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[2] Chin KC, Gohel A, Chen WZ, Elim HI, Ji W, Chong GL, et al. Goldand silver coated carbon nanotubes: an improved broad-band opticallimiter. Chem Phys Lett 2005;409(1–3):85–8.

[3] Guo DJ, Li HL. Highly dispersed Ag nanoparticles on functionalMWNT surfaces for methanol oxidation in alkaline solution. Carbon2005;43:1259–64.

[4] Harris PJF. Carbon nanotube composites. Int Mater Rev2004;49(1):31–43.

[5] Oh SD, So BK, Choi SH, Gopalan A, Lee KP, Yoon KR, et al.Dispersing of Ag, Pd, and Pt–Ru alloy nanoparticles on single-walled carbon nanotubes by gamma-irradiation. Mater Lett 2005;59:1121–4.

[6] O�Connell MJ, Bachilo SM, Huffman CB, Moore VC, Strano MS,Haroz EH, et al. Band gap fluorescence from individual single-walledcarbon nanotubes. Science 2002;297(5581):593–6.

Enhancement of Vicker�s hardness of nanoclay-supportednanotube reinforced novel polymer composites

Mei Lu a, Kin-tak Lau a,*, Wai-Yin Tam a, Kin Liao b

a Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, Chinab School of Mechanical and Production Engineering, Nanyang Technological University, Singapore

Received 30 April 2005; accepted 3 September 2005

Keywords: Carbon nanotubes; Chemical vapor deposition; Mechanical properties

Carbon nanotubes as a reinforcement engineering mate-rial in polymer composites holds the promising potentialfor low-weight composites of extraordinary mechanical,electrical, and thermal properties. The size scale, high as-pect ratio, low density, and other exceptional propertiesof nanotubes are generally advantageous when they are ap-plied in a variety of applications. However, in the case ofnanotube-reinforced polymer composites, there has onlybeen a moderate strength enhancement that is significantlybelow the theoretically predicted potential [1]. To achievethe full reinforcing potential of nanotubes, there remainstwo critical issues that have to be firstly solved, (i) the dis-persion of nanotubes in a polymer matrix and (ii) the inter-facial bonding between the nanotubes and the polymermatrix. In general, weakly interacted nanotube bundlesand aggregation of nanotubes would result in a poor dis-persion state that significantly reduces the aspect ratio ofthe reinforcement [2]. The reason for the weak interfacialbonding behavior lies in the atomically smooth, non-reac-tive surface of the nanotubes that cannot ensure efficientload transfer ability from the polymer matrix to the nano-tube lattice [3]. To solve this problem, a number of meth-

ods have been developed to maximize the benefits ofnanotubes in polymer composites, i.e. surfactant assisteddispersion [4], sonication with high power [5], in situ poly-merization [6], electric field or magnetic-induced alignmentof nanotubes [7,8], plasma polymerization [9], and surfacemodification such as inorganic coating [10], polymer wrap-ping [11], as well as protein functionalization [12].

The methods published thus far on the improvement ofmechanical properties of polymer composites have focusedon the optimization of the manufacturing process of thecomposites, i.e. with the use of prepared nanotubes. Inthe case of using nanotubes fabricated with CVD methodas reinforcement, the quality of the composite propertiesis partly negatively impacted without the complete removalof the support materials that is used to prepare nanotubes,such as silica gel, alumina, quartz, zeolites. We have uti-lized nanoclay as a support material to grown nanotubeson the surface of clay by reduced-pressure chemical vapordeposition (CVD) [13] since clay itself is a good reinforce-ment for polymer composites with its high ion exchangecapacity, high aspect ratio, ease of fabrication, and rela-tively low cost. Nanotubes thus prepared are protectedfrom aggregation by the interlayer space of nanoclay. Inaddition, nanoclay exhibits exfoliation characteristics be-cause of the nanotubes growth within its interlayer spaces.

