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Synthesis and Solution Behavior of Carbon Nanotubes Decorated with Amphiphilic Block Polyelectrolytes† Grigoris Mountrichas, Nikos Tagmatarchis, and Stergios Pispas* Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou AVenue, 11635 Athens, Greece ReceiVed: NoVember 13, 2006; In Final Form: December 20, 2006
The aqueous solubilization of carbon nanotubes (CNTs) with the aid of a block copolymer possessing one polyelectrolyte block (namely polystyrene-b-sodium (sulfamate/carboxylate polyisoprene)) is reported. The solubilization protocol, based on the co-dissolution of the copolymer and the CNTs, leads to the formation of supramolecular assemblies on the side walls of the tubes. Electron microscopy as well as infrared spectroscopy and thermogravimetric analysis were employed as meaningful probes to identify the adsorption of the polymer onto the surface of CNTs and the composition of the final hybrid material. Viscosity measurements on solutions of the copolymer decorated CNTs indicate that the polyelectrolyte effect, which is observed in the case of net polymers, is preserved in a lesser extent in the case of the copolymer/CNTs dispersions.
Introduction A large part of the scientific society has been involved in the study of the nanoworld, investigating the unique and unexpected properties of nanomaterials as well as their potential applications in science and technology. Carbon nanotubes (CNTs) is the nanomaterial that has already attracted major interest for its novel and exotic properties.1 To name only a few of the remarkable features of these nanomaterials, CNTs possess excellent thermal and electrical properties, an unusually high aspect ratio, high tensile strength, and field-effect transistor properties. However, despite the desirable properties of CNTs, their application spectrum is still rather limited due to insolubility problems in all solvents. In particular, extended van der Waals interactions between the side walls of CNTs lead to their aggregation into insoluble bundles of different length and diameter. In order to overcome such solubility problems, covalent modification of CNTs with polymer or low molecular weight organic compounds has been studied in detail.2 However, such approaches introduce extra disadvantages as covalent attachment of any material onto the skeleton of CNTs destructs their continuous π-electronic network, resulting in the loss of their unique electronic properties. Alternatively, the physical adsorption of molecules has been extensively used as a solubilization technique that keeps intact the perfect structure of the CNTs.3 The initial exfoliation of CNTs from their bundles is frequently achieved by sonication of CNTs in solvent or polymer solution.4-18 A large gamut of polymers, including polyelectrolytes, has been already used for the solubilization of CNTs via wrapping of the polymer around CNTs. Examples of such polymers include poly(vinyl pyrrolidone),4 poly(styrene sulfonate)4 and poly(metaphenylenevinylene),5 as well as some natural polymers like amylose,6 Arabic gum,7 gelatin,8 and † Part of the special issue “International Symposium on Polyelectrolytes (2006)”. * Corresponding author: Telephone: +30210-7273824. Fax: +302107273794. E-mail:
[email protected].
DNA.9 Recently, the combination of two different monomeric units along the polymer chain has been found to improve the solubilization procedure. Random copolymers like poly[(vinylidene fluoride)-co-trifluroethylene],10 poly(acrylic acidco-styrene),11 and random copolymers containing quaternized vinylpyridinium bromide moieties12 have been used as exfoliating and stabilizing molecules for CNTs dispersions. Furthermore, promising results have been obtained using block copolymers for the solubilization of CNTs because of their action as high molecular weight surfactants. Block copolymers containing pyrene moieties in one block have been presented separately by Adronov13 and Je´roˆme14 groups, and the ability of A-B-A block telomers to wrap CNTs has been presented by Kitano et al.15 Moreover, the solubilization of CNTs by micelle encapsulation, using poly(styrene-b-acrylic acid), has been studied by Kang and Taton.16 Finally, the use of block copolymers for the solubilization of CNTs has been also demonstrated by Park and co-workers using poly(styrene-bvinylpyridine)17 and Agarwal and co-workers using poly(styrene-ethylene oxide) and poly(methylmethacrylate-b-ethylene oxide).18 As far as we know, the use of block copolymers with a strong polyelectrolyte block has not yet been fully considered for the dispersion of CNTs. In order to achieve the polyelectrolyte mediated solubilization of the CNTs, we designed a block copolymer with the appropriate architecture, i.e., a linear block copolymer with one hydrophobic block, able to anchor on the nanotubes, and a second polyelectrolyte block providing electrosteric stabilization.19 Herein, we present results on the use of the particular block copolymer, consisting of polystyrene and a heparin-like, biocompatible polyelectrolyte block, as an efficient surfactant for the solubilization of CNTs. Moreover, the physical state of the self-assembled polymeric nanostructures on the side wall of CNTs has been investigated. Finally, the solution viscosity of the polymer decorated CNTs has been studied as a function of the concentration of the nanoassemble, and the behavior of this material is discussed taking into account
10.1021/jp067500k CCC: $37.00 © 2007 American Chemical Society Published on Web 03/03/2007
8370 J. Phys. Chem. B, Vol. 111, No. 29, 2007 the polyelectrolyte effect, introduced by the presence of the block copolymer chains, as well as the high aspect ratio of CNTs.
