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Preparation and characterization of conductive carbon nanotube–polystyrene nanocomposites using latex technology
Composites Science and Technology 68 (2008) 2254–2259
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Preparation and characterization of conductive carbon nanotube–polystyrene nanocomposites using latex technology Tzong-Ming Wu *, Erh-Chiang Chen Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402, Taiwan
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Article history: Received 16 July 2007 Received in revised form 8 April 2008 Accepted 9 April 2008 Available online 15 April 2008 Keywords: A. Nanostructures A. Polymers B. Electrical properties D. Transmission electron microscopy Multi-walled carbon nanotubes
a b s t r a c t This study reports the preparation of conductive multi-walled carbon nanotubes (MWCNTs)–polystyrene (PS) nanocomposites using latex technology. The MWCNTs were first well dispersed in methanol solution and then added to a suspension of the monodispersed 230 nm PS latex synthesized by emulsion polymerization with sodium dodecylsulfate anionic surfactant. Both FESEM and HRTEM images indicate that the resulting PS latices are closed to spherical dots with a particle size about 230 ± 4 nm. The HRTEM micrograph of MWCNT–PS nanocomposites shows the MWCNT is well separated and uniformly distributed between the monodispersed PS latices. The Tg of MWCNT–PS nanocomposites is gradually increased with increasing the content of MWCNT, indicating possible interaction between MWCNT and PS latices to limit the motivation of PS. The conductivities of 1.5 wt.% and 6.5 wt.% MWCNT–PS nanocomposites are more than four and ten orders in magnitude higher than that of PS without MWCNT, respectively. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Monodispersed colloidal particles have already received a wide range of attractive application in fields such as drug delivery and biodiagnostics. The ability to assemble these colloidal particles into crystalline arrays allows one to obtain useful and remarkable functionalities not only from the fundamental physics of systems with the long-range, mesoscopic order that characterizes periodic structures but also from the application of constituent materials [1–4]. All these applications are strongly dependent on the availability of colloidal spheres with tightly controlled sizes and high monodispersity (<5%). Among various colloidal particles, polystyrene (PS) colloids have potential uses in biomedical and drug delivery because the PS colloids with excellent monodispersity can be easily prepared by the classical emulsion polymerization method [5]. Carbon nanotubes (CNTs), discovered by Iijima and Ichihashi. [6], have received much attention for their use in fabricating a new classes of advanced materials due to their unique structural, mechanical and electronic properties. They have potential for use in nanotube-based composites, field emitters, nanoelectronic devices and probe tips for SPMs [7–12]. As expected for the high aspect ratio of length to diameter, experimentally introducing CNTs into a polymer matrix improves the electrical conductivity and mechanical properties of the original polymer matrix [13–16]. For example, CNTs have been incorporated into the conjugated polymers, such as poly(phenylenevinylene) and polythiophene, to * Corresponding author. Tel.: +886 4 2287 2482; fax: +886 4 2285 7017. E-mail address: [email protected] (T.-M. Wu). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.04.010
prepare composites for optoelectronic applications [17–20]. CNTs have also been used as filler into polymer matrix to exploit the superior mechanical properties of the nanotubes. For example, Coleman et al. had been successfully used CNTs as fillers to enhance the mechanical and optical properties of polymers [21– 24]. But several reports demonstrate that the improvements of the mechanical properties of CNT–polymer nanocomposites are limited due to the phase separation between the polymer matrix and CNTs [25,26]. Therefore it is still a very important subject to obtain uniform distribution of CNTs in the polymer matrix until now. The dispersion of CNT into polystyrene matrix for the fabrication of CNT–PS nanocomposites using solution blending [27–30], in situ polymerization [31–33] and melt blending [34] has naturally encouraged significant interest among researchers. For example, Safadi et al. mixed multi-walled carbon nanotubes (MWCNTs) with polystyrene using ultrasonication in a toluene solution [28]. PS solution containing well-dispersed MWCNTs by a simple sonication process were cast and spun to obtain PS–MWCNT thin film composites. They reported that there are remarkable increases in the tensile stress in the 5 wt.% MWCNT–PS nanocomposites. The conductive properties of the 5 wt.% MWCNT–PS nanocomposites are about eight orders in magnitude higher than that of pure PS matrix. Pham et al. used solution blending method to prepared single-walled carbon nanotube (SWCNT)–PS nanocomposites and studied the change of glass transition temperatures (Tg) in these nanocomposites. Their experimental data indicated the Tg of fabricated nanocomposites increases by approximately 3 °C and became much broader than that of pure PS. Alternatively,
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CNT–PS nanocomposites were fabricated by in situ polymerization of styrene in the presence of carbon nanotubes [31,32]. Their results suggested that the in situ polymerization method has more advantages to improve the dispersion of MWCNT in PS matrix, and fabricated composites with high gas sensitivity. In this paper, another approach to fabricate the self-assembly MWCNT–PS nanocomposites using latex technology has been reported. First, the MWCNT can be well dispersed in methanol solution under ultrasonication and then added to a suspension of the monodispersed PS latex synthesized by emulsion polymerization with sodium dodecylsulfate (SDS) anionic surfactant. The structure, morphology and conductivity of fabricated nanocomposites were characterized: in particular, the thermal behavior was observed and the possible degradation of this nanostructure was characterized. 2. Experimental 2.1. Synthesis of conductive MWCNT–PS nanocomposites using latex technology The MWCNTs used in this work were prepared using ethylene CVD using Al2O3 supported Fe2O3 catalysts as described in a previous study [35]. Monodispersed polystyrene (PS) latex with diameter of 230 nm was prepared by emulsion polymerization using sodium dodecylsulfate (SDS) anionic surfactant, potassium persulfate and the mixture of distilled water and methanol as emulsifier, initiator and dispersion medium, respectively. Quantitative SDS and potassium persulfate were dissolved in dispersion medium in a 250 ml two-neck flask. Then a certain amount of styrene was added under nitrogen atmosphere and rapidly mechanical stirring, and the emulsion solution was heated to 70 °C to polymerize for 8 h. The obtained SDS coated PS latex dispersion containing negative charge (Zeta-potential 30 mV) was filtrated and washed with alcohol three times to remove coagulate and soluble impurities. The monodispersed PS latex obtained was dried under a vacuum at 60 °C for 24 h. The MWCNT–PS nanocomposite was prepared using latex fabrication method. First, the MWCNT is well dispersed in methanol solution using ultrasonication process and then added to a suspension of the monodispersed 230 nm PS latex in methanol solution with constant mechanical stirring. In a typical synthesis experiment, various weight ratios of MWCNTs and monodispersed 230 nm PS particles were individually dispersed in methanol solutions and ultrasonicated over 3 h, then both transferred into a 250 ml two-neck flask under nitrogen atmosphere and rapidly mechanical stirring for 2 h. The self-assembly MWCNT–PS nanocomposites obtained were dried at 80 °C to remove any remaining methanol. 2.2. Structural analysis The Raman spectroscopy was used to characterize the structure of MWCNT. Raman spectra were recorded under a Jobin Yvon TRIAX 550 system using a He–Ne laser operating at 632.8 nm with a CCD detector. The final spectrum presented is an average of three spectra recorded at different regions over the entire range of the sample. The field-emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) were used to characterize the morphology of MWCNT–PS nanocomposite. The specimens for FESEM and HRTEM analysis were prepared using the droplet of MWCNT–PS nanocomposite in methanol solution on the surface of aluminum foil and carbon-coated copper grid and then air-dried for 2 h. FESEM was conducted at 3 kV using a JEOL JSM-6700 F field-emission instrument and HRTEM was recorded on a Hitachi HF-2000 instrument at 200 kV.
