COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 900–905 www.elsevier.com/locate/compscitech

Different types of molecular interactions in carbon nanotube/conducting polymer composites – A close analysis Anantha Iyengar Gopalan a,b,c, Kwang-Pill Lee a,b,*, Padmanabhan Santhosh a, Kyu Soo Kim a,b, Young Chang Nho d a

d

Department of Chemistry Graduate School, Kyungpook National University, Daegu 702-701, South Korea b Nano Practical Application Center, Daegu 704-230, South Korea c Department of Industrial Chemistry, Alagappa University, Karaikudi-630 003, Tamil Nadu, India Radioisotope/Radio Application Team, Korea Atomic Energy Research Institute, Daejon 305-600, South Korea Received 31 October 2005; accepted 8 February 2006 Available online 10 October 2006

Abstract Composites of polyaniline (PANI) and multi-wall carbon nanotube (MWNT) were prepared through chemical oxidative and c-radiation induced polymerization of aniline in the presence of functionalized MWNT and the composites were designated as PANI/MWNTNC(c) and PANI/MWNT-NC(c), respectively. The composites were characterized for the structure, morphology, optical and thermal properties through Raman, Fourier transform infrared (FT-IR) spectroscopy, field emission transmission electron microscopy (FETEM), UV–visible spectroscopy and thermogravimetric analysis (TGA). FETEM analysis shows the presence of PANI layer (15 nm) in PANI/MWNT-NC(c) and a PANI layer of 10 nm in the case of PANI/MWNT-NC(c). Results from Raman and FT-IR spectroscopy indicate the formation of carboxyl groups on the sidewalls of the MWNTs when the composite was prepared by c-radiation. The existence of carboxyl groups on the walls of MWNT caused induced doping in PANI for PANI/MWNT-NC(c). UV–visible spectrum of PANI/MWNT-NC(c) exhibited an additional band around 470 nm corresponding to the induced doping with carboxyl groups. This band was virtually absent in PANI/MWNT-NC(c). Thus, differences in molecular level interactions between PANI and MWNT influence the properties of the composites prepared by chemical and c-radiation polymerization. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Nanocomposites; B. Microstructure; D. Raman spectroscopy; D. Infrared (IR) spectroscopy; D. Optical microscopy; D. Thermogravimetric analysis (TGA)

1. Introduction Carbon nanotubes (CNT) posses unique electronic and mechanical properties [1,2] and are used as suitable base material for the preparation of nanocomposites [3–7]. Polyaniline (PANI) has been regarded as one of the most important conducting polymers due to its relatively facile synthesis, conductivity and environmental stability [8,9]. PANI exhibits electrical, electrochemical and optical properties that are suitable for many applications [10]. * Corresponding author. Address: Department of Chemistry Graduate School, Kyungpook National University, Daegu 702-701, South Korea. Tel.: +82 53 950 5901; fax: +82 53 952 8104. E-mail address: [email protected] (K.-P. Lee).

0266-3538/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.02.036

Formation of CNT-polymer composites is considered as a promising approach to effectively incorporate CNTs into devices and to have the advantage of exploiting the synergetic effect arising from the polymer [11–13]. Due to the formation of conducting polymer-CNT networks, CNT-polymer composites are of interest for electronic applications, including electromagnetic shielding, electrostatic dissipation, antennas and batteries [14,15]. CNT-polymer composites have been prepared with variety of polymers to suit for defined applications [16–18]. CNTs were used as conductive fillers in CNT-poly(3-octylthiophene) composites and CNT-poly(phenylenevinylene) composites have also been reported [19,20]. Film of CNT-poly(3,4-ethylenedithiophene) composites have been

