Cha

nan

com

Russell Wang, D

aAssociate Professor, Department ofbPostgraduate student, DepartmentcAssociate Professor, Department ofdProfessor, Department of Macromo

The Journal of Prosthetic

racterization of multiwalled carbon

otube-polymethyl methacrylate

posite resins as denture base materials

DS, MSD,a Junliang Tao, PhD,b Bill Yu, PhD,c andLiming Dai, PhDd

Case Western Reserve University School of Dental Medicine, and CaseWestern Reserve University School of Engineering, Cleveland, Ohio

Statement of problem. Most fractures of dentures occur during function, primarily because of the flexural fatigue ofdenture resins.

Purpose. The purpose of this study was to evaluate a polymethyl methacrylate denture base material modified withmultiwalled carbon nanotubes in terms of fatigue resistance, flexural strength, and resilience.

Material and methods. Denture resin specimens were fabricated: control, 0.5 wt%, 1 wt%, and 2 wt% of multiwalled carbonnanotubes. Multiwalled carbon nanotubes were dispersed by sonication. Thermogravimetric analysis was used to determinequantitative dispersions of multiwalled carbon nanotubes in polymethyl methacrylate. Raman spectroscopic analyses wereused to evaluate interfacial reactions between the multiwalled carbon nanotubes and the polymethyl methacrylate matrix.Groups with and without multiwalled carbon nanotubes were subjected to a 3-point-bending test for flexural strength.Resilience was derived from a stress and/or strain curve. Fatigue resistance was conducted by a 4-point bending test.Fractured surfaces were analyzed by scanning electron microscopy. One-way ANOVA and the Duncan tests were used toidentify any statistical differences (a¼.05).

Results. Thermogravimetric analysis verified the accurate amounts of multiwalled carbon nanotubes dispersed in the poly-methyl methacrylate resin. Raman spectroscopy showed an interfacial reaction between the multiwalled carbon nanotubesand the polymethyl methacrylate matrix. Statistical analyses revealed significant differences in static and dynamic loadingsamong the groups. The worst mechanical properties were in the 2 wt% multiwalled carbon nanotubes (P<.05), and 0.5 wt%and 1 wt% multiwalled carbon nanotubes significantly improved flexural strength and resilience. All multiwalled carbonnanotubes-polymethyl methacrylate groups showed poor fatigue resistance. The scanning electron microscopy resultsindicated more agglomerations in the 2% multiwalled carbon nanotubes.

Conclusions. Multiwalled carbon nanotubes-polymethyl methacrylate groups (0.5% and 1%) performed better than thecontrol group during the static flexural test. The results indicated that 2 wt% multiwalled carbon nanotubes were notbeneficial because of the inadequate dispersion of multiwalled carbon nanotubes in the polymethyl methacrylate matrix.Scanning electron microscopy analysis showed agglomerations on the fracture surface of 2 wt% multiwalled carbon nano-tubes. The interfacial bonding between multiwalled carbon nanotubes and polymethyl methacrylate was weak based on theRaman data and dynamic loading results. (J Prosthet Dent 2014;111:318-326)

Clinical Implications

Adding 0.5 wt% and 1 wt% of multiwalled carbon nanotubesimproves the flexural strength and resilience of multiwalled carbonnanotubes-polymethyl methacrylate composite resins. Fatigue testsshow that all groups with multiwalled carbon nanotubes had poorfatigue resistance. Future improvement of the material design andprocessing is needed.

Comprehensive Care.of Civil Engineering.Civil Engineering.lecular Science and Engineering.

Dentistry Wang et al

1 Diagrammatic representation of single-walled nanotubesand multiwalled carbon nanotubes. Single-walled carbonnanotube: single graphite sheet seamlessly wrapped into cy-lindrical tube; multiwalled carbon nanotube: array ofgraphite sheets concentrically nested like rings of tree trunk.

