Effect of pyrene-modified multiwalled carbon nanotubes on the properties of epoxy composites

Composites: Part A 43 (2012) 1032–1037

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Effect of pyrene-modified multiwalled carbon nanotubes on the properties of epoxy composites Jingfan Luan, Aibo Zhang ⇑, Yaping Zheng, Li Sun School of Nature and Applied Science, Northwestern Polytechnical University, Xi’an 710072, China

a r t i c l e

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Article history: Received 19 October 2011 Received in revised form 3 February 2012 Accepted 11 February 2012 Available online 19 February 2012 Keywords: A. Polymer–matrix composites (PMCs) B. Microstructures B. Electrical properties B. Mechanical properties

a b s t r a c t The block polymer of poly(styrene-b-pyrene) (PS-b-PAH) containing pyrene units was successfully applied on the surface of multiwalled carbon nanotubes (MWNTs) and the properties of nanocomposites were enhanced. The morphology of the modified MWNTs was characterized by transmission electron microscopy (TEM), and the results showed that PS-b-PAH helped effectively the MWNTs to disperse well in epoxy matrices, and these dispersed MWNTs were stabilized by the pyrene modifier. The mechanical properties of the composites, such as impact toughness and flexural strength, and the electrical conductivity of the nanocomposites, are improved significantly after the treatment of the MWNTs using PS-b-PAH. The results show that the mechanical and electrical properties of the modified MWNTs/epoxy composites with PS-bPAH are obviously superior to those of pristine MWNTs/epoxy composites. The enhanced interfacial interactions lead to good dispersion of MWNTs in epoxy matrices, thus enhancing the mechanical and electrical properties of the nanocomposites. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Carbon nanotubes (CNTs) are noted for their extraordinary mechanical, electrical, and thermal properties, and have attracted much attention for various applications such as electronic devices, field-emission display, hydrogen storage, and polymeric composites [1–4]. However, the widespread use of pristine CNTs is limited by the difficulties in processing and handling. In particular, CNTs tend to bundle together and entangle because of attractive van der Waals interactions that result in agglomeration in the presence of solvents and composites [5]. Surface modification of CNTs by functionalization or adsorption seems to be the best approach for improving their dispersion in solvents and composite materials [6,7]. There are two methods to modify the surface of CNTs, covalent functionalization and non-covalent functionalization [8]. Covalent functionalization can dramatically improve the interfacial interaction between the nanotubes and polymer matrices via direct chemical bonding, which is stronger than non-covalent interactions [9]. A notable drawback for covalent functionalization is the disruption of the surface conjugated p network, which leads to the reduction of electrical conductivity [10]. Non-covalent functionalization is an alternative method for tuning the interfacial properties of nanotubes [11]. Non-covalent functionalization can maintain the conjugated system of the CNT sidewalls and the final structure of the material. Therefore it ⇑ Corresponding author. Tel./fax: +86 29 88431688. E-mail address: [email protected] (A. Zhang). 1359-835X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2012.02.005

is more attractive than the covalent method for the maintenance of pristine structure and properties of CNTs. The non-covalent functionalization includes the application of surfactants [12–15] and polymer coating. Liu et al. modified the MWNTs noncovalently with a graft polymer of polystyrene-g-(glycidyl methacrylate-co-styrene (PS-g-(GMA-co-St)) for solubility in organic solvents [16]. Liu et al. reported the effect of non-covalently functionalized carbon nanotubes using polyethyl-enimine (PEI) in epoxy on the microstructure and properties of epoxy composites [9]. Poly(phenylene vinylene) [17] or polystyrene [18] was wrapped around the CNTs to form supermolecular complexes of CNTs. Poly(vinyl benzyloxy methyl naphthalene)-g-poly(t-butymethacrylateco-methacrylic acid) (PV BMN-g-P(tBMA-co-MAA)) was designed and synthesized to noncovalently modify the surface of MWNTs by p–p interaction for dispersing MWNTs in N66 matrices [19]. The polymer wrapping process was achieved through the van der Waals interactions and p– p stacking between CNTs and polymer chains containing aromatic rings [3]. In this paper the block polymer of PS-b-PAH containing pyrene units was synthesized by atom transfer radial polymerization (ATRP) to non-covalently modify the surface of MWNTs. It is known that the pyrene pendants in the chain of block polymers can form p–p stacking interactions with the aromatic surfaces of MWNTs. It is expected that PS-b-PAH can be used as an effective compatibilizer for dispersing MWNTs in the epoxy matrices (Scheme 1). The non-covalent functionalization leads to the improved MWNTs dispersion, and these dispersed MWNTs are stabilized by the pyrene pendants. The effectiveness of this new