A.I. Gopalan et al. / Composites Science and Technology 67 (2007) 900–905

used as the hole injecting layer in organic light emitting diodes [21]. Studies related to CNT–PANI composites have also been reported [22–26]. The method of preparation of the composite is of prime importance since subtle structural changes in the composite can ultimately influence the properties of the composites. Generally, two methods are used for the preparation of CNT-conducting polymer composites. Direct mixing of a conducting polymer with CNTs and formation of conducting polymer in the presence of CNTs are the two important approaches to prepare CNT-polymer composite. In both cases, the final product is a composite having increased conductivity over the pristine conducting polymer [24,25]. Moreover, conditions of polymerization decide the properties of the composites [27]. A comparative study on the preparation and characterization of the CNT-conducting polymer composite through these two approaches would be useful in understanding subtle differences in the properties of the composite. Polymerization can also be initiated by c-radiation. So far, a systematic study has not been made on the comparison of properties of CNT-conducting polymer composites prepared through c-radiation and chemical method. Radiolysis provides several advantages for the initiation of polymerization over a conventional chemical method. The main advantage stems out from the absence of using additional agents (like oxidizing agent) in the polymerization medium. c-radiation has been applied extensively for the initiation of polymerization, grafting of polymer chains onto polymeric backbones, modification of polymer blends and preparation of interpenetrating polymer networks. Typically, a film of transparent poly(methyl methacrylate)/SWNT composite was prepared by c-radiation [28] and the influence of c-radiation on the composites was studied [29]. In the present work, composites of multi-wall carbon nanotube (MWNT) with PANI were prepared by two initiation methods; chemical oxidative and c-radiation induced polymerization. The composites were characterized for the morphology, structure, optical and thermal properties. 2. Experimental

901

50 mg of MWNT was refluxed in 4 M HNO3 for 24 h and filtered through a polycarbonate membrane (0.2 lm pore size). The residue, MWNT–COOH (carboxylated MWNT) was washed with deionized water and dried under vacuum at 60 °C for 12 h. 50 mg of MWNT–COOH was refluxed in 100 mL of thionyl chloride at 65 °C for 24 h to get MWNT–COCl. MWNT–COCl was filtered, washed with ethanol and dried under vacuum at room temperature. To prepare the functionalized MWNTs, MWNT– COCl was refluxed with 4,4’-(9-fluorenylidene)dianiline using THF at 60 °C for 24 h. The functionalized MWNT (MWNT–FDA) was separated by filtration and dried under vacuum. 2.3. Preparation of PANI/MWNT composites The composites were prepared by two different methods; chemical oxidative and c-radiation induced polymerization. 2.3.1. Preparation of PANI/MWNT composite by chemical polymerization To a solution of aniline in 1 M HCl (10 mM), 0.005 g of MWNT–FDA in 0.5 M cetyltrimethyl ammonium bromide (CTAB) was added and sonicated for 1 h. The mixture was cooled to 4 °C using a freezing mixture. A pre-cooled (4 °C) solution of ammonium persulfate (0.1 M) in 1 M HCl was added drop wise to the mixture with constant stirring. The resulting green precipitate, PANI/MWNT-NC(c), was filtered through a sintered glass crucible and washed with 1 M HCl until the filtrate became colorless. The composites were then dried under vacuum at room temperature. 2.3.2. Preparation of PANI/MWNT composite by c-radiation induced polymerization To a solution of aniline in 1 M HCl (10 mM), 0.005 g of MWNT–FDA in 0.5 M CTAB was added and sonicated for 1 h. Nitrogen gas was bubbled through the solution (30 min) to remove oxygen and then was irradiated by c-ray (Co-60 source) to a total dose of 3 kGy under atmospheric pressure and ambient temperature. Experiments were also done with different dosages of c-radiation. The resulting precipitate, PANI/MWNT-NC(c), was filtered and washed with 1 M HCl until the filtrate became colorless. The composite was then dried under vacuum.

2.1. Materials 2.4. Characterization Aniline (Aldrich) was distilled and used. MWNTs (10– 50 nm in diameter, CNT Co. Ltd. Incheon, Korea) were rinsed with double-distilled water and dried. Cetyltrimethyl ammonium bromide (CTAB), 4,4’-(9-fluorenylidene)dianiline (FDA), hydrochloric acid of analytical grade from Aldrich were used as received. 2.2. Functionalization of MWNT The following procedure was adopted for the preparation of FDA functionalized MWNTs (MWNT–FDA).

