© Copyright 2001 American Chemical Society
AUGUST 21, 2001 VOLUME 17, NUMBER 17
Letters Self-Organization of PEO-graft-Single-Walled Carbon Nanotubes in Solutions and Langmuir-Blodgett Films Masahito Sano,* Ayumi Kamino, Junko Okamura, and Seiji Shinkai Chemotransfiguration ProjectsJST, 2432 Aikawa, Kurume, Fukuoka 839-0861, Japan Received January 24, 2001. In Final Form: May 16, 2001 Poly(ethylene oxide) (PEO), soluble in both water and many organic solvents, is grafted onto singlewalled carbon nanotubes (SWNTs), and aggregation behaviors of the resulting PEO-graft-SWNT in solutions and in Langmuir-Blodgett (LB) films are investigated. SWNTs, cleaved by acid, are dispersed relatively well in DMF and water, but poorly in chloroform and THF. PEO-graft-SWNT was synthesized by treating acid-cut SWNTs with SOCl2, followed by a reaction with monoamine-terminated PEO in a DMF and water medium. Atomic force microscopy reveals that PEO and SWNT segments take expanded and extended conformations when freshly prepared PEO-graft-SWNTs are cast from water. When PEO-graft-SWNTs are dispersed in chloroform, each SWNT segment collapses into a globular aggregate. Aging the chloroform dispersion produces self-organized structures detectable by light scattering. Langmuir-Blodgett films made from this aged solution afford a surface-micelle structure in which the coagulated collapsed SWNT core is surrounded by extended PEO patches. Addition of DMF to this chloroform solution re-expands the SWNT segments, although not completely. These results demonstrate that the conformation of SWNTs can be controlled by solvent quality as if they are ordinary hydrocarbon-based block copolymers. Yet, the conformational change is not completely reversible, and coagulation, rather than entanglement, becomes the major event even at locally concentrated regions.
Introduction Recent advances in the synthesis of single-walled carbon nanotubes (SWNTs) have allowed high-quality samples to be produced routinely,1 making SWNTs a promising component for the development of nanotechnology. In contrast to successful dry processes in which practical applications in electronics are already established,2 wet chemical approaches to functionalization of SWNTs have just begun. As an initial condition for chemical modification, SWNTs must be made soluble or dispersible in common solvents.3-5 In the case that pristine SWNTs are very long, they are cut randomly by ultrasonication in strong acids to help them disperse. Further improvement (1) Rinzler, A. G.; Liu, J.; Dai., H.; Nikolaev, P.; Huffman, C. B.; Rodrı´guez-Macı´as, F. J.; Boul, P. J.; Lu, A. H.; Heymann, D.; Colbert, D. T.; Lee, R. S.; Fischer, J. E.; Rao, A. M.; Eklund, P. C.; Smalley, R. E. Appl. Phys. A 1998, 67, 29-37. (2) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512-514.
in dispersibility has been attained by covalently linking suitable functional groups to SWNTs through carboxylic acids produced by the acid treatments.6-9 Also, aggregation of tubes needs to be controlled,10,11 because SWNTs coagulate quite easily and randomly to produce a rope, a bundle of many tubes, in solution. One (3) Boul, P. J.; Liu, J.; Mickelson, E. T.; Huffman, C. B.; Ericson, L. M.; Chiang, I. W.; Smith, K. A.; Colbert, D. T.; Hauge, R. H.; Margrave, J. L.; Smalley, R. E. Chem. Phys. Lett. 1999, 310, 367-372. (4) Ausman, K. D.; Piner, R.; Lourie, O.; Ruoff, R. S.; Korobov, M. J. Phys. Chem. B 2000, 104, 8911-8915. (5) Bahr, J. L.; Mickelson, E. T.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. Chem. Commun. 2001, 193-194. (6) Wong, S. S.; Joselevich, E.; Woolley, A. T.; Cheung, C. L.; Lieber, C. M. Nature 1998, 394, 52-55. (7) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, A. M.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95-98. (8) Liu, Z.; Shen, Z.; Zhu, T.; Hou, S.; Ying, L.; Shi, Z.; Gu, Z. Langmuir 2000, 16, 3569-3573. (9) Riggs, J. E.; Guo, Z.; Carroll, D. L.; Sun, Y.-P. J. Am. Chem. Soc. 2000, 122, 5879-5880.
