Production and Properties of Fullerene-based Materials

The discovery of fullerenes in 1980s and a method for their bulk production in 1990 provides the basis for the development of completely new carbon materials. The all-carbon cage molecules known as fullerenes, the most celebrated among which is the highly symmetric truncated icosahedral C60, are now recognized as a very large and general class of carbon molecules. The discovery of giant fullerenes in the form of nanotubes and polyhedra, has further expanded the realm of this new form of carbon. Moreover, chemical derivatives of fullerenes can be created by adding other functional groups externally or internally. Thus, fullerenes encompass a very large range of building blocks, which can be the basis for developing new materials with unique properties.

Under a contract with the New Energy and Industrial Development Organization (NEDO), an agency under the Ministry of International Trade and Industry (MITI), Japan, SRI International began a vigorous research program on the science and technology of fullerenes. The goal of our project is to produce fullerene based films that can function as high temperature lubricants or as super corrosion resistant materials. These films will be produced from simple fullerenes as well as from their derivatives including functionalized fullerenes and polymers, endohedral derivatives, and carbon nanotubes and nanopolyhedral particles. In this document we review our work on the productionand characterization of fullerene-based polymers and carbon nanotubes.

Fullerene Polymers

Wudl and coworkers discovered the facile nucleophilic addition of primary and secondary amines to C60. This reaction serves as convenient starting point for making fullerene-based polymers. We have synthesized the adduct of 2-methylaziridine (C3H7N) with C60. We chose to incorporate aziridine units because they readily react with epoxy and novolac resins to produce highly crosslinked polymers. This crosslinking proceeds without the formation of any byproducts, and is therefore particularly suited for producing high performance films by spin-, dip-, or spray-coating and other articles by reactive-injection molding techniques.

In accord with the addition of simple alkylamines, the addition of 2-methylaziridine also takes place readily in benzene solution with the N-H bond adding across a formal C-C double bond (Scheme 1). The product, which is highly soluble in chloroform, was purified by column chromatograpohy. Elemental analysis of the product corresponds to an average addition of 3 aziridine moieties per C60. Mass spectral analysis by SALI shows a large peak due to C60 with smaller peaks due to the addition of one, two, three, and four methylaziridine residues. The peak due to C60 is clearly a result of fragmentation, because the IR, and NMR data do not show the presence of any C60. The ¹³C NMR of the product is also consistent with the simple nucleophilic addition:

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Scheme 1: Addition of 2-methylaziridine to C60; for clarity, addition of only one group is shown.

We prepared polymers from the 2-methylaziridine adduct by treating it with Epon and Novolac resins. Epon resins, which are diglycidyl ethers of bisphenol-A, have epoxide structures that are opened by nucleophilic attack by the aziridine nitrogen. The aziridine ring itself is also opened in the process by the growing polyether chain. The result is a crosslinked polymer. Novolac resins are have phenolic residues that can open the aziridine ring, and because each C60 has several aziridine units, the result is again a crosslinked polymer. We have made several polymers by varying the amount of Epon (or Novolac) and the 2-methylaziridine adduct of C60. In a typical procedure, the resin and the adduct are dissolved in a mixture of toluene and dimethylformamide. The solution is spin-coated on a quartz disk, and cured at 180 C.

We have characterized the polymeric films by IR and UV-Vis spectroscopy, and by thermal gravimetric analysis. We have also cast the polymers into films by spin coating. The films with are very hard (> 9H) and amber colored. They are thermally very stable; they show minimal loss of weight up to 250 C, and can be heated up to 1000 C in argon with only about 40% weight loss. Novolac films prepared without the aziridine adduct decompose between 200 and 300 C. Further characterization of their thermal and mechanical properties is in progress.

Nanotubes and Nanopolyhedra

The discovery by Ijima that carbon tubes are abundantly produced in the arcs used for fullerene production introduced a new dimension to the fullerene field, namely nanoscale giant fullerenes that may contain as many as 1 million carbon atoms. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies of the raw samples taken from the boule that grows on the end of the negative electrode show a large variety of nanoscale particles. These particles consist of nanoscale giant fullerenes in the form of multilayered tubes, polyhedra and graphitic particles with a large range of sizes. These particles can be classified as fullerenes because the closed structure results from the introduction of 12 pentagons into hexagonal graphite lattices. At SRI, we found that single crystals of other materials, like LaC2, Gd2C3, and GdC2, can be encapsuled within the cavity of these nested polyhedral particles during the arc synthesis step. Formation of single-walled nanotubes in the soot produced when the graphite rod is doped with the transition elements Fe, Cr, and Ni has also been reported. TEM examination shows these tubes to be associated with amorphous carbon and small metal particles (2 to 20 nm).

The nanotubes are particularly interesting because their ideal geometrical form, high aspect ratios, and nearly faultless structures provides them with unique mechanical, electrical and thermal properties. These tubes are expected, on the basis of theoretical calculations, to have strongly anisotropic electrical and thermal conductivities. The mechanical properties are also interesting since the bulk moduli as well as the Youngs and bending moduli are expected to be very high simply due to the cylindrical geometry and the rigidity of the hexagonal lattice.

Our recent efforts in this area have been focused on producing samples of pure nanotubes. Methods based on density centrifugation and filtration are severely hampered by the lack of suitable solvents to disperse the particles. We have explored the use of temperature-programmed oxidation to preferentially burn-off the more reactive amorphous carbon, and leave behind samples enriched in tubes. We have had some success with this method and have produced samples essentially free form amorphous particles, but not from the multi-walled polyhedral particles. When applied to samples of single-walled nanotubes, the method affords nanotubes contaminated with metal oxides, which we have been able to remove by treatment with acids. We are now in the process of scaling up the procedure to obtain gram quantities of samples with only single-walled nanotubes. With these quantities, we will be able to make composites and test many of the physical and mechanical properties.

Principal Investigators

Ripudaman Malhotra, E-mail: <ripu@mplvax.sri.com>
Donald C. Lorents, E-mail: <lorents@mplvax.sri.com>
Rodney S. Ruoff, E-mail: <ruoff@mplvax.sri.com>

Representative Publications

R.S. Ruoff, R. Malhotra, D.L. Huestis, D.S. Tse, and D.C. Lorents, Nature 362, 140 (1993).
R.S. Ruoff, D.S. Tse, R. Malhotra, and D.C. Lorents, J. Phys. Chem. 97, 3379 (1993).
R.S. Ruoff, D.C. Lorents, B. Chan, and R. Malhotra, Science 259, 346-348 (1993).
L. Moro, R.S. Ruoff, C.H. Becker, D.C. Lorents, and R. Malhotra, J. Phys. Chem. 97, 6801 (1993).
R. Malhotra, D.F. McMillen, D.S. Tse, D.C. Lorents, R.S. Ruoff, and D.M. Keegan, Energy Fuels 7, 685 (1993).
S. Subramoney, R.S. Ruoff, D.C. Lorents, B. Chan, R. Malhotra, M.J. Dyer, and K. Parvin, Carbon 32, 507 (1994)
S. Kumar, A. Satyam, and R. Malhotra, J. Chem. Soc. Chem. Commun., 1339 (1994).

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