Single-Walled Carbon Nanotubes and Multiwalled Carbon Nanotubes

J. Phys. Chem. C 2008, 112, 10663–10667

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Single-Walled Carbon Nanotubes and Multiwalled Carbon Nanotubes Functionalized with Poly(L-lactic acid): a Comparative Study Beatriz Olalde,† Jesu´s M. Aizpurua,‡ Ainara Garcı´a,† Izaskun Bustero,† Isabel Obieta,† and Marı´a J. Jurado*,† Unidad de Salud, INASMET-TECNALIA, Paseo Mikeletegi 2, Parque Tecnolo´gico, E-20009 San Sebastia´n, Spain, and Departamento de Quı´mica Orga´nica-I, UniVersidad del Paı´s Vasco, Joxe Mari Korta R&D Center, AVenida Tolosa-72, 20018 San Sebastia´n, Spain ReceiVed: January 11, 2008; ReVised Manuscript ReceiVed: April 1, 2008

Spectroscopic (Raman) and gravimetric (acid titration, TGA) techniques were compared to determine the functionalization degree of multiwalled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) with poly(L-lactic acid) (PLLA), following a two-step oxidation/esterification process. After oxidation with HNO3, carbon nanotubes (CNTs) were activated with thionyl chloride to the corresponding acid chlorides and then grafted with PLLA. FTIR spectroscopy confirmed the formation of very similarly grafted CNTPLLA materials for both SWNT and MWNT. But, according to chemical titration and TGA results, oxidized and esterified SWNT showed a significantly higher degree of functionalization than their MWNT counterparts. Contrary to these observations, the increase of the Raman ID/IG ratio was higher for MWNT-derived materials than for the SWNT counterparts. Therefore, it was concluded that this spectroscopic technique is unsuitable for the quantitative determination of the degree of functionalization when MWNT and SWNT are compared. Introduction Since their discovery in 1991,1 carbon nanotubes (CNTs) have generated a great interest due to their unique physical, mechanical, and conductive properties.2–6 CNTs in which the wall structure consists of a single graphite sheet closed in a tubular shape are called single-walled carbon nanotubes (SWNT), while those consisting of a plurality of graphite sheets each closed into a tubular shape and nested one within the other are called multiwalled carbon nanotubes (MWNT).7 Because of its tubular shape, a carbon nanotube extends outwardly having a length that can exceed from hundred to many thousand times its diameter. Despite their extraordinary properties, most applications of carbon nanotubes are thwarted by their difficult transformation into materials that can be uniformly dispersed and easily manipulated in either organic solvents or water.8 A common strategy to overcome this problem is the noncovalent or covalent surface modification to cancel the highly hydrophobic character of CNTs caused by strong intertubular vander-Waals forces. Surfactants such as sodium dodecylbenzenesulfonate (NaDDBS),9 sodium dodecyl sulfate (SDS),10 or polymers such us poly(vinyl pyrrolidone),11 poly(m-phenylenevinylene)-co-(2,5dioctoxy-p-phenylene)vinylene,12 have been successfully used for the wrapping of CNTs by noncovalent bonding interactions. This approach has the advantage of improving solubility while preserving the integrity of the CNT surfaces but, on the other hand, it may lead to relatively unstable dispersions because of the intrinsically weak nature of the CNTs-wrapping molecule interaction. Covalent functionalization of CNT with polymers allows a good dispersion by disrupting the primary structure of the * To whom correspondence should be addressed. E-mail: mjurado@ inasmet.es. † Unidad de Salud. ‡ Universidad del Paı´s Vasco.