April 2014 319

Most fractures of dentures occurduring function, primarily from dentureresin fatigue.1-4 Flexural fatigue occursafter repeated flexing of a material; it isa mode of fracture whereby a structureeventually fails after being repeatedlysubjected to small loads that individu-ally are not detrimental to the com-ponent.5,6 The midline fracture indentures is often the result of flexuralfatigue.5,7 The reinforcement of den-ture base material has been a subjectof interest to the dental material com-munity. The fracture resistance ofdenture base polymers has been inves-tigated.2,3,8-10 Polymethyl methacrylate(PMMA) resin is the principal materialof dental prostheses. To improve theproperties of PMMA, a variety of ma-terials have been incorporated into thepolymer, including glass fibers, longcarbon fibers, and metal wires.10-14

Success has been limited.15,16 The flex-ural strength of PMMA resin such asLucitone199 (Dentsply Trubyte), whichis a high-impact resin with butadieneand styrene additives, has been evalu-ated by several investigators and hasyielded conflicting values but generallywithout significant material strength-ening.17-19

Carbon nanotubes (CNT) haveoutstanding mechanical and electricalproperties. CNTs have high mechanicalproperties with reported strengths 10 to100 times higher than steel at a fractionof the weight.20-22 CNTs are strong,resilient, and lightweight, and usuallyform stable cylindrical structures. CNTsthat have a flawless structure are clas-sified into 2 main types, namely single-walled and multiwalled CNTs (Fig. 1).Single-walled CNTs (SWCNT) consistof a single graphite sheet seamlesslywrapped into a cylindrical tube, andmultiwalled CNTs (MWCNT) have anarray of such nanotubes concentricallynested like the rings of a tree trunk. Inaddition to the exceptional mechanicalproperties associated with CNTs(elastic modulus of 1 TPa; diamond,1.2 TPa), they also have superior ther-mal and electric properties.23,24 Toexpand the potential applications ofCNTs in polymer nanocomposites, an

Wang et al

understanding is needed of the proper-ties of CNTs and the interfacial inter-actions between CNTs and the matrix.Although this requirement is no differentfrom that for conventional fiber rein-forced composites, the scale of the rein-forcement phase diameter has changedfrom micrometers to nanometers.

The addition of carbon fibers to amatrix not only gives strength and elas-ticity to the material but also improvestoughness.25 Research on nanotubecomposites has focused on polymer-CNT based materials, wherein theyexhibit mechanical properties that aresuperior to conventional polymer-basedcomposites because of their consider-ably higher intrinsic strengths andmoduli. The stress transfer efficiency canbe 10 times higher than that of tradi-tional additives.26 MWCNTs seem tohave a ‘Russian nesting doll’ structure,which has a set of patterns that consistof the same structure made in 2 halves.Inside it are a series of similar CNTs,each smaller than the last, placed oneinside the other. Each constituent tu-bule is only bonded to its neighbors byweak Van der Waals forces. This may bean issue when adding CNTs to somepolymer matrices without chemicalbonding at CNTs-polymer interfaces.27

The second concern is the uniformdispersion of CNTs into a polymermatrix. Methods of using sonic dis-membranators or chemical modifica-tion have been proposed.22,28,29 Theaddition of CNTs usually causes adeformation mode of PMMA fibers. Inpure PMMA fibers, polymer necking

occurs under increasing tension, whichresults in failure at relatively smallstrains. However, adding CNTs to apolymer may dramatically improve theresistance of the polymer to mechanicalfailure. Incorporating MWCNTs topolymer matrices may effectively bridgecracks and reduce the extent of plasticdeformation by a PMMA matrix.30,31

MWCNTs can successfully reinforcethe fracture lines by strengthening thefibrils and bridging voids to enhancethe fatigue performance of the polymer.

The effects of CNT reinforcement onthe mechanical properties of denturebase materials have not been explored.This investigation studied the effect ofMWCNT reinforcement on the me-chanical properties of a commonlyused PMMA denture base material. Thenull hypothesis was that the addition ofCNTs (MWCNTs by weight) would notimprove the mechanical properties ofthe prosthesis.

