Significant Improvement of Mechanical Properties Observed in Highly

Feb 27, 2009 - Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer, Department of Materials Science and Engineering, Beijing...
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J. Phys. Chem. C 2009, 113, 4779–4785

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Significant Improvement of Mechanical Properties Observed in Highly Aligned Carbon-Nanotube-Reinforced Nanofibers Jianying Ji,† Gang Sui,† Yunhua Yu,† Yanxin Liu,† Yuanhua Lin,‡ Zhongjie Du,† Seungkon Ryu,§ and Xiaoping Yang*,† Key Laboratory of Beijing City on Preparation and Processing of NoVel Polymer, Department of Materials Science and Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, P. R. China, State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua UniVersity, Beijing 100084, P. R. China, and Chungnam National UniVersity, 220, Gung-dong, Yuseong-gu, Daejeon 305-764, Korea ReceiVed: March 9, 2008; ReVised Manuscript ReceiVed: January 22, 2009

Continuous macroscopic aligned polyacrylonitrile (PAN) composite nanofiber sheets embedded with highly aligned PAN-grafted multiwalled carbon nanotubes (MWCNTs) have been prepared by electrospinning followed by hot-stretching. Homogeneous and highly aligned MWCNTs in the polymer matrix were obtained by hotstretching of the electrospun fibers, which led to a significant enhancement in the mechanical performance of the resulting composite nanofiber sheets. After hot-stretching, the tensile strength and modulus of an electrospun PAN nanofiber sheet (containing 2 wt % grafted MWCNTs) increased by 320.7% and 204.5%, respectively, compared with the values for the pristine PAN terpolymer. In addition we show, for the first time, that a Raman mapping method can be successfully employed to investigate the distribution and alignment of MWCNTs in nanofiber sheets. Introduction Polymer nanocomposites exhibit excellent physical and electrical properties, making them suitable for a variety of applications in biomaterials, microelectronics, and optics.1-3 In such polymer nanocomposites, carbon nanotubes (CNTs) have been widely used as fillers because of their desirable mechanical performance, high electrical conductivity, and low mass density.4 It is generally believed that the key factors in optimizing the reinforcing effects of CNTs in polymer nanocomposites are the degree of dispersion of the CNTs, their alignment, and the interface adhesion between the CNTs and the polymer matrix.5-7 Unfortunately, because of their high aspect ratio and strong van der Waals attractions, multiwalled carbon nanotubes (MWCNTs) tend to aggregate, resulting in an inhomogeneous dispersion in polymer matrixes.8 Furthermore, the surface of MWCNTs provides poor interfacial adhesion to the matrix, which limits effective load transfer from the matrix to the nanotubes.9 A covalent grafting method has been developed in an attempt to overcome these limitations.10 Many types of polymer chains can be covalently attached to the nanotube surface through “grafting to”11-14 and “grafting from”15-17 methods. These grafted polymer chains tightly cover the outer wall of the nanotubes, which leads to improved dispersion and interface adhesion.18 Electrospinning technology has also been shown to be an effective method for improving the dispersion and alignment of CNTs in polymer matrixes.19-23 Recently, Hou et al.24,25 used a rotating drum to collect electrospun composite nanofibers containing MWCNTs in an attempt to improve the alignment of MWCNTs along the fiber axis and observed significant increases in tensile strength and modulus [∼75% and 72%, * Corresponding author. Fax: +86-10-64412084. E-mail: yangxp@ mail.buct.edu.cn. † Beijing University of Chemical Technology. ‡ Tsinghua University. § Chungnam National University.