J. Luan et al. / Composites: Part A 43 (2012) 1032–1037

*

*

O O

PS-b-PAH Scheme 1. Schematic diagram of nanotube interactions with a pyrene-containing block copolymer (PS-b-PAH).The blue line is PAH unit, the green line is PS unit. (For interpretation of the references to color in this scheme legend, the reader is referred to the web version of this article.)

100

a

Weight Loss/ %

80 70 60 50 40

b

30 a.MWNTs b.PS-b-PAH/MWNTs

20 10 0

200

400

600

2.2. The preparation of samples

n

m

90

1033

800

Temperature Fig. 1. The TGA curves of (a) pristine MWNTs, and (b) PS-b-PAH modified MWNTs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

modifier for epoxy nanocomposites on the mechanical properties and conductivities of MWNTs/EP nanocomposites was investigated. To investigate the effectiveness of modification, the mechanical properties and electrical properties of unmodified MWNTs/EP nanocomposites were also examined.

2. Experimental 2.1. Chemicals and materials MWNTs (diameter 10–30 nm, 10–20 lm length, purity > 99.5 wt.%, specific gravity 2.1, purchased from Chengdu organic chemicals Co., Ltd., Chinese Academy of Sciences) were used without further purification. 1-Pyrenebutyric acid (Aldrich, 97%), 4-vinylbenzyl chloride (Aldrich, 90%), 18-crown-6 (Aldrich, 99%) and potassium carbonate were used as received. Copper (I) bromide (CuBr), ethyl 2-bromoisobutyrate (EBiB), and N,N,N0 ,N0 ,N00 pentamethyl-diethylenetri-amine (PMDETA) were used without further purification. Styrene and anisole were purified by passing through a column filled with basic alumina to remove inhibitors and other impurities. The epoxy resin E-51 used in this paper was a bisphenol A diglycidyl ether-based resin purchased from Tianjin Chemical Reagent Co., Ltd., China. Its average molecular weight is about 390 g/mol, and the epoxy value is about 0.51 g/equiv. The viscosity of the resin is 4000–6000 mPa s at 25 °C, and the density is 1.16 g/ cm3 at 20 °C. The curing agent of methylhexahydrophthalic anhydride (MHHPA, 99.5%, Italy) was used as received.