10.1021/la010126p CCC: $20.00 © 2001 American Chemical Society Published on Web 07/27/2001
5126
Langmuir, Vol. 17, No. 17, 2001
of the common methods to control aggregation is to make a compound amphiphilic so that it may self-organize to well-defined structures. Since SWNTs have a large aspect ratio of length to diameter, self-organization in two dimensions is also of interest for possible controls of their orientation. To investigate both organic and aqueous environments and to utilize Langmuir-Blodgett techniques for long SWNTs, poly(ethylene oxide) (PEO) was chosen for grafting. Self-organization of an ordinary hydrocarbon-based polymer is largely based on multi-block structures in which each block has different thermodynamic solution properties. Ordinary polymers collapse or expand depending on solvent quality or temperature that can be rationalized by the χ-parameter. Entanglement becomes the most influential event at high concentrations. Although SWNTs differ chemically from hydrocarbon-based polymers, their large Young’s moduli and maximum tensile strength12 as well as huge length-to-diameter ratio suggest that a SWNT may shrink or extend without damaging itself. On the other hand, because SWNT-solvent interactions are largely unknown and the length of SWNTs approaches macroscopic scales, it is not clear how similar SWNTs behave in solution compared with ordinary polymers. Thus, it is necessary to perceive structural responses of SWNT when solution conditions are changed. In this paper, the dispersing property of SWNTs in various solvents is investigated first. Then, dynamic light scattering (DLS) and atomic force microscopy (AFM) were used to analyze conformation and aggregation states in solution and on the surface. AFM observations have shown that our samples, even after the acid cutting, contain tubes that are a few microns long. These are considerably longer than those used by others that are only a few hundred nanometers long.7-10 In this study, centrifugation was extensively used to regulate the size of SWNTs. Experimental Section Dispersion Property. Pristine SWNTs (purchased from Tubes@Rice) were cut and etched in acids according to the reported procedure.13 Shortened SWNTs were divided into several portions of equal amounts. Each portion of SWNT was ultrasonicated (30 s in a laboratory ultrasonic cleaner, 42 kHz, 90 W) in various solvents. The amount of SWNT initially added to the solvent was 0.5 mg/mL. Immediately afterward, the dispersion was subjected to centrifugation (3500g for 10 min) and the concentration of supernatant was measured. This process was repeated over a period of time at room temperature. Synthesis of PEO-graft-SWNT. Although we followed the same synthetic scheme as previously reported,7 we paid particular attention not to let SWNTs dry or coagulate completely. The acid treatment produces carboxylic acid groups on SWNTs.14 SWNTCOOH was refluxed in neat SOCl2 for 24 h. The cooled dispersion was centrifuged at 3500g for 10 min, and only the solid sediment was retained. SWNT-SOCl was further washed several times by repeating ultrasonication in dry THF and DMF and centrifugation, followed by decantation of the supernatant solution. A small amount of SWNT-SOCl dispersed in DMF was added to monoamine-terminated PEO (molecular weight 5000) dissolved in 10 mM NaOH solution (250 mg/mL). The mixture was stirred for 12 h (10) Liu, J.; Casavant, M. J.; Cox, M.; Walters, D. A.; Boul, P.; Lu, W.; Rimberg, A. J.; Smith, K. A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1999, 303, 125-129. (11) Vigolo, B.; Pe´nicaud, A.; Coulon, C.; Sauder, C.; Pailler, R.; Journet, C.; Bernier, P.; Poulin, P. Science 2000, 290, 1331-1334. (12) Lieber, C. M. Solid State Commun. 1998, 107, 607-616. (13) 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.; RodriguezMacias, F.; Shon, Y.-S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253-1256. (14) Mawhinney, D. B.; Naumenko, V.; Kuznetsova, A.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E.Chem. Phys. Lett. 2000, 324, 213-216.
Letters
Figure 1. Dispersing properties of acid-treated SWNTs in various solvents. The filled circles are for 10 mM NaOH, and the solid curve is the result of least-squares fitting by an exponential function with a time constant of 470 h. at room temperature. The solid sediment was retained after centrifugation. It was washed repeatedly with water using centrifugation and decantation cycles until the supernatant solution was colorless and no longer foamed. Then, the dispersion was ultrasonicated, which resulted in a colored supernatant solution as well as a solid residue after centrifugation. The colored supernatant solution was used for further experiments. The black film obtained from filtering the supernatant solution showed excellent electrical conductivity. Fourier transformed infrared spectroscopy (FTIR, Nicolet 710) of the black product shows a strong peak at 1100 cm-1, which is indicative of ether oxygen. DLS (DLS-7000, Otsuka Electronics) measurements were conducted with a wavelength of 632.8 nm and a scattering angle of 90° at 30 °C. The data were analyzed by a histogram method. AFM (TopoMetrix) was operated in the noncontact mode in air at room temperature. Unless otherwise stated, all measurements were taken within a few days after preparation. Langmuir-Blodgett Films. The compound dispersed in chloroform was spread on pure water in a LB trough (FSD-50, USI System) at 20 °C. The film was compressed with a rate of 0.3 cm2/s. To characterize individual assembly, LB films were made by spreading at 20 m2/mg and transferring at 10 m2/mg on mica.