respectively, for 5% CNTs in a polyacrylonitrile (PAN) matrix]. Ko and co-workers26 also prepared continuous CNT-filled nanofibers by electrospinning and reported that the modulus improved by 120% in nanofibers containing 4 wt % singlewalled CNTs. To orient the polymer chains in an electrospun poly(ethylene oxide) (PEO) nanofiber, Kakade et al.27 applied an external electric field during the electrospinning process and obtained greatly improved orientation of the polymer chains and macroscopically aligned PEO polymer nanofibers. Although significant advances have been achieved, optimization of the macromolecular chain orientation of a polymer and alignment of CNTs in the electrospun nanofibers still remains a considerable technological challenge. Using current methods, the spray speed of polymer solutions is far higher than the speed of the receiver, which could be a flat plate, rotating drum, or copper filament drum.2,25,28 The consequent rapid formation of electrospun composite nanofibers does not allow for the sufficient stretching of MWCNTs within the available millisecond range, which leads to the entanglement of MWCNTs in the final composite nanofibers.29 A postspinning straining treatment was proposed by Wagner and co-workers29 as a way of improving the mechanical performance of such nanofibers. Postspinning in the vicinity of the glass transition temperature has also been shown to be an effective method of improving the mechanical performance of micrometer-scale fibers. Miaudet et al.30 reported that hotstretched micrometer fibers exhibited mechanical behavior different from that of CNT fibers and developed a new hotstretching technology for the fabrication of nanocomposite fibers of CNTs with poly(vinyl alocohol) (PVA).31 In this work, we describe how electrospinning followed by hot-stretching can be used to prepare composite nanofiber sheets of a grafted MWCNT/PAN terpolymer. Our results indicate that hot-stretching can improve the alignment of the MWCNTs, the orientation of the polymer chains, and the interface adhesion

10.1021/jp8077198 CCC: $40.75  2009 American Chemical Society Published on Web 02/27/2009

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SCHEME 1: Schematic View of the Chamber Employed for Hot-Stretching of the Polymer Sheets

Figure 1. FTIR spectra of (a) acid-oxidized MWCNTs, (b) PANterpolymer-grafted MWCNTs, and (c) PAN terpolymer.

between the MWCNTs and the matrix. Additionally, we show that a Raman mapping method can be successfully employed to characterize the distribution and alignment of CNTs in the nanofiber sheets. Experimental Section Materials. MWCNTs (purity, g 95%; diameter, 15-20 nm) were supplied by Shenzhen Nanotech Port Co. Ltd., Shenzhen, China, and used as received. PAN terpolymer (93.0 wt % acrylonitrile, 5.3 wt % methyl acrylate, and 1.7 wt % itaconic acid, with an average molecular weight of 100000 g/mol) was supplied by Courtaulds Co., Nottingham, Nottinghamshire, U.K. Concentrated sulfuric acic (98%), nitric acid (70%), and N,N′dimethylformamide (DMF, bp 153 °C) were purchased from Beijing Chemicals Co., Beijing, China. N,N′-Dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) were purchased from Shanghai Medpep Co., Shanghai, China. PAN-Terpolymer-Grafted MWCNTs. PAN-terpolymergrafted MWCNTs were prepared by following our previously reported procedure.32,33 Hydroxyl groups were produced on the surface of the MWCNTs by acid oxidation of the as-received MWCNTs for 1 h with a mixture of concentrated H2SO4 and HNO3 (1:1, v/v). Then, 100 mg of acid-oxidized MWCNTs was suspended in DMF (200 mL) by sonication for 30 min, after which DCC (50 mg) and DMAP (25 mg) were added to the suspension. PAN (0.5 mg) terpolymer was dissolved in DMF (5 mL), and this solution was mixed with the suspension. This suspension was then stirred for 48 h at 60 °C under nitrogen. After vacuum filtration, the resulting black product was repeatedly washed with DMF and resuspended in DMF by sonication for 30 min. The final PAN-terpolymer-grafted MWCNTs were vaccuum-dried for 8 h at 100 °C to evaporate any remaining DMF. Electrospinning and Hot-Stretching. Suspensions were prepared by ultrasonic mixing of PAN-terpolymer-grafted MWCNTs (0, 0.5, 1, 2, and 3 wt %) into a PAN terpolymer/ DMF solution for 4 h. The suspension was electrospun at room temperature (23 °C), with an applied voltage of 14 kV and a distance of 140 mm between the spinneret and the fiber collector. The electrospun composite nanofibers were collected in the form of a sheet on a 16-cm-diameter rotating drum at a rotation speed of 10.0 m/s for 3 h. The sheets (100 mm in length, 40 mm in width) were hot-stretched along the nanofiber direction by

