, nm Sonication time, min
(a) (b)
Figure 2. (a) Distribution functions of apparent hydrodynamic diameters for untreated and sonicated
Figure2.(a)Distributionfunctionsofapparenthydrodynamicdiametersforuntreatedandsonicated
CNTs and (b) apparent hydrodynamic diameter of CNTs as a function of sonication time.
CNTsand(b)apparenthydrodynamicdiameterofCNTsasafunctionofsonicationtime.
Before the sonication, the untreated CNTs show a distribution of hydrodynamic diameters
Beforethesonication,theuntreatedCNTsshowadistributionofhydrodynamicdiametersranging
ranging between 100 and 250 nm centered at 150 nm, that is, in the range of the value reported for
between100and250nmcenteredat150nm,thatis,intherangeofthevaluereportedforacid-treated
acid-treated multi walled CNT stable dispersions (150 ± 40 nm) [36]. The CNT lengths, ranging
multiwalledCNTstabledispersions(150±40nm)[36]. TheCNTlengths,rangingbetween0.2and
between 0.2 and 1 μm, can be calculated from Equation (1) considering 14 nm as the external radius
1µm,canbecalculatedfromEquation(1)considering14nmastheexternalradius(fromtheTEM
(from the TEM results) and hydrodynamic diameters reported above.
results)andhydrodynamicdiametersreportedabove.
After 15 min of sonication, the diameter drastically decreases, becoming about half its original
After15minofsonication,thediameterdrasticallydecreases,becomingabouthalfitsoriginalsize,
size, i.e., 72 nm, and it continues to progressively decrease with the sonication time. Then, for
i.e.,72nm,anditcontinuestoprogressivelydecreasewiththesonicationtime. Then,forsonication
sonication times of 90 and 120 min, the size of the CNTs is no longer altered and the value of the
timesof90and120min,thesizeoftheCNTsisnolongeralteredandthevalueofthehydrodynamic
hydrodynamic diameter remains unchanged. It is worth noting that the final value of the
diameterremainsunchanged. Itisworthnotingthatthefinalvalueofthehydrodynamicdiameter
hydrodynamic diameter reached at the maximum sonication time is about 33 nm, indicating that the
reached at the maximum sonication time is about 33 nm, indicating that the applied sonication
applied sonication treatment has a dramatic effect on the CNTs’ length and, thus, on their aspect
treatment has a dramatic effect on the CNTs’ length and, thus, on their aspect ratio. Based on
ratio. Based on Equation (1), it is possible to estimate that the final length decreases by a factor of
Equation(1),itispossibletoestimatethatthefinallengthdecreasesbyafactorofseven,consideringr
seven, considering r unaltered and the hydrodynamic radius before and after 120 min of
unalteredandthehydrodynamicradiusbeforeandafter120minofultrasonicationtime.
ultrasonication time.
3.2. UHMWPE/CNTNanocomposites: Rheology,MorphologyandThermo-OxidationBehaviour
3.2. UHMWPE/CNT Nanocomposites: Rheology, Morphology and Thermo-Oxidation Behaviour
UntreatedandsonicatedCNTshavebeenusedasnanofillersinUHMWPE-basednanocomposites
Untreated and sonicated CNTs have been used as nanofillers in UHMWPE-based
withtheaimofevaluatinghowthesonication-inducedmodificationofaCNT’saspectratioaffectsthe
nanocomposites with the aim of evaluating how the sonication-induced modification of a CNT’s
morphologyofformulatednanocompositesandtheirthermo-oxidativeresistance.
aspect ratio affects the morphology of formulated nanocomposites and their thermo-oxidative
resistance.
Rheological analyses are a powerful tool for investigating the microstructure of polymer-based
nanocomposites, since the dispersed nanoparticles are able to alter the polymer chain dynamics.