Mountrichas et al. SCHEME 1: Schematic Representation of CNTs/Amphiphilic Block Polyelectrolyte Nanoassemblies
Experimental The synthesis of the block copolymer has been realized by anionic polymerization high vacuum techniques and a postpolymerization functionalization reaction.20 The total molecular weight of the polystyrene-b-(sulfamate/carboxylate polyisoprene) copolymer is Mw ) 282 800; the molecular weight of the polystyrene block is Mw ) 12 500 (determined by taking into account the extent of the functionalization reaction and the size exclusion chromatography and NMR data of the precursor). Multiwall carbon nanotubes (MWCNTs) and shorted multiwall carbon nanotubes (s-MWCNTs) (Nanostructured and Amorphous Materials Inc.) were used without any further purification. The solubilization of CNTs was realized by co-dissolution of CNTs and a diblock copolymer in water with the aid of mild sonication. Typically, 15 mg of CNTs and 45 mg of the polymer were dissolved in 30 mL of distilled water in a sonication bath for 5 min. The ink-like solution was centrifuged to remove any insoluble material, filtered (PTFE, 0.2 µm), and washed in order to purify the CNT-based hybrid from the free polymer. Thermogravimetric measurements were performed by using a TGA-2050 instrument (TA Instruments) with a heating rate of 10 °C min-1. ATR-IR spectroscopy was done using a Fourier Transform IR spectrometer (Equinox 55 from Bruker Optics), equipped with a single reflection diamond ATR accessory (DuraSamp1IR II by SensIR Technologies). Finally, transmission electron microscopy (TEM) images were recorded with a Philips TEM 208 at an accelerating voltage of 100 kV. For the viscosity measurements at different concentrations, CannonUbbelohde dilution viscometers were used in a temperaturecontrolled bath at 30 °C (temperature stability ( 0.02 °C). Flow times for the solvent and the solutions were always higher than 120 s. The solutions were made by dissolving the samples in distilled water (conductivity less than 2 µS/cm). Solutions of lower concentration were obtained by dilution of the stock solutions within the viscometer. The measurements were stopped when the viscosity difference of polyelectrolyte solution and pure water dropped below 10%.