applying a tensile force of 1.25 N in air at 140 °C for 5.0 min. Scheme 1 shows a view of the chamber used for hot-stretching of the sheets. The temperature of the chamber was controlled by the circulation of hot air, and the tensile force was controlled by the use of loads of appropriate weights. Characterization. Fourier transform infrared (FTIR) spectrometry, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) were used to confirm the functionalization of the MWCNTs. Scanning electron microscopy (SEM, Hitachi S4700) was employed to observe the morphologies of composite nanofibers, and Image J software was used to measure the diameter of the nanofibers before and after hot-stretching. Polarized Raman spectroscopy was performed to determine the alignment of the MWCNTs in the nanofiber sheets using a TY-HR 800 Raman spectrometer with laser excitation at 532 nm. Tensile tests of the nanofiber sheets were conducted along the collected nanofiber direction using a universal testing machine (Instron 1121). Ten specimens with lengths of 20 mm and widths of 5 mm were prepared for each tensile test. The speed of the crosshead was 20 mm/min. The cross-sectional area of the sample was calculated using the weight of the sample and the densities of PAN terpolymer and MWCNTs. Results and Discussion PAN-Terpolymer-Grafted MWCNTs. After the as-received MWCNTs had been refluxed for 1 h with a mixture of concentrated H2SO4 and HNO3 (1:1, v/v), hydroxyl functional groups were produced on the surface of the MWCNTs, as confirmed by X-ray photoelectron spectroscopy and FTIR spectroscopy; the details were reported in our previous work.32,33 In the presence of DCC and DMAP, the hydroxyl groups present on the acid-oxidized MWCNTs react with the carboxylic acid groups in the PAN terpolymer to afford covalently bound polymer chains on the surface of the MWCNTs. The procedure is shown in Scheme 2. The resulting PAN-terpolymer-grafted MWCNTs were characterized by FTIR spectroscopy, TEM, and TGA. Figure 1 shows the FTIR spectra of (a) acid-oxidized MWCNTs, (b) PAN-terpolymer-grafted MWCNTs, and (c) PAN terpolymer. A broad absorption peak in Figure 1a at around 3414 cm-1 can be assigned to the OsH stretching vibration of the surface hydroxyl groups of the acid-oxidized MWCNTs. After reaction of the PAN terpolymer with the MWCNTs, the FTIR spectrum of the resulting material (Figure 1b) was very

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Figure 2. (a) TEM image of PAN-terpolymer-grafted MWCNTs and (b) TGA curves of various samples.

SCHEME 2: Procedure for the Synthesis of PAN-Terpolymer-Grafted MWCNTs

similar to that of the PAN terpolymer itself (Figure 1c). The sharp absorption peak at 2242 cm-1 can be assigned to the CtN stretching vibration of nitrile groups, the peak at 1732 cm-1 to the CdO stretching vibration, and the peak at 1452 cm-1 to a CsH bending vibration.34 The similarity of the spectra in Figure 1b,c strongly indicates the presence of PAN terpolymer in the grafted MWCNT. The obvious reduction in intensity of the OsH stretching vibration in Figure 1b compared to that in Figure 1a also suggests that the surface hydroxyl groups of the MWCNTs have undergone a grafting reaction with the PAN. Figure 2a shows TEM images of PAN-terpolymer-grafted MWCNTs. The surface of the grafted MWCNTs has a smeared appearance, and some amorphous materials can be seen to cover the surface of the MWCNTs, as indicated by the arrows. Figure 2b shows TGA traces for the various materials. The as-received MWCNTs (trace a) showed no significant weight loss up to 750 °C, whereas the neat PAN terpolymer (trace c) lost about 59% of its original weight below 640 °C. The TGA trace of the PAN-terpolymer-grafted MWCNTs (trace b) is intermediate between those of the MWCNTs and neat polymer. The weight loss from 180 to 640 °C is about 39 wt %, associated with the decomposition of the grafted polymer on the surface of the MWCNTs. The plateau in the TGA trace after 650 °C can be attributed to the MWCNTs and residues of the carbonized polymer that undergo no further decomposition. Based on these TGA traces, the amount of PAN terpolymer grafted on the MWCNTs can be estimated,35 and the weight fraction of the polymer in the grafted MWCNTs was calculated to be about 66%.

