One-Step in Situ Synthesis of Poly(methyl methacrylate)-Grafted

May 11, 2009 - To whom correspondence should be addressed. M.L. (at Qingdao University of Science and Technology): phone +86-532-8402-2814, fax +86-53...
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One-Step in Situ Synthesis of Poly(methyl methacrylate)-Grafted Single-Walled Carbon Nanotube Composites Manhong Liu,*,†,‡ Tao Zhu,‡ Zichen Li,‡ and Zhongfan Liu*,‡ College of Material Science and Engineering, Qingdao UniVersity of Science and Technology, Qingdao 266042, and College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, People’s Republic of China ReceiVed: March 12, 2009; ReVised Manuscript ReceiVed: April 23, 2009

An efficient synthesis of poly(methyl methacrylate)-grafted SWCNT composites (PMMA-SWCNTs) were developed by using in situ atom transfer radical polymerization of methyl methacrylate onto SWCNTs sonicated in o-dichlorobenzene (ODCB) in one step. Morphology studies with HRTEM and AFM and solubility changes of the PMMA-SWCNTs in CHCl3 and DMF suggest the existence of PMMA in the nanocomposites. The sonochemical reaction of ODCB and SWCNTs is critical for the formation of PMMA-SWCNTs. Systematic characterization shows that PMMA was covalently attached to the SWCNTs in PMMA-SWCNTs. Introduction Carbon nanotube-polymer composites have great potential for application in many fields, such as electronic devices and mechanical enhancements. Single-walled carbon nanotubes (SWCNTs) are attractive candidates for preparing polymeric composites due to their extremely high Young’s modulus, stiffness, flexibility, and large aspect ratios.1,2 However, SWCNTs have rarely been used successfully as electrical or mechanical inclusions in a polymer matrix, because the insolubility in solvents makes them difficult to achieve efficient dispersion. In addition, intrinsic van der Waals attraction among carbon nanotubes (CNTs), in combination with their high surface area and high aspect ratio, often leads to significant agglomeration, thus preventing efficient transfer of their superior properties to the matrix. Moreover, the nonreactive surface of CNTs results in inherently weak CNT-polymer interactions and poor interfacial adhesion, which can also lead to CNT aggregation within the matrix. Therefore, it is essential to improve the solubility of CNTs and the CNT-polymer interactions. Chemical functionalization of CNTs enables the change of their solution properties for ease of dispersion and subsequent understanding of the chemistry of these nanomaterials. Covalent chemical modifications have been extensively studied and have afforded means for attaching various chemicals to CNT surfaces, such as sidewall halogenation, hydrogenation, cycloadditions, radical additions, electrophilic additions, addition of inorganic compounds, ozonolysis, mechanochemical functionalizations, plasma activation, nucleophilic additions, and grafting of polymers.3 A potential issue of covalent modification is that the treatment of CNTs under extremely harsh conditions may cause damages in CNT framework and properties. Therefore, new methods for covalent CNT modification under mild conditions to preserve the valuable properties of CNTs are desirable. For instance, although the CNTs are known to be electron-deficient substances, there are reports using electrophilic * To whom correspondence should be addressed. M.L. (at Qingdao University of Science and Technology): phone +86-532-8402-2814, fax +86-532-8402-2814, e-mail [email protected]. Z.L.: phone +8610-6275-7157, fax +86-10-6275-7157, e-mail [email protected]. † Qingdao University of Science and Technology. ‡ Peking University.