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2.3. Physical properties Thermal analysis of the samples was obtained using a Perkin– Elmer PYRIS Diamond differential scanning calorimeter (DSC) under a nitrogen atmosphere. All specimens were weighted in the range of 5–6 mg and were carried from room temperature to 150 °C at a scanning rate of 10 °C /min. Thermal stabilities of these samples were operated using Perkin–Elmer thermogravimetric analysis (TGA)/differential thermal analyzer (DTA) and all experiments were performed under a nitrogen atmosphere at a purge rate of 100 ml/min. The samples were performed from room temperature until 700 °C at a scanning rate of 10 °C /min. The isothermal degradation at 360, 370, 380, 400 and 420 °C were also studied. The samples of monodispersed 230 nm PS particles, MWCNT and MWCNT–PS nanocomposites were pressed into pellet under 20 MPa. DMA experiments were performed on a Perkin–Elmer instrument DMA 7e apparatus equipped with a film tension clamp. The instrument was programmed to measure G0 (storage modulus) over the range of 60–140 °C at 5 °C/min heating rate and 10 Hz constant frequency. The conductivity at room temperature using four probe methods was measured by a programmable DC voltage/current detector. Each data shown here is the mean value of the measurement from at least three samples and the experimental error using standard deviation of these data is obtained. 3. Results and discussion Fig. 1 shows FESEM and HRTEM images of the monodispersed PS latex synthesized by emulsion polymerization with surfactant SDS. From the FESEM data, the average diameter of PS latex is about 230 nm and its size distribution is highly uniform. Closer inspection of HRTEM images of PS latex shown in Fig. 1b reveals that the resulting PS latices are closed to spherical dots with a particle size deviation of less than 2% (230 ± 4 nm). The Raman spectrum of the purified MWCNTs containing two characteristic peaks is shown in Fig. 2. One peak at 1580 cm1 (G mode) is the Raman-allowed phonon high-frequency mode and the other at 1355 cm1 (D mode) is a disordered-induced peak, which may originate from the defects in the curved graphene sheets and tube ends. The TEM images of prepared MWCNT (insert in Fig. 2) clearly shows the diameter of about 10–15 nm hollow cores of nanotubes and total size of 20–40 nm of nanotubes. Fig. 3 indicate the HRTEM images of 1.5 wt.% and 5 wt.% MWCNT–PS nanocomposites prepared using latex technology. Both PS latex and MWCNT each dispersed in methanol solution and ultrasonicated for 4 h and then solution mixed with constant mechanical stirring result in better dispersion of the MWCNT in PS latex. These results reveal that the resulting nanocomposites have tube-like and spherical morphology representing the MWCNT and PS latex, respectively. The MWCNTs wrapped by PS latex can be clearly observed. There are several PS latices adsorbed on the interface of MWCNT, which is also observed by Choi and coworkers [30]. From the TEM results, the MCWNT is well separated and uniformly distributed in the monodispersed PS latices. The formation mechanism of MWCNT–PS nanocomposites is believed to involve possible interactions between negative charges of SDS coated PS latex (Zeta-potential 30 mV) and slightly positive charges of MWCNT (Zeta-potential 3 mV) with surfactant SDS using ultrasonication process, which is also reported by previous investigations [36,37]. Therefore, it is possible to allow the MWCNTs wrap up in PS latices. The glass transition temperature (Tg) of monodispersed PS particle and fabricated MWCNT–PS nanocomposites was measured on DSC in nitrogen atmosphere. Fig. 4 shows the DSC thermal curves of the MWCNT–PS nanocomposites with various MWCNT ratios. As seen from Fig. 4, the Tg of PS is 99.1 °C and then gradually in-
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Fig. 1. (a) FESEM and (b) HRTEM image of monodispersed PS latex prepared emulsion polymerization. Fig. 3. HRTEM image of (a) 1.5 wt.% and (b) 5 wt.% MWCNT–PS nanocomposites prepared by latex technology.
Fig. 2. Raman-spectroscopy of MWCNT (inserted HRTEM image of MWCNT).
creases with increasing the content of MWCNT. As the addition of 6.5 wt.% MWCNT into monodispersed PS latices, the Tg increases up to 105.7 °C. Detail Tg of monodispersed PS particle and MWCNT–PS
Fig. 4. DSC thermal analysis of (a) monodispersed PS particle, (b) 1.5 wt.% MWCNT– PS, (c) 3.5 wt.% MWCNT–PS, (d) 5 wt.% MWCNT–PS and (e) 6.5 wt.% MWCNT–PS nanocomposites.