Morphology of PAN/MWCNT Composite Nanofibers. As shown in Figure 3a,b, the as-spun nanofibers were partially aligned along the collecting direction, and the alignment of the fibers became closer to parallel after hot-stretching. Also, the diameters of the original as-spun fibers (100-220 nm) were significantly reduced, to 60-190 nm, by hot-stretching. As seen from the inset in Figure 3b, part of a carbon nanotube was exposed at one end of some nanofibers. This nanotube had a larger diameter (about 50 nm) than the original MWCNTs (15-20 nm), and its surface was rougher than that of the original CNTs; this indicates that some polymer becomes strongly attached to the MWCNTs and that there is a strong interfacial adhesion between the nanotubes and the PAN terpolymer matrix in the composite nanofiber. This strong adhesion originates from the PAN terpolymer molecule chains grafted on the surface of the MWCNTs, which act as a link between the nanotubes and the bulk PAN matrix. Alignment of MWCNTs in the Composite Nanofiber Sheets. Raman spectroscopy was used to analyze the changes in alignment of the composite nanofiber after hot-stretching. Figure 4 shows polarized Raman spectra of the 2 wt % grafted MWCNT/PAN composite nanofiber sheet recorded (a) before and (b) after hot-stretching, in which the VV curve represents the spectrum when the polarization of the incident laser is parallel to the nanofiber axis and the VH curve represents the spectrum when the polarization of the incident laser is normal to the nanofiber axis. Before hot-stretching (Figure 4a), strong absorption peaks appeared at 1349.76 and 1580.75 cm-1, which can be assigned

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Figure 3. SEM images of 2 wt % grafted MWCNT/PAN terpolymer composite nanofiber sheets: (a) low-magnification and (b) high-magnification SEM images of the original electrospun nanofiber; (c) low-magnification and (d) high-magnification SEM images after hot-stretching. The inset in b is a high-magnification SEM image from a fractured surface of an electrospun nanofiber.

Figure 4. Polarized Raman spectra of 2 wt % grafted MWCNT/PAN composite nanofiber sheets (a) before and (b) after hot-stretching.

to the D and G bands, respectively, of the MWCNTs. The G band is attributed to an in-plane E2g zone-center mode, and the D band is ascribed to disorder-induced modes in the MWCNTs. The shoulder peak at 1620.79 cm-1 is usually denoted as the D′ band and has also been attributed to disorder-induced features in the MWCNTs; a similar band was also observed in polyureafunctionalized MWCNTs.36 Figure 4b shows the polarized Raman spectra of the hot-stretched composite nanofiber sheets. Both the G and D bands shift to higher wavenumbers after hotstretching, especially the G band (shift of ∼3.08 cm-1). This blue shift indicates that there is an interfacial interaction between

the MWCNTs and the surrounding polymer (probably via the COOH groups in the PAN polymer). Similar behavior was observed for stretched single-walled carbon nanotube/polyisobutylene composites when compared to the unstretched composites.37 There is an obvious difference in the intensity of the G band for different polarization directions after hotstretching, which can be correlated to the alignment of CNTs in the composites. Normally, the degree of CNT alignment can be evaluated by the depolarization factor R, the ratio of the peak intensities of the G band in the two polarization directions, i.e., parallel (VV configuration) and perpendicular (VH configura-

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Figure 5. Polarized Raman mapping for a nanofiber sheet of a 2 wt % grafted MWCNT/PAN terpolymer composite: (a) sketch of polarized Raman mapping scanning points; (b) polarized Raman mapping spectra of areas. 11-15 shown in a; (c) polarized Raman mapping spectra of areas 3, 8, 13, 18, and 23 shown in a; (d) R values for the data in b and c.

tion), to the fiber axis.38,39 Based on the data shown in Figure 4, the R values were 1.03 and 2.95 for samples before and after hot-stretching, respectively. The marked increase in R can be ascribed to the better alignment of CNTs after hot-stretching. Raman mapping is known to be an effective technique for providing detailed information regarding the distribution of a specific compound in a composite material.40 For example, Nepal et al.41 used this method to evaluate the spatial distribution of SWCNTs in biopolymer layers. To obtain more detailed information regarding the distribution and alignment of the MWCNTs in the composite nanofiber sheets, we employed a polarized Raman mapping technique; this is the first such application of this technique. The whole mapping area was as large as 10 µm × 10 µm, with increments of at least 2 µm considering the beam light-spot diameter (∼2 µm). The beam scanning time was chosen as 100 s for the VV configuration and 200 s for the VH configuration. As shown in Figure 5a, the polarized spectra of areas 11-15 correspond to scanning points along the nanofiber direction, whereas the spectra of areas 3, 8, 13, 18, and 23 were taken along the perpendicular to the nanofiber direction. Figure 5b,c shows typical polarized Raman