Raman spectra of the composites were recorded using SPEX, Ramalog 9I/Coherenet, Innova 90–5 Laser Raman Spectrophotometer in the region of 50 cm 1 to 4000 cm 1. Fourier transform infrared (FT-IR) spectra were recorded using a Bruker IFS 66v FTIR spectrophotometer using KBr pellets. The morphology of the composites was examined by field emission transmission electron microscope (FETEM)–JOEL JEM-2000EX with a field emission gun operated at 200 kV. UV–visible spectra were recorded using Shimadzu UV–visible spectrophotometer.

902

A.I. Gopalan et al. / Composites Science and Technology 67 (2007) 900–905

Thermogravimetric analysis was performed using TA 2950 Hi-Res TGA at a heating rate of 10 °C/min under nitrogen atmosphere. 3. Results and discussion Our main interest in the present work is to identify the differences in the molecular level interactions between PANI and MWNT in PANI/MWNT-NC(c) and PANI/ MWNT-NC(c). Interestingly, results from FT-IR, Raman, UV–visible spectroscopy and thermal analysis of these composites clearly demonstrate that there are differences in the structural, electrical and thermal properties between PANI/MWNT-NC(c) and PANI/MWNT-NC(c). And, the differences are expected to arise from the changes in molecular level interactions between PANI and MWNT. 3.1. Raman spectroscopy Fig. 1 displays the Raman spectra of the composites, PANI/MWNT-NC(c) and PANI/MWNT-NC(c). For understanding the interactions between MWNT–FDA and PANI in the composites, the bands in the Raman spectrum of MWNT–FDA (Fig. 1d) are assigned. Raman spectrum of MWNT–FDA (Fig. 1d) displays three bands. The appearance of D-line at 1352 cm 1 corresponds to the amorphous carbon and disorder induced line [30]. The band at 1582 cm 1 corresponds to G-line representing the in-plane stretching E2g mode and a shoulder around 1600 cm 1 is assigned for D’-line (disorder line) [31]. For a comparative purpose, HCl doped PANI was prepared in the absence of MWNT–FDA and Raman spectrum of PANI–HCl was recorded (Fig. 1a) and compared with PANI/MWNT-NC(c) (Fig. 1c) and PANI/ MWNT-NC(c) (Fig. 1b). At first, there is a striking difference in the intensity of the band around 1480 cm 1 between PANI/MWNTNC(c) (Fig. 1b) and PANI/MWNT-NC(c) (Fig. 1c) in

comparison to the band observed for PANI–HCl (Fig. 1a). The band around 1480 cm 1 in PANI–HCl corresponds to C–C stretching of benzenoid ring in aniline units of PANI [32]. A remarkable decrease in the peak intensity of the band around 1480 cm 1 for PANI/ MWNT-NC(c) and PANI/MWNT-NC(c) informs that PANI/MWNT-NC(c) and PANI/MWNT-NC(c) have lower portions of benzenoid units in PANI than in PANI–HCl. Comparatively, the decrease in the intensity of the 1480 cm 1 band is more for PANI/MWNT-NC(c) than noticed for PANI/MWNT-NC(c). Hence, significant portions of benzenoid units present in PANI backbone of PANI/MWNT-NC(c) might be converted into quinoid forms. Otherwise, PANI/MWNT-NC(c) is more doped than PANI/MWNT-NC(c). It is now important to clarify the following question. Why does PANI/MWNT-NC(c) have more quinoid units than in PANI/MWNT-NC(c). We have obtained evidences for the formation of carboxyl groups on the surface of MWNT when polymerization was performed with c-radiation (evidence is presented in the later part of discussion). The carboxyl groups thus generated at the surface of MWNT induce doping of PANI chains to result more quinoid units in PANI/MWNT-NC(c). A clue for the formation of carboxyl groups at the side walls of MWNT was obtained from the close analysis of Raman spectrum of PANI/MWNT-NC(c) (Fig. 1c). It can be seen that D band (1352 cm 1) of MWNT present in PANI/MWNT-NC(c) is more intense than the band observed for PANI/MWNTNC(c) (Fig. 1b) and MWNT–FDA (Fig. 1d). This may be due to the increase in the defect sites of MWNT in PANI/ MWNT-NC(c). We attribute the increase in the intensity of D-line of MWNT in PANI/MWNT-NC(c) as due to the generation of carboxyl groups by breaking few of C– C bonds in MWNT. Thus, we attribute that carboxyl groups were additionally generated in MWNT when PANI/MWNT-NC(c) was prepared. As a result of it, there can be differences in the interactions between MWNT and PANI for the PANI/MWNT-NC(c). 3.2. FT-IR spectroscopy