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Table 1 Glass transition temperatures (Tg), onset temperature of degradation (Tonset), activation energies (Ea), electrical conductivity and dynamic mechanical properties (G0 ) at 60 °C of monodispersed PS particle and MWCNT–PS nanocomposites Sample PS particle 1.5 wt.% MWCNT–PS 3.5 wt.% MWCNT–PS 5 wt.% MWCNT–PS 6.5 wt.% MWCNT–PS
Tg ( °C) 99.1 101 101.7 102.9 105.7
Tonset (°C)
Ea (kJ/mol)
383.5 ± 0.3 384.7 ± 0.3 386.4 ± 0.3 386.6 ± 0.3 387.8 ± 0.3
nanocomposites are listed in Table 1. These results indicate that the presence of MWCNT induces the interaction of PS latex. Therefore it is necessary to provide more thermal energy to overcome the interaction for further transitional and rotational motions of PS molecules in the glassy region, whereas the Tg of nanocomposites increases as MWCNT content increases. Thermal stability of monodispersed PS particle and MWCNT–PS nanocomposites was measured using thermogravimetric analysis (TGA)/differential thermal analyzer (DTA) in nitrogen atmosphere. Fig. 5 shows the TGA/DTA thermal curves of the MWCNT–PS nanocomposites with various MWCNT contents. From the DTA data, the peak position of maximum degradation was slightly shifted to high temperature by adding the MWCNT into PS latex. The TGA profiles of MWCNT–PS nanocomposites shows similar tendency, and the onset temperature of degradation (Tonset) can be determined from these curves by extrapolating from the curve at the peak of degradation back to the initial weight of the polymer. As seen from this figure, the Tonset of monodispersed PS latex is about 383.5 ± 0.3 °C. The onset temperature of weight loss of 1.5 wt.% MWCNT–PS nanocomposites is about 384.7 ± 0.3 °C, which may be due to the thermal degradation of organic polystyrene decomposition. With continuously increasing content of MWCNTs, the onset temperature of PS decomposition increases from 384.7 ± 0.3 °C for 1.5 wt.% PS–MWCNT nanocomposites to 387.8 ± 0.3 °C for 6.5 wt.% PS–MWCNT nanocomposites. Detail Tonset of monodispersed PS particle and MWCNT–PS nanocomposites are also listed in Table 1. All results show that introducing the MWCNT into PS latex system can enhance the thermal stability of PS–MWCNT nanocomposites due to the interaction between the MWCNT and PS lattices and the formation of a barrier of MWCNT which inhibit mass transfer and provide thermal insulation to shield the under-
190.9 ± 1.7 187.0 ± 0.6 193.2 ± 1.1 198.2 ± 0.3 201.3 ± 0.7
Conductivity (S/cm) 13
1.0 10 6.0 108 4.9 106 2.1 104 4.9 104
G0 (MPa) 10,400 14,400 19,300 20,700 22,300
lying polymer from the heat source [38]. From above data, the residual at 500 °C is considered to be MWCNTs since PS has been completely decomposed at 450 °C. In order to understand the effect of MWCNT on the thermal behavior of nanocomposites, the isothermal thermal degradation can be performed to study the degradation kinetics of MWCNT– PS nanocomposites. The method of isothermal kinetic parameter proposed by Freeman and Carroll has widely been used to determine the order and the activation energy of the degradation [39]. The rate of degradation may be written as
dW ¼ kd W n dt
ð1Þ
where kd is the rate constant; W is the weight remaining, %; n is the order of the reaction. If assuming first order decomposition, then ln W ¼ ln W 0 kd t
ð2Þ
where W0 is the initial weight. Fig. 6a and b shows the plot of ln W versus t for monodispersed PS particle and 1.5 wt.% PS–MWCNT nanocomposites isothermal degradation at 360, 370, 380, 400 and 420 °C. It is found that all isothermal degradation data contains a straight line which means that the degradation can be regarded as a first order decomposition with a steady rate constant. The rate constants, kd, were obtained as the slopes of the curves in Fig. 6. All plots of ln W versus t for 3.5 wt.%, 5 wt.% and 6.5 wt.% MWCNT–PS nanocomposites show similar tendency and the degradation can be assigned to be the first order decomposition with a steady rate constant. Additionally, DEa ð3Þ kd ¼ A exp RT where A is the pre-exponential factor; DE is the total activation energy; R is the universal gas constant; T is the absolute temperature (K). Arrhenius plots of ln kd against 1/T for monodispersed PS particle and MWCNT–PS nanocomposites are shown in Fig. 7, and are approximately linear. The activation energy can be determined from the slope of plots and is dependent on the content of MWCNT. The activation energy slightly increases with the presence of 1.5 wt.% MWCNT in MWCNT–PS nanocomposites and then decreases with increasing MWCNT content. The result indicates that the addition of 1.5 wt.% MWCNT into PS probably induces the more interaction between the SDS coated PS latices and MWCNT. The addition of more MWCNT into PS latex induces more steric hindrance, which is expected to obtain a high DE due to the limited Table 2 Dynamic mechanical properties of monodispersed PS particle and MWCNT–PS nanocomposites G0 (MPa)
Fig. 5. TGA analysis of MWCNT–PS nanocomposites (inserted DTA curves of (a) PS particle, (b) 1.5 wt.% MWCNT–PS, (c) 3.5 wt.% MWCNT–PS, (d) 5 wt.% MWCNT–PS and (e) 6.5 wt.% MWCNT–PS nanocomposites).
PS particle 1.5 wt.% MWCNT–PS nanocomposites 3.5 wt.% MWCNT–PS nanocomposites 5 wt.% MWCNT–PS nanocomposites 6.5 wt.% MWCNT–PS nanocomposites
60 °C
135 °C
10400 14400 19300 20700 22300
52.9 160 1350 1490 6320
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Fig. 7. Arrhenius plots of ln kd against 1/T for monodispersed PS particle and MWCNT–PS nanocomposites.