spectra along the nanofiber direction and the perpendicular direction, respectively. It can be seen that the intensities of the G band in the VV configuration change very slightly and, in both configurations, the R values are also almost constant (Figure 5d). We also measured other scanning points along these two directions as marked in Figure 5a. The results indicated that the distribution of MWCNTs in the sheet is homogeneous and the degree of alignment of the MWCNTs in the sheet is high. Mechanical Properties of Composite Nanofiber Sheets. Tensile testing of the nanofiber sheets was conducted along the collected nanofiber direction. Figure 6 shows stress-strain curves of composite nanofiber sheets with various loadings of MWCNTs. The average tensile strength and modulus of the pure PAN terpolymer nanofiber sheet were found to be about 71.9 MPa and 2.2 GPa, respectively. When 2.0 wt % grafted MWCNTs was incorporated into the nanofiber sheet, the tensile strength and modulus increased to about 114.8 MPa and 3.2 GPa (see Figures 7 and 8), respectively, which indicates that the presence of the grafted MWCNTs led to an obvious reinforcement of the PAN nanofiber sheets. This behavior can be ascribed to the homogeneous dispersion of the grafted

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Figure 6. Typical stress-strain curves for (a) electrospun pure PAN terpolymer nanofiber sheets; (b) electrospun PAN terpolymer nanofiber sheets with 2 wt % loading of grafted MWCNTs; (c) hot-stretched pure PAN terpolymer nanofiber sheets; (d-f) hot-stretched PAN terpolymer nanofiber sheets with 1, 2, and 3 wt % loadings of grafted MWCNTs, respectively.

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Figure 8. Experimental and calculated values of tensile strength for various grafted MWCNT/PAN terpolymer nanofiber sheet samples: calculated results for (a) hot-stretched and (b) original electrospun samples; experimental data for (c) hot-stretched and (d) original electrospun samples.

Normally, for a unidirectional discontinuous nanofiberreinforced composite system, the modified Halpin-Tsai equation7,29,42 can be used to estimate the composite longitudinal modulus, EL

EL )

Figure 7. Experimental and calculated values of the modulus for various grafted MWCNT/PAN terpolymer nanofiber sheet samples: calculated results for (a) hot-stretched and (b) original electrospun samples; experimental data for (c) hot-stretched and (d) original electrospun samples.

MWCNTs and the strong interfacial interaction between the MWCNTs and the PAN terpolymer in the composite nanofibers. After hot-stretching, the tensile strength and modulus of the pure PAN terpolymer fiber sheet were about 215.9 MPa and 3.6 GPa, respectively. These values represent significant increases (of ∼200.3% and 63.6%, respectively) as compared to the pristine nonstretched sample, which can mainly be attributed to the unidirectional alignment and tight stacking of composite nanofibers in the sheets and the high degree of orientation of macromolecular chains in the nanofibers after hot-stretching. The hot-stretched composite nanofiber sheets with 2 wt % grafted MWCNTs also exhibited marked increases in tensile strength of ∼163.5% (from 114.8 to 302.5 MPa) and in modulus of ∼109.3% (from 3.2 to 6.7 GPa), which can be ascribed to the improved alignment of MWCNTs within the sheet and the enhanced interfacial interaction between the nanotubes and the polymer matrix. Similar behavior was also observed for the tensile strength and modulus of composite nanofiber sheets with varying loadings of grafted MWCNTs (Figures 7 and 8).

1 + 2(lf /df)ηLVf Em 1 - ηLVf

(1)

In eq 1, ηL ) [(Ef/Em) - 1]/[(Ef/Em) + 2(lf/df)], where Em and Ef are the tensile modulus values of the matrix and nanotube, respectively, and lf/df is the aspect ratio of the nanotube. The volume fraction, Vf, can be estimated from the weight fractions and densities of MWCNTs and PAN, respectively. Here, we take the density of MWCNTs as 1.8 g · cm-3, the density of PAN as 1.12 g · cm-3, the aspect ratio of the nanotubes as 55, and the modulus (Ef) of the MWCNTs as 450 GPa.29 Thus, we can calculate the composite modulus by eq 1, and the values are shown in Figure 7a,b. As previously reported, a model based on a composite with short aligned fibers can be used to predict the tensile strength of a PAN fiber reinforced with nanotubes29,42

σcomposite ) (1 - σfr/2lfτ)σfVf + σm(1 - Vf)