MATERIAL AND METHODS

Test specimens were fabricated withthe denture base resin, Lucitone199original shade (Dentsply Intl). TheMWCNTs as received from the manu-facturer (Designed Nanotubes LLC)were added to the measured acrylicmonomer at 0.5% wt/wt, 1% wt/wt,and 2% wt/wt in a glass beaker.The liquid monomer was then ultra-sonically mixed for 20 minutes (ModelUP400S; Hielscher Ultrasonics GmbH).Manufacturer’s instructions were fol-lowed with a powder-liquid ratio of

320 Volume 111 Issue 4

21 g (32 mL):10 mL and a mixing timeof 20 seconds with a vacuum mixer.Liquid monomer with and withoutMWCNTs was added to the powderand mixed for 20 seconds to ensure thewetting of all powder particles. The mixwas covered for 9 minutes at roomtemperature and allowed to reachpacking consistency. The mix waspacked with conventional dentureflasks (Hanau Type; Whip Mix Corp).Specimens were fabricated in a rectan-gular mold prepared in standard den-ture flasks by using a template thatmeasured 70�40�3 mm. The closedflasks, tightened with spring clamps,were polymerized in a water bath for 9hours at 71�C and cooled for 30 mi-nutes in water at 26�C. The flask wasbench cooled before deflasking. Thespecimens were removed from theflasks and cleaned of stone particles.After deflasking, each mold was cut toobtain the specimens of 70�10�3 mmfor the flexural strength test. The spec-imens were sequentially polished withsilicon carbide paper (1000, 800, and600 grit) to achieve smooth edges.

The flexural strength was determinedby using the 3-point bending test asspecified by the International Organiza-tion for Standardization specification20795-1:2008.32 Four groups were pre-pared at 0%, 0.5%, 1%, and 2% ofMWCNTs, with 7 specimens per group.The specimens were stored in distilledwater at room temperature for 2 weeksbefore mechanical tests with a universaltesting machine (Sintech Renew 1121;Instron Engineering Corp). Before eachtest, the specimen thickness and widthweremeasuredwith a digital micrometer.A standard 3-point bending device wasattached to the machine and connectedto a computer. The testing parameterwas set with a load of 4.448 kN and acrosshead speed of 0.5 mm/min.

The flexural strength (S) was calcu-lated from the following formula:

S¼3 FL/2 bd2,

where: S, flexural strength in MPa; F,the load at break in N; L, 50 mm, thespan of specimen between supports; b,

The Journal of Prosthetic Dentis

width of each specimen; and d, thick-ness of each specimen.

Based on the load-displacement(F-D) curves of the 3-point bendingtest, mechanical properties were deter-mined. The load and displacement atthe yield point was taken as the yieldload (FY) and the yield displacement(DY). Stiffness was calculated based on5% of the strain.

The moment of inertia about thebending axis (I) and the thickness (t) ofeach specimen was used to calculateresilience, resilience¼0.5 (yield stress �yield strain).33 The fatigue test of thedifferent groups was conducted with aBionix II test system (MTS Inc). Pre-defined cyclic loading was applied tothe 4-point supported beam until thebeam was fractured. The cycle count tofracture was termed as the fatigueresistance.

The loading condition was con-trolled by the stroke of the tip of theactuator. First, after the beam wasplaced on the lower 2-point supports,the loading tip was lowered to contactthe top surface of the beam. Once goodinitial contact was achieved, displace-ment of the loading tip was set at zero,and the center of the cyclic deflectionwas set as 3 mm; the loading functionwas then defined as a sine wave loadingwith a frequency of 5.7 Hz and adeflection of 2 mm. The applied forcesand deflections of the loading tip werecontrolled by a computer. The numberof the cycle count was recorded as fa-tigue resistance when the specimen wasfractured, at which point the contactforce began to show gross reduction.Seven MWCNTs-PMMA specimens ofeach group were tested. All specimenswere stored in distilled water at roomtemperature for 2 weeks before testing.