Fig. 6. Isothermal degradation of (a) monodispersed PS particle and (b) 1.5 wt.% MWCNT–PS nanocomposites at various degradation temperatures.
transportation ability of polymer chains during degradation processes. But the addition of more amounts of MWCNT into PS caused the decreasing ratio of SDS to MWCNT could reduce the interaction ability between the SDS coated PS latices and MWCNT (a lower DE), the DE of MWCNT–PS nanocomposites decreases as MWCNT content increases from 1.5 wt.% to 6.5 wt.%. Detail activation energy of monodispersed PS particle and PS–MWCNT nanocomposites are summarized in Table 1. The dynamic storage modulus G0 of neat PS matrix and PS– MWCNT nanocomposites is measured at a temperature range of 60–140 °C. The data of storage modulus at 60 °C are listed in Table 2. At 60 °C, the storage modulus of PS matrix is 1.04 107 Pa, which decreases with the increasing temperature; at 135 °C it drops to 52.9 103 Pa. This is attributed to insufficient thermal energy to overcome the potential barrier for transitional and rotational motions of segments of the polymer molecules in the glassy region, while above the glass-transition temperature (Tg), the thermal energy becomes comparable to the potential energy barriers to the segmental motions. For 1.5 wt.% PS–MWCNT nanocomposite, significant enhancement of G0 can be seen in the lower temperature range, indicating the addition of 1.5 wt.% MWCNT
into PS matrix have strong influence on the elastic properties of the PS matrix. Below Tg, the enhancement of G0 is about 38.5% increase as compared to that of the neat PS. By adding more MWCNT into PS matrix, the storage modulus G0 continuously increases with the presence of 3.5 wt.%, 5.0 wt.% and 6.5 wt.% MWCNT in PS– MWCNT nanocomposites. The enhancement of G0 of PS–MWCNT nanocomposites as compared to virgin PS matrix is 85.6%, 99% and 114% for 3.5 wt.%, 5.0 wt.% and 6.5 wt.% PS–MWCNT nanocomposites, respectively. These results indicate the reinforcement effects of PS–MWCNT nanocomposite are predominated by the presence of MWCNT and possible interaction between the cMWCNT and PS. These results indicate that the presence of MWCNT in PS–MWCNT nanocomposites is possibly confined and retarded the segmental chain motion of PS at the interface. Exfoliation of the MWCNT at the nanoscale level may be the possible cause for a phenomenal increase in the storage modulus for the PS–MWCNT nanocomposites. The electrical conductivities of monodispersed PS particle and MWCNT–PS nanocomposites were measured using the standard Van Der Pauw dc four-probe method [40]. The electrical conductivity of MWCNT–PS nanocomposites as a function of MWCNT content in the PS shows that the electrical conductivity with MWCNT content in the PS latex was in accordance with a percolation theory [41]. The conductivities for MWCNT and PS at room temperature were 20 and 1013 S/cm, respectively. As the addition of 1.5 wt.% MWCNT into PS, the conductivities at room temperature dramatically increase from 1013 S/cm to 6 108 S/cm. According to the percolation theory, the percolation threshold for the MWCNT–PS nanocomposites is between 1.0 and 1.5 wt.% because a rapid increase in the electrical conductivity occurs when the MWCNT content exceeds 1.0 wt.%. With the continuous increase in the content of MWCNT, the conductivities at room temperature gradually increase from 6 108 S/cm for 1.5 wt.% MWCNT–PS nanocomposites to 4.9 104 S/cm for 6.5 wt.% MWCNT–PS nanocomposites. These data is slightly lower than that prepared through in situ polymerization of styrene with MWCNT, graphite or metal powder [33,36,42]. This result is probably due to the PS is covalently linked to carbon black or metal powder during the in situ polymerization procedure as well as the higher conductivity of graphite or metal powder compared to that of MWCNT. Detail electrical conductivities of monodispersed PS particle and MWCNT–PS nanocomposites are also summarized in Ta-
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ble 1. Nevertheless, the conductivities of fabricated composites with very low MWCNT content at room temperature are more than four orders in magnitude higher than those of PS without MWCNT. This result is perhaps due to the presence of MWCNTs containing a large aspect ratio and surface area, so serving as a conducting path between the insulated PS domains and increasing the conductivity with increasing MWCNT content. 4. Conclusions The MWCNT–PS nanocomposites have successfully prepared using latex technology by mixing the monodispersed PS latex synthesized by emulsion polymerization and MWCNTs in methanol solution using ultrasonication process. Both FESEM and HRTEM images indicate that the resulting PS latices are closed to spherical dots with a particle size about 230 nm and a standard deviation of less than 2% (230 ± 4 nm). The HRTEM micrograph of nanocomposites shows the MWCNT is well separated and uniformly distributed among the monodispersed PS latices. The conductivities of 1.5 wt.% and 6.5 wt.% MWCNT–PS nanocomposites are more than four and ten orders in magnitude higher than that of PS without MWCNT, respectively. That is because conductive MWCNT serves as a conducting path between insulated PS domains, increasing the conductivity with increasing MWCNT content. Acknowledgements The authors would like to thank the National Science Council for financially supporting this research under Contract No. NSC95-2218-E-005-004. References [1] Pieranski P. Colloidal crystals. Contemp Phys 1983;24:25–53. [2] Nagai K, Ohashi T, Kaneko R, Taniguchi T. Preparation and applications of polymeric macrospheres having active ester groups. Colloids Surf A 1999;153:133–6. [3] Zhang J, Chen Z, Wang Z, Zhang W, Ming N. Preparation of monodisperse polystyrene spheres in aqueous alcohol system. Mater Lett 2003;57:4466–70. [4] Chen Y, Ford T, Materer NF, Teeters D. Conversion of colloidal crystals to polymer nets: turning latex particles inside out. Chem Mater 2001;13: 2697–704. [5] Piirma, editor. Emulsion polymerization. NewYorkNew York: Academic; 1982. [6] Iijima S, Ichihashi T. Single-shell carbon nanotubes of l nm diameter. Nature 1993;363:603–5. [7] Fan S, Chapline MG, Franklin NR, Tombler TW. Self-oriented regular arrays of carbon nanotubes and their field emission properties. Science 1999;283: 512–4. [8] Dai H, Hafner JH, Rinzler AG, Colbert DT, Smalley RE. Nanotubes as nanoprobes in scanning probe microscopy. Nature 1996;384:147–50. [9] Frank S, Poncharal P, Wang ZL, de Heer WA. Carbon nanotube quantum resistors. Science 1998;280:1744–6. [10] Tans SJ, Verschueren ARM, Dekker C. Room-temperature transistor based on single carbon nanotube. Nature 1998;393:49–51. [11] Ajayan PM, Stephan O, Colliex C, Trauth D. Aligned carbon nanotube arrays formed by cutting a polymer resin–nanotube composite. Science 1994;265: 1212–6. [12] Wong EW, Sheehan PE, Lieber CM. Nanobeam mechanics: elasticity, strength and toughness of nanorods and nanotubes. Science 1997;277:1971–5. [13] Schadler LS, Giannaris SC, Ajayan PM. Load transfer in carbon nanotube epoxy composites. Appl Phys Lett 1998;73:3842–4. [14] Wu TM, Lin YW, Liao CS. Preparation and characterization of polyaniline/ multi-walled carbon nanotube composites. Carbon 2005;43:734–40.