an ink-like appearance. Sonication promotes solubilization of the components at room temperature. The present preparation protocol is simple and different from those reported in the literature so far,4-18 because it does not involve the use of cosolvents or CNTs addition to preformed solutions of the polymer. Interesting features arise from the TEM investigation of the block copolymer decorated CNTs. In Figure 1a, the fine dispersion of the nanotubes is visualized; upon higher magnification, the existence of several hemi-micelles along the CNTs side-walls is observed, in addition to the roughening of the nanotubes surface. The later observation can be ascribed to the adsorption of the polymer (Figure 1b, c, respectively). To the best of our knowledge, this is the first time that adsorbed micelles are observed onto CNTs through interactions with the micellar core. The above observation could be possibly explained by the solubilization protocol, which involves codissolution of the copolymer and the CNTs in water. In particular, the polymeric micelles are formed in the presence of the CNTs, which force the adsorption of the whole micellar structure on the side-walls of the nanotubes. The coexistence of copolymer chains and CNTs in the final material has been also demonstrated by IR spectroscopy. The ATR-IR spectra of the net polymer, pristine CNTs, as well as of polymer decorated CNTs are presented in Figure 2. In the polymer decorated CNTs, the presence of a wrapped polymer is verified as the vibrations due to the net polymer (i.e., broad bands at 1040 and 1144 cm-1 can be correlated with the SO3
Results and Discussion It is well-known that annealed polyelectrolytes are very soluble in water because of inter- and intramolecular electrostatic repulsion among the ionic sites.21 Moreover, the ability of structures containing aromatic units to “stick” on the side wall of CNTs has been also reported.22 Thus, because our goal was to solubilize CNTs through noncovalent π-π stacking interactions, a block copolymer containing both planar aromatic moieties and ionic sites was synthesized. Polystyrene was considered as an ideal candidate of the one block, because it is highly hydrophobic and possesses an aromatic ring at every monomeric unit. On the other hand, the criteria, which have been used for the choice of the polyelectrolyte block, were the high water solubility and the high charge density. Sodium sulfamate/carboxylate polyisoprene is a novel polyelectrolyte, which fulfills the above criteria; also, it is biocompatible, and it has anticoagulant activity.23 The diblock copolymer was prepared by a post-polymerization reaction on a polystyreneb-polyisoprene precursor. The protocol followed for the solubilization of CNTs is depicted in Scheme 1. A direct codissolution of the copolymer and the CNTs in water resulted in hybrid dispersions very stable for a long period of time having
Figure 1. Representative TEM images showing (a) the polymer decorated multiwalled carbon nanotubes, (b) the presence of hemimicelles onto CNTs, and (c) the roughening of the CNTs surface.
Synthesis/Solution Behavior of CNTs
Figure 2. ATR-IR spectra of polymer (solid line), MWCNTs (dot line), and polymer decorated CNTs (dashed line).
Figure 3. TGA graph of CNTs (dotted line), block copolymer (solid line), and polymer decorated CNTs (dashed line).
group symmetric and asymmetric stretching vibration) are preserved in the spectrum of the hybrid material. The up-turn of the baseline toward higher wavenumbers is attributed to the absorption characteristics of the CNTs in the mid-IR region due to electronic contributions.24 Although qualitative observations of the above system have been performed by TEM and ATR-IR, a quantitative investigation concerning the composition of the hybrid has been realized by TGA. The thermograph of the polymer decorated CNTs, as well as the pure individual components of the final material, is given in Figure 3. It is obvious that almost half of the sample (55.2 wt %) consists of the block copolymer. The above result indicates that a large amount of copolymer is needed in order to achieve water solubility of CNTs. Furthermore, the polymer decomposition during TGA experiments in the presence of CNTs follows a different pathway as compared with the net polymer (i.e., Figure 4); however, we cannot offer a possible explanation for the above observation with the data at hand. The solution behavior of the polymer decorated CNTs is of great interest because it is well-established that for materials with high aspect ratios (such as rigid rod-like nanostructures) it is possible to induce changes in the solution viscosity, and they could possibly serve as drag reducing additives during turbulent flows.25 In this context, viscometry studies on the polymer decorated CNTs solutions were performed revealing interesting features. In Figure 4, the viscosity results are
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Figure 4. ηsp vs sample concentration for the polymer (squares) and polymer decorated s-CNTs (inverse triangles).