substitution reactions to achieve less destructive functionalization of CNTs.4-6 After further derivatization through hydrolysis and acylation, the resulting ester-functionalized CNTs behave as organic macromolecules. Polymers can be covalently attached to CNTs by using the “grafting to” or “grafting from” approaches.3 In the “grafting to” approach, polymer is first prepared and then reacted with the functional groups of CNTs. The limitation of this technique is that attachment of a small number of chains hinders diffusion of additional macromolecules to the surface, thereby leading to low grafting density. The “grafting from” approach improves this problem to some extent, in which the nanotube surface is first covalently attached with initiators and the resulting nanotube-based macroinitiators are then exposed to monomers. The attachment of atom transfer radical polymerization (ATRP) initiators to CNTs has been used to graft different polymers to CNTs successfully.7-10 However, these ATRP methods all require several steps to produce the nanotube-based macroinitiators. A one-step attachment of polystyrene to SWCNTs was achieved by using living anionic polymerization, but it requires stringent no-water and no-oxygen reaction conditions.11 Here we report a simple and efficient method to prepare CNT-polymer composites in mild conditions and in one step. Polymer-grafted SWCNT composites were obtained by in situ ATRP of monomers in SWCNTs dispersed in o-dichlorobenzene (ODCB) by ultrasonication. This method should be suitable for a wide range of monomers as well as chemically functionalized CNTs, and has potential for the mass production of CNT-polymer composites with strong interfacial adhesion. Ultrasonication has been found useful in the dispersion of pristine CNTs in organic solvents and chemical functionalization of CNTs.12-15 Sonication could separate large bundles of CNTs into small bundles or single tubes, so that the CNTs can better contact and react with the reagents. The fact that sonication can cause structural damage has been known16,17 for some time. Some solvents (e.g., ODCB and chlorobenzene) are found to interact with CNTs irreversibly through sonochemical reactions.14,15 Haddon et al.15 found that the sonochemical decomposition and polymerization of ODCB is necessary for the stabilization of SWCNTs in ODCB. The decomposition products of sonicated ODCB interact irreversibly with SWCNTs, and the sonication

10.1021/jp902234v CCC: $40.75  2009 American Chemical Society Published on Web 05/11/2009

Poly(methyl methacrylate)-Grafted SWCNT Composites SCHEME 1

can irreversibly disrupt the electronic and molecular structure of SWCNTs. Previously we also showed that peroxy organic acids are covalently attached to SWCNTs under ultrasonication.18,19 We therefore reasoned that sonication of SWCNTs in halogen-containing organic solvents could generate ATRP initiators onto CNTs for in situ polymerization of monomers to form CNT-polymer composites. To test this possibility, SWCNTs grown by chemical vapor deposition were purified and annealed.18,20 The purified SWCNTs (p-SWCNTs) were first sonicated in ODCB at room temperature. Methyl methacrylate (MMA) was then added, and polymerization was initiated by the SWCNTs at 60 °C under ATRP conditions. After washing, the poly-MMA-SWCNT composites (PMMA-SWCNTs) were obtained. The general strategy for synthesizing PMMASWCNTs via in situ ATRP of MMA onto SWCNTs sonicated in o-ODCB in one step is described in Scheme 1. Samples before and after polymerization were characterized with various microscopic and spectroscopic analysis. Experimental Section Materials. CuBr and 1,1,4,7,7-pentamethyldiethylenetriamine (PMDETA) were purchased from Acros and used as received. Methyl methacrylate (MMA) (A.R., Beijing Chemicals Co.) was distilled under reduced pressure over CaH2. Other organic reagents or solvents were purchased from Beijing Chemicals Co. and used as received. Preparation and Purification of SWCNTs. SWCNTs were produced by using the chemical vapor deposition (CVD) method, as described in details elsewhere.18,20 With the CVD method, impurities in the as-prepared SWCNTs were mainly amorphous carbon and catalyst particles. The catalyst (support and metal particles) was removed by procedures as follows: sonicating the as-prepared SWCNTs in 37 wt % hydrochloric acid for 30 min, leaving them in acid overnight, and then diluting with deionized water followed by filtration through a 0.1 µm membrane filter (Gelman). The obtained SWCNTs soot was subsequently heated in air for 1 h at 500 °C to oxidize the amorphous carbon. Finally, purified SWCNTs (denoted as p-SWCNTs) were obtained by annealing in N2 at 1000 °C for 30 min to remove the oxygen-based functional groups generated from the thermal treatment. Preparation of PMMA-SWCNTs. (a) Preparation of PMMA-SWCNTs by ATRP. Four milligrams of p-SWCNTs was sonicated in 5 mL of o-dichlorobenzene (ODCB) in a dry glass tube at room temperature for 2 h. After sonication, 10 mg of CuBr, 15 µL of PMDETA, and 3 mL of MMA were added. After the mixture was degassed three times, the tube was sealed under vacuum and then kept in a thermostated water bath at 60 °C for 24 h. It was observed that the viscosity of the reaction mixture increased dramatically at the end of the reaction. The well-dispersed black-brown mixture was released from the tube and subsequently diluted with CHCl3, filtered through a 0.1 µm membrane filter (Gelman), and washed with CHCl3. The residue was resuspended, sonicated, and then filtered. Any physically adsorbed species were removed after washing the residue with CHCl3 several times. CH3OH was added to the filtrate of the last time, and no precipitate was found indicating that free