0008-6223/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.carbon.2005.09.007

* Corresponding author. Tel.: +852 2766 7730; fax: +852 2365 4703.E-mail address: mmktlau@polyu.edu.hk (K.-t. Lau).

Letters to the Editor / Carbon 44 (2006) 381–392 383

Without the traditional removal of the clay support, the pe-culiar network structure of nanotube–nanoclay could bedirectly incorporated into the polymer matrix and theresulting composite can be expected to exhibit enhancedmechanical properties combining the advantages of bothnanotubes and the exfoliated nanoclay structures.

The nanoclay-supported nanotubes (as see Fig. 1) weredirectly dispersed into the low viscosity, ARALDITE GY251 epoxy resin with strong sonication (output power of800 W, output frequency of 20 kHz) for 1 h. The CIBAHY 956 amine hardener was then added into the nano-clay–nanotube (NT–NC)/epoxy mixture by stirring. Thecuring reaction was processed at room temperature for24 h. The NT–NC/epoxy composites samples with varyingnanotube concentration of 0.5, 1, 2, and 4 wt.% were fabri-cated for Vicker�s hardness measurement. The compositessamples with 2 wt.% nanotubes produced by MgO-supported CVD method, 2 wt.% nanoclay, and 2 wt.%nanotube/nanoclay (3:4) mixture (NT/NC) were also pre-pared for comparison purpose. The proportion of nano-tubes and nanoclay in the NT/NC mixture wasdetermined by the carbon yield calculated from the initialweight of nanoclay support and the final weight of theproduct of nanotubes and nanoclay.

The Vicker�s hardness for 2 wt.% nanotubes, 2 wt.%nanoclay, 2 wt.% NT–NC, 2 wt.% NT/NC reinforcedepoxy composites, and a pure epoxy sample are shown inFig. 2. All the four types of fillers exhibit reinforcement ef-fects in the epoxy matrix. The most noticeable feature ofFig. 2 is that the hardness of 2 wt.% NT–NC/epoxy com-posites increased by a much wider range than that of theother three composites. The average Vicker�s hardnesses

Fig. 1. FE-SEM image of CNTs-nanoclay composite (a) and schematicillustration of the growth mechanism of this composite from theprecipitation of Co(OH)2 colloidal particles (b) [13].

10

12

14

16

18

2 wt%NT/NCmixture2 wt%NT-NC2 wt%NC2 wt%NT

Vic

ker'

s h

ard

nes

s n

o.

Epoxy

Fig. 2. Vicker�s hardness of pure epoxy, 2 wt.% nanotubes/epoxy, 2 wt.% nanoclay/epoxy, 2 wt.% NT–NC/epoxy, and 2 wt.% NT/NC/epoxy composites.

384 Letters to the Editor / Carbon 44 (2006) 381–392

based on ten measurements of each sample are tabulated inTable 1. For nanotube/epoxy composites, the averageVicker�s hardness is slightly higher than that of nanoclay/epoxy composites due to the relatively higher aspect ratioand modulus of nanotubes. However, the hardness valuesof nanotube/epoxy composites scattered more widely thanthose of nanoclay/epoxy composites, ranging from 11.8 to14.2. This can be attributed to the fact that it is more dif-ficult for nanotubes to achieve a good dispersion state ina polymer matrix by a simple sonication method due tothe strong van der Waals interaction between nanotubes.However, in the NT–NC/epoxy composites, the additionof NT–NC composites into epoxy resulted in an improve-ment of 40% in the hardness value compared to the pureepoxy. This significant improvement can be attributed tothe in situ growth of nanotubes on nanoclay layers beforethey are incorporated into epoxy, effectively connectingnanotubes and nanoclay layers to form a new type of fillers

instead of their independent existence as two separatedphases.