Figure 5. ηsp/c vs sample concentration for the polymer (squares) and polymer decorated s-CNTs (triangles).
presented for the s-MWCNTs/polymer hybrid sample (ηsp ) (ηo - ηs)/ηs where ηo is the solution viscosity and ηs is the viscosity of the solvent). In this plot, a relative decrease of the viscosity in the sample containing the CNTs is observed. The above decrease could be attributed to the presence of the CNTs, which force the creation of superstructures with high aspect ratios. It has to be noticed that the measured flow times for the MWCNTs/polymer sample were identical with those of pure water (for the same concentration with the s-MWCNT/polymer sample), indicating that the presence of elongated structures reduces the viscosity of the copolymer containing solution. Moreover, the slope of both curves (water and CNT/polymer solution) is the same, indicating the existence of the same hydrodynamic interaction among the dissolved species. It is worth mentioning that if the nanotubes are fully polymer decorated; electrostatic interactions among polyelectrolyte chains will be dominant in both cases. Finally, it is well-established that aqueous polyelectrolyte solutions show the polyelectrolyte effect;21c i.e., upon decreasing polyelectrolyte concentration in salt free solutions, the values for the ratio ηsp/c are increased. In Figure 5, the ratio ηsp/c vs concentration for the polymer decorated CNTs and net copolymer is plotted. The polyelectrolyte effect is clearly demonstrated in the case of the net polymer. However, in the case of the copolymer/CNTs hybrid sample, only a slight increase is
8372 J. Phys. Chem. B, Vol. 111, No. 29, 2007 observed upon dilution of the solution. The aforementioned upturn could be attributed to the polyelectrolytic character of the nanoassembly, which is preserved independently of the CNTs addition due to the presence of the block polyelectrolyte. However, the polyelectrolyte effect is significantly reduced, probably due to the stiffness and the high aspect ratio of the nanotubes, which probably considerably contribute to the rheological behavior of the solutions, through alignment distribution of the rod-like objects. Conclusions The ability of a polystyrene-b-(sodium sulfamate/carboxylate polyisoprene) block copolymer with one polyelectrolyte block to solubilize CNTs has been demonstrated. The adsorption of the copolymer on the nanotubes side-walls and the formation of the copolymer/CNT nanohybrid was probed by TEM, ATRIR and TGA. Electron microscopy images indicate the existence of supramolecular structures (hemi-micelles) onto CNTs, the creation of which is probably explained by the dissolution protocol. The viscosity data from copolymer/CNT dispersions revealed that the polyelectrolyte effect is preserved in the composite material, although the absolute value of the viscosity has been significantly reduced demonstrating the potential use of copolymer/CNTs-based nanohybrids as viscosity modifiers. Acknowledgment. This work, conducted as part of the award “Functionalization of Carbon Nanotubes Encapsulating Novel Carbon-based Nanostructured Materials” made under the European Heads of Research Councils and European Science Foundation EURYI (European Young Investigator) Awards scheme, was supported by funds from the Participating Organizations of EURYI and the EC Sixth Framework Programme. Experimental assistance on TGA measurements by Dr. D. Tsiourvas is acknowledged. References and Notes (1) (a) Introduction to Nanotechnology; Poole, C. P., Owens, F. J., Eds; Wiley-Interscience: Weinheim, Germany, 2003. (b) Nanophysics and Nanotechnology: An Introduction to Modern Concepts in Nanoscience; Wolf, E. L., Ed.; John Wiley & Sons: New York, 2004. (c) Acc. Chem. Res. 2002, 35, 997. (2) For recent reviews, see (a) Balasubramanian, K.; Burghard, M. Small 2005, 1, 180. (b) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. ReV. 2006, 106, 1105. (3) (a) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838. (b) Nakashima, N.; Tomonari, Y.; Murakami, H. Chem.