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9671 PMMA formed when submitting MMA to ATRP was removed after the complete extraction. Finally the sample was dried in vacuum and referred to as “PMMA-SWCNTs”. (b) Peparation of PMMA-SWCNTs by Free Radical Polymerization Initiated by 2,2-Azobisisobutyronitrile (AIBN). Four milligrams of p-SWCNTs was sonicated in 5 mL of ODCB in a dry glass tube at room temperature for 2 h. After sonication, 0.5 wt % AIBN of the amount of monomer and 3 mL of MMA were added to the tube. The polymerization condition and the following treatments were the same as those in the preparation of PMMA-SWCNTs by ATRP. (c) Mixing PMMA-Br with SWCNTs. PMMA-Br was prepared by ATRP of MMA in chlorobenzene with ethyl 2-bromoisobytarate (EBB) as the initiator and CuBr/PMDETA as the catalyst. The feed molar ratio was [MMA]/[EBB]/[CuBr]/ [PMDETA] ) 100/1/1/1, [MMA] ) 2 M in chlorobenzene. The polymerization was conducted at 60 °C for 6 h; PMMA-Br (MnGPC ) 6500, Mw/Mn ) 1.28) was obtained in 60% yield. Four milligrams of p-SWCNTs was sonicated in 5 mL of ODCB in a dry glass tube at room temperature for 2 h. After sonication, 10 mg of CuBr, 15 µL of PMDETA, and 0.2 g of PMMA-Br prepared with ATRP were added. After the mixture was degassed three times, the tube was sealed under vacuum and then kept in a thermostated water bath at 60 °C for 24 h. The following treatment was the same as those in the preparation of PMMA-SWCNTs by ATRP. To determine the effect of sonication, similar experiment was performed without sonication. Instruments and Measurements. High-resolution transmission electron microscopy (HRTEM) analysis was performed with a TECNAI-F30 (Philips) TEM under 300 kV. Raman characterization was conducted on a Renishaw System 1000 Raman imaging system (Renishaw, U.K.) equipped with a 632.8 nm, 25 mW He-Ne laser. The spectra were obtained by collecting at least 10 different points in each sample. UV/vis/ NIR absorption spectra were recorded in double-beam mode with a UV/vis/NIR scanning spectrophotometer (Shimadzu, UV3101 PC). The IR spectra were obtained by using an AVATAR360 Fourier transform infrared (FTIR, Nicolet) spectrometer. AFM characterization was carried out on a Nanoscope III (Digital Instrument, USA) in tapping mode. The SWCNT samples for AFM were dispersed in a proper solvent and spin coated on a Si substrate. TGA was performed at a heating rate of 10 deg/min. XPS spectra were obtained with an Axis Ultra spectrometer (Kratos, UK). A mono Al KR (1486.6 eV) X-ray source was used at a power of 225 W (15 kV, 15 mA). To compensate for surface charge effects, binding energies were calibrated by using C1s hydrocarbon peak at 284.8 eV. Results and Discussion SWCNTs grown by the CVD method20 were purified with hydrochloric acid to remove catalyst, followed by oxidizing the amorphous carbon at 500 °C in air, and finally annealed in N2 at 1000 °C. The purified SWCNTs (p-SWCNTs) were first sonicated in ODCB, and then the reaction was implemented by initiating MMA to polymerize under ATRP conditions described in the Experimental Section. After washing, the composites (PMMA-SWCNTs) were dried and characterized with various methods to validate the functionalization of SWCNTs. Various microscopic and spectroscopic techniques were used to characterize the PMMA-SWCNT composites prepared under sonication in ODCB. HRTEM images showed that PMMASWCNTs existed as bundles or as individual tubes (Figure 1a), and enwrapping substances on the wall of the SWCNTs were