Herein, we performed linear viscoelastic measurements to infer the influence of the different aspect
ratios of CNTs subjected to sonication treatment on the relaxation processes of UHMWPE
macromolecules. In Figure 3a, the trend of complex viscosity as a function of frequency for neat
UHMWPE and all CNT-containing nanocomposites is reported. The complex viscosity of the neat
polymer matrix exhibits a very short Newtonian plateau, only at the lowest investigated frequencies,
Materials 2018,11,383 7of14
Rheologicalanalysesareapowerfultoolforinvestigatingthemicrostructureofpolymer-based
nanocomposites,sincethedispersednanoparticlesareabletoalterthepolymerchaindynamics.Herein,
weperformedlinearviscoelasticmeasurementstoinfertheinfluenceofthedifferentaspectratiosof
CNTssubjectedtosonicationtreatmentontherelaxationprocessesofUHMWPEmacromolecules.
In Figure 3a, the trend of complex viscosity as a function of frequency for neat UHMWPE and all
CNT-containing nanocomposites is reported. The complex viscosity of the neat polymer matrix
exhibitsaveryshortNewtonianplateau,onlyatthelowestinvestigatedfrequencies,followedbya
Materials 2018, 11, x 7 of 14
distinctpower-lawtrendforthewholeinvestigatedfrequencyrange. Suchmarkedlynon-Newtonian
behafvoilolouwrecda nbyb ae daitsttriinbcut tpeodwteor-tlhaewv terernydh fiogrh thme owlehcoulela irnvweestiigghatteodf ftrheequmenactryi xra,nwghe.o Ssuecmh amcarrokmedollye cules
requniroenl-oNnegwrteolnaixanat bioenhatvimiouers cbaenc abue saettroifbuthteedl atorg theec ovenrtye nhtigohf menotlaecnugllaerm weenigtsh.t Tofh tehae dmdaitnrigx,o wfhuonster eated
andmsoancricoamteodlecCuNlesT srebqruiinreg sloanbgo ruetlaaxnatiionnc rteimasees obefctahuesec oomf tphele lxarvgies ccoosnitteyntv oafl ueenstainngltehmeewnths.o Tlehet ested
frequaedndcinyg roafn ugnet,rebautetds aigndn isfiocnaicnatteddi fCfeNrTens cberisngesm aebroguet awnh inecnrecaosme opf athrien cgomthpelevxa vriisocuossitnya vnaoluceosm inp osite
the whole tested frequency range, but significant differences emerge when comparing the various
samples. Untreated CNTs have a more marked effect on the complex viscosity curve, causing an
nanocomposite samples. Untreated CNTs have a more marked effect on the complex viscosity curve,
importantincreaseoftheη*valueswithrespecttothoseofneatmatrixandthedisappearanceofthe
causing an important increase of the η* values with respect to those of neat matrix and the
Newtonianplateauatlowfrequencies. NanocompositescontainingsonicatedCNTsshowcomplex
disappearance of the Newtonian plateau at low frequencies. Nanocomposites containing sonicated
viscosityvalueshigherthanthoseofneatmatrix,buttheeffectofthenanofillersisprogressivelyless
CNTs show complex viscosity values higher than those of neat matrix, but the effect of the nanofillers
pronouncedwithincreasingsonicationtime. Theobtainedresultcanbeexplainedconsideringthat
is progressively less pronounced with increasing sonication time. The obtained result can be
usuaellxyplaaimneodr ecopnrsoindoeruinngc ethdaitn ucsrueaalsley oa fmthoreec pormonpoluexncveids cinocsriteyasien odfi cthaete csoamnpilnexc rveiascsoinsigtyi minpdoicrattaens caeno fthe
tube–intcurbeeasiinntge irmacptoiortnasncine odfi tchtae ttiunbge–thtuebreh ienoteloragcitcioanlsb ienh daivcitoatuinrg[ 3t7h]e. rheological behaviour [37].
