COMPOSITESSCIENCE AND

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Composites Science and Technology 67 (2007) 3331–3337

TECHNOLOGY

Fabrication and mechanical properties of well-dispersedmultiwalled carbon nanotubes/epoxy composites

Peng Guo, Xiaohong Chen, Xinchun Gao, Huaihe Song *, Heyun Shen

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 100029 Beijing, PR China

Received 29 September 2006; received in revised form 26 January 2007; accepted 21 March 2007Available online 30 March 2007

Abstract

Multiwalled carbon nanotubes (MWCNTs)/epoxy nanocomposites were fabricated by using ultrasonication and the cast moldingmethod. In this process, MWCNTs modified by mixed acids were well dispersed and highly loaded in an epoxy matrix. The effects ofMWCNTs addition and surface modification on the mechanical performances and fracture morphologies of composites were investi-gated. It was found that the tensile strength improved with the increase of MWCNTs addition, and when the content of MWCNTs load-ing reached 8 wt.%, the tensile strength reached the highest value of 69.7 MPa. In addition, the fracture strain also enhanced distinctly,implying that MWCNTs loading not only elevated the tensile strength of the epoxy matrix, but also increased the fracture toughness.Nevertheless, the elastic modulus reduced with the increase of MWCNTs loading. The reasons for the mechanical property changesare discussed.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Carbon nanotubes; A. Polymer-matrix composites (PMCs); B. Mechanical properties

1. Introduction

Ever since the paper on carbon nanotubes (CNTs) waspublished by Iijima [1], there has been an increasing interestin the composites of carbon nanotubes and polymers.CNTs, as a novel crystalline carbon form, are unique nano-structured materials with remarkable physical and mechan-ical properties [2,3], such as high tensile strength andYoung’s modulus [4,5], high thermal conductivity [6] aswell as high current density. These excellent propertiesinspire interest in using CNTs as fillers in polymer compos-ite systems in order to enhance the electrical conduction,thermal transport, and mechanical properties of the origi-nal polymer materials [7,8].

Among various polymers, high strength epoxy systemsare very important materials for aircraft, space shuttle,electronics products, and many other industrial applica-

0266-3538/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compscitech.2007.03.026

*

Corresponding author.E-mail address: songhh@mail.buct.edu.cn (H. Song).

tions. Nanotube-reinforced epoxy systems hold the prom-ise of delivering superior composite materials with highstrength, light weight, and multi-functional features, andhave attracted great attention. Allaoui et al. [9] investigatedthe influence of multiwalled carbon nanotubes (MWCNTs)in a rubbery epoxy matrix, and found that the addition ofMWCNTs up to 4 wt.% could lead to a significant increasein strength and Young’s modulus. But, the mentionedpreparation method, solution mixing did not lead to anadequate distribution of CNTs in the epoxy matrix.Agglomerates in the dimension of several tens of microm-eters could still be detected by a light microscope. Liaoet al. [10] produced the epoxy-based nanocomposites byusing sonication and surfactants to disperse single-wall car-bon nanotubes (SWCNTs) and investigated their thermo-mechanical properties. It was shown that the incorporationof SWCNTs led to a reduction of the glass transition tem-perature. Miygawa and Drzal [11] reported that the storagemodulus of the epoxy-based nanocomposites containingfluorinated SWCNTs increased, but the glass transition

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temperature decreased linearly with an increase in the con-tent of CNTs. Recently, a mechanical reinforcement ofepoxy resins by the use of CNTs was reported by Zhuet al. [12,13]. They made a significant progress in improvingthe CNTs dispersion and matrix adhesion by adding1 wt.% alkylamino-functionalized SWCNTs to the epoxy,which showed an increased strength and Young’s modulus.The functionalization increased the polarity and directlyincorporated the SWCNTs into the epoxy network.

To date, although great progress has been made in thedevelopment of epoxy-based nanocomposites with CNTsas a reinforcing structural element, due to the usage of dif-ferent epoxy resin, different processing techniques andparameters, as well as the choice of CNTs with differentpre-treatments, the reported results showed diversity andit is difficult to compare them.

