Small miracles--turning nanodreams into reality: high-performance composites from carbon nanotubes--an integrated approach for success.

Up until very recently, the majority of efforts to make high-performance composites from CNT involved a simple physical mixing of the nanotubes into a matrix with the hope that some remarkable new composite would result. Unfortunately, little to no enhancement of composite properties was observed. The reason for this stems from the structure of the nanotubes themselves. CNT are fully aromatic with [sp.sup.2] hybridized sidewalls, giving them low chemical reactivity, very low solubility in most solvents, and weak affinity for most common composite matrices. In addition, just as graphene sheets prefer stacking to form graphite, CNT have a strong inter-tube attraction that causes them to bundle into thick rope-like structures (see Figure 2). The energy to de-bundle two SWNT, for example, is about 12 kilocalories (0.5 eV) per nanometre of length, which is a significant amount of energy considering SWNT can be several microns long. Therefore, to take full advantage of CNT in composite applications one must first overcome the bundling force in order to uniformly disperse the CNT as well as increase the interaction between the CNT and the matrix. Both of these goals can be reached through proper chemical modification of the CNT themselves, which is now recognized as the key to fully leveraging carbon nanotubes' remarkable properties. This chemistry can be accomplished in a number of ways, including wrapping the CNT with a polymer chain, non-covalent [pi]-[pi] stacking to the delocalized aromatic network, and direct covalent ([sp.sup.3]) functionalization to carbon atoms in the nanotubes. Our group at the NRC has developed extensive expertise in the covalent attachment of SWNT to various matrices through the linking of tailored functional moieties to SWNT sidewalls. Several strategies have been developed and proven that offer excellent flexibility and control over this process. One method we favour is to perform chemistry on reduced (negatively charged) SWNT because the nanotubes are naturally exfoliated during the process, and it substantially reduces the time and cost of the functionalization. (12) Figure 3 illustrates the effectiveness of our methods for CNT-composites. The first two panels show covalently functionalized SWNT dispersed in an epoxy resin, while the third shows the result of physical mixing unmodified SWNT.

[FIGURE 3 OMITTED]

An integrated approach for success

In order to counter the challenges discussed above and to discover what is ultimately possible with carbon nanotubes, we have adopted an integrated approach to the development of CNT-composites. In this approach we exert strict control over every step of the development process, including SWNT synthesis, purification, characterization, functionalization, and integration with the matrix. In this way we are able to exercise quality control and traceability at every stage. This approach is already proving successful with recent demonstration that the fracture toughness in epoxy resins can be improved by more than 60 percent with the addition of as little as 0.16 weight-percent of functionalized SWNT.

Future of CNT composites

There have been high expectations and many promises regarding CNT-composites, with few being fulfilled as yet. Fortunately, the CNT community is now realizing that reliability, quality, standardization, and chemistry are essential to making concerted progress. Through our experience we now know that with an integrated approach it is possible to make high-performance composite materials with CNT. Just how far the performance of these materials can be pushed remains to be seen. One prediction we can make is that the next few years will bring great advancements to the field and move us even closer to developing the ultimate multifunctional composites.

References

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(2.) Min-Feng Yu, Oleg Lourie, Mark J. Dyer, Katerina Moloni, Thomas F. Kelley, and Rodney S. Ruoff, "Strength and breaking Mechanism of Multiwalled Carbon Naotubes Under Tensile Load," Science 87, (January 2000) pp. 637-640.

(3.) Philippe Poncharal, Claire Berger, Yah Yi, Z. L. Wang, and Walt A. de Heer, "Room Temperatura Ballistic Conduction in Carbon Nanotubes," Journal of Physical Chemistry B 106, 47 (November 2002) pp. 12104-12118.

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(6.) Tae-Youl Choi, Dimos Poulikakos, Joy Tharian, and Urs Sennhauser, "Measurement of the Thermal Conductivity of Individual Carbon Nanotubes by the Four-Point Three-[omega] Method," Nanoletters 6, 8 (August 2006) pp. 1589-1593.

(7.) Jonathan N. Coleman, Umar Khan, and Yurii K. Gun'ko, "Mechanical Reinforcement of Polymers Using Carbon Nanotubes," Advanced Materials 18, 6 (March 2006) pp. 689-706.

(8.) Jean-Paul Salvetat, Sanjib Bhattacharyya, and R. Byron Pipes, "Progress on Mechanics of Carbon Nanotubes and Derived Materials," Journal of Nanoscience and Nanotechnology 6, 7 (2006) pp. 1857-1882.

(9.) Erik T. Thostenson, Chunyu Li, and TsuWei Chou, "Nanocomposites in context," Composites Science and Technology 65, 3-4 (March 2005) pp. 495-516.

(10.) Christopher T. Kingston and Benoit Simard, Analytical Letters 36, 15 (2003) pp. 3139-3145.

(11.) Christopher. T. Kingston, Zygmunt J. Jakubek, Stephane Denommee and Benoit Simard, Carbon 42, 8-9 (2004) pp. 1657-1664.

(12.) Yadienka Martinez-Rubi, Jingwen Guan, Shuqiong Lin, Christine Scriver, Ralph E. Sturgeon and Benoit Simard, "Rapid and Controlabte Covalent Functionalization of Single-Walled Carbon Nanotubes at Room Temperature," Chemical Communication, 2007, DOI: 10.1039/b712299c.

Stephane Denommee, MCIC, is a technical officer with expertise with nanomaterials.

Jingwen Guan is a research officer with expertise in the chemistry of SWNT

Christopher Kingston is a research officer with expertise in the synthesis and characterization of SWNT.

Yadienka Martinez-Rubi is an NSERC post-doctoral fellow with expertise in the chemistry of SWNT

Benoit Simard, FCIC, is principal research officer and group leader of the Molecular and Nano-Material Architectures Group at the National Research Council Canada's Steacie Institute for Molecular Sciences (SIMS).