J. Phys. Chem. C 2007, 111, 2379-2385
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A General Approach to Chemical Modification of Single-Walled Carbon Nanotubes with Peroxy Organic Acids and Its Application in Polymer Grafting Manhong Liu,†,‡ Yanlian Yang,† Tao Zhu,*,† and Zhongfan Liu*,† Beijing National Laboratory for Molecular Sciences, Center for Nanoscale Science and Technology, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, and College of Material Science and Engineering, Qingdao UniVersity of Science and Technology, Qingdao 266042, People’s Republic of China ReceiVed: August 26, 2006; In Final Form: NoVember 29, 2006
The previous work on the functionalization of single-walled carbon nanotubes (SWCNTs) with peroxytrifluoroacetic acid under ultrasonication is extended to other peroxy organic acids including m-chloroperbenzoic acid and 2-bromo-2-methylperpropionic acid (BMPPA). Systematic characterization of the treated SWCNTs shows that oxygen-based functional groups and the ester groups with Cl or Br substituents were introduced to the nanotubes, demonstrating that the peroxy organic acid treatment is a general approach to chemical modification of SWCNTs. The extent of functionalization depends on the acidity, oxidizability, and concentration of the peroxy organic acids. Furthermore, the attached reactive moieties allow the chemical coupling of SWCNTs to other materials. By using BMPPA-SWCNTs as the initiator, poly(methyl methacrylate) was grafted to SWCNTs through atom transfer radical polymerization.
Introduction Chemical modification of carbon nanotubes is essential for various applications.1-4 Chemical functionalization of carbon nanotubes allows the change of their solution properties and subsequent understanding of the chemistry of these nanomaterials.5 For covalent chemical modifications, defect sites can be generated at the nanotube ends and sidewalls by different oxidizing reagents, including the mixtures of sulfuric and nitric acids,6 piranha solution (sulfuric acid and hydrogen peroxide),6 ozone,7-9 and concentrated H2SO4 containing (NH4)2S2O8 and P2O5 followed by H2SO4 and KMnO4.10 These oxidative processes are capable of generating a variety of oxygen-based functional groups, such as aldehyde, ketone, alcohol, and carboxylic groups.7 Oxidatively treated SWNTs were subsequently derivatized into amide or ester groups.6,11-15 Direct chemical functionalization of the sidewalls has been achieved by fluorination,16,17 derivatization with aryl diazonium salts via thermally induced reaction,18 and attachment of substituted phenyl groups.19,20 Other examples include reactions with nitrenes21 and radicals,21-24 1,3-dipolar cycloaddition,25,26 and electrophilic addition of chloroform.27 Covalent approaches also include the addition of inorganic compounds to carbon nanotubes, such as osmium tetroxide,28 a nitronium (NO2+) salt,29 or a metal complex.30 Functionalization of carbon nanotubes with polymers is of importance for the modulation of solution properties and physicochemical properties of carbon nanotubes.5 The functionalization by using the “grafting from” approach has been reported by several groups.5,31-34 One modification approach involves the attachment of atom transfer radical polymerization (ATRP) initiators to carbon nanotubes. These initiators were found to be active in the polymerization of various monomers. Different polymers, such as poly(n-butyl methacrylate),31 poly(methyl methacrylate),32,33 poly(tert-butyl acrylate),33a poly† ‡
Peking University. Qingdao University of Science and Technology.