1.E+08 1.E+06 Increasing
Increasing sonication time
sonication time UHMWPE
1.E+07 UHMWPE/CNTs-0
UHMWPE/CNTs-15
Complex viscosity, Pa*sec11..EE++0056 UUUUHHHHMMMMWWWWPPPPEEEE////CCCCNNNNTTTTssss----369100020 Storage Modulus, Pa 11..EE++0045 SUUUUaHHHHmMMMMpWWWWlePPPPEEEE///CCCNNNTTTsss---013 50 S l o p2000e....0673v4501alue UUUUUUUHHHHHHHMMMMMMMWWWWWWWPPPPPPPEEEEEEE//////CCCCCCNNNNNNTTTTTTssssss------113690250000
1.E+04 UHMWPE/CNTs-60 1.00
UHMWPE/CNTs-90 1.30
1.E+03 1.E+03
1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02
Frequency, rad/sec Frequency, rad/sec
(a) (b)
FiguFreig3u.re(a 3). C(ao) mCopmlepxlevxis vciossciotsyitayn adnd(b ()bS) tSotroargageem moodduulluuss ((iinn tthhee iinnsseett tthhee vvaaluluese sofo tfhteh leowlo wfrefqrueqenuceyn cyG(cid:48)
G′ slopes are reported) as a function of frequency for neat UHMWPE and all CNT-containing
slopesarereported)asafunctionoffrequencyforneatUHMWPEandallCNT-containingnanocomposites.
nanocomposites.
In oIunr ocuars eca,steh, ethhe ighhigha sapsepcetct rarattiioo ooff uunnttrreeaatteedd CCNNTsT sfafvaovros rtshet heestaebsltiasbhlmisehnmt oefn tinotefrtiunbtee rtube
interianctetiroanctsioanns danhde nhceencaed addidnigngt htheen naannoofifilllleerrss ccaauussees sana nalateltreartiaotnio onf othfet hpoelypmoleyrm chearinc hdayinnamdyicnsa, mics,
resulrteinsugltiinngr hieno lrohgeioclaolgbiceahl abveihoauvrioduifrf edriefnfetretontt htoa ttohfant eoaft nUeHatM UWHPMEW.APEs.t hAes sothnei csaotnioicnattiiomne tiinmcer eases,
increases, the CNTs aspect ratio diminishes and the nanofillers are not able to form interconnected
theCNTsaspectratiodiminishesandthenanofillersarenotabletoforminterconnectedstructures
structures that modify the macromolecular dynamics. As a result, the effect of adding the nanofillers
that modify themacromoleculardynamics. As a result, the effectof adding the nanofillerson the
on the complex viscosity curve is progressively less significant as the sonication time increases, and
complexviscositycurveisprogressivelylesssignificantasthesonicationtimeincreases,andtheη*
the η* trend of the nanocomposites becomes similar to that of neat matrix. Similar conclusions can be
trendofthenanocompositesbecomessimilartothatofneatmatrix. Similarconclusionscanbedrawn
drawn looking at the trends of storage modulus as a function of frequency reported in Figure 3b. As
lookfinarg aas tntehaet UtrHenMdWsPoEf sist ocorangceernmeod,d aut lluowsa fsreaqufuenncciteiso nneaotf mfraetqriuxe enxchyibritesp thoer tteydpiicnalF hiogmuroepo3lby.mAers-faras
neatlUikHe tMermWinPaEl bisehcaovnicoeurrn weidth, asctalloinwg fprreoqpueretniecsi eosf anpepartomximatartiexlye xGh′ iαb ωit2s, stheee vtaylpuiecsa llishteodm ino tphoel iynmseet r-like
termiinn Failgbuereh a3bv.i oTuher awdidtihtioscna olfi nugntpreraotpede ratnieds soofniacpatperdo CxNimTas tcealuyseGs (cid:48)aαn iωnc2r,esaesee ovfa tlhuee Gs′l visatleudesi inn tthhee inset