2.2.1. Preparation of block polymer of PS-b-PAH by ATRP Vinyl pyrene-butyric ester monomer was synthesized using the procedure in [20]. The block polymer of PS-b-PAH was produced by ATRP. The ratio of [pyrene]:[PS-Br]:[CuBr]: [PMDETA] is 100:1:1:1:1 in anisole. Here, [PS-Br] = [CuBr] = [PMDETA] = 0.024 M, [pyrene] = 2.4 M. Vinyl pyrene-butyric ester (1.8178 g, 4.5 mmol), Ps-Br(Mn = 5352, 0.24 g, 0.045 mmol), CuBr (0.0065 g, 0.045 mmol), PMDETA (0.0078 g, 0.045 mmol), and anisole(1.875 mL) were added to a 25 mL Schlenk flask. The resulting mixture was deoxygenated by three freeze–pump-thaw cycles and reacted at 110 °C. The polymerization was stopped at reserved time. The mixture was then diluted with THF, filtered through a neutral aluminum oxide column to remove the copper catalyst, precipitated in methanol, and dried in a vacuum to a constant mass. 2.2.2. Preparation of modified MWNTs/EP composites Solutions of PS-b-PAH block polymer solutions (Mn = 17035, 0.0063 g, 3.7  107 mol) were prepared in 5 mL THF. 1 mg MWNTs was added to the solution, and then the solutions were sonicated for 2 h in a water bath. The resulting mixtures were centrifuged at 5000 rpm for 3 min, filtered through glass wool and then dried in a vacuum to a constant mass of modified MWNTs. The dried MWNTs were dispersed in the EP resin. The mixture was sonicated in a water bath for 1 h at room temperature. After that the curing agent of MHHPA was added into the solution during the stirring. The weight ratio of EP resin and MHHPA was 100:70. Then the mixture was poured into a mold and cured in a vacuum oven at 150 °C for 2 h, and then at 200 °C for 2 h. The cured MWNTs/epoxy resin was cooled to the room temperature and stripped from the mold. 2.3. Measurements The morphology was investigated using transmission electron microscopy (TEM). The sheet samples were prepared with the thickness of 1 mm for the rheological and morphological measurements. The flexural strength of MWNTs/epoxy composites was measured using a tensile testing machine of CMT 5105. The specimens with dimensions of 80 mm long  15 mm wide  2.7 mm thick were loaded in three-point bending until failure with a support span of 51 mm at a constant cross-head speed of 2.0 mm/min. The specimens for the impact test were cut into size of 80 mm  10 mm  3.5 mm. The impact tests were performed on a machine (Zbc-4) at room temperature with an impact energy of 4.0 J. The impact fracture surfaces of specimens were examined using a scanning electronic microscopy (SEM, Stereoscan 250MK3, Cambridge) after etching these fracture surfaces for 1 week and coating with gold. Thermogravimetry analysis (TGA) measurements were performed with TGAQ50 (USA) from room temperature to 800 °C at a scan rate of 10 °C/min in a nitrogen atmosphere. Dielectric properties of dielectric constant, loss and conductance were tested at frequencies from 102 Hz to 106 Hz using a Novocontrol broadband dielectric spectrometer from a German supplier. The size of the testing samples was 2 mm  10 mm  10 mm. 3. Results and discussion 3.1. The effect of pyrene containing polymers on the morphology of MWNTs The PS-b-PAH block polymers were used to modify the MWNTs by an ultrasound method to get non-covalent modification MWNTs. The TGA was done in N2 flow, and the polymer attached on MWNTs was decomposed to form amorphous carbon. The

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(a)

100nm

(b)

100nm

(a’)

(b’)

16.9nm 27.7nm

27.7nm

20nm

20nm

Fig. 2. TEM images of (a, a0 ) pristine MWNTs, (b, b0 ) PS-b-PAH modified MWNTs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(a)

(b)

50nm

50nm

0.5μm

0.5μm

Fig. 3. TEM photograph of 0.6 wt.% MWNTs/EP composites (a) pristine MWNTs/EP, and (b) PS-b-PAH modified MWNTs/EP.

TGA curves (Fig. 1) shown that the content of MWNTs and amorphous carbon are about 30.2 wt.%. The morphology of the modified MWNTs was investigated by TEM (Fig. 2). It can be seen that the pristine MWNTs behave bundles while the MWNTs modified with the PS-b-PAH block polymer do not form such dense bundles. These less entangled structures are caused by the strong binding affinity between the pyrene pendants and the MWNT sidewalls. In other words, the PS-b-PAH block polymer is obviously efficient at dispersing MWNTs as individual nanotubes or smaller bundles. Polymer modification prevented MWNTs from aggregating in bundles and enhanced their dispersion. The diameter of pristine MWNTs is about 27.7 nm (as shown in Fig. 2a0 ). The thickness of the wrapped polymer layer is about 16.9 nm (shown in Fig. 2b0 ).