styrene,35a,36,37 and polystyrene-block-poly(tert-butyl acrylate),35b were successfully attached to carbon nanotubes by using ATRP. However, these methods by ATRP involve several steps to produce the nanotube-based macroinitiators. We recently reported a new and simple method for chemical modification of SWCNTs with peroxytrifluoroacetic acid (PTFAA).38 Purified SWCNTs were treated with in situ generated PTFAA under ultrasonication in mild conditions. Oxygen-based functional groups and trifluoroacetic groups were introduced to the nanotubes through covalent bonds. In the meantime, the SWCNTs were shortened into well-dispersed nanotubes with lengths of ca. 300 nm. After treatment, the resultant SWCNTs are more soluble in polar solvents. To further understand the mechanisms and to explore the applicability of this approach, here we describe the treatment of SWCNTs with other peroxy organic acids. Peroxy organic acids containing Cl or Br substituents, m-chloroperbenzoic acid (MCPBA) and 2-bromo-2-methylperpropionic acid (BMPPA), were used to functionalize SWCNTs. Systematic characterization of treated nanotubes showed that different kinds of peroxy organic acids display similar behaviors but the modification extents depend on the acidity, oxidizability, and concentration of the peroxy organic acids. In addition, the covalently attached ester groups could enable the chemical coupling of SWCNTs with other materials. The 2-bromopropionate group on SWCNTs introduced by BMPPA treatment was found to serve as an initiator for living radical polymerization of methyl methacrylate (MMA), forming PMMA-SWCNTs through grafting polymerization. Using our method, the SWCNT initiator could be obtained in one step. Our method should facilitate covalent attachment of polymers to carbon nanotubes and thus expand the scope of applicability of the resultant composites. Experimental Section Materials. MCPBA, CuBr, 1,1,4,7,7-pentamethyldiethylenetriamine (PMDETA), and ethyl 2-bromoisobutyrate (2-EBiB)
10.1021/jp065539j CCC: $37.00 © 2007 American Chemical Society Published on Web 01/19/2007
2380 J. Phys. Chem. C, Vol. 111, No. 6, 2007 were purchased from Acros and used as received. MMA (AR, Beijing Chemicals Co.) was distilled under reduced pressure over CaH2. Trifluoroacetic anhydride, 2-bromo-2-methylpropionic acid, and other organic reagents or solvents were supplied by Beijing Chemicals Co. and used as received. The CVD growth and the purification of SWCNTs are described in detail elsewhere,38,39 and the resultant purified SWCNT sample is referred to as “p-SWCNTs”. The PTFAA treatment of SWCNTs is described in detail in ref 38, and the product is referred to as “F-SWCNTs”. MCPBA Treatment of SWCNTs. MCPBA was commercially available and used directly in the experiments. Typically, 15 mg of p-SWCNTs was suspended in 10 mL of CH2Cl2. The mixture was sonicated at 0 °C, and a solution containing 10 g of MCPBA and 90 mL of CH2Cl2 was added within 1 h. The mixture was then sonicated for 12 h at 35 °C. The suspension was then diluted with ethanol, filtered through a 0.1 µm membrane filter (Gelman), and washed with ethanol. The residue was resuspended and sonicated in ethanol and then filtered. Washing with ethanol was repeated several times to remove any physically adsorbed species. Finally the resultant product was dried in an oven at 120 °C overnight and is referred to as “MCPBA-SWCNTs”. BMPPA Treatment of SWCNTs. A 15 mg sample of p-SWCNTs was suspended in 10 mL of CH2Cl2, and 20 mL of 60% H2O2 was added. The mixture was sonicated at 0 °C, and a solution containing 25 g of 2-bromo-2-methylpropionic acid and 40 mL of CH2Cl2 was added to the mixture within 1 h. The following treatments were the same as those in MCPBA treatment of SWCNTs. The resultant product is referred to as “BMPPA-SWCNTs”. Synthesis of Poly(methyl methacrylate)-Grafted SWCNTs. In a typical run, 4 mg of BMPPA-SWCNTs was dispersed in 5 mL of dichlorobenzene (DCB) in a glass tube, and 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 an oil bath of 60 °C for 12 h. The tube was then broken to release the suspension, which was then diluted with CHCl3, filtered through a 0.1 µm membrane filter (Gelman), and washed with CHCl3. The residue was resuspended and sonicated in CHCl3 and then filtered. Washing with CHCl3 was repeated several times to remove any physically adsorbed species. Finally the sample was dried in vacuum and is referred to as “PMMA-SWCNTs”. Characterization of SWCNTs. 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, UV-3101 PC). The IR spectra were obtained by using an AVATAR-360 Fourier transform infrared (FTIR, Nicolet) spectrometer. AFM characterization was performed on a Nanoscope III (Digital Instrument) in tapping mode. The SWCNT samples for AFM were dispersed in the proper solvent and spun coat on a Si substrate. The nanotube lengths were measured from the AFM images. The length distribution histogram of the nanotubes was obtained on the basis of the measurements of about 200-300 nanotubes. XPS spectra were obtained using an Axis Ultra spectrometer (Kratos, U.K.). 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 using
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Figure 1. IR spectra of SWCNTs: (a) p-SWCNTs, (b) BMPPASWCNTs, (c) N2 annealed BMPPA-SWCNTs.
