Characterization of multiwalled carbonnanotube-polymethyl methacrylate

Rus PhD,c andLimCas , and CaseWe d, Ohio

Statement of prob l fatigue ofdenture resins.

cience and Engineering.fatigue resistance.processing is need

aAssociate Professor, Department of ComprebPostgraduate student, Department of CivilcAssociate Professor, Department of Civil EndProfessor, Department of Macromolecular SThe Journal of Prosthetichensive Care.Engineering.gineering.Adding 0.5 wt% and 1 wt% of multiwalled carbon nanotubesimproves the exural strength and resilience of multiwalled carbonnanotubes-polymethyl methacrylate composite resins. Fatigue testsshow that all groups with multiwalled carbon nanotubes had poor

Future improvement of the material design anded.Purpose. The purpose of this study was to evaluate a polymethyl methacrylate denture base material modied withmultiwalled carbon nanotubes in terms of fatigue resistance, exural 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 exural 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 (a.05).

Results. Thermogravimetric analysis veried 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 signicant differences in static and dynamic loadingsamong the groups. The worst mechanical properties were in the 2 wt% multiwalled carbon nanotubes (P

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 ber rein-forced composites, the scale of the rein-forcement phase diameter has changedfrom micrometers to nanometers.

The addition of carbon bers 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 are

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 thebrils and bridging voids to enhancethe fatigue performance of the polymer.

The effects of CNT reinforcement onthe mechanical properties of denture

otutbn

April 2014 319Most fractures of dentures occurduring function, primarily from dentureresin fatigue.1-4 Flexural fatigue occursafter repeated exing 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 exuralfatigue.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 bers, longcarbon bers, and metal wires.10-14

Success has been limited.15,16 The ex-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 conicting values but generallywithout signicant 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 awless structure are clas-sied 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, anWang et alsuperior to conventional polymer-basedcomposites because of their consider-ably higher intrinsic strengths andmoduli. The stress transfer efciency 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 modica-tion have been proposed.22,28,29 Theaddition of CNTs usually causes adeformation mode of PMMA bers. Inpure PMMA bers, polymer necking

1 Diagrammatic representatiand multiwalled carbon nanonanotube: single graphite sheelindrical tube; multiwalled cargraphite sheets concentricallybase 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).Manufacturers instructions were fol-lowed with a powder-liquid ratio of

n of single-walled nanotubesbes. Single-walled carbonseamlessly wrapped into cy-on nanotube: array ofested like rings of tree trunk.

320 Volume 111 Issue 421 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 dentureasks (Hanau Type; Whip Mix Corp).Specimens were fabricated in a rectan-gular mold prepared in standard den-ture asks by using a template thatmeasured 70403 mm. The closedasks, tightened with spring clamps,were polymerized in a water bath for 9hours at 71C and cooled for 30 mi-nutes in water at 26C. The ask wasbench cooled before deasking. Thespecimens were removed from theasks and cleaned of stone particles.After deasking, each mold was cut toobtain the specimens of 70103 mmfor the exural strength test. The spec-imens were sequentially polished withsilicon carbide paper (1000, 800, and600 grit) to achieve smooth edges.

The exural strength was determinedby using the 3-point bending test asspecied by the International Organiza-tion for Standardization specication20795-1:2008.32 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 exural strength (S) was calcu-lated from the following formula:

S3 FL/2 bd2,

where: S, exural strength in MPa; F,the load at break in N; L, 50 mm, thespan of specimen between supports; b,The Journal of Prosthetic Dentiswidth 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, resilience0.5 (yield stress yield strain).33 The fatigue test of thedifferent groups was conducted with aBionix II test system (MTS Inc). Pre-dened 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 deectionwas set as 3 mm; the loading functionwas then dened as a sine wave loadingwith a frequency of 5.7 Hz and adeection of 2 mm. The applied forcesand deections 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 eld emission SEM.Raman spectroscopic analysis wasconducted by using a visible laser lightwith the wavelength of 514 nm (Titansapphire laser, Jobin-Yvon HR640tryspectrometer) tted 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 conrmation of theamount of MWCNTs in PMMA wereevaluated with thermogravimetric anal-ysis (TGA). TGA measurements wereperformed with a thermogravimetricanalyzer from 20C to 800C. Theheating rate was 10C/min with a ni-trogen gas ow at 60 cm3/min rate.