inFiwguhroele3 ibn.veTshtiegaatdedd iftrieoqnueonfcuyn rtarnegaete; dmoarnedovseorn, ifcoar taeldl iCnvNeTstsigcaatueds ensaannocionmcrpeoassiteeso, fththee teGrm(cid:48) vinaallu esin
thewbehhoalveiionuvr etsetnigdas tteod dfirseaqpupeeanrc yanrda nthgee ;dmepoernedoevnecre, foof rGa′l lonin fvreeqstuiegnactye datn laonwo cfroemqupeonsciiteess b,ethcoemteersm inal
behawveioaukerrt. eHnodwsetvoerd, iassa palpreeaadrya nnodtitcheed dine ptheen adneanlycseiso offG th(cid:48)eo ncofmrepqleuxe nvicsycoastitylo cwurfvreesq, ua esnigcnieifsicbaenct omes
effect of the CNT sonication time on the nanocomposites’ rheological behaviour can be noticed. For
weaker. However,asalreadynoticedintheanalysisofthecomplexviscositycurves,asignificanteffect
the nanocomposite containing untreated CNTs, the low frequency storage modulus is almost
independent of frequency, suggesting a transition from liquid-like to solid-like rheological behaviour
for this sample. This non-terminal behaviour can be attributed to the arrangement of CNTs in
interconnected structures with the formation of a semi-3D network of nanoparticles that restrains the
polymer relaxation process. Nanocomposites containing CNTs treated for low sonication times, i.e.,
CNT-15 and CNT-30, show similar solid-like rheological behaviour, although the values of the low
Materials 2018,11,383 8of14
oftheCNTsonicationtimeonthenanocomposites’rheologicalbehaviourcanbenoticed. Forthe
nanocompositecontaininguntreatedCNTs,thelowfrequencystoragemodulusisalmostindependent
offrequency,suggestingatransitionfromliquid-liketosolid-likerheologicalbehaviourforthissample.
Thisnon-terminalbehaviourcanbeattributedtothearrangementofCNTsininterconnectedstructures
with the formation of a semi-3D network of nanoparticles that restrains the polymer relaxation
process. NanocompositescontainingCNTstreatedforlowsonicationtimes,i.e.,CNT-15andCNT-30,
showsimilarsolid-likerheologicalbehaviour,althoughthevaluesofthelowfrequencyslopeofthe
Materials 2018, 11, x 8 of 14
storagemodulusarehigherthanthatofCNT-0containingnanocomposite. Otherwise,therheological
behafvrieoquurenocfyt hsleonpae noof ctohme sptoosraitgees mcoondtualiunsin agreC hNigThsetrr ethaatend thfoatr olof nCgNtTim-0e csobnetacianmineg pnraongorceosmsivpoelsyites.i milar
tothOatthoefrnweisaet, mthae trrhixeowloigthicatlh beeihnacvrieoausre oof fthteh neasnoonciocmatpioosnitteism coen,itnaidniicnagt CinNgTtsh tarteathteed lfoowr loansgp eticmtersa tioof
became progressively similar to that of neat matrix with the increase of the sonication time, indicating
thesonicatedCNTsdoesnotallowtheCNTstophysicallybridgeandtoformthepercolationnetwork.
that the low aspect ratio of the sonicated CNTs does not allow the CNTs to physically bridge and to
To sum up, as the sonication time increases, the aspect ratio of the CNTs diminishes due
form the percolation network.