nanotubes and the functionalization is stable and controllable.13–15 This functionalization could be divided in “grafting to” or “grafting from” method. Typically, the “grafting to” method comprises amidation or esterification reactions between the amine or hydroxy groups of functionalized polymers and the carboxylic groups created onto the CNT surface by oxidation with sulfuric acid, nitric acid16,17 or mixture thereof.18 Suitable polymers for such a modification include poly(ethylene glycol) (PEG),19 poly(methyl methacrylate) (PMMA),20 poly(vinyl alcohol) (PVA),21 or poly(ethylene-co-vinyl alcohol) (EVOH).22 The “grafting from” technique relies on the growth of polymers from the CNT surface by the in situ polymerization of monomers promoted by immobilizing initiators. Different polymers have been grown on the nanotube surfaces, for instance, polystyrene (PS),23,24 poly(methyl methacrylate) (PMMA),25 and poly(4vinylpyridine) (PVP)26 have been attached on CNTs by free radical polymerization. There are some previous works on the noncovalent and covalent combination of PLLA with MWNTs, but not with SWNTs. For instance, Zang et al. described the preparation and characterization of a PLLA and MWNT composite through a solution mixing without any functionalization.27 Chen et al. obtained MWNT covalently bonded to PLLAs of different molecular weight by a “grafting to” esterification reaction.28 The same authors also proposed a “grafting from” synthesis of MWNT-PLLA by ring-opening polymerization of L-lactide onto oxidized MWNT initiators.29 Herein we report a comparative study of the covalent “grafting to” attachment of PLLA to MWNT and SWNT, following a two-step oxidation/esterification process. The determination of the relative degree of CNT surface alteration for each chemical step is accomplished by using different approaches: on the one hand, a combination of acid-base titration30 and thermogravimetric analysis and, on the other hand, the ID/IG ratio provided by Raman spectroscopy.

10.1021/jp800266j CCC: $40.75  2008 American Chemical Society Published on Web 06/28/2008

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Olalde et al.

Experimental Section Materials. The CVD MWNTs (over 80% pure) were purchased from Sun Nanotech Co. Ltd. (Jiangxi, China) and had an average diameter of 10-30 nm and a length of 1-10 µm. The SWNTs (over 90% pure) were supplied by Thomas Swan and Co. Ltd., having diameters about 2 nanometers and lengths of several microns. Poly(L-Lactic acid) (PLLA) with an inherent viscosity of 0.90-1.2 dL/g and an average molecular weight and polydispersity (Mw/Mn) of 104 000 and 1.80, respectively (measured by gel-permeation chromatography) was purchased from Absorbable Polymers (Pelham, AL) and was used as received. Nitric acid, thionyl chloride (SOCl2), anhydrous N,N-dimethylformamide (DMF), dichloromethane (CH2Cl2), and triethylamine (TEA) were purchased from Sigma Aldrich and were used as received without further purification. Method. Carboxyl-functionalized CNTs were prepared by oxidation of CNT with HNO3.13 CNTs (300 mg) and HNO3 (65%, 200 mL) were mixed in a 500 mL flask equipped with a condenser. The flask was then immersed in an ultrasonic bath under reflux (100 °C) for 3 h. After cooling down to room temperature, the reaction mixture was filtered in vacuo through a polycarbonate filter of 0.2 µm pore size (Millipore). The solid was dispersed in 500 mL distilled water sonicated in an ultrasound bath for 30 min and filtered again, repeating this washing several times until complete removal of the residual HNO3. Finally, the CNT-CO2H samples were oven-dried overnight at 60 °C. CNT-CO2H (100 mg), SOCl2 (10 mL), and anhydrous DMF (1 mL) were refluxed for 24 h with stirring under nitrogen atmosphere in a 100 mL flask equipped with a condenser. After cooling to room temperature, the excess SOCl2 was evaporated in a rotavapor to afford crude CNT-COCl (100 mg), which was resuspended in a solution of PLLA (500 mg) in CH2Cl2 (15 mL). Upon complete solution of the polymer, Et3N (1 mL) was carefully dropped over the mixture (exothermic reaction) and the dispersion was stirred for 48 h at 70 °C under nitrogen atmosphere. The resulting organic phase was washed successively with NaOH (1 M) and distilled water, poured into an additional 200 mL of CH2Cl2, sonicated in an ultrasound bath for 30 min and filtered in vacuo through a polytetrafluoroethylene filter of 0.2 µm pore size (Millipore). The washing step was repeated several times until complete elimination of free PLLA, and the CNT-PLLA materials were oven-dried overnight at 45 °C. Characterization. The Raman spectra for the carbon nanotubes were obtained using a RENISHAW in via Raman microscope with a laser light wavelength of 514 nm. Fourier transform infrared (FTIR) spectra were recorded from CNT samples embebed in KBr pellets on a Magna-IR Spectrometer 750 (from Nicolet) with a monochromatic IR-ray source (EverGlo mid-IR), placed in front of the detector (DTGS-KBr). Thermogravimetric (TGA) experiments were carried out in argon-diluted air atmosphere (90% Ar/10% air) using a TGA 92 16.18 SETARAM equipment over 10 mg samples heated at a scan rate of 10 °C/min from 25 to 1200 °C. Results and Discussion Figure 1 shows the FTIR spectra of the functionalized CNT-PLLA materials. After the grafting reaction any unreacted PLLA was removed thoroughly from the products by washing with dichloromethane and, therefore, the peaks observed in each spectrum were attributed to MWNT or SWNT structures covalently bonded to the PLLA polymer. Accordingly, all the expected characteristic absorption bands for hydrocarbon and ester functional groups were unambiguously identified in both