From these studies, we can see that the content of CNTsused in the matrix was usually small and the dispersioncould achieve homogeneity, but the use of solvent pro-longed the processing time, and perhaps affected the capa-bilities of the epoxy resin, too. In this study, our intentionis focused on how to increase the content and the homoge-neous dispersion of MWCNTs without a solvent in thematrix. As a result, some evidence is given which indicatean improvement of the strength and toughness of the com-posites. The influence of different contents of MWCNTsloading on the dispersion and interfacial adhesion withepoxy resin and then on the mechanical properties of nano-tube-based composites were investigated.

2. Experimental

2.1. Materials and experimental details

The multiwalled carbon nanotubes, used in this studysynthesized by the chemical vapor deposition (CVD), weresupplied by Tsinghua University – Nanfeng NanoparticleTechnology Center, these have a diameter of 5–60 nm

Fig. 1. Tensile test sample

and a length of 0.5–500 lm. Epoxy resin E-51, a low viscos-ity (viscosity 6 2.5 Pa S at 40 �C) type diglycidyl ether ofbiphenol-A resin, and a hardener (2-ethylic-4-methyl imid-azole, EMMZ) used as the matrix were obtained fromTianjin Chem. Reagent Co. The pristine CNTs were com-posed of many aggregates with different sizes. The aggre-gates would be obstacles to uniform dispersion ofMWCNTs in the epoxy resin. So epoxy composites con-taining well-dispersed MWCNTs were prepared by the fol-lowing procedures: One gram of the crude MWCNTs waspurified by 150 ml 20 wt.% dilute nitric acid in a water bathfor 24 h at 40 �C in order to remove the unwanted amor-phous carbon and metal catalyst. Then the purifiedMWCNTs were chemically modified by mixed acids:0.5 g of the purified MWCNTs were suspended in 80 mlof a 3:1 (vol/vol) mixture of concentrated sulfur acid(98%) and nitric acid (67%) and sonicated for 5 h at roomtemperature. The resultant acid-treated MWCNTs werethen washed with distilled water until pH 7, and dried at80 �C in a vacuum oven. After this, the modifiedMWCNTs were manually suspended in epoxy resin andthen sonicated for 4 h using a high-power (600 W) ultra-sonic machine. During the last 10 min of ultrasonication,a few drops of curing agent EMMZ were added into thesystem. Thereafter, the blend was degassed for 3 h in a vac-uum oven, cast into a stainless steel mold and was cured at120 �C in an oven under atmospheric pressure for 4 h. Theshape and size of the mold is shown in Fig. 1. Finally, 10dog-bone shaped flat composite specimens were obtained.

2.2. Characterization of MWCNTs and their epoxy

composites

A transmission electron microscope (TEM, H800) andFourier transform infrared spectroscopy instrument(FTIR, Nexus670) were used to measure the morphologyand surface chemistry of crude and modified MWCNTs.The mechanical properties of CNTs/epoxy composites

geometry (unit: mm).

Fig. 2. TEM images of MWCNTs: (a) crude MWCNTs and (b) acid-treated MWCNTs.

Fig. 3. FTIR spectra of the crude and acid-treated MWCNTs.

P. Guo et al. / Composites Science and Technology 67 (2007) 3331–3337 3333

were measured by an InstronTM 1185 mechanical testingmachine. Tensile tests were carried out at 25 �C with a con-stant cross-head rate of 2 mm/min. The choice of this quitelow loading rate is to consider the brittle character of com-posites. Eight to ten specimens were tested for each sampleand the average value was obtained from the data of thesemeasured specimens (Fig. 1). The thickness of samples is3 mm ± 0.5 mm. The fracture surface of MWCNTs/epoxycomposites with different nanotube contents was observedby a high resolution scanning electron microscope(HRSEM, FEI XL-30). The morphology and dispersedstate of mixed-acid treated MWCNTs in the epoxy matrixwere performed by a high resolution transmission electronmicroscope (HRTEM, JEOL TEM-3010) using the super-thin cutting technique.