to shortening, and this issue implies that the sonicated CNTs are no longer able to hinder the
To sum up, as the sonication time increases, the aspect ratio of the CNTs diminishes due to
polymer chain relaxation processes. To confirm this feature, stress relaxation measurements were
shortening, and this issue implies that the sonicated CNTs are no longer able to hinder the polymer
performed and the obtained results are reported in Figure 4, where the relaxation modulus of
chain relaxation processes. To confirm this feature, stress relaxation measurements were performed
invesatnigda ttheed onbatnaionceodm rpesouslittse saries creopmoprtaerde dint oFitghuarte o4f, UwHheMreW thPeE r.eTlahxeatmioond muloudsuGlu(st )oof finCvNesTt-igcoatnetda ining
nanoncaonmocpoomsipteossitisesh iigsh ecromthpaanretdh attoo ftnheaat tomf aUtrHixManWdP,Es.i mTihlaer lymtooduwluhsa tGw(at)s oobf seCrNveTd-cothnrtoaiunginhgl inear
visconealnaosctiocmmpoesaisteusr iesm hiegnhtesr, ththaen tehnaht aonf nceeamt menattroixf aGn(dt,) siismmilaorlrye top rwohnaot uwnacse odbsfeorrveudn tthrreoautegdh lCinNeaTrs and
progvreisscsoievleaslyticd mimeainsuisrhemesenwtsi,t hthien cenrehaasnicnegmCenNt Tofs Gon(ti)c iast imonoreti mpreosn.oFuuncrethde fromr ournetr,ewathedil eCaNtTssh oanrtd times
progressively diminishes with increasing CNT sonication times. Furthermore, while at short times
thestressrelaxationbehaviourisquitesimilarfornanocompositesandunfilledmatrix,atlongtimes
the stress relaxation behaviour is quite similar for nanocomposites and unfilled matrix, at long times
different relaxation kinetics can be observed. In particular, neat UHMWPE and nanocomposites
different relaxation kinetics can be observed. In particular, neat UHMWPE and nanocomposites
containingCNTssonicatedforlongtimesrelaxlikealiquid,indicatingthatthepresenceoftreated
containing CNTs sonicated for long times relax like a liquid, indicating that the presence of treated
CNT-60,CNT-90andCNT-120doesnotaffectthechainrelaxationspectrumofUHMWPE.Incontrast,
CNT-60, CNT-90 and CNT-120 does not affect the chain relaxation spectrum of UHMWPE. In
addinguntreatedandCNTssonicatedforshortperiodssignificantlymodifiedthepolymerchains’
contrast, adding untreated and CNTs sonicated for short periods significantly modified the polymer
relaxcahtaioinns’a rnedlatxhaetionna nanodco tmhep noasnitoecsomshpoowsieteds ashposweuedd oa- psoseliudd-oli-kseolbide-hliakve iboeuhra.vInioduere. dIn,dtheeeds, ttrhees sstrreelsasx esto
aneqreulialxibesr ituom anv aeqluueilirbartihuemr vthaalune troatzheerro thaannd ttoh zeeoroc caunrdr etnhcee ooccfutrhriesnpceh eonf othmise pnhoennocamnenboena cttarnib buet edto
thefoatrtmribautitoedn toof tahet hforerme-adtiiomne onfs aio tnhraeles-udipmeernstsriuoncatul rseupoefrCstNruTctsuwrei tohf iCnNUTHs wMiWthiPnE U.HMWPE.
1.E+06
1.E+05
Increasing
sonication time
1.E+04
a
G(t), P 1.E+03 UUHHMMWWPPEE/CNTs-0
UHMWPE/CNTs-15
1.E+02
UHMWPE/CNTs-30
UHMWPE/CNTs-60
1.E+01 UHMWPE/CNTs-90
UHMWPE/CNTs-120
1.E+00
1.E+00 1.E+01 1.E+02 1.E+03 1.E+04
Time, sec
Figure 4. Stress relaxation behaviour of neat UHMWPE and all CNT-containing nanocomposites.
Figure4.StressrelaxationbehaviourofneatUHMWPEandallCNT-containingnanocomposites.