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Figure 1. (a) HRTEM image of PMMA-SWCNTs; (b) AFM image of p-SWCNTs; (c) AFM image of PMMA-SWCNTs; (d) cross-section along trace 1-1 shown in panel b and 2-2 shown in panel c.

observed, which are likely to be PMMA. With longer time of electron beam irradiation during the TEM observation, the nanotube disappeared in the p-SWCNT sample.21 By contrast, in the PMMA-SWCNTs-1 sample, surrounding polymers were left untouched after the disappearance of the nanotube. Atomic force microscopy (AFM) showed that the height of PMMASWCNT composites is larger than that of p-SWCNTs (Figure 1b-d), indicating that the attachment of PMMA increases the thickness of tube walls. The height of the PMMA-SWCNTs was found to differ along the tube, suggesting that MMA polymerizes on the SWCNTs to various extents at different sites. Moreover, the PMMA-SWCNTs showed a relatively good solubility in CHCl3 (46 g/m3) and a poor solubility in DMF, whereas p-SWCNTs can hardly be dispersed in CHCl3. These changes in morphology and solubility suggest the existence of PMMA in the composites. Figure 2 presents the IR spectral changes of the SWCNT sample caused by MMA polymerization. The IR spectrum of PMMA-SWCNTs exhibited the characteristic stretching band of the CdO group of PMMA at ∼1735 cm-1 and the typical C-H stretching absorptions of alkyl substituents at ∼2978, ∼2918, and ∼2847 cm-1. Moreover, a new peak at 1635 cm-1 emerged, which did not exist in the spectrum of p-SWCNTs or PMMA, and was thus attributed to the covalent interaction between the nanotube and the polymer in the PMMA-SWCNT composites.22,23 The IR of PMMA-SWCNTs annealed in N2 was similar to that of p-SWCNTs, indicating that PMMA in the composites decomposed completely at high temperature and the annealed SWCNTs appeared pristine.24,25 Microscopic techniques of HRTEM and AFM and IR spectral characterization combined with solubility changes indicated that PMMA existed in PMMA-SWCNTs, which may be noncovalent blends or covalent grafted to SWCNTs, or the coexistence of both possiblities.

Figure 2. IR spectra of samples: (a) p-SWCNTs, (b) PMMA-SWCNTs, (c) PMMA-Br, and (d) N2-annealed PMMA-SWCNTs.

The Raman spectrum offers useful information concerning the structural changes of SWCNTs, especially changes due to the significant sidewall modification.24-26 As seen in Figure 3, there were four bands for the p-SWCNTs located at 151.6, 194.8, 216.0, and 254.4 cm-1, respectively, which are the typical modes of SWCNTs termed the radial breathing modes (RBM). The RBM is directly related to the diameter of the tubes by the following relation,27,28 ω ) 223.5/d + 12.5, where d is the tube diameter in nanometers and ω the radial breathing mode in wavenumbers. Thus, the diameters of the tubes are 1.6, 1.2, 1.1, and 0.9 nm, respectively. Krupke29 reported that all metallic SWCNTs have RBM frequencies in the range between 218 and 280 cm-1, and the semiconducting SWCNTs in the range between 175 and 213 cm-1. Therefore, most of the tubes made by our method belong to the semiconducting SWCNTs. As seen in Figure 3, the D line at ∼1326 cm-1 is diagnostic of disruptions in the hexagonal framework of the SWCNTs, and the G line at ∼1588 cm-1 corresponds to the characteristic

Poly(methyl methacrylate)-Grafted SWCNT Composites

Figure 3. Raman spectra of samples: (a) p-SWCNTs, (b) PMMASWCNTs, and (c) N2-annealed PMMA-SWCNTs.