the C1s hydrocarbon peak at 284.8 eV. TGA was performed at a heating rate of 10 °C/min. Results and Discussion SWCNTs used in these experiments were grown by the CVD method.39 After removal of the catalyst by hydrochloric acid and oxidization of the amorphous carbon at 500 °C in air, they were annealed in N2 at 1000 °C. The resultant p-SWCNTs were treated with three kinds of peroxy organic acids under ultrasonication. After repeated washing, the samples were dried and characterized with various methods to validate the functionalization of SWCNTs. In our previous study, PTFAA-treated samples were characterized with various methods, and the results showed that, in addition to oxygen-based functional groups, trifluoroacetic groups were covalently attached to the SWCNTs. Moreover, these modified SWCNTs were shortened to ca. 300 nm length in the same step of functionalization, resulting in exfoliation of nanotube ropes to yield small bundles and individual nanotubes. The functionalized SWCNTs were easily dispersed in polar solvents such as dimethylformamide, water, and ethanol.38 For MCPBA- and BMPPA-treated SWCNTs, spectral data similar to those of PTFAA-treated SWCNTs were obtained, indicating that SWCNTs are also chemically modified by these peroxy organic acids. Characteristic features for Cl, Br, and the benzyl group were also evident in various spectral data of the samples treated by the corresponding acids. Raman and TGA analysis showed that the modification extent varies with different peroxy organic acids. Detailed results and comparisons are as follows. Figures 1 and 2 give the IR spectra of the SWCNT samples treated with BMPPA and MCPBA, respectively. The two bands around 1569 and 1152 cm-1 are attributed to the graphitic structure of carbon nanotubes. There were no other obvious bands in the spectrum of p-SWCNTs (Figures 1a and 2a), indicating that the p-SWCNTs contain no functional groups. Similar to the IR spectrum of F-SWCNTs,38 it was found that the CdO absorbance band developed around 1732 cm-1 (Figures 1b and 2b) is indicative of the production of carboxylic acid groups and/or ester groups. The two new bands assigned to the stretching mode of quinone groups were located at 1670 and 1450 cm-1 for BMPPA-SWCNTs (Figure 1b) and at 1650 and 1439 cm-1 for MCPBA-SWCNTs (Figure 2b). The band at 1210 cm-1 (for BMPPA-SWCNTs in Figure 1b) or at 1265 cm-1 (for MCPBA-SWCNTs in Figure 2b) may be evidence
Chemical Modification of SWCNTs
Figure 2. IR spectra of SWCNTs: (a) p-SWCNTs, (b) MCPBASWCNTs, (c) N2 annealed MCPBA-SWCNTs.