Fig. 1. Raman spectra of (a) PANI–HCl, (b) PANI/MWNT-NC(c) ([APS] = 50 mM), (c) PANI/MWNT-NC(c) (c-irradiation dose: 3 kGy), and (d) MWNT–FDA. Wt. of MWNT–FDA = 5 mg; [Aniline] = 10 mM; Medium [CTAB]: 0.5 M; [HCl]: 1 M.

FT-IR spectroscopy provides clear evidences for the formation of carboxyl groups in MWNT during c-radiation induced polymerization. Fig. 2a presents the FT-IR spectrum of PANI–HCl. The bands around 820, 1130, 1298, 1496, and 1580 cm 1 noticed for PANI–HCl are assigned for bending of C–H (out-of-plane) on benzene ring (B) pdisubstituted, bending of C–H (in-plane), vibration of the B–NH+ @ Q structure [33], stretching of Caromatic–N, stretching of N-benzenoid (B)–N ring and stretching of N @ Q @ N ring, respectively [34]. Few striking differences in FT-IR spectral characteristics were noticed between PANI/MWNT-NC(c), PANI/ MWNT-NC(c) and PANI–HCl. Firstly, FT-IR spectrum of PANI/MWNT-NC(c) (Fig. 2c) showed the C @ O stretching band corresponding to the carboxyl group was

A.I. Gopalan et al. / Composites Science and Technology 67 (2007) 900–905

d

% Transmittance

c

b

a

1600

1400

1200

1000

800

600

400

Wavenumbers (cm-1)

Fig. 2. FT-IR spectra of (a) PANI–HCl, (b) PANI/MWNT-NC(c) ([APS] = 50 mM), (c) and (d) PANI/MWNT-NC(c) with different cirradiation dose (c: 0.5 kGy and d: 5 kGy). Wt. of MWNT–FDA = 5 mg; [Aniline] = 10 mM; Medium [CTAB]: 0.5 M; [HCl]: 1 M.

1

found around 1630 cm . This band was virtually absent in the spectrum of PANI/MWNT-NC(c) (Fig. 2b) and PANI–HCl (Fig. 2a) and provide support for the formation of carboxyl groups when the composite was prepared by c-radiation. PANI/MWNT-NCs(c) were also prepared with two different c-radiation dosages (0.5 kGy and 5 kGy). PANI/MWNT-NC(c) prepared with 5 kGy of cradiation (Fig. 2d) showed a more intense band for the C @ O stretching band corresponding to the carboxyl group (1630 cm 1) than noticed for the composite prepared with a c-radiation dose of 0.5 kGy (Fig. 2c). The observation also favors the formation of carboxyl groups during the preparation of composites with c-radiation. Now, it becomes clear that PANI/MWNT-NC(c) had carboxyl groups whereas PANI/MWNT-NC(c) did not have carboxyl groups. As a result of it, molecular level interactions

