13655
2007, 111, 13655-13658 Published on Web 08/29/2007
Coating of Inner and Outer Carbon Nanotube Surfaces with Polymers in Supercritical CO2 Evgeniya H. Lock,*,† Wilson Merchan-Merchan,‡ James D’Arcy, Alexei V. Saveliev, and Lawrence A. Kennedy Department of Mechanical and Industrial Engineering, UniVersity of Illinois at Chicago, Chicago, Illinois 60607 ReceiVed: July 19, 2007; In Final Form: August 17, 2007
Polymer coating and filling of carbon nanotubes (CNTs) and CNT arrays under supercritical conditions is reported in this letter. As a supercritical solvent, supercritical carbon dioxide was chosen due to its unique chemical and physical characteristics allowing effective mass transfer. Flame synthesized and commercially obtained CNTs in powder form, as well as CNT arrays of high purity and alignment were employed to study the polymer coating of inner and outer CNT surfaces. The influence of polymerization process parameters was investigated. The outcome of this work is the development of a novel, fast and efficient method for coating and filling of CNTs and CNT arrays with polymers.
Although the coating of the outer carbon nanotube (CNT) surface can be performed in aqueous solutions and other organic solvents,1,2 the modification of the inner surface is much more complex. Different techniques have been explored for transport into CNTs or encapsulation of materials inside of CNT. Some of the reported methods are based on gas-phase diffusion,3,4 liquid-phase capillary action from solution5,6 or direct filling from molten media,7 collisions of accelerated atoms on tubes,8 ion exchange,9 or through the guidance of external force.10 In all cases, the issues of wettability of CNT surface, controllability, and simplicity of the proposed method need to be taken into consideration. The suggested method for filling and coating of carbon nanotubes employed in this study is the application of supercritical carbon dioxide as a reaction medium. Having a zero surface tension, the supercritical fluid (SCF) allows complete penetration into the nanotube cavity. With critical temperature (31 °C) close to the ambient and an easily achieved critical pressure of 74 bar, supercritical CO2 is an attractive solvent for the polymerization process. Compared with liquids, the higher diffusivity in SCF allows increased reaction rates when the diffusion limitations are great. Furthermore the lower viscosity of SCF in comparison to the liquids can reduce the solventcage effects and therefore increase the initiator efficiency. Since the solvent power of SCF is directly related to its density, a large variation in solubility can be achieved by simply changing the pressure and the temperature of the system. Another important feature of supercritical CO2 polymerization is plasticization, which results in the lowering of the polymer’s glass transition temperature. Moreover, by simple pressure reduction, * To whom correspondence should be addressed. E-mail: evgeniya.
[email protected]. † Present address: NRC Postdoctoral Associate, Plasma Physics Division, Naval Research Laboratory, Washington, DC 20375. ‡ Present address: Assistant Professor, School of Aerospace and Mechanical Engineering, University of Oklahoma, Norman, OK 73019.
10.1021/jp075684c CCC: $37.00
phase change is observed, and the supercritical fluid is transformed to a gas and exits the system without leaving any residues. It has been reported in the literature that different methods of synthesizing CNTs can yield structures containing unique physical characteristics and morphologies. Therefore, in the present study, CNTs synthesized by different techniques have been employed. The flame generated nanotubes with average diameter of 30 nm and length of 30 µm were grown in our laboratory.11 They had closed tips (cavities) and empty compartments that resembled bamboo-like structures. Two commercially available multiwall CNTs with open tips from Pyrograf, Inc. (purity 95%, dia 100 nm, L ) 30 µm) and from Nanolab (dia 15 nm, L ) 5 µm) also were used. Both were CVD grown, but the Pyrograph Inc. samples were heat treated since this improves the polymer adhesion to CNTs walls.12,13 The present study also shows the ability to coat CNTs forming arrays. CNT arrays were grown in our laboratory on the surface of a catalytic support, and details can be found in prior works.14 Three different sets of experiments were conducted, all with varying experimental procedure. In the first set, CNTs (Figure 1a) were coated and encapsulated with polystyrene and polyaniline. For this particular case, the monomers styrene and acrilonitrile (Sigma Aldrich) and the polymerization initiator benzoyl peroxide (Sigma Aldrich) were employed. Approximately 0.5 mg of CNTs was dispersed in a solution containing 5 mL of monomer and 68 mg of initiator and sonicated for approximately 10 min. Only 0.4 mL of this solution was placed in a specifically designed TEM grid holder, shown in Figure 1, so that the TEM grid was immersed in the solution. Then the grid holder was positioned in the reaction chamber, and the latter was sealed. Liquid carbon dioxide was compressed to 100 bar pressure by high-pressure pump (Thar Technology, model P-50) and was introduced into the reaction chamber (dia ) 8 mm and V ) 6 cm3) maintained at constant T ) 40 °C and P ) 100 bar. The CNTs and monomer and initiator solution was left to © 2007 American Chemical Society
13656 J. Phys. Chem. C, Vol. 111, No. 37, 2007
Figure 1. Schematics of experimental conditions for polymer coating, filling, and encapsulation of CNTs: (a) CNTs were immersed in the monomer and initiator solution, (b) half of the CNT arrays were immersed in the solution, (c) CNTs dry dispersed on a TEM grid, (d) CNT arrays as grown on solid support. In cases c and d, the solution was not in contact with the CNTs.