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Preparation and characterization of conductive carbon nanotube–polystyrene nanocomposites using latex technology Tzong-Ming Wu *, Erh-Chiang Chen Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402, Taiwan
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Article history: Received 16 July 2007 Received in revised form 8 April 2008 Accepted 9 April 2008 Available online 15 April 2008 Keywords: A. Nanostructures A. Polymers B. Electrical properties D. Transmission electron microscopy Multi-walled carbon nanotubes
a b s t r a c t This study reports the preparation of conductive multi-walled carbon nanotubes (MWCNTs)–polystyrene (PS) nanocomposites using latex technology. The MWCNTs were first well dispersed in methanol solution and then added to a suspension of the monodispersed 230 nm PS latex synthesized by emulsion polymerization with sodium dodecylsulfate anionic surfactant. Both FESEM and HRTEM images indicate that the resulting PS latices are closed to spherical dots with a particle size about 230 ± 4 nm. The HRTEM micrograph of MWCNT–PS nanocomposites shows the MWCNT is well separated and uniformly distributed between the monodispersed PS latices. The Tg of MWCNT–PS nanocomposites is gradually increased with increasing the content of MWCNT, indicating possible interaction between MWCNT and PS latices to limit the motivation of PS. The conductivities of 1.5 wt.% and 6.5 wt.% MWCNT–PS nanocomposites are more than four and ten orders in magnitude higher than that of PS without MWCNT, respectively. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Monodispersed colloidal particles have already received a wide range of attractive application in fields such as drug delivery and biodiagnostics. The ability to assemble these colloidal particles into crystalline arrays allows one to obtain useful and remarkable functionalities not only from the fundamental physics of systems with the long-range, mesoscopic order that characterizes periodic structures but also from the application of constituent materials [1–4]. All these applications are strongly dependent on the availability of colloidal spheres with tightly controlled sizes and high monodispersity (<5%). Among various colloidal particles, polystyrene (PS) colloids have potential uses in biomedical and drug delivery because the PS colloids with excellent monodispersity can be easily prepared by the classical emulsion polymerization method [5]. Carbon nanotubes (CNTs), discovered by Iijima and Ichihashi. [6], have received much attention for their use in fabricating a new classes of advanced materials due to their unique structural, mechanical and electronic properties. They have potential for use in nanotube-based composites, field emitters, nanoelectronic devices and probe tips for SPMs [7–12]. As expected for the high aspect ratio of length to diameter, experimentally introducing CNTs into a polymer matrix improves the electrical conductivity and mechanical properties of the original polymer matrix [13–16]. For example, CNTs have been incorporated into the conjugated polymers, such as poly(phenylenevinylene) and polythiophene, to * Corresponding author. Tel.: +886 4 2287 2482; fax: +886 4 2285 7017. E-mail address: [email protected] (T.-M. Wu). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.04.010
prepare composites for optoelectronic applications [17–20]. CNTs have also been used as filler into polymer matrix to exploit the superior mechanical properties of the nanotubes. For example, Coleman et al. had been successfully used CNTs as fillers to enhance the mechanical and optical properties of polymers [21– 24]. But several reports demonstrate that the improvements of the mechanical properties of CNT–polymer nanocomposites are limited due to the phase separation between the polymer matrix and CNTs [25,26]. Therefore it is still a very important subject to obtain uniform distribution of CNTs in the polymer matrix until now. The dispersion of CNT into polystyrene matrix for the fabrication of CNT–PS nanocomposites using solution blending [27–30], in situ polymerization [31–33] and melt blending [34] has naturally encouraged significant interest among researchers. For example, Safadi et al. mixed multi-walled carbon nanotubes (MWCNTs) with polystyrene using ultrasonication in a toluene solution [28]. PS solution containing well-dispersed MWCNTs by a simple sonication process were cast and spun to obtain PS–MWCNT thin film composites. They reported that there are remarkable increases in the tensile stress in the 5 wt.% MWCNT–PS nanocomposites. The conductive properties of the 5 wt.% MWCNT–PS nanocomposites are about eight orders in magnitude higher than that of pure PS matrix. Pham et al. used solution blending method to prepared single-walled carbon nanotube (SWCNT)–PS nanocomposites and studied the change of glass transition temperatures (Tg) in these nanocomposites. Their experimental data indicated the Tg of fabricated nanocomposites increases by approximately 3 °C and became much broader than that of pure PS. Alternatively,
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CNT–PS nanocomposites were fabricated by in situ polymerization of styrene in the presence of carbon nanotubes [31,32]. Their results suggested that the in situ polymerization method has more advantages to improve the dispersion of MWCNT in PS matrix, and fabricated composites with high gas sensitivity. In this paper, another approach to fabricate the self-assembly MWCNT–PS nanocomposites using latex technology has been reported. First, the MWCNT can be well dispersed in methanol solution under ultrasonication and then added to a suspension of the monodispersed PS latex synthesized by emulsion polymerization with sodium dodecylsulfate (SDS) anionic surfactant. The structure, morphology and conductivity of fabricated nanocomposites were characterized: in particular, the thermal behavior was observed and the possible degradation of this nanostructure was characterized. 2. Experimental 2.1. Synthesis of conductive MWCNT–PS nanocomposites using latex technology The MWCNTs used in this work were prepared using ethylene CVD using Al2O3 supported Fe2O3 catalysts as described in a previous study [35]. Monodispersed polystyrene (PS) latex with diameter of 230 nm was prepared by emulsion polymerization using sodium dodecylsulfate (SDS) anionic surfactant, potassium persulfate and the mixture of distilled water and methanol as emulsifier, initiator and dispersion medium, respectively. Quantitative SDS and potassium persulfate were dissolved in dispersion medium in a 250 ml two-neck flask. Then a certain amount of styrene was added under nitrogen atmosphere and rapidly mechanical stirring, and the emulsion solution was heated to 70 °C to polymerize for 8 h. The obtained SDS coated PS latex dispersion containing negative charge (Zeta-potential 30 mV) was filtrated and washed with alcohol three times to remove coagulate and soluble impurities. The monodispersed PS latex obtained was dried under a vacuum at 60 °C for 24 h. The MWCNT–PS nanocomposite was prepared using latex fabrication method. First, the MWCNT is well dispersed in methanol solution using ultrasonication process and then added to a suspension of the monodispersed 230 nm PS latex in methanol solution with constant mechanical stirring. In a typical synthesis experiment, various weight ratios of MWCNTs and monodispersed 230 nm PS particles were individually dispersed in methanol solutions and ultrasonicated over 3 h, then both transferred into a 250 ml two-neck flask under nitrogen atmosphere and rapidly mechanical stirring for 2 h. The self-assembly MWCNT–PS nanocomposites obtained were dried at 80 °C to remove any remaining methanol. 2.2. Structural analysis The Raman spectroscopy was used to characterize the structure of MWCNT. Raman spectra were recorded under a Jobin Yvon TRIAX 550 system using a He–Ne laser operating at 632.8 nm with a CCD detector. The final spectrum presented is an average of three spectra recorded at different regions over the entire range of the sample. The field-emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) were used to characterize the morphology of MWCNT–PS nanocomposite. The specimens for FESEM and HRTEM analysis were prepared using the droplet of MWCNT–PS nanocomposite in methanol solution on the surface of aluminum foil and carbon-coated copper grid and then air-dried for 2 h. FESEM was conducted at 3 kV using a JEOL JSM-6700 F field-emission instrument and HRTEM was recorded on a Hitachi HF-2000 instrument at 200 kV.