A homogeneous polymer layer is clearly observed on the surface of the MWNTs. This can be contributed to the strong p–p stacking from the pyrene pendants of PAH units. 3.2. The effect of non-covalent modification MWNTs on the dispersibility in EP composites The effect of non-covalent modification MWNTs on the dispersion in EP composites is demonstrated in Fig. 3. The high-resolution transmission electron microscopy (HRTEM) image of pristine MWNTs dispersed in EP matrices is shown in the inset of Fig. 3a, and the image of the PS-b-PAH modified MWNTs dispersed in EP matrices is shown in the inset of Fig. 3b.

J. Luan et al. / Composites: Part A 43 (2012) 1032–1037

160

Flexural strength (MPa)

between the conjugated polymers of PS-b-PAH and the MWNTs can indeed help to disperse the modified MWNTs in the epoxy matrices, enabling a more efficient load transfer from the matrices to the MWNTs, and allowing a uniform load distribution. On the other hand, the agglomerates reduce the reinforcing effects of the MWNTs because they act as flaws in the resin. The reinforcement potential of the MWNTs can only be activated if effective load transfer between the surrounding epoxy matrices and the MWNTs and reverse is possible. This is why the poorly dispersed MWNTs/ epoxy composites have inferior mechanical properties compared to the well-dispersed ones. In general, a good dispersion of the nanofillers is the prerequisite for an efficient exploitation of the potential benefit from CNTs as structural reinforcement [21].

pristine MWNTs/EP PS-b-PAH modified MWNTs/EP

150

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140 130 120 110 100 90 0.0

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Flexural modulus (GPa)

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24 22 20 18 16 14 12 10 0.0

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1035

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Content of MWNTs (wt%) Fig. 4. The effect of MWNTs content on the mechanical properties (a) flexural strength, (b) flexural modulus, and (c) impact toughness. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Comparing the morphology of PS-b-PAH modified MWNTs in epoxy matrices (Fig. 3b) with that of pristine MWNTs (Fig. 3a), we see that the PS-b-PAH modified MWNTs remain uniformly dispersed without preferred alignment over the whole curing process, while the pristine MWNTs start re-agglomeration upon application of the curing temperature, indicating a beneficial effect of PS-b-PAH functionalization on the stability of MWNT dispersion even at a curing temperature. The strong p–p stacking interactions

3.3. The effect of non-covalent modified MWNTs on the mechanical properties of EP composites Fig. 4a and b demonstrates that both the flexural strength and modulus of the PS-b-PAH modified MWNTs/EP composites are improved significantly comparing to the pristine MWNTs/EP composites with about 0.6 wt.% MWNT content. The flexural strength is found to increase by 46.69% in comparison with the neat epoxy resin at room temperature. The relatively large improvement in flexural strength can be explained by the modification of MWNTs with pyrene containing polymers. The pyrene pendants in the chain of copolymers can form p–p stacking interactions with the MWNTs p-electrons, which served as a multivalent anchor to grasp the sidewall of the MWNTs. The interaction between the pyrene pendants and the MWNTs sidewalls is greatly enhanced. At the same time, the PS chain acts as a tail into the composites, and plays a role to prevent crack propagation and extending. Fig. 4c is the effect of MWNT content on the impact toughness of MWNTs/EP composites. The results show that when the MWNT content is increased to 0.6 wt.%, the impact toughness of pristine MWNTs/EP composites and PS-b-PAH modified MWNTs/EP composites is improved by 33.09% and 127.94%, respectively, in comparison with the neat epoxy resin. However the improvement in impact fracture toughness with surfactant-treated CNTs/EP composites is about 60% compared to the neat epoxy [14]. Obviously, the improvement in impact strength of PS-b-PAH modified MWNTs/EP composites is substantially higher than that of surfactant treated MWNTs/EP composites. The morphologies of impact fracture surface of the nanocomposites with 0.6 wt.% MWNTs are shown in Fig. 5. There are significant differences in morphologies of the pristine and PS-b-PAH modified MWNTs nanocomposites. The pristine MWNTs nanocomposites have a relatively smooth fracture surface with small-size, repetitive spatulate patterns (shown in Fig. 5a). The PS-b-PAH modified MWNTs nanocomposites displayed much rough and fluctuant morphology with large, elongated radial crack patterns (shown in Fig. 5b). It is well known that the fracture morphology with elongated radial crack patterns corresponded to a higher crack growth resistance of the composite. We compared the same MWNT content nanocomposites. There are little pristine MWNTs in Fig. 5a, indicating that there is agglomeration for the pristine MWNTs nanocomposites. From Fig. 5b, we see that the PS-b-PAH modified MWNTs dispersed well. These observations further confirm that the modifier of PS-b-PAH acts as a dispersant of MWNTs in epoxy matrices. 3.4. The effect of non-covalent modification MWNTs on the conductivity of EP composites Fig. 6 shows the electrical conductivities of the pristine MWNTs/ EP composites and the PS-b-PAH modified MWNTs/EP composites. A percolation transition can be observed in these plots. According to the percolation theory, the conductivity exhibits a transition behav-