3. Results and discussion

3.1. Texture and surface characteristics of treated

MWCNTs

The crude MWCNTs were composed of some impuritiessuch as metallic catalyst particles and amorphous carbons.These impurities should be removed, because they wouldbe obstacles to improving the mechanical properties ofthe composites. The morphologies of the crude andmixed-acid treated MWCNTs are shown in Fig. 2a andb, respectively. From the two images we can see that thecrude MWCNTs were aggregated in tangles (Fig. 2a) andindividual nanotubes are present in Fig. 2b, implying thatthe acid-treated MWCNTs were not aggregated and wouldbe well dispersed owing to the increased polarity from thefunctional groups through the chemical treatment. TheFTIR spectra in Fig. 3 verified this case. By acid treatment,the multiwalled carbon nanotubes exhibit distinct absorp-tions in wavenumbers of 3436 cm�1 (the O–H stretches ofterminal carboxyl groups), 1632 cm�1(the carboxyl group(C@O)) and 1115 cm�1 (the C–O stretching), respectively.These peaks are not present or show weak absorption incrude MWCNTs. Therefore, it is clear that the chemicaltreatment of the MWCNTs by concentrated aqueous acidswould make the surface of MWCNTs get attached polarfunctional groups such as –OH, –C–O, which could leadto an improvement of the MWCNTs/epoxy resin interfa-cial interaction. So, MWCNTs can be easily integrated intothe matrix structure via reaction with epoxy resin and con-sequently become an integral part of the matrix polymerstructure rather than a separate filler [13].

3.2. Effect of MWCNTs loading on the mechanical

properties of epoxy composites

In an early stage of composite preparation, the pristineMWCNTs were used to act as enforcement fillers and itwas found that both the tensile strength and elastic modu-lus of this composite decreased 10–20% compared withthose of the genuine resin in the MWCNTs addition range

1–3 wt.%. When the content of MWCNTs is above 3 wt.%,the viscosity of the mixture increased greatly which leads todifficulty in dispersion using ultrasonic treatment. Theseresults indicated that because of the lack of effective coher-

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ence of pristine MWCNTs with epoxy resin, the unmodi-fied MWCNTs are not favourable as the reinforcing agentfor the preparation of nanocomposites. Fig. 4 shows theinfluence of MWCNTs loading on the mechanical perfor-mances of epoxy composites. From Fig. 4 we can see thatthe general tendency is that, both the tensile strength andthe fracture strain increased with the addition ofMWCNTs, while Young’s modulus reduced at the sametime. When the content of MWCNTs reached 8 wt.%, thetensile strength exhibited a maximum value of 69.7 MPa,and the fracture strain reached the highest value of19.9%, too. The above mentioned relative improvementof epoxy resin in strength by adding CNTs can beexplained by the high specific mechanical property and spe-cific surface area of the MWCNTs [14]. Here, MWCNTsplay the role of an enhancing framework.

In order to have a better view of the effect of MWCNTson the mechanical behavior of the composites, we presentthe stress–strain curves of the tested samples in Fig. 5. Itcan be seen that, the stress–strain curves exhibit conversedtypes from the hard and brittle (the broken character of

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Fig. 4. The effects of MWCNTs loading content (wt.%) on the mechanical perf(b) and fracture strain (c).

thermosetting resin) to the firm and tough fractures. More-over, due to the non-linear relationship between stress andstrain, it is difficult for us to calculate the value of absorp-tion energy during the fracture processing. However, thearea under the stress–strain curve can be used to character-ize the value of absorption energy before the factureoccurred, and judge the materials’ toughness. FromFig. 5, it can be seen that, as the MWCNTs contentincreased from 5 wt.% to 8 wt.%, the areas under thestress–strain curves increased clearly, suggesting that thetoughness of the composites increased moderately owingto the addition of MWCNTs, especially when the contentof MWCNTs reached the highest value at 8 wt.%.

From Figs. 4 and 5, the fact that the fracture strainincreased and Young’s modulus reduced simultaneouslywith the addition of CNTs indicated that the addition ofMWCNTs could increase the toughness of the composites.An explanation for the behavior can be found by consider-ing the concentric structure, etched surface and shortenedlength of MWCNTs via mixed-acid treatment. Because aweak van der Waals force exists between individual graph-

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Fig. 5. Stress–strain curves of MWCNTs/epoxy resin composites.

Non-deformationin matrix before loading

Fiber

Matrix

Deformation in the matrix unequally during loading processing and after loading

Fig. 7. Deformation of short fibers in the composite.