903

between MWNT and PANI in these composites, PANI/ MWNT-NC(c) and PANI/MWNT-NC(c) are different. Also, the band corresponding to the stretching of B–NH+ @ Q (1130 cm 1) showed variation in position and intensity between the PANI/MWNT-NC(c) and PANI/MWNT-NC(c). Typically, the band noticed for PANI–HCl around 1114 cm 1 showed a shift for PANI/ MWNT-NC(c). Hence, we anticipate that B–NH+ @ Q groups in PANI might have interactions with MWNT. Grafting of PANI chains onto MWNT through reactions with –NH groups present in FDA is anticipated. Interestingly, the band of B–NH+ @ Q showed a band around 1124 cm 1 and a shoulder at 1080 cm 1 for PANI/ MWNT-NC(c). Hence, in PANI/MWNT-NC(c), besides grafting of PANI chains to MWNT, additional doping was also expected through interactions with carboxyl groups generated by c-radiation. Also, FT-IR spectrum of PANI/MWNT-NC(c) showed a band around 1190 cm 1 which is assigned for the stretching modes of carboxyl groups [35]. It is to be noted that the band observed at 1190 cm 1 for PANI/MWNT-NC(c) was virtually absent for PANI/MWNT-NC(c). 3.3. Morphology Field emission transmission electron microscopy (FETEM) was used to ascertain the morphology of PANI/MWNT-NC(c) and PANI/MWNT-NC(c). FETEM images of the composites informed that a layer of PANI was formed over MWNT in both cases. PANI-MWNT composite prepared by polymerizing aniline in the presence of MWNT has been reported to have a microstructure in which MWNT was presented as entrapped mass in PANI matrix [36]. In the present work, formation of a layer of PANI on the surface of MWNT was noticed for PANI/ MWNT-NC(c) (Fig. 3a) and PANI/MWNT-NC(c) (Fig. 3b). FETEM images thus provide evidences for the grafting of PANI chains onto MWNT. It is clear from the FETEM images that both composites have a layer of PANI on the surface of MWNT. However, a thicker layer of PANI (15 nm) was noticed for PANI/MWNT-NC(c) than noticed for PANI/MWNT-NC(c) (10 nm).

Fig. 3. FETEM images of (a) PANI/MWNT-NC(c) ([APS] = 50 mM), and (b) PANI/MWNT-NC(c) (c-irradiation dose: 3 kGy). Wt. of MWNT– FDA = 5 mg; [Aniline] = 10 mM; Medium [CTAB]: 0.5 M; [HCl]: 1 M (Scale bar: 5 nm).

904

A.I. Gopalan et al. / Composites Science and Technology 67 (2007) 900–905

3.4. UV–visible spectroscopy

Table 1 Data from thermogravimetric analysis

UV–visible spectroscopy was utilized to understand the electronic states of PANI in PANI/MWNT-NC(c) and PANI/MWNT-NC(c). Fig. 4 displays the UV–visible spectra recorded for the aqueous dispersion of PANI/MWNTNC(c) and PANI/MWNT-NC(c). Clearly, the electronic states of PANI in these composites are different. PANI/ MWNT-NC(c) exhibited a band around 380 nm (Fig. 4b) corresponding to the polaronic state of PANI. The polaronic band noticed for PANI/MWNT-NC(c) was found to be hypsochromically shifted in comparison to PANI– HCl (Fig. 4a). The HCl doped PANI showed a characteristic band for the polaronic transition around 420 nm [37] (Fig. 4a). Hence, we attribute the shift in the position of polaronic state of PANI in PANI/MWNT-NC(c) is due to the existence of PANI chains connected to MWNT. UV–visible spectrum of PANI/MWNT-NC(c) (Fig. 4c) showed entirely different electronic bands than noticed with PANI/MWNT-NC(c). UV–visible spectrum of PANI/ MWNT-NC(c) exhibited two bands, one at 370 nm and another around 470 nm. The distinct difference in the electronic state for PANI/MWNT-NC(c) is expected to arise from the doping induced electronic state by the interactions between imine sites of PANI and carboxyl groups in MWNT. Hence, UV–visible spectral data of PANI/ MWNT-NC(c) also provide evidence for the additional molecular interactions between MWNT and PANI in PANI/MWNT-NC(c). To ascertain the origin of bands, PANI/MWNT-NC(c) was neutralized by treating with aqueous ammonia and the UV–visible spectrum of the neutralized PANI/MWNTNC(c) was recorded (Fig. 4d). The neutralized PANI/ MWNT-NC(c) showed two bands around 360 and