of the formation of epoxide groups although the assignment is not clearly confirmed as the 1100-1300 cm-1 region is also characteristic of other bonds such as C-O- from esters and alcohols, or still C-C. These data suggest that oxygen-based groups, such as carboxylic acid groups (-COOH) and epoxide groups, were present on the SWCNT surface. It is noted that the 2918 and 2847 cm-1 peaks appeared in the IR spectrum of BMPPA-SWCNTs (Figure 1b), which are assigned to the stretching mode of alkyl groups, indicating that the 2-bromo2-methylpropionic ester group may be introduced on SWCNTs. Similarly, in the IR spectrum of MCPBA-SWCNTs (Figure 2b), the peaks in the 3100-2900 cm-1 region are attributed to the C-H stretching vibrations of substituted benzene and the peaks at 877, 805, and 712 cm-1 are evidence of the out-of-plane C-H wag vibrations of meta-substituted benzene.40 These results indicate that m-chlorobenzoic ester may be connected to SWCNTs. Also there is a broad band at ∼3300 cm-1 indicating the OH stretching vibration. (This band is not shown in Figures 1 and 2. The IR spectrum including this region of one example (MCPBA-SWCNTs) is included in the Supporting Information.) The analyses of IR spectra further support the mechanism we hypothesized before:38 there are two possible routes that may result in defect sites on SWCNTs by peroxy organic acid treatment. First, the epoxides form through a facile 1,3-dipolar cycloaddition reaction to the double bonds in the SWCNTs when peroxy organic acid is attached to the SWCNTs. In the presence of acidic reagents, the epoxy moiety is then cleaved to yield hydroxide and a 2-bromo-2-methylpropionic ester group or an m-chlorobenzoic ester group. Second, the collapse of cavitation bubbles in ultrasonication produces microscopic domains of high temperature, leading to localized sonochemistry that attacks the surface of the SWCNT tubes and leaves an open hole on the tube side. Sonication in the presence of an oxidizing acid would lead to subsequent attacks at the defect sites to form oxygenbased functional groups such as organic oxoacids. The formed ester groups may also be attacked under ultrasonication, leading to other oxygen-based functional groups. In addition, from Figures 1c and 2c, the same phenomenon could be observed, that the new peaks from the modification process in BMPPA-SWCNTs or MCPBA-SWCNTs disappeared after annealing in N2 at 1000 °C for 0.5 h, indicating that the oxygen-based functional groups are removed and the annealed samples appear to be intact like the pristine SWCNTs.38,41,42 The Raman spectra of the SWCNT samples treated with BMPPA are shown in Figure 3. There was no dramatic change in the Raman spectra before and after modification. It can be seen that radial breathing modes (RBMs) in the region of 170270 cm-1 were not greatly affected by covalent modification, although the peak intensity decreased to various extents. The
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2381
Figure 3. Raman spectra of SWCNTs: (a) p-SWCNTs, (b) BMPPASWCNTs, (c) N2 annealed BMPPA-SWCNTs.
Figure 4. Raman spectra of SWCNTs: (a) p-SWCNTs, (b) MCPBASWCNTs, (c) N2 annealed MCPBA-SWCNTs.
intensity of the D line increased a little after BMPPA oxidation. As calculated from the spectra, the intensity ratio of the G line to the D line (IG/ID) only decreased from 15.5 (p-SWCNTs) to 10.1 (BMPPA-SWCNTs), suggesting a low density of sp3hybridized carbon “defect” sites formed on the carbon nanotube walls due to the addition of the functional groups. These results implied that a low degree of functionalization by BMPPA treatment was achieved, which was also confirmed by both TGA and XPS data as indicated below. The BMPPA-treated sample was then annealed in N2, and the Raman spectrum of the annealed sample was restored to that of p-SWCNTs (Figure 3c), indicating that the nanotube sidewalls remain intact. Raman characterization of SWCNTs modified with MCPBA is shown in Figure 4. The increase in the relative intensity of the disorder mode can be attributed to an increased number of sp3-hybridized carbons in the nanotube framework and can be taken as a rough measure of the degree of functionalization. In the Raman spectra, the relative intensities of the three main modes were largely altered after MCPBA modification. IG/ID decreased from 15.5 (p-SWCNTs) to 2.0 (MCPBA-SWCNTs). The multiple peaks seen in the RBM region correspond to the distribution of tube diameters in the sample. In contrast to the tangential mode at ∼1590 cm-1, the intensities of the RBMs (170-270 cm-1) of MCPBA-SWCNTs were quite different from those of pSWCNTs: Only one peak in the RBM region remained after MCPBA oxidation. Similar results were obtained with FSWCNTs, where the bands in the RBM region decreased to various extents, and in particular, the band at 194.8 cm-1 disappeared, indicating that SWCNTs with various tube diameters reacted with PTFAA, although the degrees vary. The reason why tubes with various diameters reacted differently remains unknown yet. It was reported43 that the change of the
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Figure 6. TGA data for SWCNTs: (a) p-SWCNTs, (b) BMPPASWCNTs, (c) MCPBA-SWCNTs. Figure 5. UV/vis/NIR spectra of SWCNTs: (a) p-SWCNTs, (b) BMPPA-SWCNTs, (c) MCPBA-SWCNTs. Inset: Expanded view of the semiconductor transition region.