The new filler with the network structure can be dis-persed in the matrix with both a good separation of nano-tubes by nanoclay layers and the full exfoliation ofnanoclay. The advantages of nanotubes and nanoclay canbe manifested and combined well to harden the polymermatrix. In addition, the interconnection of nanotubes andnanoclay layers may form mechanical interlocking withinthe matrix, allowing for the effective load transfer fromthe matrix to the nanotubes and nanoclay. Accordingly,the novel NT–NC fillers are superior to either of the indi-vidual nanotubes or nanoclay fillers. The novel filler is alsodifferent with fillers obtained by a simple mixture of nano-tubes and nanoclay of the same proportion ratio, evidentfrom the only 21% improvement in the hardness of NT/NC/epoxy composite. Although the hardness of the com-posites with the mixture of NT/NC as fillers is higher than

Table 1Average Vicker�s hardness and percent improvement of pure epoxy, 2 wt.% nanotubes/epoxy, 2 wt.% nanoclay/epoxy, 2 wt.% NT–NC/epoxy, and 2 wt.%NT/NC/epoxy composites

Sample Average Vicker�s hardness no. Percent improvement (%)

Pure epoxy 10.8 02 wt.% NT/epoxy 12.9 192 wt.% NC/epoxy 12.5 162 wt.% NT–NC composite/epoxy 15.1 402 wt.% NT/NC mixture/epoxy 13.1 21

10

12

14

16

18

4 wt%NT-NC2 wt%NT-NC1 wt%NT-NC0.5 wt%NT-NC

Vic

ker'

s h

ard

nes

s n

o.

Epoxy

Fig. 3. Vicker�s hardness of NT–NC/epoxy composites with different NT–NC concentration.

Letters to the Editor / Carbon 44 (2006) 381–392 385

those of the exclusive nanotubes or nanoclay fillers, thereinforcement effect is still not considered significant dueto the poor interaction between nanotubes and nanoclayin the mixture.

The Vicker�s hardness of NT–NC/epoxy composites withvarying the NT–NC concentration is also investigated withresults shown in Fig. 3 and Table 2. The hardness of theNT–NC/epoxy composites starts out to increase with theincrease in the concentration of NT–NC. However, noadditional improvement in the hardness is observed afterthe hardness peaked at 40% improvement at 2 wt.% NT–NC loading, i.e. the hardness of the composites at 4 wt.%NT–NC loading is measured to be comparable to the hard-ness at 2 wt.% NT–NC loading. Although the number ofreinforcement fill increased at higher loading, the continuedincrease of NT–NC above 2 wt.% would not result in an in-crease in hardness due to the poor dispersion of NT–NC athigher loading. The improvements are not as dramatic for0.5 and 1 wt.% NT–NC/epoxy composites samples, butthe hardness values are relatively more concentrated thanthose of the samples at higher loading. This is an indicationthat the dispersion state with lower NT–NC addition isbetter than that with higher NT–NC addition.

In summary, the addition of 2 wt.% nanoclay-supportedcarbon nanotubes into the epoxy matrix led to a 40%improvement in the Vicker�s hardness of the composites.The considerable improvement is attributed to the in situgrowth of nanotubes on nanoclay layers, which allowsfor the separation of nanotubes, the exfoliation of nanoclayin the polymer matrix, as well as the possibility for mechan-ical interlocking to occur within the matrix. The hardnessenhancement in the nanoclay–nanotubes/epoxy compositesmotivated additional research on the strength, thermalproperties, dynamic mechanical behavior of the polymercomposites, as well as metal or ceramic composites filledwith this novel nano-reinforcement. The results of thesestudies will soon be reported in coming publications.

Acknowledgments

The authors gratefully acknowledge the support of theResearch Grant Council of Hong Kong (B-Q856).

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Table 2Average Vicker�s hardness and percent improvement of 2 wt.% NT–NC/epoxy composites with different NT–NC concentration

Sample Average Vicker�s hardness no. Percent improvement (%)

Pure epoxy 10.8 00.5 wt.% NT–NC composite/epoxy 11.9 101 wt.% NT–NC composite/epoxy 12.5 162 wt.% NT–NC composite/epoxy 15.1 403 wt.% NT–NC composite/epoxy 14.7 36

386 Letters to the Editor / Carbon 44 (2006) 381–392