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2.3. Physical properties Thermal analysis of the samples was obtained using a Perkin– Elmer PYRIS Diamond differential scanning calorimeter (DSC) under a nitrogen atmosphere. All specimens were weighted in the range of 5–6 mg and were carried from room temperature to 150 °C at a scanning rate of 10 °C /min. Thermal stabilities of these samples were operated using Perkin–Elmer thermogravimetric analysis (TGA)/differential thermal analyzer (DTA) and all experiments were performed under a nitrogen atmosphere at a purge rate of 100 ml/min. The samples were performed from room temperature until 700 °C at a scanning rate of 10 °C /min. The isothermal degradation at 360, 370, 380, 400 and 420 °C were also studied. The samples of monodispersed 230 nm PS particles, MWCNT and MWCNT–PS nanocomposites were pressed into pellet under 20 MPa. DMA experiments were performed on a Perkin–Elmer instrument DMA 7e apparatus equipped with a film tension clamp. The instrument was programmed to measure G0 (storage modulus) over the range of 60–140 °C at 5 °C/min heating rate and 10 Hz constant frequency. The conductivity at room temperature using four probe methods was measured by a programmable DC voltage/current detector. Each data shown here is the mean value of the measurement from at least three samples and the experimental error using standard deviation of these data is obtained. 3. Results and discussion Fig. 1 shows FESEM and HRTEM images of the monodispersed PS latex synthesized by emulsion polymerization with surfactant SDS. From the FESEM data, the average diameter of PS latex is about 230 nm and its size distribution is highly uniform. Closer inspection of HRTEM images of PS latex shown in Fig. 1b reveals that the resulting PS latices are closed to spherical dots with a particle size deviation of less than 2% (230 ± 4 nm). The Raman spectrum of the purified MWCNTs containing two characteristic peaks is shown in Fig. 2. One peak at 1580 cm1 (G mode) is the Raman-allowed phonon high-frequency mode and the other at 1355 cm1 (D mode) is a disordered-induced peak, which may originate from the defects in the curved graphene sheets and tube ends. The TEM images of prepared MWCNT (insert in Fig. 2) clearly shows the diameter of about 10–15 nm hollow cores of nanotubes and total size of 20–40 nm of nanotubes. Fig. 3 indicate the HRTEM images of 1.5 wt.% and 5 wt.% MWCNT–PS nanocomposites prepared using latex technology. Both PS latex and MWCNT each dispersed in methanol solution and ultrasonicated for 4 h and then solution mixed with constant mechanical stirring result in better dispersion of the MWCNT in PS latex. These results reveal that the resulting nanocomposites have tube-like and spherical morphology representing the MWCNT and PS latex, respectively. The MWCNTs wrapped by PS latex can be clearly observed. There are several PS latices adsorbed on the interface of MWCNT, which is also observed by Choi and coworkers [30]. From the TEM results, the MCWNT is well separated and uniformly distributed in the monodispersed PS latices. The formation mechanism of MWCNT–PS nanocomposites is believed to involve possible interactions between negative charges of SDS coated PS latex (Zeta-potential 30 mV) and slightly positive charges of MWCNT (Zeta-potential 3 mV) with surfactant SDS using ultrasonication process, which is also reported by previous investigations [36,37]. Therefore, it is possible to allow the MWCNTs wrap up in PS latices. The glass transition temperature (Tg) of monodispersed PS particle and fabricated MWCNT–PS nanocomposites was measured on DSC in nitrogen atmosphere. Fig. 4 shows the DSC thermal curves of the MWCNT–PS nanocomposites with various MWCNT ratios. As seen from Fig. 4, the Tg of PS is 99.1 °C and then gradually in-
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Fig. 1. (a) FESEM and (b) HRTEM image of monodispersed PS latex prepared emulsion polymerization. Fig. 3. HRTEM image of (a) 1.5 wt.% and (b) 5 wt.% MWCNT–PS nanocomposites prepared by latex technology.
Fig. 2. Raman-spectroscopy of MWCNT (inserted HRTEM image of MWCNT).
creases with increasing the content of MWCNT. As the addition of 6.5 wt.% MWCNT into monodispersed PS latices, the Tg increases up to 105.7 °C. Detail Tg of monodispersed PS particle and MWCNT–PS
Fig. 4. DSC thermal analysis of (a) monodispersed PS particle, (b) 1.5 wt.% MWCNT– PS, (c) 3.5 wt.% MWCNT–PS, (d) 5 wt.% MWCNT–PS and (e) 6.5 wt.% MWCNT–PS nanocomposites.