Figure 1. FTIR spectra of (a) MWNT-PLLA and (b) SWNT-PLLA.

samples, and those observed for MWNT-PLLA were in good agreement with reported data.28 For instance, in the carbonyl area, bands at 1738 and 1624 cm-1 were observed for MWNT-PLLA and at 1726 and 1632 cm-1 for SWNT-PLLA, that were attributed to CdO stretching vibrations of carboxyl and carbonyl groups. Weak broad bands appearing at 1100, 1037 cm-1 and 1180,1025 cm-1 were associated to C-O stretching vibrations of MWNT-PLLA and SWNT-PLLA respectively.31 In addition, the band at 1577 and 1572 cm-1 for each sample was assigned to CdC double bonds located near the newly formed ester groups on the CNT surface.32,33 In both samples, at 1383 for MWNT-PLLA and at 1391 cm-1 for SWNT-PLLA there is a phenol characteristic vibration band due to the delocalization of “π” electrons that behave like aromatic compounds.34 Finally, the spectra show -CH3 asymmetric bending bands (1453 and 1463 cm-1 for MWNT-PLLA and SWNT-PLLA samples, respectively) and C-H stretching modes at high frequencies (2880-3005 cm-1).31 Raman spectroscopy is routinely used to study the structural modifications of the nanotube walls arising from the introduction of defects caused by the attachment of different chemical species.35 The Raman spectra of CNT show three significant regions: (a) the radial breathing mode (RBM) which is dependent on the tube diameter and only appears in the low frequency region of SWNT spectra, (b) the tangential mode, also known as the G band, and (c) the disorder mode D band.36 The D to G band intensity ratio (ID/IG) provides an estimation of the CNT functionalization degree: the higher the ID/IG ratio, the larger the proportion of carbonaceous species and the degree of functionalization.37 While Raman ID/IG ratio is a reliable indicator to compare the wall functionalization degree of the same type of CNTs, it remains to be established whether this ratio can be used for the quantitative comparison of different types of CNTs and, more particularly, of SWNT and MWNT. Figure 2 shows the Raman spectra of three MWNT species: pristine MWNT, oxidized MWNT and PLLA-modified MWNT. The raw MWNT (Figure 2a) presents an ID/IG ratio over 0.65, which increased to 2 for MWNT-CO2H (Figure 2b) and to 2.2 after the esterification with PLLA (Figure 2c). The significant increase of the ID/IG ratio after the oxidation of pristine MWNT to MWNT-CO2H was primarily attributed to the defect sites introduced in the CNT by the partial degradation of the outer wall to carboxylic groups. Application of the titration method of Hu et al.30 using NaHCO3 as a base, afforded a 1.0% mole percentage of carboxylic acid groups for MWNT-CO2H, which was in good agreement with the three-fold increase observed for the ID/IG ratio after the oxidation step. In contrast to this behavior, no significant ID/IG ratio change was observed after the grafting of PLLA to the surface of MWNT, suggesting that

SWNTs and MWNTs Functionalized with Poly(L-lactic acid)

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Figure 2. Raman spectra of (a) pristine MWNT, (b) MWNT-CO2H and (c) MWNT-PLLA.

Figure 3. Raman spectra of (a) raw SWNT, (b) SWNT-CO2H and SWNT-PLLA.

the structural integrity of the CNT wall was fully preserved during the esterification reaction. Figure 3 shows the Raman spectra of pristine SWNT (panel a), the oxidized SWNT (panel b) and the SWNT functionalized with PLLA (panel c). The intensity ratios of D band to G band of pristine SWNT was over 0.16, while the ID/IG ratio reached values of 0.38 and 0.40 for SWNT-CO2H and SWNT-PLLA, respectively. While the general trend in increasing the ID/IG ratio after the oxidation and esterification reactions was similar for SWNT and MWNT, the former displayed only a 2.2-fold increase after the oxidation step to SWNT-CO2H. In addition, under titration conditions identical to those previously used for MWNT-CO2H, the single-walled counterpart presented almost a double mole percentage of free carboxylic acid groups: 1.9%. Table 1 summarizes the ID/IG ratio for the pristine CNT, CNT-CO2H and CNT-PLLA and the titration results. As deduced from Raman spectra analysis, the surface modification was more pronounced in MWNTs than in the SWNTs after the oxidation step. However, these results were in sharp contrast with the trend observed from titration of carboxylic acid groups