RBM frequencies indicates the covalent functionalization in the sidewall of SWCNTs, which disrupted the electronic structure in tubular SWCNTs. Therefore, the changes in the RBM region after MCPBA oxidation indicate that the bonding of functional groups to the carbon atoms on SWCNTs changes the overall symmetry and bonding structure of the SWCNTs, wherein significant amounts of sp2 carbons have been converted to sp3 carbons. The changes in the Raman spectra are in accordance with the information from the IR spectra, which showed that there were functional groups formed during the functionalization. Moreover, the intensities of the radial breathing modes for the annealed sample (Figure 4c) were only partially restored after annealing at 1000 °C in N2, indicating that the SWCNTs were destroyed to some extent after MCPBA treatment and N2 annealing. Comparing the Raman results of SWCNTs treated with three peroxy organic acids, we can conclude that the amounts of covalently attached moieties to the nanotube framework are different. The functionalization degree increases in the order BMPPA, PTFAA, and MCPBA. This trend is consistent with that obtained from TGA analysis, which will be discussed below. We used UV/vis/NIR to confirm the changes of the carbon nanotubes resulting from the modification. The UV/vis/NIR spectra collected for p-SWCNTs in DMF, BMPPA-SWCNTs, and MCPBA-SWCNTs in ethanol are shown in Figure 5. With UV/vis/NIR it is possible to detect two absorption bands around 1400 and 1560 nm (curve a in the inset of Figure 5) caused by the electron transfers within the van Hove singularities in p-SWCNTs.44 The location of the van Hove peaks of SWCNTs is related to the diameter of the nanotubes.45,46 Our results indicate that there were not many metallic SWCNTs in the sample. We focused on the two transitions to investigate whether the electronic properties of the SWCNTs remain unchanged or are disrupted after functionalization. From the spectra of BMPPA-SWCNTs and MCPBA-SWCNTs, we noticed that the van Hove transitions, characteristic of unperturbed SWCNTs, vanished. This may be attributed to the carbon rehybridization from sp2 to sp3 since the π electrons in the highest occupied molecular orbitals (HOMOs) are no longer available. Therefore, the absence of electronic bands centered at 1400 and 1650 nm in treated samples demonstrates the occurrence of SWCNT functionalization. This is in accordance with Raman characterization results as well. The optical characteristics of the SWCNT solution, monitored by absorbance at 500 nm, obey Beer’s law
TABLE 1: XPS Data of SWCNT Samples atomic concn (%) sample
C1s
O1s
Br3d
p-SWCNTs BMPPA-SWCNTs MCPBA-SWCNTs
97.79 95.95 90.8
2.21 3.49 8.78
0.55
Cl2p
0.42
with respect to relative concentrations.47 Calculated from the slope, the solubility of BMPPA-SWCNTs and MCPBASWCNTs in ethanol was about 44 and 79 g m-3, respectively. Thermal gravimetric analyses of derivatized SWCNTs were used to evaluate the extent of functionalization. The percentage weight loss curves indicate that the overall weight loss during the process was 4% for p-SWCNTs, 12% for BMPPASWCNTs, and 27% for MCPBA-SWCNTs (Figure 6). The weight loss of p-SWCNTs is due to degassing and the evaporation of residual solvent. Excluding this part of the weight loss, there was net weight loss of ca. 8% for BMPPA-SWCNTs and ca. 23% for MCPBA-SWCNTs, revealing the decomposition of the functional groups on nanotubes. The net weight loss may be due to the evolution of CO, CO2, halogen, or their combinations. The weight loss of halogen could be neglected according to the low atomic percentage of halogen identified by XPS (see Table 1). Assuming that all net weight losses were due to the evolution of CO2 or CO, we can calculate that approximately 2.2-3.4% of the BMPPA-SWCNT carbons and 6.3-9.7% of the MCPBA-SWCNT carbons were functionalized. Taking into account the evolution of the halogen did not change these numbers significantly. As expected, MCPBA-SWCNTs resulted in a higher degree of functionalization (thus more loss of weight) compared to BMPPA-SWCNTs. As for F-SWCNTs (the net weight loss is 12%), there were about 3.3-5.1% carbon atoms functionalized by PTFAA treatment.38 The order of the functionalization degree of nanotubes for the three peroxy organic acids is MCPBA > PTFAA > BMPPA. To elucidate the surface state of p-SWCNTs before and after modification, XPS analyses were conducted, and the results are summarized in Table 1 (XPS spectra are provided in the Supporting Information). Changes in the O1s peaks in the XPS spectra could be seen for the pristine and modified SWCNTs. After acid treatment, the relative peak intensity of O1s to C1s increased, indicating an increase of the oxygen content in SWCNTs which is associated with the formation of oxygenbased functional groups on the nanotube surface. Analysis of the chlorine region in the XPS spectrum of MCPBA-SWCNTs showed a peak at 200.4 eV from Cl (2p). Clark et al.48 reported that the binding energy of C-Cl in chlorobenzene was 200.1 eV. This binding energy is typical of
Chemical Modification of SWCNTs
Figure 7. Analysis of SWCNTs by AFM: (a) p-SWCNTs, (b) MCPBA-SWCNTs, (c) BMPPA-SWCNTs.
chlorine in an organic C-Cl bond, suggesting that m-chlorobenzoic ester may have been introduced to carbon nanotubes. Similarly, it was found that the binding energy of Br (3d) in the spectrum of BMPPA-SWCNTs was 70.5 eV, which is identical to that of an organic C-Br bond,49 indicating 2-bromo2-methylpropionic ester groups may exist in BMPPA-SWCNTs. AFM was used for the characterization of derivatized SWCNTs. A typical AFM image of the MCPBA-SWCNTs is shown in Figure 7b. As a control, the image of the starting materials is given in Figure 7a. The shortening of SWCNTs after MCPBA treatment is obvious from these AFM images. Height profiles of these images showed that the formation of bundles took place even after functionalization. However, the average thickness of the bundles was reduced after MCPBA modification. From the AFM image of BMPPA-SWCNTs (Figure 7c), we were able to verify that the length of the
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2383 nanotubes remained unaltered, which means that the SWCNTs were not significantly shortened after BMPPA treatment, consistent with the above Raman results. By analyzing the data obtained with various characterization methods, we concluded that the modification extent of the SWCNTs was different with three kinds of peroxy organic acids. AFM analysis of the starting nanotubes and the products revealed that the functionalized nanotubes were shortened by MCPBA or PTFAA oxidation, whereas the length of SWCNTs did not change much upon BMPPA modification. From Raman characterization, we found significant distortion of the extended π-electronic structure in F-SWCNTs and MCPBA-SWCNTs. In contrast, the electronic structure of BMPPA-SWCNTs was not disrupted significantly during the modification process. TGA data elucidated that there are 3.3-5.1% functionalized carbons in F-SWCNTs, 6.3-9.7% in MCPBA-SWCNTs, and 2.2-3.4% in BMPPA-SWCNTs. These results indicated that these peroxy organic acids are all effective for functionalizing carbon nanotubes but their modification ability varies. The modification ability of MCPBA is the strongest, and that of PTFAA is moderate. BMPPA can be considered as a mild modification agent for SWCNTs compared with the other two. These results are related to the power of oxidizability and the relative acidity of the peroxy organic acids. Substituents near the carboxyl group affect the acidity and oxidizability of the peroxy organic acids.50 The acidity and the oxidizability increase in the order BMPPA, MCPBA, and PTFAA by considering the inductive, resonance, and field effect in their structures. Therefore, it is expected that BMPPA has the lowest modification extent for SWCNTs. However, among the three peroxy organic acids, PTFAA should have the strongest ability to modify SWCNTs theoretically, but experimentally it is not as strong as MCPBA. This discrepancy may be