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Fig. 5. SEM photographs of impact fracture surfaces of 0.6 wt.% MWNTs/EP composites (a) pristine MWNTs, and (b) PS-b-PAH modified MWNTs.

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MWNTs (wt%)

Fig. 6. The conductivity and percolation threshold of MWNTs/EP composites in frequency of 102 Hz (a) pristine MWNTs/EP, and (b) PS-b-PAH modified MWNTs/EP. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

ior in the vicinity of percolation threshold and can be expressed as [22,23]:

r ¼ r0 ðp  pc Þt for p > pc

ð1Þ

where r is conductivity, r0 is a constant, p is filler concentration, pc represents the percolation threshold, and t corresponds to critical exponents. For the PS-b-PAH modified MWNTs/EP composites, the best fit of Eq. (1) yields a percolation threshold of 0.184 wt.%, and a critical exponent t of 1.928 (inset of Fig. 6b). A very small percolation threshold can be obtained in epoxy composites filled with PS-b-PAH modified MWNTs. However, for the pristine MWNTs/ EP composites, the percolation threshold is 0.695 wt.%, and the critical exponent t is 1.735 (inset of Fig. 6a). It is obvious that the critical exponent t of the PS-b-PAH modified MWNTs/EP composites is larger than that of pristine MWNTs/EP composites, which can be attributed to the well-dispersed dispersion of PS-b-PAH modified MWNTs in epoxy matrices. According to the percolation theory, the conductivity of composite is mainly affected by the exponent t when the content of MWNTs closes to or exceeds the percolation threshold. Since the t in PS-b-PAH modified MWNTs/ EP is larger than that of pristine MWNT composites, this leads to larger conductivity. The greater conductivity of PS-b-PAH modified MWNTs/EP composites is likely due to the role of pyrene pendants in the chain of PS-b-PAH, as the aromatic ring not only can help to improve the dispersion of MWNTs in epoxy matrices, but also can be a bridge to transport electrons, which is helpful to improve the conductivity. On the other hand, it was observed that the percolation threshold was significantly reduced in the presence of the non-covalent modification of MWNTs with PS-b-PAH block polymers, indicating that the PS-b-PAH modified MWNT network in