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Table 1 Glass transition temperatures (Tg), onset temperature of degradation (Tonset), activation energies (Ea), electrical conductivity and dynamic mechanical properties (G0 ) at 60 °C of monodispersed PS particle and MWCNT–PS nanocomposites Sample PS particle 1.5 wt.% MWCNT–PS 3.5 wt.% MWCNT–PS 5 wt.% MWCNT–PS 6.5 wt.% MWCNT–PS
Tg ( °C) 99.1 101 101.7 102.9 105.7
Tonset (°C)
Ea (kJ/mol)
383.5 ± 0.3 384.7 ± 0.3 386.4 ± 0.3 386.6 ± 0.3 387.8 ± 0.3
nanocomposites are listed in Table 1. These results indicate that the presence of MWCNT induces the interaction of PS latex. Therefore it is necessary to provide more thermal energy to overcome the interaction for further transitional and rotational motions of PS molecules in the glassy region, whereas the Tg of nanocomposites increases as MWCNT content increases. Thermal stability of monodispersed PS particle and MWCNT–PS nanocomposites was measured using thermogravimetric analysis (TGA)/differential thermal analyzer (DTA) in nitrogen atmosphere. Fig. 5 shows the TGA/DTA thermal curves of the MWCNT–PS nanocomposites with various MWCNT contents. From the DTA data, the peak position of maximum degradation was slightly shifted to high temperature by adding the MWCNT into PS latex. The TGA profiles of MWCNT–PS nanocomposites shows similar tendency, and the onset temperature of degradation (Tonset) can be determined from these curves by extrapolating from the curve at the peak of degradation back to the initial weight of the polymer. As seen from this figure, the Tonset of monodispersed PS latex is about 383.5 ± 0.3 °C. The onset temperature of weight loss of 1.5 wt.% MWCNT–PS nanocomposites is about 384.7 ± 0.3 °C, which may be due to the thermal degradation of organic polystyrene decomposition. With continuously increasing content of MWCNTs, the onset temperature of PS decomposition increases from 384.7 ± 0.3 °C for 1.5 wt.% PS–MWCNT nanocomposites to 387.8 ± 0.3 °C for 6.5 wt.% PS–MWCNT nanocomposites. Detail Tonset of monodispersed PS particle and MWCNT–PS nanocomposites are also listed in Table 1. All results show that introducing the MWCNT into PS latex system can enhance the thermal stability of PS–MWCNT nanocomposites due to the interaction between the MWCNT and PS lattices and the formation of a barrier of MWCNT which inhibit mass transfer and provide thermal insulation to shield the under-
190.9 ± 1.7 187.0 ± 0.6 193.2 ± 1.1 198.2 ± 0.3 201.3 ± 0.7
Conductivity (S/cm) 13
1.0 10 6.0 108 4.9 106 2.1 104 4.9 104
G0 (MPa) 10,400 14,400 19,300 20,700 22,300
lying polymer from the heat source [38]. From above data, the residual at 500 °C is considered to be MWCNTs since PS has been completely decomposed at 450 °C. In order to understand the effect of MWCNT on the thermal behavior of nanocomposites, the isothermal thermal degradation can be performed to study the degradation kinetics of MWCNT– PS nanocomposites. The method of isothermal kinetic parameter proposed by Freeman and Carroll has widely been used to determine the order and the activation energy of the degradation [39]. The rate of degradation may be written as
dW ¼ kd W n dt
ð1Þ
where kd is the rate constant; W is the weight remaining, %; n is the order of the reaction. If assuming first order decomposition, then ln W ¼ ln W 0 kd t
ð2Þ
where W0 is the initial weight. Fig. 6a and b shows the plot of ln W versus t for monodispersed PS particle and 1.5 wt.% PS–MWCNT nanocomposites isothermal degradation at 360, 370, 380, 400 and 420 °C. It is found that all isothermal degradation data contains a straight line which means that the degradation can be regarded as a first order decomposition with a steady rate constant. The rate constants, kd, were obtained as the slopes of the curves in Fig. 6. All plots of ln W versus t for 3.5 wt.%, 5 wt.% and 6.5 wt.% MWCNT–PS nanocomposites show similar tendency and the degradation can be assigned to be the first order decomposition with a steady rate constant. Additionally, DEa ð3Þ kd ¼ A exp RT where A is the pre-exponential factor; DE is the total activation energy; R is the universal gas constant; T is the absolute temperature (K). Arrhenius plots of ln kd against 1/T for monodispersed PS particle and MWCNT–PS nanocomposites are shown in Fig. 7, and are approximately linear. The activation energy can be determined from the slope of plots and is dependent on the content of MWCNT. The activation energy slightly increases with the presence of 1.5 wt.% MWCNT in MWCNT–PS nanocomposites and then decreases with increasing MWCNT content. The result indicates that the addition of 1.5 wt.% MWCNT into PS probably induces the more interaction between the SDS coated PS latices and MWCNT. The addition of more MWCNT into PS latex induces more steric hindrance, which is expected to obtain a high DE due to the limited Table 2 Dynamic mechanical properties of monodispersed PS particle and MWCNT–PS nanocomposites G0 (MPa)
Fig. 5. TGA analysis of MWCNT–PS nanocomposites (inserted DTA curves of (a) PS particle, (b) 1.5 wt.% MWCNT–PS, (c) 3.5 wt.% MWCNT–PS, (d) 5 wt.% MWCNT–PS and (e) 6.5 wt.% MWCNT–PS nanocomposites).
PS particle 1.5 wt.% MWCNT–PS nanocomposites 3.5 wt.% MWCNT–PS nanocomposites 5 wt.% MWCNT–PS nanocomposites 6.5 wt.% MWCNT–PS nanocomposites
60 °C
135 °C
10400 14400 19300 20700 22300
52.9 160 1350 1490 6320
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Fig. 7. Arrhenius plots of ln kd against 1/T for monodispersed PS particle and MWCNT–PS nanocomposites.
Fig. 6. Isothermal degradation of (a) monodispersed PS particle and (b) 1.5 wt.% MWCNT–PS nanocomposites at various degradation temperatures.