Scanning electronmicroscopy (SEM)was performed on fractured specimenswith a Hitachi scanning electron mi-croscope (model S3200N). Images wereacquired in the secondary electronmode with thermal field emission SEM.Raman spectroscopic analysis wasconducted by using a visible laser lightwith the wavelength of 514 nm (Titansapphire laser, Jobin-Yvon HR640

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spectrometer) fitted with a charge-coupled device (CCD) detector. Thelaser with 50 mW was used to line andfocus on each specimen throughout theprocedure. Each specimen was tested at3 different positions. Scattering datawere acquired in a backscattering ge-ometry. The thermal stability ofeach MWCNTs-PMMA composite resinspecimens and confirmation of theamount of MWCNTs in PMMA wereevaluated with thermogravimetric anal-ysis (TGA). TGA measurements wereperformed with a thermogravimetricanalyzer from 20�C to 800�C. Theheating rate was 10�C/min with a ni-trogen gas flow at 60 cm3/min rate.

Data derived from the control andthe experimental groups on flexuralstrength, resilience, and fatigue resis-tance test were analyzed and comparedby using 1-way ANOVA and the Dun-can test to identify any statistical dif-ferences at (a¼.05).

RESULTS

The TGA curves of pure MWCNTs;pure Lucitone199 PMMA; and 0.5%,1% and 2% MWCNTs-PMMA compos-ite resin specimens are shown inFigure 2. Pure PMMA resin started todegrade in a nitrogen atmosphere at300�C and was completely degradedat 600�C. For MWCNT-PMMA speci-mens, the percentage weight ofMWCNTs was detected by TGA above630�C after the PMMA was completelydegraded, and the results accuratelyreflected the amount of MWCNTs foreach specimen. The weights of theMWCNTs remain fairly constant attemperatures beyond 630�C (Fig. 1,insert).

The use of Raman spectroscopy inthis study was to examine the purity ofthe MWCNTs and any interfacial re-actions between the MWCNTs and thePMMA. Three arrays of the Ramanspectra, which consist of pureMWCNTs,pure PMMA, and 2% MWCNTs inPMMA are shown in Figure 3. For theMWCNTs alone (Fig. 3, bottom of the 3arrays), 2 unique peaks of graphiticstructures were detected by Raman

Wang et al

3 Raman spectra of pure multiwalled carbonnanotubes, pure polymethyl methacrylate, and2% multiwalled carbon nanotubes in polymethylmethacrylate resin.

2 Thermogravimetric analysis curves of pure multiwalledcarbon nanotubes. Pure Lucitone199 polymethyl methacry-late and 0.5%, 1%, and 2% multiwalled carbon nanotubes-polymethyl methacrylate composite resins.

April 2014 321

spectroscopy; one was located at 1306cm-1 designated to the D band (sp3

atomic orbital of carbon), and the sec-ond peak was located at 1594 cm-1

designated to the G band (sp2 atomicorbital of carbon) associated withstretching motions of tangential C-Cbonds. G band represents stable C-Cbond inside MWCNTs and D band in-dicates defect graphitic structures orunstable C-C bonds. The array of thecombined 2% MWCNTs and PMMAspecimen show different intensities andlocations of D and G band peaks, whichconfirms the interactions betweenMWCNTs and the PMMAmatrix. Similar

Wang et al

patterns of Raman shifts of D and Gbands of 0.5%, 1%, and 2% MWCNTs inPMMA are shown in Figure 4. The Dband intensities decreased as the per-centage of MWCNTs increased. The de-creases in D band peaks in Figure 4resulted from the interactions of unsta-ble carbons with PMMA polymers. Thepeak ratios of D and G band changedfrom 0.93 for 0.5%MWCNTs to 0.84 for2% MWCNTs. The results from theRaman spectroscopy analysis of theMWCNTs used in this study are identicalto those of McNally et al.22

The results of the flexural strengthtest are shown in Figure 5. Compared

with the control group, flexural strengthincreased by an average of 9.3% with0.5% and 3.9% with 1% MWCNTs-PMMA groups. However, an 11.4%decrease in flexural strength was notedfor the 2% MWCNTs-PMMA group.The plot of resilience values for allgroups is shown in Figure 6. There wasan average 15.9% increase in resiliencein the 0.5 wt% MWCNTs-PMMA groupand 29% in the 1 wt% group. However,there was a 10.7% decrease in the 2 wt%MWCNTs-PMMA group when com-pared with the control group.