TABLE 1: Relative Raman ID/IG Intensities for Pristine CNT, CNT-CO2H, and CNT-PLLA and Free Carboxylic Acid Group Percentages for CNT-CO2H sample MWNTa MWNT-CO2Hb MWNT-PLLAc SWNTd SWNT-CO2He SWNT-PLLAf

ID/IG ratio 0.65 2.00 2.20 0.16 0.38 0.40

titration CO2H groups (% mol) 1.0 1.9

a Pristine MWNT. b Oxidized MWNT. c MWNT functionalized with PLLA. d Pristine SWNT. e Oxidized SWNT. f SWNT functionalized with PLLA.

after oxidative modification, which suggested SWNTs as the more damaged compounds. While a rational of this observation is unclear at present, some acidic structures other than carboxylic acids (e.g., lactones, phenols, carbon net vacancies) or grain boundaries lowering the crystal symmetry of the nanotube lattice, could be at the origin of the apparent overestimation of the wall modification measured by the Raman ID/IG ratio for

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Figure 4. Thermogravimetric analysis (TGA) weight lost curve for (a) raw MWNT and (b) PLLA-grafted MWNT.

Olalde et al. the relative extent of the modification generated in the external wall of the nanotubes, depending on their multiwalled or singlewalled nature. On the basis of titration results and TG analysis, it was concluded that both partial oxidation of the CNTs surface to carboxylic groups and incorporation of PLLA were appreciably higher for SWNT than for MWNT. The mol percentage of carboxylic groups for oxidized MWNT was estimated to be 1.0% and the quantity of PLLA bonded was around 20 wt %, while for SWNT these values were 1.9% and 30 wt %, respectively. On the other hand, the Raman ID/IG ratio was found to be not a valid indication to compare the degree of chemical functionalization of MWNT and SWNT. Actually, higher ID/IG ratio variations were observed after the oxidation step for MWNT (from 0.65 to 2.00) than for SWNT (from 0.16 to 0.38), despite the lower functionalization degree of the former compounds. Therefore, it was concluded that the Raman ID/IG ratio not only accounted for the relative amount of carboxylic acid groups present in the CNT structures, but also for any structural defect lowering the crystal symmetry of the CNT lattices. Acknowledgment. A grant from “Fundacio´n Centros Tecnolo´gicos In˜aki Goenaga” to B.O. is acknowledged. The authors thank the Basque Government (PI-2004-2 and Etortek: Nanomaterials) for financial support.

Figure 5. Thermogravimetric analysis (TGA) weight lost curve for (a) raw SWNT and (b) PLLA-grafted SWNT.

MWNT derivatives. Whatever the reason, no significant variation of the ID/IG ratio was observed after the esterification of both types of CNT-CO2H to the corresponding CNT-PLLAs. To clear up the discrepancy in the determination of the functionalization degree of CNTs using titration values or Raman data, the PLLA-grafted MWNT and SWNT samples were submitted to TG analysis (Figures 4 and 5). Because of the large differences between the polymer degradation and CNT combustion temperatures, this technique is known to provide a very reliable quantification of the relative weight of polymers grafted to nanotubes. As expected, the thermographs of the raw MWNT and SWNT showed a single combustion slope (a), while the CNT-PLLAs displayed a first weight loss due to the PLLA degradation in the range of 270-300 °C and a second weight loss at higher temperature due to the combustion of CNTs. This first weight loss occupied a 20% weight portion for MWNT-PLLA and 30% for SWNT-PLLA, indicating that the amount of PLLA chains covalently attached to the CNT surface was higher in the case of SWNT. These values correlated satisfactorily with the free carboxylic acid mole percentages determined by acid-base titration, but not with the observed Raman ID/IG ratio increases. Therefore, as TG analysis was considered an accurate indication of the actual degree of functionalization of the CNTs, it was concluded that the Raman ID/IG ratio must to be used with caution to make quantitative comparisons of the wall modification degrees of SWNT and MWNT derivatives. Conclusions MWNT and SWNT showed a practically identical chemical reactivity when they were transformed into the corresponding CNT-PLLA hybrids through an oxidation/esterification process. In both cases, the covalent bonding of PLLA to the nanotube wall was unequivocally ascertained by FTIR and Raman spectroscopy. However, appreciable differences were found in

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