Composite

Concentration of aniline (mM)

Amount of PANI (%)

PANI/MWNT-NC(c)a

10 15 20 25

73.07 77.04 79.27 83.18

PANI/MWNT-NC(c)b

10 15 20 25

46.59 53.26 56.54 60.06

2.5

2.5 2

2.0 Abs

1.5 1

d

1.5

Abs

0.5

650 nm, which are characteristics of emeraldine base form of PANI [38]. The existence of the band around 470 nm in the neutralized PANI/MWNT-NC(c) clearly informed that PANI existed in the doped state even in the neutralized form. Otherwise, the carboxyl groups generated on the surface of MWNT cause doping of PANI units even in the neutralized state. 3.5. Thermogravimetric analysis Thermogravimetric analysis was used to estimate the amount of PANI grafted onto MWNT in the composites. Both PANI/MWNT-NC(c) and PANI/MWNT-NC(c) showed a two stage weight loss, one at 160 °C and another around 320 °C, corresponding to the removal of dopant and degradation of backbone units, respectively [39,40]. These thermal transitions are similar to PANI–HCl. Hence, thermogram of the composites provides evidence for the presence of PANI chains with MWNT. The amount of PANI grafted to MWNT in the composites was determined from the weight loss of the composites at 400 °C (Table 1). Thermal data of the composites informed that PANI/MWNT-NC(c) contained more amount of PANI than PANI/MWNT-NC(c). Also, the amount of PANI increases with increase in the concentration of aniline used for polymerization. The presence of more amount of PANI in PANI/MWNT-NC(c) is consistent with the observation made through FETEM (Fig. 3b).

0

1.0

4. Conclusions

c 0.5

300 400 500 600 700 800 Wavelength (nm)

a b

0 300

Wt. of MWNT–FDA = 5 mg; Medium [CTAB]: 0.5 M; [HCl]: 1 M. a c-irradiation dose: 3 kGy. b [APS] = 50 mM.

400

500 600 Wavelength (nm)

700

800

Fig. 4. UV–vis spectra of (a) PANI-HCl, (b) PANI/MWNT-NC(c) ([APS] = 50 mM), (c) PANI/MWNT-NC(c) (c-irradiation dose: 3 kGy), (d) neutralized PANI/MWNT-NC(c); [Aniline] = 10 mM; wt. of MWNT–FDA = 5 mg; Medium [CTAB]: 0.5 M; [HCl]: 1 M.

Composites of PANI and MWNT were prepared through chemical oxidative and c-radiation induced polymerization methods. The composites are found to have different molecular level interactions between the components. The carboxyl groups that are formed on the sidewalls of the MWNTs in PANI/MWNT-NC(c) induce doping in PANI. The differences in molecular level interactions between MWNT and PANI in the composites prepared by chemical or c-radiation polymerization impart modifications in the electronic and thermal properties of the composites.