A1g, E1g, and E2g modes of graphene sheet. The Raman spectrum in Figure 3a indicates that the p-SWCNTs have long-range onedimensional (1-D) order with few defect sites along the sidewall.30 Figure 3b shows the Raman spectrum of the SWCNTs after MMA polymerization. It is interesting to notice that the intensity of the D-line is substantially increased after functionalization. The intensity ratio of the G line to the D line (IG/ID) decreased from 15.5 for p-SWCNTs to 9.2 for PMMASWCNTs, indicating that the number of sp3-hybridized carbon “defect” sites on the SWCNT walls was increased after MMA polymerization in PMMA-SWCNTs. In addition, the intensities of four Raman bands in the radial breathing modes region (170-270 cm-1) which correspond to the distribution of SWCNT diameters decreased to various extents after MMA polymerization and, in particular, the band at 194.8 cm-1 disappeared, indicating that SWCNTs with various tube diameters were covalent functionalized after polymerization with varied reacting degrees. It was reported30 that the change of the radial breathing modes indicated covalent functionalization in the sidewall of SWCNTs, which disrupted the electronic structure in tubular SWCNTs. Consistent with the IR results, the Raman spectrum of N2-annealed PMMASWCNTs was also restored to approximately that of p-SWCNTs (Figure 3c) after annealing at 1000 °C in N2, indicating that heating functionalized nanotubes in an inert atmosphere removes the organic moieties and restores the pristine nanotube structure, which is also found by other researchers.24,25 Raman spectral changes in the radial breathing modes region and little decrease of IG/ID indicated that functionalized SWCNTs after polymerization may have little damage on the CNT framework. Further evidence for functionalization of SWCNTs came from the UV/vis/NIR spectra (Figure 4). The location of the van Hove peaks of SWCNTs is relative to the diameter of the nanotubes.31,32 The observed peaks are due to overlapping van Hove transitions from all nanotube sizes that are present. The UV-vis-NIR absorption spectrum of p-SWCNTs obtained by the CVD method in DMF (Figure 4a) is lacking the peak at 550 nm, indicating that the presence of metallic SWCNTs is not much in the sample. This result was consistent with the Raman characterization. The spectrum of p-SWCNTs is similar to that of laser-oven-grown SWCNTs, although it is different from the spectrum of SWCNTs obtained from the gas-phase decomposition of CO (HiPco process).33 It is clear that the diameter size distributions of SWCNTs differ considerably with different preparation methods. The spectrum of p-SWCNTs (Figure 4a) has some characteristics of the van Hove electronic transition features of p-SWCNTs. The absorption bands around 1400 and 1560 nm (curve a in the inset of Figure 4) were attributed to

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Figure 4. UV-vis-NIR spectra of samples: (a) p-SWCNTs in DMF, (b) PMMA-SWCNTs in CHCl3, and (c) PMMA-SWCNTs in PhCl. Inset: Expanded view of the semiconductor transition region.

TABLE 1: TGA Data Analysis of Samples Made from Polymerization with p-SWCNTs experiment

method

sonication before reaction

p-SWCNTs PMMA-Br PMMA-SWCNTs-1 PMMA-SWCNTs-2 PMMA-SWCNTs-3b PMMA-SWCNTs-4b PMMA-SWCNTs-5c

none ATRP ATRPa ATRPa ATRPa ATRPa radical polymerization

no no yes no yes no yes

weight weight loss loss (%) (%)d 3.6 98 49.7 12.4 25.8 7.2 29.2

46.1 8.8 22.2 3.6 25.6

a

ATRP conditions: polymerization with p-SWCNTs as initiator; temperature 60 °C; time 24 h; 10 mg, 0.07 mmol of CuBr; 15 mL, 0.07 mmol of PMDETA; 3 mL, 0.024 mol of MMA; 5 mL of ODCB. b With 0.2 g of PMMA-Br, without MMA. c Radical polymerization conditions: 0.5% weight AIBN to MMA was added. d The weight loss of PMMA-SWCNTs excluded that of p-SWCNTs.

the first van Hove singularity in semiconducting CNTs, indicating most of the carbon atoms in p-SWCNTs with tubular structure are sp2-hybridized. These two bands vanished in the UV/vis/NIR spectrum of PMMA-SWCNTs (Figure 4b), indicating that the electronic structure of SWCNTs was disrupted during the modification process.24-26 This can be explained by the conversion of sp2-hybridized carbon atoms to functionalized sp3 atoms. This result is consistent with the increased disorder mode in the Raman spectrum. Both the complete absence of the van Hove singularity in the absorption spectrum and the increased disorder mode in the Raman spectrum are indicative of an interruption of the extended π-electronic structure in PMMA-SWCNTs. Taken together, the spectroscopic results support that PMMA is covalently attached to SWCNTs with no significant damage to the CNT framework. To examine the effect of sonication in the preparation of PMMA-SWCNT composites, thermal gravimetric analyses (TGA) were performed on SWCNTs, PMMA-Br, and SWCNTpolymer composites prepared with and without sonication (Table 1). The purified SWCNTs lost less than 3.6% of their weight at 800 °C under nitrogen. Under the same conditions, the composite sample prepared with sonication before polymerization (PMMA-SWCNTs-1) lost 49.7% of its weight, whereas the one without sonication (PMMA-SWCNTs-2) lost only 12.4% of its weight. Excluding the weight loss of p-SWCNTs, the weight loss of the composites can be considered as the weight of PMMA covalently attached to CNTs (vide infra), because free PMMA was removed from the nanocomposites by extensive washing with CHCl3. Therefore, the amount of the polymer grafted onto the SWCNTs has increased from 8.8% to 46.1% when ATRP was operated with the sonicated