The equipment used for the 4-pointbending fatigue test is shown inFigure 7A. A dynamic loading of fatigueresistance was determined by a typicalloading curve, as shown in Figure 7B.When the beam was fractured, thecontact forces between the beam andthe loading tip were reduced andremained at a low level. The results ofthe fatigue resistance test for the 4groups are shown in Figure 8. Incontrast, fatigue resistance in allexperimental groups decreased signifi-cantly (92.6% to 99.7%).

One-way ANOVA and the Duncantest showed statistical differencesamong the control and experimentalgroups (0.5%, 1%, and 2%) at a Pvalue <.05. The level of MWCNTssignificantly affected the PMMA’smaximum flexural strength (P<.001).When 0.5% of MWCNTs was used,maximum flexural strength (P<.001)improved significantly compared withthe control group. Resilience improvedwhen MWCNTs were added to PMMAat the 0.5% and 1% level (P¼.017).When MWCNTs levels went from 1% to2% (P¼.008), resilience weakened. Aclear reverse effect of MWCNTs on thefatigue test (P<.001) was observed asthe wt% of MWCNTs increased.

The morphology and extent ofdispersion of MWCNTs in the PMMAmatrix was studied by SEM on fracturedsurfaces. The fractured surface at lowmagnification (�500) if shown inFigure 9 and bundled MWCNTsand typical fractured surfaces ofPMMA with 0.5% MWCNTs viewedperpendicular to the fractured surface

120

105

90

75

60

45

30

15

00.0 0.5 1.0 1.5 2.0

CNT Content (%)

Max

. Fle

xura

l Str

engt

h (M

Pa)

5 Flexural strength of polymethyl methacrylate with 0%,0.5%, 1%, and 2% multiwalled carbon nanotubes.

4 Raman shifts of 0.5%, 1%, and 2% multiwalled carbonnanotubes-polymethyl methacrylate resins with differentD-G band ratios.

322 Volume 111 Issue 4

(�25 000) are shown in Figure 10. TheMWCNTs can be clearly identified andare uniformly dispersed as singlenanotubes. MWCNTs protrude fromthe fractured surface without PMMAcoating. The center diameter of theMWCNTs and the concentric arrange-ment of the nanotubes are clearlyevident. The overall diameter of theMWCNTs is approximately 25 to 35nm. With 2% MWCNTs, more aggre-gates of varying dimensions areobserved. In some instances, thePMMA seems to wrap around theMWCNTs (Fig. 11).

The Journal of Prosthetic Dentis

DISCUSSION

This study was designed to investi-gate the potential applications ofCNTs-PMMA composite resin as adental base material. The hypothesisthat the addition of MWCNTs wouldnot improve the mechanical propertiesof the MWCNTs-PMMA compositeresin was rejected based on the resultsof both static and dynamic loadingtests. The purpose of using TGA in thisstudy was to determine the dispersionof MWCNTs in the PMMA matrixquantitatively. The results confirm the

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accurate amounts of MWCNTs inPMMA matrix for each group. ThePMMA denture resin alone degradesfrom 380�C to 580�C based on themanufacturer’s data. The data showthat the PMMA was completelydegraded at 600�C. As shown inFigure 2, the addition of MWCNTs inPMMA slightly improved the thermalstability of the PMMA resin because ofthe right shifts of TGA curves of 0.5%,1%, and 2% MWCNTs-PMMA speci-mens higher than 500�C. However,TGA did not provide information on thequalitative distributions of MWCNTsin the PMMA matrix, namely, homoge-nous or inhomogeneous dispersion ofMWCNTs.

SEM was used to examine the qual-itative analysis of MWCNTs dispersed inthe PMMA matrix. Microscopic obser-vations across the fractured surfaceindicated that the MWCNTs were welldistributed and dispersed in the PMMAmatrix when 0.5% and 1% were used.Once the MWCNTs reached 2% byweight, more agglomerations werenoted. Microvoids on fractured surfacesalso were detected by SEM. TheMWCNTs-PMMA nanocomposite resinin this study was prepared by using high-power ultrasonic vibration. The resultsfrom SEM observations suggest that thepreparation method in this study needsfurther improvement. The SEM resultsmight explain the reason why the 2%MWCNTs-PMMA group had the lowestvalues in both static and dynamicloading tests. Results of studies haveshown that MWCNTs tend to agglom-erate when processed into polymerssuch as CNTs-polymer compositeresins.22,29 Individual nanotubes aredifficult to separate during mixing. Thecurrent mixing method has limitations.