In order to gain insight into the state of dispersion of untreated and sonicated CNTs within
pInolyomrdeer rmtaotrgixaeisn, minosripghhotloingitcoalt hobesesrtvaatetioonfs dthisrpouegrshi oonptoicfalu mnitcrreoastceodpya nwderseo cnaircriaetde douCt NanTds inw ithin
polyFmigeurrme 5a trreipxreess,emntoatripvhe oolpotgiciacla mloicbrsoegrrvapathiso narse trherpoourtgehd o(tphtei ceamlbmedicdreods cCoNpyTsw aeprpeeacra rarsi ethde oduatrka ndin
Figupreha5ser)e. pIrt eisse nevtaidtievnet ofrpotmic atlhme oicprtoicgarla opbhsseravraetiroenps otrhtaetd t(hteh ediesmtribbeudtidone dofC bNoTths aupnptreeaatreads atnhde dark
sonicated CNTs is quite uniform and homogeneous, although in the nanocomposite containing
phase). Itisevidentfromtheopticalobservationsthatthedistributionofbothuntreatedandsonicated
untreated CNTs, some agglomerates can be observed. The sonication treatment, thus, is beneficial in
CNTsisquiteuniformandhomogeneous,althoughinthenanocompositecontaininguntreatedCNTs,
enhancing the extent of CNT dispersion. According to the literature, the shortening of CNTs
someagglomeratescanbeobserved. Thesonicationtreatment,thus,isbeneficialinenhancingthe
facilitates their dispersion within the polymer matrix due to the decreasing degree of physical
interaction between the CNTs themselves. Indeed, as the length of the CNTs diminishes, the contact
area between nanofillers becomes lower and decreases the number of physical entanglements
between nanotubes [38,39].
Materials 2018,11,383 9of14
extentofCNTdispersion.Accordingtotheliterature,theshorteningofCNTsfacilitatestheirdispersion
withinthepolymermatrixduetothedecreasingdegreeofphysicalinteractionbetweentheCNTs
themselves.Indeed,asthelengthoftheCNTsdiminishes,thecontactareabetweennanofillersbecomes
loweranddecreasesthenumberofphysicalentanglementsbetweennanotubes[38,39].
Materials 2018, 11, x 9 of 14

(a) (b) (c)
Figure 5. Representative optical micrographs of: (a) UHMWPE/CNT-0; (b) UHMWPE/CNT-60;
Figure 5. Representative optical micrographs of: (a) UHMWPE/CNT-0; (b) UHMWPE/CNT-60;
(c) UHMWPE/CNT-120.
(c)UHMWPE/CNT-120.
The effect of the sonication-induced shortening of CNTs and of the consequent modification of
Tthhee neafnfeocfitlloerf tdhisetrsiobnutiicoant ioonn -tihned tuhceermdos-hooxirdteantiivneg boehfaCvNioTusr oafn UdHoMftWhePcEo-bnasseeqdu neanntomcoomdpifiosciatetsio nof
thenwanaos fiinllveerstdigisattreidb tuhtriooungohn thteh aentahleyrsmis oof- othxeid eavtoilvuetiobne htiamvei oouf rFToIfRU sHpeMctrWa cPoEll-ebcatesde donn naenaot cmomatrpixo sites
and nanocomposites during an oxidative treatment performed in an air oven at 120 °C. In Figure 6,
wasinvestigatedthroughtheanalysisoftheevolutiontimeofFTIRspectracollectedonneatmatrix
andnthaen oFcToImR psopseicttersa dfourr innegaat nUoHxMidWatPivEe atrneda tamll eCntNpTe-crofonrtamineidngi nnaannoaciormopvoesnitaest 1a2r0e ◦sCho.wInnF. iIgnu re6,
particular, the progress of the degradative processes was followed through the monitoring of the
theFTIRspectraforneatUHMWPEandallCNT-containingnanocompositesareshown. Inparticular,
carbonyl index (CI). The CI refers to the area of the complex peak in the FTIR spectrum in the range
theprogressofthedegradativeprocesseswasfollowedthroughthemonitoringofthecarbonylindex