Results and Discussion Dispersing Property of Acid-Treated SWNT. It is important to realize that most of SWNTs are not individually dispersed in solution, but exist as ropes. Also, tubes with a larger number of defects tend to produce many short tubes by acid treatments.13 Accordingly, absolute values of the dispersed amount depend heavily on the quality of pristine samples and the strength of centrifugation. Nevertheless, as long as SWNTs are taken from the same batch after the acid treatments, relative differences caused by solvents can be used as a measure of dispersing quality. Figure 1 shows the temporal changes of the dispersed amount in 10 mM NaOH solution as well as the amounts immediately after dispersion in organic solvents at room temperature. SWNTs are dispersed relatively well in DMF and water, in agreement with other reports,6,10,11,13 but coagulate rapidly in THF and CHCl3. Although SWNTs coagulate gradually in water, the process is very slow with a time constant of 470 h. Contrary to a previous report,15 we did not see an increase in viscosity or gelation that indicated entanglement, probably due to the different pristine samples and preparation methods used by us and others. PEO-graft-SWNT in Water. Figure 2 displays AFM images of PEO-graft-SWNTs that were cast on mica from aqueous dispersion. Contrast was amply enhanced to show (15) Shaffer, M. S. P.; Windle, A. H. Macromolecules 1999, 32, 68646866.
Letters
Langmuir, Vol. 17, No. 17, 2001 5127
Figure 3. Surface pressure-area isotherm of PEO-graftSWNT. The inset shows that of monoamine-terminated PEO.
Figure 2. AFM images of (a) an extended tube and (b) a collection of tubes obtained from a cast film of PEO-graft-SWNT from an aqueous dispersion on mica. Contrast is enhanced to make thin PEO films visible.
very thin patches. The predominantly found feature is the extended straight chains that are 1-2 nm thick, corresponding to single or double SWNT chains (Figure 2a). A faint cloudlike feature, only a few angstroms thick, attached to a long extended chain is most likely to be the PEO segments, because we have not seen this feature in cast films of unreacted SWNTs.16 By imaging cast films of pure PEO solution, we have noted that PEO chains tend to collect and condense into islands of round patches having uniform heights. Thus, the faint feature results probably because only a small number of PEO chains are involved. This small grafting density is also a reason that FTIR could not discern the amide peak. Although we could not identify the exact grafting sites, previous works using the same reaction scheme indicate that they are likely to be at the open ends of SWNTs.7,13 When many PEO-graftSWNTs are gathered, however, some chains still form bundles and globular aggregates (Figure 2b). Both figures show that PEO segments are imaged larger than what we expect from molecular weight 5000 chains even when an enlarging effect of AFM tip convolution is considered. It (16) The PEO film swells in water or DMF. Swelling makes the film thicker, whereas softening allows the AFM tip to penetrate deeper. The film elastic property is also altered. Thus, the thickness of objects involving PEO measured by AFM is subjected to considerable errors. Contrarily, a hard material like SWNT gives a height with reasonable accuracy.