Another critical issue associatedwith the properties of the experimentalMWCNTs-PMMA composite resins isthe interfacial interaction-wetting be-tween the polymer and the MWCNTs.Ideally, with good interfacial bondingbetween the PMMA and the MWCNTs,any load applied to the polymer matrixshould be transferred to the nanotubes.In this study, Raman spectroscopy was

Wang et al

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.00.0 0.5 1.0 1.5 2.0

CNT Content (%)

Res

ilien

ce (

J/m

m3 )

6 Resilience values of polymethyl methacrylate resin with0%, 0.5%, 1%, and 2% multiwalled carbon nanotubes.

7 A, Setup for 4-point bending fatigue test. B, Typicalloading curve as function of loading vs cycle count forfatigue test.

April 2014 323

Wang et al

used to examine the existence of inter-facial bonding, and static and dynamicloading tests were used to qualify theinterfacial bonding between MWCNTsand the PMMA matrix.

Raman spectroscopy is called the‘fingerprint’ technique in that everychemical gives a unique Raman signalor spectrum and offers several advan-tages for microscopic analysis. Becauseit is a scattering technique, specimensdo not need to be fixed or sectioned.Raman spectra can be collected from asmall volume (<1 mm in diameter).Typically, in Raman spectroscopy,high-intensity laser radiation withwavelengths in either the visible or near-infrared regions of the spectrum ispassed through a sample. Photonsfrom the laser beam are absorbed bythe molecules, exciting them to a higherenergy state. If the molecules relax backto the vibrational state where theystarted, then the reemitted energy canbe detected and recorded.

The spectrum of the Raman-scattered light depends on the molec-ular constituents present and theirstate, which allows the spectrum to beused for material identification andanalysis. This can be used to gain in-formation about the sample composi-tion and structure determination interms of chemical groups present andalso its purity. The frequency differencesbetween the excitation radiation andthe Raman scattered radiation arecalled the Raman shift. Raman shiftsare reported in units of wavenumber(cm-1) and are defined as D (cm-1)¼(1/lo - 1/lR), where D is the Raman shift,lo is the laser wavelength, and l R is theRaman radiation wavelength. Eachmolecule has a unique characteristicarray of light produced by the Ramaneffect, which can be plotted by intensityagainst the Raman shift on a Ramanspectrum.

For CNTs, Raman spectroscopy candetect 2 unique peaks of graphitic struc-ture: one is located at 1306 cm-1 desig-nated to the D band (sp3 atomic orbitalof carbon); the second peak is located at1594 cm-1 designated to the G band (sp2

atomic orbital of carbon) associatedwith

1 000 000100 000

10 0001000

10010

10.1

0.011E–31E–41E–51E–61E–7

0.0 0.5 1.0 1.5 2.0CNT content (%)

Fati

gue

Num

ber

8 Fatigue resistance of polymethyl methacrylate resin with0%, 0.5%, 1%, and 2% multiwalled carbon nanotubes.

9 Low magnification scanning electron microscopy oftypical fracture surface of multiwalled carbon nanotubes-polymethyl methacrylate composite. Scale bar¼50 mm.

324 Volume 111 Issue 4

the stretching motions of tangential C-Cbonds. A higher D band from CNTs in-dicates higher amorphous carbon atoms,which associate with crystallographicdefects related to graphitic structures.

The observed shift in the Ramanpeaks of MWCNTs-PMMA in Figure 3indicates the presence of interfacial in-teractions between MWCNTs and thepolymer matrix. In Figure 4, the decreaseof the ratios of the intensity of D- Ramanpeak and G- Raman peak (ID/IG)with the increase of MWCNTs contentsuggests that the polymer-nanotubes

The Journal of Prosthetic Dentis

interactions had reduced the amor-phous graphitic structures (D band) byinterfacial reactions between MWCNTsand the PMMA matrix.