1850–1600 cm−1, thus reflecting the formation of carboxylic acids, ketones, esters and lactones, which
(CI).Tarhee thCeI mreafienr sprtoodtuhcetsa roef athoef UthHeMcoWmPpEl ethxeprmeaok-oixnidtahteioFnT [I4R0,4s1p]e. cTthrue mcalicnultahteedr aCnIg vea1lu8e5s0 f–o1r6 n0e0act m−1,
thusmreafltreicxt ianngd tahlel ffoorrmmualatitoedn noafncoacrobmopxoysliitcesa cairde sr,ekpeotrotende sin, eFsitgeurrse a7n, dasl aac ftuonncetsio,nw ohfi cthhe atrheertmheo-main
produocxtidsaotfiotnh etimUeH, aMloWngP wEitthh ethrem voa-louxesid oaf ttihoen in[4d0u,c4t1io].n Ttihmeec, adlecfuinleadte ads CthIe vtiamluee nsefeodredn efoart CmI aretraicxheasn dall
formuthlae tveadlunea onfo fcivoem [p42o]s. itesarereportedinFigure7,asafunctionofthethermo-oxidationtime,along
withthevLaeltu uess noofwth ceoninsidduerc ttihoen thtiemrmeo,-doexfiidnaetdivea sbethhaevtiiomure onfe UeHdeMdWfoPrEC aIndre naacnhoecsotmhpeovsaitleuse aot sfhfiovrte [42].
oxidation times (i.e., until 72 h). Neat UHMWPE shows a sudden increase of carbonyl formation,
Letusnowconsiderthethermo-oxidativebehaviourofUHMWPEandnanocompositesatshort
indicating that the degradation of neat matrix begins in the early stage of the oxidative treatment.
oxidation times (i.e., until 72 h). Neat UHMWPE shows a sudden increase of carbonyl formation,
The nanocomposite containing untreated CNTs shows improved thermo-oxidative resistance, with
indicating that the degradation of neat matrix begins in the early stage of the oxidative treatment.
an increase of the induction time compared to that of neat matrix, and this result can be ascribed to
ThenanocompositecontaininguntreatedCNTsshowsimprovedthermo-oxidativeresistance,with
the well-documented radical scavenging activity of CNTs. Indeed, it is known from the literature that
aninCcrNeaTsse, boefcathuseei onfd tuhecitri opnectuimliaer ecloemctrpoanrice dstrtuoctthuraet, oafren aebalte mto aptrroixte,cat npdolythmiserrse asgualitncsat ndebgeraadsactrivibee dto
the wpehlel-ndoomceunma,e nactetidngr aadsi caanltsiocxaivdeanngtsi nagndac tiinvtietryruopftiCngN Tths.e Icnhdaeine dp,riotpisagkantioown ndufrroinmg tphoelylimteerra ture
that dCeNgrTasd,atbioenca [u4s3e,44o].f Aths efiarr pase cthueli anraneolceocmtrpoonsiictess tcrouncttauinrein,ga rseonaicbalteedt oCNpTros taercet cpoonlcyemrneedrs, tahge ainst
degrabduailtdi-vuep pcuhrevneos mofe CnaI ,reavcetailn ag maasrkaendt iionxfliudeannctes oaf nthde iCnNteTr rsuopnitcinatgionth teimceh oanin thper ooxpiadgataitoino nratdeu; ring
polymspeercdifeicgarllayd, aatsi oa nfu[n4c3t,i4o4n] .oAf tshefa sroansictahtieonn atinmoec,o amn pinocsrietaessec oofn tthaei nstianbgilsizoinnigc aatcetidonC oNf TCsNaTrse ccaonn bcee rned,
observed. According to the literature, the radical scavenging activity of CNTs can be amplified
thebuild-upcurvesofCIrevealamarkedinfluenceoftheCNTsonicationtimeontheoxidationrate;
through the increase in the number of structural defects on the CNT surface [45]. The last can be
specifically,asafunctionofthesonicationtime,anincreaseofthestabilizingactionofCNTscanbe
explained considering that the antioxidant feature of CNTs is attributed to the presence of acceptor-