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ene shells of the MWCNTs, slipping between the shellscould easily occur. For this reason, when loading on thecomposite, the MWCNTs in the polymer-matrix can bedrawn layer by layer and exhibit toughness to a certainextent. Most significantly, through mixed-acid treatment,not only many polar functional groups were generated onthe surface of MWCNTs, but also the outer graphene layersof MWCNTs were easily destroyed by oxidation to someextent. The latter would further weaken the interaction ofgraphene layer and make it slip easily between the shells ofMWCNTs. In Fig. 6, the HRSEM image indicated thatstructural relaxation happened at the surface of MWCNTswhen they were pulled from matrix. Some of the ends ofdeformed MWCNTs in the matrix were peeled off and formthe cone-shape morphology. Previously, several researchershave reported that carbon nanotubes can be cut by mixed-acid treatment [15]. Indeed, in our experiment, it was foundthat the aspect ratio of MWCNTs decreased via concentra-tion H2SO4/HNO3 oxidation, which can be seen fromFig. 2b. Therefore, the presence of shortened carbon nano-tubes should be one of the contributions for the enhance-

Fig. 6. HRSEM image of MWCNTs drawn from the matrix.

ment of composite toughness. Fig. 7 illustrates adeformation model of short fibers in the composite. Accord-ing to the toughening mechanism of short fibers [16], whenloading on the composite, matrix and fibers can deform atthe same time. Because of Ef (modulus of fiber)� Em (mod-ulus of matrix), fiber can confine the matrix deformationaround nanotubes. As a consequence, the resin matrix can-not be directly broken when it reaches the fracture limit.That is to say, the composite will not crack until the nano-tubes cannot sustain the increasing force loading. The inter-face radical stress controls the gliding behavior. The shortaspect ratio of nanotubes is favorable for improving thenon-uniform distribution of axial stress and dramaticallydecreasing shearing force at the end of short fiber [16].Besides, it is supposed that with the composite breaking,nanotubes broke with the surrounding matrix in the mean-time. The flexible nature of nanotubes may contribute theenhancement of toughness of composite, too.

3.3. Morphology of the composite interface and dispersion of

MWCNTs in epoxy matrix

The fracture morphologies of MWCNTs/epoxy com-posites are shown in Fig. 8. We can see that at the edge

Fig. 8. HRSEM image of the fracture surface of MWCNTs/epoxycomposites.

Fig. 9. HRSEM images of the fracture surfaces of MWCNTs/epoxy composites for MWCNT contents of (a) 0 wt.%, (b) 1 wt.%, (c) 2 wt.%, and (d)8 wt.%.

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of the white points there are some dark marks of pits. Thepull-outs exactly happened in some areas, while in themost areas the nanotubes were still firmly in the matrix.

Fig. 10. HRTEM image of composites containing 8 wt.% MWCNTs.

This phenomenon reveals the great effect of MWCNTssurface groups on the interfacial bonding. Only if CNTsfirmly adhere to the epoxy matrix, the load transfers fromepoxy resin to the CNTs. In addition, the white points inFig. 9b–d represent the MWCNTs dispersed in the epoxyresin. With the increasing content of nanotubes, the whitespots increase. From the images we can see that theMWCNTs were well dispersed in the matrix withoutany aggregations of MWCNTs regardless of the contentof the CNTs. In Fig. 10, the TEM image of compositescontaining 8 wt.% MWCNTs also exhibited the well-dis-persed state of carbon nanotubes in the polymer matrix.The mechanical properties have been improved simulta-neously because the stress concentration can be easily alle-viated by the well-dispersed MWCNTs. ComparingFig. 9a and b–d, the composites containing MWCNTsexhibit a significantly rougher fracture surface comparedwith the neat epoxy, indicating a toughness effect of thenanofillers.

4. Conclusions

The well-dispersed MWCNTs/epoxy composites weresuccessfully fabricated by means of a cast molding method.The mechanical properties of MWCNTs/epoxy compositeswere improved significantly with the addition of CNTs.

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The combination of chemical functionalization ofMWCNTs and high energy sonication is a useful approachto improve the dispersion state of the CNTs and the inter-facial adhesion to the epoxy matrix. The most significantimprovements of strength (+11.65%) and especially frac-ture strain at break (+127.8%) were attained with acid-treated MWCNTs at an 8 wt.% content.

Here, the introduced sonication technique gave a disper-sion of the nanotubes in an epoxy resin. A further increaseof the mechanical properties of nanotube/epoxy compos-ites can be expected by a variation of processing parame-ters. This is the way to be followed to take advantage ofthe MWCNTs characteristics and properties for the nextgeneration of composite materials.

Acknowledgement

This work was supported by the Program for New Cen-tury Excellent Talents in the University (NCET-04-0122).

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