Mountrichas et al. Lett. 2002, 6, 638. (c) Liu, P. Eur. Polym. J. 2005, 41, 2693. (4) O’Connell, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265. (5) (a) Star, A.; Stoddart, J. F.; Steuerman, D.; Diehl, M.; Boukai, A.; Wong, E. W.; Yang, X.; Chung, S. W.; Choi, H.; Heath, R. J. Angew. Chem., Int. Ed. 2001, 40, 1721. (b) Star, A.; Liu, Y.; Grant, K.; Ridvan, L.; Stoddart, J. F.; Steuerman, D. W.; Diehl, M. R.; Boukai, A.; Heath, J. R. Macromolecules 2003, 36, 553. (c) McCarthy, B.; Coleman, J. N.; Czerw, R.; Dalton, A. B.; Byrne, H. J.; Tekleab, D.; Iyer, P.; Ajayan, P. M.; Blau, W. J.; Caroll, D. L. Nanotechnology 2001, 12, 187. (6) Star, A.; Steuerman, D. W.; Heath, J. R.; Stoddart, J. F. Angew. Chem., Int. Ed. 2002, 41, 2508. (7) Bandyopadhyaya, R.; Nativ-Roth, E.; Regev, O.; Yerushalmi-Rozen, R. Nano Lett. 2002, 2, 25. (8) Takahashi, T.; Tsunoda, K.; Yajima, H. Ishii, T. Chem. Lett. 2002, 7, 690. (9) Gigliotti, B.; Sakizzie, B.; Bethune, D. S.; Shelby, R. M.; Cha, J. N. Nano Lett. 2006, 6, 159. (10) El-Hami, K.; Matsushige, K. Chem. Phys. Lett. 2003, 368, 168. (11) Dror, Y.; Pyckout-Hintzen, W.; Cohen, Y. Macromolecules 2005, 38, 7828. (12) Sinani, V. A.; Gheith, M. K.; Yaroslavov, A. A.; Rakhnyanskaya, A. A.; Sun, K.; Mamedov, A. A.; Wicksted, J. P.; Kotov, N. A. J. Am. Chem. Soc. 2005, 127, 3463. (13) Bahun, G. J.; Wang, C.; Adronov, A. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1941. (14) Lou, X.; Daussin, R.; Cuenot, S.; Duwez, A. S.; Pagnoulle, C.; Detrembleur, C.; Bailly, C., Je´roˆme, R. Chem. Mater. 2004, 16, 4005. (15) Kitano, H.; Tachimoto, K.; Nakaji-Hirabayashi, T.; Shinohara, H. Macromol. Chem. Phys. 2004, 205, 2064. (16) Kang, Y.; Taton, T. A. J. Am. Chem. Soc. 2003, 125, 5650. (17) Shin, H.; Min, B. G.; Jeong, W.; Park, C. Macromol. Rapid Commun. 2005, 26, 1451. (18) Cotiuga, I.; Picchioni, Francesco, F.; Agarwal, U. S.; Wouters, D.; Loos, J.; Lemstra, P. J. Macromol. Rapid Commun. 2006, 27, 1073. (19) (a) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academia Press: New York, 1983. (b) Sung, A. M.; Piirma, I. Langmuir 1994, 10, 1393. (20) Pispas, S. J. Polym. Sci, Part A: Polym. Chem. 2006, 44, 606. (21) (a) Forster, S.; Schmidt, M. AdV. Polym. Sci. 1995, 120, 51. (b) Radeva, T. Ed. Physical Chemistry of Polyelectrolytes; Marcel & Dekker Inc., New York, 2001. (c) Kulicke, W. M.; Clasen, C. Viscosimetry of Polymers and Polyelectrolytes; Springer: Berlin, 2004. (22) Zorbas, V.; Smith, A. M.; Xie, H.; Ortiz-Acevedo, A.; Dalton, A. B.; Dieckmann, G. R.; Draper, R. K.; Baughman, R. H.; Musselman, I. H. J. Am. Chem. Soc. 2005, 127, 12323. (23) Sederel, L. C.; van der Does, L.; van Duijl, J. F.; Beugeling, T.; Bantjes, A. J. Biomed. Mater. Res. 1981, 15, 819-827. (24) Kim, U. J.; Furtado, C. A.; Liu, X.; Chen, G.; Eklund, P. C. J. Am. Chem. Soc. 2005, 127, 15437. (25) (a) Kulicke, W. M.; Kotter, M.; Grager, H. AdV. Polym. Sci. 1989, 89, 1. (b) Singh, R. P., Ed. Encyclopedia of Fluid Mechanics; Gulf Publishing: Gulf Huston, TX 1990; Vol. 9, Chapter 14.