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SWCNTs. The 8.8% polymer formation on SWCNTs without ultrasonification (PMMA-SWCNTs-2) can be explained by the fact that nanotubes can act as radical scavengers and the growing polymer radicals initiated by heat may attach themselves to the defective carbon surface of the nanotubes. It was reported that CNTs are capable of acting as antioxidants34 and that radical species produced by benzoyl peroxide can be used to functionalize the sidewalls of CNTs.12 The amount of the polymer grafted onto the SWCNTs has much increased up to 46.1% when ATRP was operated with the sonicated SWCNTs, which may be related to the changes of SWCNTs after sonication in ODCB. The decomposition of organic solvents and water under ultrasonication is well-documented in the literature.35 The decomposition products of certain haloaromatic solvents are also known to polymerize when subjected to ultrasonication.36,37 Haddon et al.15 found that ultrasonic dispersions of SWCNTs in ODCB lead to the formation of a sonopolymer, which adheres to the nanotube surface. They believed that sonication in ODCB irreversibly damages the integrity of the SWCNT structure.38,39 When SWCNTs are sonicated in organic solvents, the local point pressure and temperature could reach as high as 100 MPa and 5000 K, respectively.40 In the widely accepted “hot-spot” theory, the decomposition of organic solvents is believed to occur through a radical pathway.41 SWCNTs have previously been observed to act as chain-terminating surfaces when a polymerization reaction is conducted in their presence.42,43 Haddon et al.15 proposed that the polymer undergoes termination and becomes immobilized on the nanotube surface. Koshio et al.14 suggested that organic molecules (such as monochlorobenzene and PMMA) are decomposed at the hot spots, and reactive species are formed. At the same time, the sidewalls of the SWCNTs are damaged,17 and carbon-dangling bonds form as a result of the high temperature and pressure. The reactive species produced by decomposition of monochlorobenzene reacted easily with the reactive sites formed on the sidewalls and at the open ends of SWCNTs. We include in this classification that ODCB could decompose into active substance under high pressure and temperature, and defects could form at the sidewall and the ends of CNTs. The organic active substance could react with the defect sites,14 leading to the formation of Cl-containing initiators that are suitable for ATRP. To elucidate if any Cl-containing initiators that are suitable for ATRP formed, the surface state of SWCNTs after ultrasonication in ODCB was analyzed by XPS. Analysis of the chlorine region in the XPS spectrum of SWCNTs after ultrasonication in ODCB showed a peak at 200.5 eV from Cl (2p). Clark et al.44 reported that the binding energy of C-Cl in chlorobenzene was 200.1 eV. This binding energy is typical of chlorine in an organic C-Cl bond, suggesting that organic C-Cl containing groups have been introduced to CNTs. Due to the above analysis, the 46.1% weight gain on SWCNTs with ultrasonification (PMMA-SWCNTs-1) may be obtained with two approaches. First, polymer formation on SWCNTs can be explained by the fact that nanotubes can act as radical scavengers and the growing polymer radicals initiated by heat, AIBN, or ATRP initiators may attach themselves to the defective carbon surface of the nanotubes. Second, the initiating capacity of functionalized SWCNTs consists of putting them in contact with CuBr and PMMA-Br (obtained by ATRP) because of the much larger weight losses (37.3% of weight lost) with sonication before polymerization (PMMA-SWCNTs-1) than without sonication (PMMA-SWCNTs-2).