Data derived from the control andthe experimental groups on exuralstrength, 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 (a.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 at300C and was completely degradedat 600C. For MWCNT-PMMA speci-mens, the percentage weight ofMWCNTs was detected by TGA above630C after the PMMA was completelydegraded, and the results accuratelyreected the amount of MWCNTs foreach specimen. The weights of theMWCNTs remain fairly constant attemperatures beyond 630C (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 RamanWang et al

so

April 2014 3212 Thermogravimetric analysicarbon nanotubes. Pure Lucitspectroscopy; 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, whichconrms the interactions betweenMWCNTs and the PMMAmatrix. Similar

3 Raman spectra of purnanotubes, pure polymet2% multiwalled carbon nmethacrylate resin.

late and 0.5%, 1%, and 2% mupolymethyl methacrylate compo

Wang et alcurves of pure multiwalledne199 polymethyl methacry-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 exural strengthtest are shown in Figure 5. Compared

e multiwalled carbonhyl methacrylate, andanotubes in polymethyl

ltiwalled carbon nanotubes-site resins.with the control group, exural strengthincreased by an average of 9.3% with0.5% and 3.9% with 1% MWCNTs-PMMA groups. However, an 11.4%decrease in exural 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 signi-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

322 Volume 111 Issue 4(25 000) are shown in Figure 10. TheMWCNTs can be clearly identied 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).

120

105

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5 Flexural strength of polymet0.5%, 1%, and 2% multiwalled

4 Raman shifts of 0.5%, 1%, ananotubes-polymethyl methacryD-G band ratios.

The Journal of Prosthetic DentisDISCUSSION

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 conrm the

1.0 1.5 2.0ontent (%)

hyl methacrylate with 0%,carbon nanotubes.

nd 2% multiwalled carbonlate resins with different

tryaccurate amounts of MWCNTs inPMMA matrix for each group. ThePMMA denture resin alone degradesfrom 380C to 580C based on themanufacturers data. The data showthat the PMMA was completelydegraded at 600C. 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 500C. 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 aredifcult 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 wasWang 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 alused 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 thengerprint 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 xed or sectioned.Raman spectra can be collected from asmall volume (

5324 Volume 111 Issue 41 000 000100 000

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0.0 0.CN

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berthe 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

8 Fatigue resistance of polym0%, 0.5%, 1%, and 2% multiwa

9 Low magnication scanningtypical fracture surface of multpolymethyl methacrylate compo

The Journal of Prosthetic Dentis1.0 1.5 2.0T content (%)interactions had reduced the amor-phous graphitic structures (D band) byinterfacial reactions between MWCNTsand the PMMA matrix.

The Raman data in Figures 3 and 4conrmed 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 exural strength test.

ethyl methacrylate resin withlled carbon nanotubes.

electron microscopy ofiwalled carbon nanotubes-site. Scale bar50 mm.

tryThe 0.2% group had signicantly 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 colormodications of MWCNTs can bedone by attaching other functionalgroups, which would change the colorof CNTs. This is beyond the scope ofWang et al

April 2014 325this 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

10 Bundled multiwalled carbofracture surface of polymethyl mmultiwalled carbon nanotubes.

11 Micrograph of fracture surfnanotubes-polymethyl methacrimage shows agglomeration muand arrows are single brils muScale bar1 mm.

Wang et althereby 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.

Int J Prosthodont 1992;5:315-20.4. Stafford GD, Lewis TT, Huggett R. Fatigue of

n nanotubes and typicalethacrylate resin with 0.5%Scale bar1 mm.

ace of 2% multiwalled carbonylatespecimen. Center of theltiwalled carbon nanotubesltiwalled carbon nanotubes.testing of denture base polymers. J OralRehabil 1982;9:139-54.

5. Diaz-Arnold AM, Vargas MA, Shaull K,Laffoon JE, Qian F. Flexural and fatiguestrengths of denture base resin. J ProsthetDent 2008;100:47-51.

6. Fujii K. Fatigue properties of acrylic denturebase resins. Dent Mater J 1989;8:243-59.

7. Darbar UR, Huggett R, Harrison A. Denturefracture: a survey. Br Dent J 1994;176:342-5.

8. Vuorine AM, Dyer SR, Lassila LV, Vallittu PK.Effect of rigid rod polymer ller on mechan-ical properties of poly-methyl methacrylatedenture base material. Dent Mater 2008;24:708-13.

9. Kim SH, Watts DC. The effect of reinforce-ment with woven E-glass bers on the impactstrength of complete dentures fabricatedwith high-impact acrylic resin. J ProsthetDent 2004;91:274-80.

10. Jagger DC, Jagger RG, Allen SM, Harrison A.An investigation into the transverse andimpact strength of high strength denturebase resins. J Oral Rehabil 2002;29:263-7.

11. Franklin P,WoodDJ, BubbNL. Reinforcementof poly (methyl methacrylate) denture basewith glass ake. Dent Mater 2005;24:365-70.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 exural 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

Copyright 2014 by the Editorial Council forThe Journal of Prosthetic Dentistry.

A

b istse ta 0is

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Characterization of multiwalled carbon nanotube-polymethyl methacrylate composite resins as denture base materialsMaterial and methodsResultsDiscussionConclusionsReferences