explained by the different concentrations of the two peroxy organic acids in the modification reaction. The concentration of MCPBA purchased from Acros is about 70%, whereas the concentration of the in situ generated PTFAA is no more than 20%. (PTFAA was generated in situ by adding 60% H2O2 to trifluoroacetic anhydride. The highest concentration of PTFAA determined in situ by 19F NMR was obtained when the volume ratio of trifluoroacetic anhydride to H2O2 was between 10:1 and 10:2.) The concentration of PTFAA in the modification reaction is much less than that of MCPBA. Therefore, the extent of modification is influenced not only by the acidity and oxidizability but also by the concentration of the peroxy organic acids. Since the above data support that 2-bromo-2-methylpropionic ester groups exist in BMPPA-SWCNTs, we next examined whether such groups could mediate the chemical coupling of SWCNTs with other materials. The 2-bromopropionate group is an excellent initiator for ATRP. We therefore tried to use BMPPA-SWCNTs as an SWCNT initiator to polymerize MMA through ATRP. Nanocomposites were achieved by ATRP of methyl methacrylate in DCB at 60 °C with CuBr/PMDETA as the catalyst and BMPPA-SWCNTs as the initiator. The TGA data of resultant nanocomposites, SWCNTs, and homo-PMMA under nitrogen are shown in Table 2. The purified SWCNTs lost less than 3.6% of their weight at 800 °C under nitrogen. Under the same conditions, the nanotube-based macroinitiators without PMMA (BMPPA-SWCNTs) lost 11.8%, whereas the PMMAgrafted SWCNTs (PMMA-SWCNTs-1) lost 29.1%. Therefore, the relative amount of grafted polymer of the nanocomposite calculated from the TGA data was ca. 17%. This weight loss can be considered as the weight of PMMA covalently attached
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TABLE 2: TGA Data Analysis of Samples Made from Atom Transfer Radical Polymerization with p-SWCNTsa experiment
initiator (amt, mmol)
MMA amt (mol)
weight loss (%)
p-SWCNTs BMPPA-SWCNTs PMMA PMMA-SWCNTs-1 PMMA-SWCNTs-2 PMMA-SWCNTs-3 PMMA-SWCNTs-4b
none none none BMPPA-SWCNTs (16 × 10-4) BMPPA-SWCNTs (16 × 10-4) + 2-EBiB (0.068) p-SWCNTs (16 × 10-4) p-SWCNTs
none none none 0.024 0.024 0.024 preprepared PMMA
3.6 11.8 98 29.1 42 12.4 7.2
a Polymerization conditions: 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 dichlorobenzene. b With 0.2 g of PMMA-Br, without MMA.
to carbon nanotubes, because free MMA was removed from the nanocomposites by extensive washing with CHCl3. Three more control experiments were carried out to support the above conclusion. Because ATRP with surface-bound initiators results in an extremely low concentration of the deactivating CuBr2 species in solution,51-53 monomer conversion and polymerization should be boosted with the addition of free initiators. In one experiment (PMMA-SWCNTs-2), we added free initiator 2-EBiB to the reaction, and the amount of grafted PMMA was indeed increased to 30%. In the second experiment (PMMASWCNTs-3), unfunctionalized SWCNTs were used as ATRP initiators under conditions identical to those of the BMPPASWCNT-initiated polymerizations. The content of the polymer in this sample calculated from the TGA data was 8%, which is less than 17% loss in sample PMMA-SWCNTs-1. The 8% polymer formation on unfunctionalized nanotubes 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 carbon nanotubes are capable of acting as antioxidants54 and that radical species produced by benzoyl peroxide can be used to functionalize the sidewalls of carbon nanotubes.21 In the third experiment (PMMA-SWCNTs-4), we mixed PMMA preprepared by ATRP (PMMA-Br) with SWCNTs under the same reaction conditions described above, and ca. 4% polymer weight loss of the mixed sample was found, suggesting that PMMA-Br may be reactive under these conditions and further polymerization is terminated by unfunctionalized SWCNTs. The above results clearly show that BMPPA-SWCNT initiators were active in the polymerization of methyl methacrylate and that unfunctionalized carbon nanotubes may decrease the