is likely that unreacted PEO chains are entangled with grafted chains strongly enough that they remain attached even after repeated washing. This strong physisorption occurs only with the grafted chains, since physisorbed PEO chains without anchoring sites tend to aggregate by themselves, as evidenced by a phase-separated feature seen in a cast film of a physical mixture of PEO and unmodified SWNTs. Thus, the present sample contains both PEO-graft-SWNTs and physisorbed PEO chains that are strongly entangled with grafted PEO chains. At any rate, no self-organized structures are found in water soon after the synthesis. Since water is a good solvent for PEO and also a good dispersing solvent for SWNTs, both PEO and SWNT segments expand and do not satisfy the selforganization conditions. Self-Assemblies in CHCl3 and Langmuir-Blodgett Films. PEO-graft-SWNTs could be collected from the water dispersion because they could not go through a 0.2 µm Teflon filter. The collected compound was dispersed in CHCl3, and the supernatant solution after centrifugation was retained. The compound was now sent through the Teflon filter to give a colored filtrate. AFM images of a cast film on mica show small globular aggregates of several tens of nanometers to a few hundred nanometers, and no features similar to those in Figure 2 were seen. This indicates that SWNTs have collapsed in CHCl3. The same chloroform solution was spread on the water surface to obtain the surface pressure-area isotherm, as shown in Figure 3. The pressure increases gradually until 7 mN/m at 0.4 m2/mg and then rapidly afterward upon compression. Because the SWNT segments account for most of the mass and PEO alone has a collapse pressure near 7 mN/m (see the insert), the isotherm until 0.4 m2/ mg is mostly governed by the grafted PEO segments. The low molecular weight PEO tends to dissolve in water at high compression. Thus, the rapid increase past this area is caused by compression of the SWNT segments. These isothermal behaviors suggest that PEO-graft-SWNT chains are not entangled, but segregated strongly. This is supported by AFM observations that LB films on mica consist of small globular aggregates as seen in the cast film and not a continuous film. Quite often, these small aggregates form ring-shaped islands with very thin patches of PEO around. LB films made at smaller molecular areas simply produced more ring-shaped islands. No bundles were observed. Thus, once SWNT segments are collapsed in CHCl3, they do not re-expand when they are spread on a water surface. The above results indicate that the collapsed state prevents PEO-graft-SWNTs from forming bundles. This implies that PEO-graft-SWNTs have an opportunity to self-assemble. So, we let the chloroform solution stand still at room temperature for 3 months. DLS of this aged
5128
Langmuir, Vol. 17, No. 17, 2001
Letters
Figure 6. AFM image of a LB film made from spreading the CHCl3 with a DMF dispersion of PEO-graft-SWNT used to obtain the data as shown in Figure 4c. Although this strip feature is reproducibly found, the surface micelle feature shown in Figure 5 still dominates on the same surface.
Figure 4. Size distributions obtained by dynamic light scattering: (a) aged PEO-graft-SWNT dispersion in CHCl3; (b) dispersion of part a immediately after addition of 0.5% DMF; (c) same dispersion as in part b 2 h later.
Figure 5. AFM image of a LB film made from spreading the CHCl3 dispersion of PEO-graft-SWNT used to obtain the data as shown in Figure 4a.
solution indicates the presence of aggregates with a hydrodynamic diameter of 60 ( 15 nm in solution (Figure 4a). The LB film made from this aged solution gives the AFM image of Figure 5. Small globular aggregates seen from the fresh solution sample can no longer be seen. A protruded core is surrounded by thin films, forming a round “surface micelle” structure. The average core volume directly measured from AFM images is 1.4 × 10-4 µm3, which is close to the volume of a sphere with a diameter of 60 nm. Thus, the surface micelle consists of a coagulated aggregate of collapsed SWNT segments surrounded by expanded PEO chains. The PEO segments appear larger
than those expected by the same reason as discussed above. Despite repeated AFM imagings on separately prepared samples, we could not resolve individual SWNTs in the core region. This is explained by the presence of soft PEO films covering the top surface of aggregated SWNT cores. This model suggests that the micelles with a similar structure are assembled in solution by aging. Knowing that DMF is a good dispersing solvent, 0.5 wt % of DMF is added to the above chloroform solution. DLS shows that the previous 60 ( 15 nm aggregates expand and broaden to 90 ( 55 nm immediately after the addition of DMF (Figure 4b) and then to 150 ( 105 nm 2 h later (Figure 4c). At the same time, small aggregates are emerging. Corresponding LB films contain not only the surface micelles, as before, but also feature strips, as depicted in Figure 6. The strips have a uniform thickness of 1 nm, a width of 90 nm, and a length in the range 300-700 nm.16 The height and width distributions are quite narrow. The strip structure was not seen in a cast film of PEO-graft-SWNT in DMF, indicating that the strips resulted from partial re-expansion of once collapsed chains. We think that the strips are related to the small aggregates detected by DLS. These results indicate that the collapsed SWNT segments re-expand by addition of DMF and small portions are released into the solution. The re-expansion is not to the full extent. In fact, replacing CHCl3 by DMF does not bring the once collapsed chains to extended forms. Heating and prolonged ultrasonication often lead to coagulation. Conclusion We have shown that the conformation of SWNTs can be controlled by solvent quality and the self-organized structures can be manipulated as if PEO-graft-SWNT is an ordinary block copolymer. On the other hand, certain differences were found. The dispersion is only kinetically stable. Coagulation, rather than entanglement, is the major factor even at locally concentrated regions. The structural change is not completely reversible. It is difficult to redisperse and re-extend once the SWNT segments are coagulated and collapsed. These points should be considered carefully when a wet chemical approach is applied to functionalization of SWNTs. LA010126P