A.I. Gopalan et al. / Composites Science and Technology 67 (2007) 900–905

Acknowledgements This work was supported by the Nuclear R&D program, Ministry of Science and Technology, Korea. The authors acknowledge the help of Centre for high volt transmission microscope, Korea Basic Science Institute, Daejon, Korea for recording the FETEM images. References [1] Rao CNR, Satishkumar BC, Govindaraj A, Nath M. Nanotubes. Chem Phys Chem 2001;2:78–105. [2] Baughman RH, Zakhidov A, de Heer WA. Carbon nanotubes-the route toward applications. Science 2002;297:787–92. [3] Fan S, Chapline MG, Franklin NR, Tombler TW, Cassell AM, Dai H. Self-oriented regular arrays of carbon nanotubes and their field emission properties. Science 1999;283:512–4. [4] Frank S, Poncharal P, Wang ZL, de Heer WA. Carbon nanotube quantum resistors. Science 1998;280:1744–6. [5] Tans SJ, Verschueren ARM, Dekker C. Room-temperature transistor based on a single carbon nanotube. Nature 1998;393:49–52. [6] Dai H, Hafner JH, Rinzler AG, Colbert DT, Smalley RE. Nanotubes as nanoprobes in scanning probe microscopy. Nature 1996;384: 147–50. [7] Modi A, Koratkar N, Lass E, Wei B, Ajayan PM. Miniaturized gas ionization sensors using carbon nanotubes. Nature 2003;424:171–4. [8] MacDiarmid AG, Chiang JC, Halpern M, Huag WS, Mu Sl, Somasiri LD, et al. ‘Polyaniline’-interconversion of metallic and insulating forms. Mol Cryst Liq Cryst 1985;121:173–80. [9] Skotheim TA, Elsenbaumer RL, Reynolds JR. In: Handbook of Conducting Polymers. New York: Marcel Dekker; 1997. [10] Neoh KG, Kang ET, Tan KL. Spectroscopic studies of protonation, oxidation and light irradiation of polyaniline solutions. Polymer 1992;33:2292–8. [11] Hamon MA, Chen J, Hu H, Chen Y, Itkis ME, Rao AM, et al. Dissolution of single-walled carbon nanotubes. Adv Mater 1999;11:834–40. [12] Riggs JE, Guo Z, Carroll DL, Sun YP. Strong luminescence of solubilized carbon nanotubes. J Am Chem Soc 2000;122:5879–80. [13] Hughes M, Chen GZ, Shaffer MS, Fray DJ, Windle AH. Electrochemical capacitance of a nanoporous composite of carbon nanotubes and polypyrrole. Chem Mater 2002;14:1610–3. [14] Sandler J, Shaffer MSP, Prasse T, Bauhofer W, Schulte K, Windle AH. Development of a dispersion process for carbon nanotubes in an epoxy matrix and the resulting electrical properties. Polymer 1999;40:5967–71. [15] Dalton AB, Collins S, Munoz E, Razal JM, Ebron VH, Ferraris JP, et al. Super-tough carbon-nanotube fibres. Nature 2003;423:703. [16] Xiao Q, Zhou X. The study of multi-walled carbon nanotube deposited with conducting polymer for supercapacitor. Electrochim Acta 2002;48:575–80. [17] Lota K, Khomenko V, Frackowiak E. Capacitance properties of poly(3,4-ethylenedioxythiophene)/carbon nanotubes composites. J Phys Chem Solids 2004;65:295–301. [18] Carrara S, Bavastrello V, Ricci D, Stura E, Nicolini C. Improved nanocomposite materials for biosensor applications investigated by electrochemical impedance spectroscopy. Sens Actuat B 2005;109:221–6. [19] Musa I, Baxendale M, Amaratunga GAJ, Eccleston W. Properties of regioregular poly(3-octylthiophene)/multi-wall carbon nanotube composites. Synth Met 1999;102:1250. [20] Coleman JN, Curran S, Dalton AB, Davey AP, Mc Carthy B, Blau W, et al. Physical doping of a conjugated polymer with carbon nanotubes. Synth Met 1999;102:1174–5. [21] Woo HS, Czerw R, Webster S, Carroll DL, Park JW, Lee JH. Organic light emitting diodes fabricated with single wall carbon

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39] [40]