epoxy matrices is formed at lower MWNT concentration than that of pristine MWNTs. Li et al. produced CNT/epoxy nanocomposites with percolation thresholds varying in the range from 0.1 to > 1 wt.% by controlling the dispersion state and aspect ratio of CNTs [24]. They noted that the critical factors determining the percolation threshold were: (i) aspect ratio of CNTs; (ii) disentanglement of CNT agglomerates on the nanoscopic scale; and (iii) uniform distribution of individual CNTs or CNT agglomerates on microscopic scale. In our study a significant decrease in the percolation threshold concentration of PS-b-PAH modified MWNTs/EP is attributed to homogeneous dispersion of MWNTs in epoxy matrices. Good spatial dispersion of MWNTs in epoxy matrices can help MWNTs form a continuously electrically conductive path. If MWNTs disperse homogeneously in polymer matrices, the composites will have high electrical conductivity, which is desirable for electromagnetic shielding and electrostatic materials. 4. Conclusion The surface of MWNTs was non-covalently modified by the PSb-PAH block polymer, which was synthesized using the ATRP technique. The PS-b-PAH modified MWNTs form a homogenous dispersion without preferred alignment in epoxy matrices, while pristine MWNTs are easy to form bundles in epoxy matrices. The PS-b-PAH block polymer modified MWNTs/EP composites also have substantially better mechanical properties than those of the pristine MWNTs/EP composites. When compared with neat epoxy for the case with 0.6 wt.% MWNT content, the impact toughness of the pristine MWNTs/EP composites and PS-b-PAH modified MWNTs/ EP were enhanced by 33.09% and 127.94%, and the flexural strength was increased by 12.11% and 46.69%, respectively. This

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indicates that non-covalent modification MWNTs with block polymer of PS-b-PAH could also significantly improve the mechanical properties of MWNTs/EP. The morphologies of impact fracture surface of the nanocomposites with 0.6 wt.% MWNTs were investigated by SEM. These observations indicated that the dispersion of MWNTs was enhanced significantly due to the non-covalent modification of PS-b-PAH, and the improved dispersion of MWNTs was stabilized and maintained after the epoxy resin was fully cured. The percolation threshold was significantly reduced when using the PS-b-PAH modified MWNTs instead of the pristine MWNTs in EP composites. The percolation threshold of the PS-b-PAH modified MWNTs/EP composites was 0.184 wt.%, and the critical exponent t was 1.928. Whereas, the percolation threshold of the pristine MWNTs/EP composites was 0.695 wt.%, and the critical exponent t was 1.735. This meant that well dispersed suspensions of MWNTs in epoxy resin form a spatial structure of agglomerates which results in a lower percolation threshold of 0.184 wt.%.

Acknowledgments This work is supported by the Aerospace Technology Innovation Foundation (CASC200906), National Natural Science Foundation (51073129), Aeronautical Science Foundation of China (2010ZF53060) and supported by NWPU Graduate Student Entrepreneurship Seed Fund (Z2011007).

References [1] Mohamed A, Derrick D, Merlin T, Fielding J, Nyairo E, Price G. Magnetically processed carbon nanotube/epoxy nanocomposites: morphology, thermal, and mechanical properties. Polymer 2010;51:1614–20. [2] Eitan A, Jiang K, Dukes D, Andrews R, Schadler LS. Surface modification of multiwalled carbon nanotubes: toward the tailoring of the interface in polymer composites. Chem Mater 2003;15(16):3198–201. [3] Ma PC, Naveed AS, Maromb G, Kim JK. Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: a review. Compos: Part A 2010;41:1345–67. [4] Coleman JN, Khan U, Blau WJ, Gun’ko YK. Small but strong: a review of the mechanical properties of carbon nanotube–polymer composites. Carbon 2006;44(9):1624–52. [5] Beate K, Gudrun P, Sven P, Pötschke P. Correlation of carbon nanotube dispersability in aqueous surfactant solutions and polymers. Carbon 2009;47(3):602–12.