observed. Accordingtotheliterature,theradicalscavengingactivityofCNTscanbeamplifiedthrough
like localized states, which results from lattice defects on CNTs. As probed through Raman analysis,
theincreaseinthenumberofstructuraldefectsontheCNTsurface[45]. Thelastcanbeexplained
the content of structural defects progressively increases with the sonication time; therefore, the
consiadmerpinlifgietdh aatntthioexiadnatniot xaicdtiaonnt ofef astounriecaotefdC CNNTsTsi scaatnt rbibe uattetrdibtuotetdh etop rtheseeirn hceigohfera cacmeoputonrt- loifk leatltoiccea lized
statesd,ewfehctisc hcormespualrtesdf rtoo munltartetaitceedd neafneocttusboens. CANftTers .7A2 hs pofr othbeerdmtoh-oroxuidgahtivRea tmreaantmaennat,l yasni su,ntehxepceoctnetde ntof
structiunvraelrsdioenfe cotfs tphreo CgrIe bssuiivlde-luypi nccurrevaesse socwcuitrhs.t hTehes osnloicpaet ioofn tthiem Ce;I tchuerrveefos rfeo,rt hneeaatm UpHliMfieWdPaEn taionxdi dant
actionnaonfocsoomnipcoastieted-cConNtaTinsincagn CbNeTa-0tt rdibecurteeadsest,o itnhdeicirathinigg hae rreadmucotuionnt ooff tlhaett ircaeted oeff efcotrsmcaotimonp aorfe d to
untreoaxtieddizneda nsopteucbieess. .IAn fcteornt7r2asht, othfet hnearnmooco-omxpidosaittievs ectornetaatimnineng t,soannicuanteedx pCeNctTesd eixnhviebrits iao nfaosftetrh eCI
degradation kinetic and the growth of the CI values increases as a function of the CNT sonication
build-upcurvesoccurs. TheslopeoftheCIcurvesforneatUHMWPEandnanocomposite-containing
time. The obtained results could be explained considering that, as reported in the literature, the
CNT-0 decreases, indicating a reduction of the rate of formation of oxidized species. In contrast,
stabilizing action of CNTs can be compromised if local increases of the temperature at the
thenanocompositescontainingsonicatedCNTsexhibitafasterdegradationkineticandthegrowth
polymer/nanoparticle interface occur [46]. In our case, the oxidation treatment is carried-out at high
of the CI values increases as a function of the CNT sonication time. The obtained results could
temperature (i.e., 120 °C); therefore, after a certain period of exposure, nanocomposites are no longer
be exapbllaei ntoe ddiscsoipnastied tehrei nthgerthmaatl, eanserrgeyp aonrtde tdhei nlotchael tleimtepreartautruer,e tshtaertsst atob iilniczrienagsea, cctaiuosninogf inC tNurTns acna n be
acceleration of degradative phenomena.
Materials 2018,11,383 10of14
compromisediflocalincreasesofthetemperatureatthepolymer/nanoparticleinterfaceoccur[46].
Inourcase,theoxidationtreatmentiscarried-outathightemperature(i.e.,120◦C);therefore,aftera
certainperiodofexposure,nanocompositesarenolongerabletodissipatethethermalenergyandthe
localtemMapteeriraalst 2u01r8e, 1s1t, ax r tstoincrease,causinginturnanaccelerationofdegradativepheno10m ofe 1n4a .

UHMWPE/CNTs-15
bance, a.u.
bsor
A
120h
0h
1800 1750 1700 1650
Figure 6. FTIR spectra collected during thermo-oxidative treatment for neat UHMWPE and CNT-
Figure 6. FTIR spectra collected during thermo-oxidative treatment for neat UHMWPE and
containing nanocomposites.
CNT-containingnanocomposites.
The worsening of the thermo-oxidative resistance of the nanocomposites as a function of the
CNT sonication time could be ascribed to the lower thermal conductivity of shorter CNTs with