(2)

where r is the radius of the nanotube, τ is the tube-matrix interfacial shear strength, and rm is the strength of the matrix. Here, we use the values σf ) 30 GPa, σm ) 30 MPa, r ) 8.5 nm, lf ) 1000 nm, and τ ) 150 MPa, as previously reported.43 The values of the tensile strength calculated for various nanofiber sheets are illustrated in Figure 8a,b. As shown in Figures 7 and 8, the experimental results are in good agreement with the calculated data for the grafted MWCNT/PAN composites when the loading of grafted MWCNTs is below 2 wt %. However, when the loading of MWCNTs increased to 3 wt %, the experimental values of the tensile strength and modulus deviated from the calculated results; this can mainly be ascribed to the poor dispersion of highly concentrated MWCNTs in the nanofibers, caused by the high viscosity of the electrospinning solution. As shown in Figures 7 and 8, all of the hot-stretched samples exhibited superior mechanical properties, which indicates that the alignment of the carbon nanotubes is improved greatly during hot-stretching and demonstrates that a combination of

Highly Aligned Carbon-Nanotube-Reinforced Nanofibers electrospinning and hot-stretching is a successful way to obtain high-performance composite nanofiber sheets. Conclusions Hot-stretching is a facile and effective method for enhancing the mechanical properties of grafted MWCNT/PAN terpolymer composite nanofiber sheets. Macrostructure analysis shows that, after hot-stretching, electrospun nanofibers exhibit excellent macroscopic alignment. Polarized Raman mapping spectroscopy is a very useful technique for characterizing the improved alignment and distribution of MWCNTs in the PAN terpolymer matrix. Compared with the properties of the pristine PAN terpolymer, the tensile strength and modulus of hot-stretched grafted MWCNT/PAN terpolymer composite nanofiber sheets increased by as much as 320.7% and 204.5%, respectively, with a 2 wt % loading of MWCNTs in the PAN matrix. Acknowledgment. The authors acknowledge financial support from National Natural Science Foundation of China (Nos. 50503004 and 30471907), the European Aeronautic Defence and Space Company EADS (France2007-DCR/SP-001189-1), the National High Technology Research and Development Program of China (2007AA03Z351), and the Program of New Century Excellent Talents (NCET) of Universities in China. References and Notes (1) Ji, Y.; Li, B.; Ge, S.; Sokolov, J. C.; Rafailovich, M. H. Langmuir 2006, 22, 1321. (2) Greiner, A.; Wenforff, J. H. Angew. Chem., Int. Ed. 2007, 46, 2. (3) Li, D.; Xia, Y. AdV. Mater. 2004, 16, 1151. (4) Baughman, R. H.; Zakhidov, A. A.; Heer, W. A. Science 2002, 297, 787. (5) Coleman, J. N.; Khan, U.; Gun’ko, Y. K. AdV. Mater. 2006, 18, 689. (6) Moniruzzaman, M.; Winey, K. I. Macromolecules 2006, 39, 5194. (7) Liu, L.; Barber, A. H.; Nuriel, S.; Wagner, H. D. AdV. Funct. Mater. 2005, 15, 975. (8) Ramesh, S.; Shan, H.; Haroz, E.; Billups, W. E.; Hauge, R.; Adams, W. W.; Smalley, R. E. J. Phys. Chem. C 2007, 111, 17827. (9) Ye, H.; Lam, H.; Titchenal, N.; Gogotsi, Y.; Ko, F. Appl. Phys. Lett. 2004, 85, 1775. (10) Lin, Y.; Zhou, B.; Shiral, K.; Liu, A. P.; Allard, L. F.; Sun, Y. P. Macromolecules 2003, 36, 7199. (11) Lin, Y.; Hill, D. E.; Bentley, J.; Allard, L. F.; Sun, Y. P. J. Phys. Chem. B 2003, 107, 10453. (12) Qu, L.; Lin, Y.; Hill, D. E.; Zhou, B.; Wang, W.; Sun, X.; Kitaygorodskiy, A.; Suarez, M.; Connell, J. W.; Allard, L. F.; Sun, Y. P. Macromolecules 2004, 37, 6055. (13) Baskaran, D.; Dunlap, J. R.; Mays, J. W.; Bratcher, M. S. Macromol. Rapid Commun. 2005, 26, 481.

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