The Raman data in Figures 3 and 4confirmed the interfacial reactions be-tween MWCNTs and PMMA but didnot give quantitative or qualitative infor-mation on interfacial bonding. Becausesome degree of interfacial bonding waspresent between MWCNTs and PMMA,0.5% and 1% MWCNTs groups hadbetter results than those of the controlgroup for the static flexural strength test.

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The 0.2% group had significantly lowervalues than the control group accordingto the static loading test, which likelyresulted from the inhomogeneousdispersion of MWCNTs in the PMMAmatrix. The negative effect of MWCNTson PMMA resin, as shown by the dy-namic fatigue test, was indicative of poorload transfer between polymer andMWCNTs. The crack propagationmightintensify along numerous MWCNTs-PMMA interfacial locations.

Reported results on CNTs-polymercomposite resin properties are scat-tered, depending on factors such as thetypes of CNTs (SWCNTs or MWCNTs),their morphology, diameter, length,and processing method. Other factorsinclude the types of matrix and theinterfacial interaction between CNTsand the matrix. Material design andprocessing are important steps indeveloping new denture based mate-rials, followed by laboratory and clin-ical tests. Data from the study did notsupport the working hypothesis; thissuggests that the system needs im-provement. To address the materialdesign issue, a future plan includesmodifying the surface of MWCNTs topromote better interfacial interactionsto polymers. Chemical, electrochemical,or plasma treatment could be useful toplace different organ-functional groupsonMWCNTs. Adding carboxyl or amidefunctional groups to MWCNTs wouldbe a logical approach to facilitateMWCNTs-PMMA interfacial reactions.Another approach would be to use anoxidation or silanization process tomodify the surface of CNTs with a low-pressure oxygen plasma treatment. Formaterial processing of MWCNTs toPMMA, the future addition of surfac-tants to PMMA monomereMWCNTsduring mixing may provide a better so-lution to the homogenous dispersion ofMWCNTs to a polymer material.

The color of CNTs can be deter-mined by the amounts of conjugateddouble bonds of carbons. Various colormodifications of MWCNTs can bedone by attaching other functionalgroups, which would change the colorof CNTs. This is beyond the scope of

Wang et al

10 Bundled multiwalled carbon nanotubes and typicalfracture surface of polymethyl methacrylate resin with 0.5%multiwalled carbon nanotubes. Scale bar¼1 mm.

11 Micrograph of fracture surface of 2% multiwalled carbonnanotubes-polymethyl methacrylatespecimen. Center of theimage shows agglomeration multiwalled carbon nanotubesand arrows are single fibrils multiwalled carbon nanotubes.Scale bar¼1 mm.

April 2014 325

this article and would be important fora future study. Other alternatives are touse SWCNTs, which are transparent oruse fewer MWCNTs in the PMMAmatrix.

Further study is needed to improvethe dispersion of MWCNTs into com-mercial denture base systems and

Wang et al

thereby allows the effects of MWCNTson the denture base materials to betested as they are prepared in clinicalsituations. Future investigations willalso center on enhancing the bondingof MWCNTs to the PMMA denturebase material to reduce denturefailures.

CONCLUSIONS

TGA showed accurate quantitativedispersions of MWCNTs in the PMMAmatrix. Raman spectroscopy showedthat an interfacial reaction occurredbetween MWCNTs and the PMMAmatrix. The results from the static anddynamic loading test suggested that theinterfacial bonding between MWCNTsand PMMA was weak and in need ofimprovement. The addition of 0.5%and 1% MWCNTs improved the PMMAresin flexural strength and resiliencebut not the 2% MWCNTs because ofpoor dispersion of MWCNTs based onSEM observations. MWCNT adverselyaffected the fatigue resistance ofMWCNT-PMMA composite resins, withfatigue resistance deteriorating withhigher concentrations of MWCNTs.

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Corresponding author:Dr Russell WangDepartment of Comprehensive CareCase Western Reserve University School ofDental Medicine10900 Euclid AveCleveland, OH 44106-4905E-mail: rxw26@case.edu

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