Liu et al. To further test if active initiators for ATRP are introduced to SWCNTs after sonication in ODCB, we incubated SWCNTs with the preprepared polymer PMMA-Br under the same ATRP conditions. PMMA-Br participates in ATRP in the presence of initiators,45 so it can be grafted to SWCNTs if there are initiators on SWCNTs. Indeed, when the SWCNTs were sonicated in ODCB and then reacted with PMMA-Br, a 22.2% net weight loss was observed (PMMA-SWCNTs-3, Table 1), indicating PMMA-Br was grafted to the SWCNTs. By contrast, only a 3.6% weight loss was dectected for SWCNTs not treated with sonication in ODCB (PMMA-SWCNTs-4). In another control experiment (PMMA-SWCNTs-5), the composite was prepared by common radical polymerization, using AIBN as initiator and under sonication. The amount of PMMA grafted to SWCNTs (25.6% weight loss) was significantly less than that grafted by the ATRP method (46.1% weight loss). Altogether, these results suggest initiators suitable for ATRP are introduced onto SWCNTs after sonication in ODCB, and the initiators can initiate the grafting of monomeric MMA as well as the polymeric PMMA-Br to SWCNTs. Conclusion PMMA-SWCNTs have been successfully prepared by using in situ ATRP of MMA with sonicated SWCNTs in ODCB in mild conditions and in one step. The polymer is covalently attached to the SWCNTs in the nanocomposites. This is a facile method for preparing CNT-polymer composites with mild conditions that can be suitable for various monomers and mass production. Acknowledgment. Financial support from the National Natural Science Foundation of China (NSFC 50821061), the Ministry of Science and Technology (MOST 2007CB936203, 2006CB932403, 2006CB932602), and the Outstanding Youth Promotive Foundation of Shandong (2008BS09009) is acknowledged. We thank Dr. Jianqiang Meng for technical help with ATRP experiments. References and Notes (1) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Nature 1996, 381, 678–680. (2) Dalton, A. B.; Collins, S.; Munoz, E.; Razal, J. M.; Ebron, V. H.; Ferraris, J. P.; Coleman, J. N.; Kim, B. G.; Baughman, R. H. Nature 2003, 423, 703. (3) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006, 106, 1105–1136. (4) Tagmatarchis, N.; Georgakilas, V.; Prato, M.; Shinohara, H. Chem. Commun. 2002, 2010–2011. (5) Balaban, T. S.; Balaban, M. C.; Malik, S.; Hennrich, F.; Fischer, R.; Ro¨sner, H.; Kappes, M. M. AdV. Mater. 2006, 18, 2763–2767. (6) Tian R.; Wang, X.; Xu, Y.; Li, S.; Wan, L.; Li, M.; Cheng, J. J. Nanopart. Res.DOI 10.1007/s11051-008-9516-7. Published Online Sept 26, 2008. (7) Qin, S.; Qin, D.; Ford, W. T.; Resasco, D. E.; Herrera, J. E. J. Am. Chem. Soc. 2004, 126, 170–176. (8) Kong, H.; Gao, C.; Yan, D. J. Am. Chem. Soc. 2004, 126, 412– 413. (9) Yao, Z.; Braidy, N.; Botton, G. A.; Adronov, A. J. Am. Chem. Soc. 2003, 125, 16015–16024. (10) Baskaran, D.; Mays, J. W.; Bratcher, M. S. Angew. Chem., Int. Ed. 2004, 43, 2138–2142. (11) Viswanathan, G.; Chakrapani, N.; Yang, H.; Wei, B.; Chung, H.; Cho, K.; Ryu, C. Y.; Ajayan, P. M. J. Am. Chem. Soc. 2003, 125, 9258– 9259. (12) Holzinger, M.; Vostrowsky, O.; Hirsch, A.; Hennrich, F.; Kappes, M.; Weiss, R.; Jellen, F. Angew. Chem., Int. Ed. 2001, 40, 4002–4005. (13) Ausman, K. D.; Piner, R.; Lourie, O.; Ruoff, R. S.; Korobov, M. J. Phys. Chem. B 2000, 104, 8911–8915. (14) Koshio, A.; Yudasaka, M.; Zhang, M.; Iijima, S. Nano Lett. 2001, 1, 361–363.

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