concentration of the persistent radicals in the reaction system, thus diminishing the degree of polymerization. The fact that BMPPA-SWCNTs could be used as initiators in ATRP indicates that 2-bromo-2-methylpropionic ester groups were indeed covalently attached to SWCNTs upon BMPPA modification. Conclusion We reported a general modification approach of SWCNTs with peroxy organic acids and the possibility of further modification of SWCNTs by polymer grafting. The peroxy organic acid modified samples were systematically characterized by FTIR, XPS, Raman, UV/vis/NIR, TGA, and AFM. The results indicated the conversion of a certain amount of sp2hybridized carbon atoms to oxidized sp3 atoms. The oxygenbased functional groups and the ester groups with F, Cl, or Br substituents were introduced to the nanotubes for all three modifications. The modification extent varied with different peroxy organic acids and was dependent on the acidity, oxidizability, and concentration of the peroxy organic acids. The ability to modify SWCNTs to various extents may provide flexilibility for different applications. AFM results showed that
the SWCNTs were shortened and well-dispersed after PTFAA or MCPBA treatment. The length of SWCNTs did not change much after BMPPA modification. TGA results elucidated that the functionalized carbon atoms are different in F-SWCNTs, MCPBA-SWCNTs, and BMPPA-SWCNTs. The order of the functionalization degree of nanotubes for the three peroxy organic acids is MCPBA > PTFAA > BMPPA. Furthermore, the BMPPA-SWCNTs could be used as initiators in the atom transfer radical polymerization of methyl methacrylate to achieve polymer grafting on carbon nanotubes via a simple procedure. Acknowledgment. Financial support from the National Natural Science Foundation of China (NSFC Grants 60301001, 90406024, and 20473004) and the Ministry of Science and Technology (MOST Grant 2001CB6105) is gratefully acknowledged. M.L. acknowledges support by the Outstanding Youth Promotive Foundation of Shandong (Grant 2005BS11009). Y.Y. gratefully acknowledges the State Key Laboratories of Transducer Technology and of Colloid and Interface Chemistry of the Educational Ministry at Shandong University for financial support. T.Z. acknowledges the project sponsored by SRF for ROCS, MOE, and partial support from the Beijing Key Lab for Nanophotonics and Nanostructure. We also thank Prof. Zichen Li and Dr. Jianqiang Meng for their help with the ATRP experiments. Supporting Information Available: Full-range IR spectrum of MCPBA-SWCNTs (Figure 1s) and XPS spectra of SWCNTs before and after peroxy organic acid treatment (Figure 2s). This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Tasis, D.; Tagmatarchis, N.; Georgakilas, V.; Prato, M. Chem.s Eur. J. 2003, 9, 4000. (2) Rao, C. N. R.; Satishkumar, B. C.; Govindaraj, A.; Nath, M. ChemPhysChem 2001, 2, 78. (3) Gao, B.; Yue, G. Z.; Qiu, Q.; Cheng, Y.; Shimoda, H.; Fleming, L.; Zhou, O. AdV. Mater. 2001, 13, 1770. (4) Hafner, J. H.; Cheung, C. L.; Oosterkamp, T. H.; Lieber, C. M. J. Phys. Chem. B 2001, 105, 743. (5) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006, 106, 1105. (6) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y.-S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. (7) Banerjee, S.; Wong, S. S. J. Phys. Chem. B 2002, 106, 12144. (8) Mawhinney, D. B.; Naumenko, V.; Kuznetsova, A.; Yates, J. T.; Liu, J.; Smalley, R. E. J. Am. Chem. Soc. 2000, 122, 2383. (9) Cai, L.; Bahr, J. L.; Yao, Y.; Tour, J. M. Chem. Mater. 2002, 14, 4235. (10) Kovtyukhova, N. I.; Mallouk, T. E.; Pan, L.; Dickey, E. C. J. Am. Chem. Soc. 2003, 125, 9761. (11) Fu, K.; Huang, W.; Lin, Y.; Riddle, L. A.; Carroll, D. L.; Sun, Y. P. Nano Lett. 2001, 1, 439. (12) Hamon, M. A.; Chen, J.; Hu, H.; Chen, Y.; Itkis, M. E.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. AdV. Mater. 1999, 11, 834. (13) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y. A.; Rao, M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95.
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