1037

[6] Liu HH, Wang N, Hou LC, Peng WW, Jin YM, Zhang XX. 4-substituted benzoic acids functionalized multi-walled carbon nanotubes in mild polyphosphoric acid/phosphorous pentoxide. Adv Mater Res 2011;217–218:768–73. [7] Assali M, Leal MP, Fernández I, Pablo RG, Baati R, Khiar N. Improved noncovalent biofunctionalization of multi-walled carbon nanotubes using carbohydrate amphiphiles with a butterfly-like polyaromatic tail. Nano Res 2010;3(11):764–78. [8] Fu JW, Huang XB, Huang YW, Zhang JW, Tang Z. One-pot noncovalent method to functionalize multi-walled carbon nanotubes using cyclomatrix-type polyphosphazenes. Chem Commun 2009;9:1049–51. [9] Liu L, Etik KC, Liao KS, Lance AH, Bergbreiter HD, Grunlan JC. Comparison of covalently and noncovalently functionalized carbon nanotubes in epoxy. Macromol Rapid Commun 2009;30:627–32. [10] Coleman JN, Khan U, Gun’ko YK. Mechanical reinforcement of polymers using carbon nanotubes. Adv Mater 2006;18(6):689–706. [11] Zhou W, Lv S, Shi W. Preparation of micelle-encapsulated single-wall and multi-wall carbon nanotubes with amphiphilic hyperbranched polymer. J Eur Polym 2008;44(3):587–601. [12] Gong X, Liu J, Baskaran S, Voise RD, Young JS. Surfactant-assisted processing of carbon nanotube/polymer composites. Chem Mater 2000;12(4):1049–52. [13] Grossiord N, Loos J, Regev O, Koning CE. Toolbox for dispersing carbon nanotubes into polymers to get conductive nanocomposites. Chem Mater 2006;18(5):1089–99. [14] Geng Y, Liu MY, Li J, Jing Li, Shi XM, Kim JK. Effects of surfactant treatment on mechanical and electrical properties of CNT/epoxy nanocomposites. Composites Part A 2008;39:1876–83. [15] Vaisman L, Wagner HD, Marom G. The role of surfactants in dispersion of carbon nanotubes. Adv Colloid Interf Sci 2006;128–130:37–46. [16] Liu YT, Zhao W, Huang ZY, Gao YF, Xie XM, Wang XH, et al. Noncovalent surface modification of carbon nanotubes for solubility in organic solvents. Carbon 2006;44:1581–616. [17] McCarthy B, Coleman JN, Czerw R, Dalton AB, Carroll DL, Blau WJ. Microscopy studies of nanotube–conjugated polymer interactions. Synth Met 2001;121:1225–6. [18] Hill DE, Lin Y, Rao AM, Allard LF, Sun YP. Functionalization of carbon nanotubes with polystyrene. Macromolecules 2002;35:9466–71. [19] Kyung TK, Won HJ. Non-destructive functionalization of multi-walled carbon nanotubes with naphthalene-containing polymer for high performance Nylon66/multi-walled carbon nanotube composites. Carbon 2011;49:819–26. [20] Bahun GJ, Wang C, Andronov A. Solubilizing single-walled carbon nanotubes with pyrene-functionalized block copolymers. J Polym Sci Part A: Polym Chem 2006;44:1941–51. [21] Florian HG, Malte HG, Fiedler WB, Schulte K. Influence of different carbon nanotubes on the mechanical properties of epoxy matrix composites – a comparative study. Compos Sci Technol 2005;68(15–16):2300–13. [22] Michael D, Abdelhak HS, Philippe H, Frédérick R. Transport properties in heterogeneous compacted granular media made of carbon nanotubes and potassium bromide. Appl Phys Lett 2009;94(23):231910–3. [23] Allaoui A, Hoa SV, Pugh D. The electronic transport properties and microstructure of carbon nanofiber/epoxy composites. Compos Sci Technol 2008;68(2):410–6. [24] Li J, Ma PC, Chow WS, To CK, Tang BZ, Kim JK. Correlations between percolation threshold, dispersion state and aspect ratio of carbon nanotube. Adv Funct Mater 2007;17:3207–15.