transportation ability of polymer chains during degradation processes. But the addition of more amounts of MWCNT into PS caused the decreasing ratio of SDS to MWCNT could reduce the interaction ability between the SDS coated PS latices and MWCNT (a lower DE), the DE of MWCNT–PS nanocomposites decreases as MWCNT content increases from 1.5 wt.% to 6.5 wt.%. Detail activation energy of monodispersed PS particle and PS–MWCNT nanocomposites are summarized in Table 1. The dynamic storage modulus G0 of neat PS matrix and PS– MWCNT nanocomposites is measured at a temperature range of 60–140 °C. The data of storage modulus at 60 °C are listed in Table 2. At 60 °C, the storage modulus of PS matrix is 1.04 107 Pa, which decreases with the increasing temperature; at 135 °C it drops to 52.9 103 Pa. This is attributed to insufficient thermal energy to overcome the potential barrier for transitional and rotational motions of segments of the polymer molecules in the glassy region, while above the glass-transition temperature (Tg), the thermal energy becomes comparable to the potential energy barriers to the segmental motions. For 1.5 wt.% PS–MWCNT nanocomposite, significant enhancement of G0 can be seen in the lower temperature range, indicating the addition of 1.5 wt.% MWCNT
into PS matrix have strong influence on the elastic properties of the PS matrix. Below Tg, the enhancement of G0 is about 38.5% increase as compared to that of the neat PS. By adding more MWCNT into PS matrix, the storage modulus G0 continuously increases with the presence of 3.5 wt.%, 5.0 wt.% and 6.5 wt.% MWCNT in PS– MWCNT nanocomposites. The enhancement of G0 of PS–MWCNT nanocomposites as compared to virgin PS matrix is 85.6%, 99% and 114% for 3.5 wt.%, 5.0 wt.% and 6.5 wt.% PS–MWCNT nanocomposites, respectively. These results indicate the reinforcement effects of PS–MWCNT nanocomposite are predominated by the presence of MWCNT and possible interaction between the cMWCNT and PS. These results indicate that the presence of MWCNT in PS–MWCNT nanocomposites is possibly confined and retarded the segmental chain motion of PS at the interface. Exfoliation of the MWCNT at the nanoscale level may be the possible cause for a phenomenal increase in the storage modulus for the PS–MWCNT nanocomposites. The electrical conductivities of monodispersed PS particle and MWCNT–PS nanocomposites were measured using the standard Van Der Pauw dc four-probe method [40]. The electrical conductivity of MWCNT–PS nanocomposites as a function of MWCNT content in the PS shows that the electrical conductivity with MWCNT content in the PS latex was in accordance with a percolation theory [41]. The conductivities for MWCNT and PS at room temperature were 20 and 1013 S/cm, respectively. As the addition of 1.5 wt.% MWCNT into PS, the conductivities at room temperature dramatically increase from 1013 S/cm to 6 108 S/cm. According to the percolation theory, the percolation threshold for the MWCNT–PS nanocomposites is between 1.0 and 1.5 wt.% because a rapid increase in the electrical conductivity occurs when the MWCNT content exceeds 1.0 wt.%. With the continuous increase in the content of MWCNT, the conductivities at room temperature gradually increase from 6 108 S/cm for 1.5 wt.% MWCNT–PS nanocomposites to 4.9 104 S/cm for 6.5 wt.% MWCNT–PS nanocomposites. These data is slightly lower than that prepared through in situ polymerization of styrene with MWCNT, graphite or metal powder [33,36,42]. This result is probably due to the PS is covalently linked to carbon black or metal powder during the in situ polymerization procedure as well as the higher conductivity of graphite or metal powder compared to that of MWCNT. Detail electrical conductivities of monodispersed PS particle and MWCNT–PS nanocomposites are also summarized in Ta-
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ble 1. Nevertheless, the conductivities of fabricated composites with very low MWCNT content at room temperature are more than four orders in magnitude higher than those of PS without MWCNT. This result is perhaps due to the presence of MWCNTs containing a large aspect ratio and surface area, so serving as a conducting path between the insulated PS domains and increasing the conductivity with increasing MWCNT content. 4. Conclusions The MWCNT–PS nanocomposites have successfully prepared using latex technology by mixing the monodispersed PS latex synthesized by emulsion polymerization and MWCNTs in methanol solution using ultrasonication process. Both FESEM and HRTEM images indicate that the resulting PS latices are closed to spherical dots with a particle size about 230 nm and a standard deviation of less than 2% (230 ± 4 nm). The HRTEM micrograph of nanocomposites shows the MWCNT is well separated and uniformly distributed among the monodispersed PS latices. The conductivities of 1.5 wt.% and 6.5 wt.% MWCNT–PS nanocomposites are more than four and ten orders in magnitude higher than that of PS without MWCNT, respectively. That is because conductive MWCNT serves as a conducting path between insulated PS domains, increasing the conductivity with increasing MWCNT content. Acknowledgements The authors would like to thank the National Science Council for financially supporting this research under Contract No. NSC95-2218-E-005-004. References [1] Pieranski P. Colloidal crystals. Contemp Phys 1983;24:25–53. [2] Nagai K, Ohashi T, Kaneko R, Taniguchi T. Preparation and applications of polymeric macrospheres having active ester groups. Colloids Surf A 1999;153:133–6. [3] Zhang J, Chen Z, Wang Z, Zhang W, Ming N. Preparation of monodisperse polystyrene spheres in aqueous alcohol system. Mater Lett 2003;57:4466–70. [4] Chen Y, Ford T, Materer NF, Teeters D. Conversion of colloidal crystals to polymer nets: turning latex particles inside out. Chem Mater 2001;13: 2697–704. [5] Piirma, editor. Emulsion polymerization. NewYorkNew York: Academic; 1982. [6] Iijima S, Ichihashi T. Single-shell carbon nanotubes of l nm diameter. Nature 1993;363:603–5. [7] Fan S, Chapline MG, Franklin NR, Tombler TW. Self-oriented regular arrays of carbon nanotubes and their field emission properties. Science 1999;283: 512–4. [8] Dai H, Hafner JH, Rinzler AG, Colbert DT, Smalley RE. Nanotubes as nanoprobes in scanning probe microscopy. Nature 1996;384:147–50. [9] Frank S, Poncharal P, Wang ZL, de Heer WA. Carbon nanotube quantum resistors. Science 1998;280:1744–6. [10] Tans SJ, Verschueren ARM, Dekker C. Room-temperature transistor based on single carbon nanotube. Nature 1998;393:49–51. [11] Ajayan PM, Stephan O, Colliex C, Trauth D. Aligned carbon nanotube arrays formed by cutting a polymer resin–nanotube composite. Science 1994;265: 1212–6. [12] Wong EW, Sheehan PE, Lieber CM. Nanobeam mechanics: elasticity, strength and toughness of nanorods and nanotubes. Science 1997;277:1971–5. [13] Schadler LS, Giannaris SC, Ajayan PM. Load transfer in carbon nanotube epoxy composites. Appl Phys Lett 1998;73:3842–4. [14] Wu TM, Lin YW, Liao CS. Preparation and characterization of polyaniline/ multi-walled carbon nanotube composites. Carbon 2005;43:734–40.
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