soak in supercritical CO2 for 2 h. Subsequently, the pressure was reduced and supercritical CO2 was transformed to the gas phase and exhausted. Finally, the TEM grid was removed and placed in an oven to initiate the polymerization at 100 °C for polystyrene and at 87 °C for polyacrylonitrile, respectively (their glass transition temperatures). Varying the pressure release time from approximately 2 to 5 min did not affect the obtained results. In the second set (Figure 1b), two different kinds of experiments with carbon nanotube arrays were conducted depending on the monomer solution age. In both cases, half of the wire on which CNT arrays were grown was immersed in
Letters the monomer solution. In the first experiment, a freshly made solution of 5 mL of styrene and 68 mg of BPO was used, and 0.2 mL of this solution was placed in the grid holder, so that the CNT arrays did not have contact with the monomer solution. A one week old solution was used in the second experiment; thus, the solution was partially polymerized. Then the foregoing procedure was followed except that the soaking time was increased to 3 h. In the third set of experiments, the supercritical carbon dioxide was used not only to transport monomer molecules into CNT cavities but also as a polymerization medium. Approximately 0.1 mg of Pyrograph’s CNTs were dry dispersed on the TEM grid, and 0.2 mL of monomer and initiator solution was placed on the bottom of the grid holder with no contact to the CNTs (Figure 1c). The reaction chamber T ) 40 °C and P ) 100 bar were held constant for 1 h, and then the temperature was raised to 60 °C at the same pressure. The sample was treated for an additional 2 h. Finally, the temperature was reduced to room temperature, and then the pressure was released. No further treatment at atmospheric conditions was required. Last, experiments with CNTs arrays were conducted as well with a week old monomer and initiator solution (Figure 1d). In this test, the CNT arrays did not have contact with the monomer solution. A temperature of 60 °C and a pressure of 120 bar were held constant for 4 h. The soaking time was 4 h and was sufficient for polymerization; no postprocessing of the sample was needed. All experiments showed that supercritical carbon dioxide was an excellent medium for coating of inner and outer CNT surfaces with polymers. In the first set of experiments, coating with different uniformity was observed in all nanotubes. The flame generated (Figure 2a,b) and Pyrograph’s nanotubes (Figure 2e,f) had uniform coatings of approximately 11 and 20 nm, respec-
Figure 2. (a and b) Coating of flame generated CNT [11] with polystyrene; (c and d) coating and encapsulation of Nanolab’s CNTs with polystyrene; (e and f) encapsulation and single coating of Pyrograph’s CNTs with polystyrene; (g and h) double coating of Pyrograph’s CNTs with polyanyline; (i and j) one step polymerization of Pyrograph’s nanotubes with polystyrene in supercritical conditions. Numbers in the TEM images signify (1) polystyrene coating, (2) CNT wall, (3) encapsulated polystyrene, and (4) polyaniline coating.
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J. Phys. Chem. C, Vol. 111, No. 37, 2007 13657
Figure 3. SEM images of CNT arrays coated with polystyrene: (a and e) untreated CNT arrays; (b and f) CNTs impregnation in polystyrene matrix using the initial solution of monomer and initiator; (c, d, g, and h) coating and filling of CNT arrays with polystyrene using the initial solution of monomer, polymer, and initiator.