905

nanotubes dispersed in a hole conducting buffer: The role of carbon nanotubes in a hole conducting polymer. Synth Met 2001;116:369–72. Li XH, Wu B, Huang JE, Zhang J, Liu ZF, Lin LH. Fabrication and characterization of well-dispersed single-walled carbon nanotube/ polyaniline composites. Carbon 2002;41:1670–3. Feng W, Bai XD, Lian YQ, Liang J, Wang XG, Yoshono K. Wellaligned polyaniline/carbon-nanotube composite films grown by in situ aniline polymerization. Carbon 2003;41:1551–7. Cochet M, Maser WK, Benito AM, Callejas MA, Martı´nez MT, Benoit JM, et al. Synthesis of a new polyaniline/nanotube composite: in situ polymerisation and charge transfer through site-selective interaction. Chem Commun 2001:1450–1. Zengin H, Zhou W, Jin J, Czerw R, Smith Jr DW, Echegoyen L, et al. Carbon nanotube doped polyaniline. Adv Mater 2002;14:1480–3. Sainz R, Benito AM, Martı´nez MT, Galindo JF, Sotres J, Baro AM, et al. Soluble self-aligned carbon nanotube/polyaniline composites. Adv Mater 2005;17:278–81. Baibarac M, Baltog I, Lefrant S, Mevellec JY, Chauvet O. Polyaniline and carbon nanotubes based composites containing whole units and fragments of nanotubes. Chem Mater 2003;15:4149–56. Clayton LM, Sikder AK, Kumar A, Cinke M, Meyyappan M, Gerasimov TG, et al. Transparent poly(methyl methacrylate)/ single-walled carbon nanotube (PMMA/SWNT) composite films with increased dielectric constants. Adv Funct Mater 2005;15: 101–6. Muisener PAR, Clayton L, Angelo JD, Harmon JP, Sikder AK, Kumar A, et al. Effects of gamma radiation on poly(methyl methacrylate)/single-wall nanotube composites. J Mater Res 2002;17:2507–13. Saito R, Dresselhaus G, Dresselhauss MS. Physical Properties of Carbon Nanotubes. London: Imperial College Press; 1998. Filho AGS, Jorio A, Samsonidze GG, Dresselhaus G, Saito R, Dresselhaus MS. Raman spectroscopy for probing chemically/physically induced phenomena in carbon nanotubes. Nanotechnology 2003;14:1130–9. Cochet M, Louarn G, Quillard S, Buisson JP, Lefrant S. Theoretical and experimental vibrational study of emeraldine in salt form Part II. J Raman Spectrosc 2000;31:1041–9. Ping ZB, Nauer GE, Neugebauer H, Theiner J, Neckel A. Protonation and electrochemical redox doping processes of polyaniline in aqueous solutions: Investigations using in situ FTIR-ATR spectroscopy and a new doping system. J Chem Soc Faraday Trans 1997;93:121–9. Trchova M, Stejskal J, Prokes J. Infrared spectroscopic study of solid-state protonation and oxidation of polyaniline. Synth Met 1999;101:840–1. Mawhinney DB, Naumenko V, Kuznetsova A, Yates Jr JT, Liu J, Smalley RE. Infrared spectral evidence for the etching of carbon nanotubes: Ozone oxidation at 298 K. J Am Chem Soc 2000;122:2383–4. Nascimento GMD, Corio P, Novickis RW, Temperini MLA, Dresselhaus MS. Synthesis and characterization of single-wallcarbon-nanotube-doped emeraldine salt and base polyaniline nanocomposites. J Polym Sci Part A 2005;43:815–22. Scherr EM, MacDiarmid AG, Manohar SK, Masters JG, Sun Y, Tang X, et al. Polyaniline: Oriented films and fibers. Synth Met 1991;41:735–8. Yue J, Wang ZH, Cromack KR, Epstein AJ, MacDiarmid AG. Effect of sulfonic acid group on polyaniline backbone. J Am Chem Soc 1991;113:2665–79. Kulkarni VG, Camphell LD, Mathew WR. Thermal stability of polyaniline. Synth Met 1989;30:321–5. Wei Y, Hsueh KF. Thermal analysis of chemically synthesized polyaniline and effects of thermal aging on conductivity. J Polym Sci Part A 1989;27:4351–63.