Long Carbon Nanotubes Grown on the Surface of Fibers for Hybrid Composites

C ARBON nanotubes (CNTs) have been the focus of considerable research since Iijima [1] confirmed their structure in 1991. In addition to the exceptional electronic and thermal properties associated with CNTs, they have also been reported to possess exceptional mechanical properties: for example, an extensional modulus of approximately 1 TPa and fracture strength of approximately 130 GPa for multiwalled CNTs (MWCNTs) [2]. Exploiting such electrothermomechanical properties toward the development of macroscopic structural materials has been the subject of considerable research. The most direct structural application for CNTs is as reinforcement for traditional composite materials, and most of the work in this direction has focused on dispersing and aligning CNTs (both singleandmultiwalled CNTs) in polymericmatrices to reinforce the matrix. The four main processing factors that influence the mechanical properties of the final composite are dispersion and alignment of the CNTs within the matrix, adhesion between the CNTs andmatrix, and the CNT length. All have been formidable challenges to the realization of aligned-CNT composites. Dispersion is important because CNTs tend to agglomerate when dispersed in a polymeric resin. These aggregates are not well adhered to the polymer and can also act as stress concentrators, reducing the final performance of the composite [3]. Alignment of the CNTs is needed to increase the effectiveness of the reinforcement; for example, a molecular dynamics model developed by Odegard et al. [4] showed that Young’s modulus of a CNT/polyimide composite can be increased by a factor of 3when theCNTs are oriented parallel to the direction of the load. The formation of aggregates reduces the effective area of contact between the nanotubes and the polymer, hence reducing the adhesion between the twomaterials. CNT aggregates also reduce the aspect ratio of the reinforcement, lowering the reinforcing effect, and can further act as inclusions (dry entangled CNTs with no epoxy binding the aggregate together) that reduce the strength of the reinforced polymer. Aggregates are a further difficulty for singleand double-walled CNTs because such CNTs can be held tightly in hexagonally packed bundles/ropes. The two most commonly used methods to embed CNTs into a polymer matrix, sonication [5] and calendering [6], do not completely prevent the formation of aggregates and usually damage the CNTs. Using these methods, moderate improvements in matrix mechanical properties are typically observed due to the low CNT volume fractions and generally poor dispersion and alignment. Beyond a certain volume fraction (usually around 3%), no further improvements in the composite mechanical properties are achieved using the methods previously mentioned. At higher volume fractions with these methods, dispersion and alignment deteriorate significantly. Good dispersion, alignment, and adhesion are completely necessary to take full advantage of the mechanical properties of the CNTs. Others have used chemical vapor deposition (CVD) processes to grow densely packed carpets, or forests, of well-aligned carbon nanotubes, which can be wet by polymer solutions [7] to create aligned-CNT composite films. This avoids most of the problems associated with mixing CNTs and polymers. One process used to impregnate the CNTs relies on dissolving the polymer using a lowviscosity solvent. Although this process is perfectly valid for microfabrication, it is not considered to be feasible for large-scale applications, which necessitate large substrate areas and rapid processing. CNT/polymer composites that have been developed so far can improve the mechanical properties of the matrix and open a whole range of multifunctional applications for nanocomposite thin films. However, in terms of structural applications, these CNT/ polymer composites cannot compete with traditional continuousfiber composite materials. The combination of CNTs, polymer matrices, and advanced fibers to create so-called hybrid composites is seen as a practical approach to deriving structural/multifunctional benefits from CNTs. Perhaps the most promising results to date are related to the improvement in the fiber/matrix interface by growing CNTs on the surface of the fibers. Several studies have shown that growing carbon nanotubes on the surfaces of fibers by CVDmethods [8] significantly increases the surface area overwhich to transfer load (e.g., from 1.77 to 17:2 m=g Presented as Paper 1854 at the 47th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Newport, RI, 1–4 May 2006; received 6 March 2007; accepted for publication 3 March 2008. Copyright © 2008 by E. J. García and B. L. Wardle. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to theCopyright Clearance Center, Inc., 222RosewoodDrive,Danvers,MA01923; include the code 0001-1452/ 08 $10.00 in correspondence with the CCC. Graduate Student, Department of Aeronautics andAstronautics, Building 41-317, 77 Massachusetts Avenue; [email protected]. Student Member AIAA (Corresponding Author). Graduate Student, Department of Mechanical Engineering, Building 3-470, 77 Massachusetts Avenue. BoeingAssistant Professor, Department ofAeronautics andAstronautics, Building 33-314, 77 Massachusetts Avenue; [email protected]. AIAA JOURNAL Vol. 46, No. 6, June 2008