tively. Coating of the Nanolab’s CNTs (Figure 2c,d) varied from 4 to 30 nm. These results suggest that the secondary treatment of the Pyrograph’s CNTs allowed for higher coating thickness and uniformity. Thus, these CNTs were chosen for further experiments. Encapsulation of the polymeric material was observed only when the CNTs have open cavities, suggesting that diffusion through open tips is the governing transport mechanism. Penetration of material through the CNTs walls was not observed, possibly due to relatively low experimental temperature. It should be pointed out that flame generated CNTs have a bamboo-like structure with carbon layers forming nanocompartments in the central part of their cavities. These structures have clear graphitic origin that resembles a CNT wall (Figure 2a,b). Based on the performed calculations, the amount of polymer deposited inside the nanotube is by a factor of 8 higher than the amount deposited as an outside coating. This might be due to stronger adhesive forces between the monomer solution and the graphitic layer inside the CNT channel compared to the outside surface interaction. This finding is consistent with a theoretical calculation by Kondratyuk et al.15 on adsorption of alkanes on CNTs. The other possible explanation involves the effect of the highly negative curvature of the inner nanotube surface. Due to the physical characteristics of the CNTs, it is possible that there is a preferential condensation of monomer/ initiator solution occurring on the negatively curved inner CNT surface as compared to the positively curved outer surface. By repeating the polymerization procedure for polystyrene using the monomer aniline, a second polyaniline coating (Figure 2g,h) was achieved. The polyacrylonitrile film was uniform and had a thickness of 5 nm, which was 2 times lower than the polystyrene film due to different solubilities of the starting monomers in supercritical CO2. Results for CNT arrays suggested that due to the very high solubility of the monomer in supercritical CO2, when monomer and initiator solution is used for the CNT arrays’ polymerization, the individual CNTs are completely impregnated in a polymer matrix (Figure 3b,f). This caused charging when the material was placed in the scanning electron microscope, and therefore, no further analysis of coating of individual CNTs was possible. However when the monomer and initiator solution is replaced with partly polymerized solution, i.e., a week old solution, then its solubility in supercritical CO2 is decreased and the polymer coating of the CNT arrays was easily observed (Figure 3c,d).
In the third set of experiments, one step coating and encapsulation was achieved directly in supercritical conditions reducing the total sample processing time by more than a factor of 3. Furthermore, it was shown that there was no need for direct immersion of the carbon nanotubes (Figure 2i,g) and CNT arrays (Figure 3g,h) in the monomer solution that is unlike any other conventional filling method. Figure 3h shows the space filling and coating of a specific bundle of CNTs with polymeric material, arrows 1 and 2 highlight a compressed CNT bundle composite and its tips, respectively. Decreasing the soaking time from 2 to 1 h did not affect the coating and encapsulation of the CNTs. Due to the strong plasticization effect, the glass transition temperature of the polystyrene was lowered and a T ) 60 °C was sufficient for polymerization to occur. In the experiments with CNT arrays using a partly polymerized solution, direct exposure to supercritical CO2 at 60 °C also resulted in excellent CNT coating (Figure 2 i,j). This works shows a feasible and controllable technique for polymer coating and filling of CNTs under supercritical conditions. The properties of the supercritical CO2 offer potential advantages for unique process designs allowing separation of sample and monomer solution, reduction of sample treatment times, and excellent coating uniformity of CNTs and CNT arrays. Acknowledgment. The authors thank Mr. John Roth from the UIC Research Resource Center for his valuable suggestions that helped in the completion of this work. References and Notes (1) Seeger, T.; Redlich, P.; Grobert, N.; Terrones, M.; Walton, D. R.; Kroto, H. W.; Ruele, M. Chem. Phys. Lett. 2001, 339, 41. (2) O’Connel, 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. (3) Skoulidas, A. I.; Sholl, D. S.; Johnson, J. K. J. Chem. Phys. 2006, 124, 1. (4) Lee, K. H.; Sinnott, S. B. J. Phys. Chem. B 2004, 108, 9861. (5) Koga, K.; Gao, G. T.; Tanaka, H.; Zeng, X. C. Nature 2001, 412, 802. (6) Kim, B. M.; Qian, S.; Bau, H. H. Nano Lett. 2005, 5, 873. (7) Ajayan, P. M.; Iijima, S. Nature 1993, 361, 333. (8) Majumder, M.; Chopra, N.; Andrews, R.; Hinds, B. J. Nature 2005, 438, 44. (9) Wang, S.; Choi, D.; Yang, S. AdV. Mater. 2002, 14, 1311.
13658 J. Phys. Chem. C, Vol. 111, No. 37, 2007 (10) Korneva, G.; H. Ye.; Gogotsi, Y.; Halverson, D.; Friedman, G.; Bradley, J.; Kornev, K. Nano Lett. 2005, 5, 879. (11) Merchan-Merchan, W.; Saveliev, A. V.; Kennedy, L. A.; Fridman, A. A. Chem. Phys. Lett. 2002, 354, 20. (12) Ebbesen, T. W.; Ajayan, P. M.; Hiura, H.; Tanigari, K. Nature 1994, 367, 519.
Letters (13) Kim, Y. A.; Muramatsu, H.; Hayashi, T.; Endo, M.; Terrones, M.; Dresselhaus, M. S. Chem. Phys. Lett. 2004, 398, 87. (14) Merchan-Merchan, W.; Saveliev, A. V.; Kennedy, L. A. Carbon 2004, 42, 599. (15) Kondratyuk, P.; Wang, Y.; Johnson, J. K.; Yates, J. T. J. J. Phys. Chem. B 2005, 109, 20999.
