Hierarchical Composites Reinforced with Carbon Nanotube Grafted Fibers: The Potential Assessed at the Single Fiber Level Hui Qian, †,‡ Alexander Bismarck, ‡ Emile S. Greenhalgh, § Gerhard Kalinka, 4 and Milo S. P. Shaffer* ,† Department of Chemistry, Imperial College London, London SW7 2AZ, U.K.; The Polymer and Composite Engineering (PaCE) Group, Department of Chemical Engineering, Imperial College London, London SW7 2AZ, U.K.; The Composites Centre, Imperial College London, London SW7 2AZ, U.K.; and BAM-Federal Institute for Materials Research and Testing, DiVision V.6, D-12205 Berlin, Germany ReceiVed September 27, 2007. ReVised Manuscript ReceiVed December 3, 2007 The feasibility of reinforcing conventional carbon fiber composites by grafting carbon nanotubes (CNTs) onto the fiber surface has been investigated. Carbon nanotubes were grown on carbon fibers using the chemical vapor deposition (CVD) method. Iron was selected as the catalyst and predeposited using the incipient wetness technique before the growth reaction. The morphology of the products was characterized using scanning electron microscopy (SEM), which showed evidence of a uniform coating of CNTs on the fiber surface. Contact angle measurements on individual fibers, before and after the CNT growth, demonstrated a change in wettability that can be linked to a change of the polarity of the modified surface. Model composites based on CNT-grafted carbon fibers/epoxy were fabricated in order to examine apparent interfacial shear strength (IFSS). A dramatic improvement in IFSS over carbon fiber/epoxy composites was observed in the single fiber pull-out tests, but no significant change was shown in the push-out tests. The different IFSS results were provisionally attributed to a change of failure mechanism between the two types of tests, supported by fractographic analysis. Introduction Carbon nanotubes (CNTs) are excellent candidates for a new generation of high-strength, high-stiffness materials due to their low density, high aspect ratio, and intrinsically superior mechanical properties to conventional materials. 1,2 Although promising results have been obtained, obtaining absolute improvements over existing high-performance ma- terials has proved challenging. Interest is, therefore, growing in the improvement of conventional high-performance com- posites using CNTs, in particular, the development of hierarchical composite materials based on CNT-grafted fibers. 3,4 The intention is that the presence of CNTs in the matrix may alleviate many of the drawbacks of conventional fiber composites, especially longitudinal compression and interlaminar properties. A small number of reports 5,6 have looked at CNTs randomly dispersed into thermosetting matrices prior to infiltration, but viscosity and self-filtration issues severely limit the concentration of CNTs that can be incorporated. Grafting the nanotubes onto the conventional fiber surface, on the other hand, has the potential to provide higher loadings of CNTs with a radial orientation that may be optimal for transverse reinforcement. The fiber/matrix interface has been the subject of numerous studies over the past few decades. Efforts have been made to improve the interfacial adhesion using various methods, 7–11 by either enhancing the chemical activity of the fiber surface or increasing the surface area. The magnitude of improve- ment in the interfacial shear stress (IFSS) of carbon fiber/ epoxy composites has been very variable, depending on the fiber and matrix combinations; increases in the range 17-217% have been reported when using thermal treat- ments 8 and electrochemical oxidations 9 to modify the carbon fibers. Grafting carbon fibers with CNTs is likely to improve the fiber-matrix interfacial strength, which will enhance the adhesion and thus improve the composite delamination resistance. Unlike conventional approaches to improving IFSS, the nanotube should offer additional benefits. For example, reinforcement radial to the carbon fibers, extending into the surrounding matrix, will inhibit fiber microbuckling, which is a critical composite failure mode under compressive loading. 12 * Corresponding author. E-mail: m.shaffer@imperial.ac.uk. † Department of Chemistry, Imperial College London. ‡ Department of Chemical Engineering, Imperial College London. § The Composites Centre, Imperial College London. 4 BAM-Federal Institute for Materials Research and Testing. (1) Harris, P. Int. Mater. ReV. 2004, 49, 31–43. (2) Coleman, J. N.; Khan, U.; Blau, W. J.; Gun’ko, Y. K. Carbon 2006, 44, 1624–1652. (3) Thostenson, E. T.; Li, W. Z.; Wang, D. Z.; Ren, Z. F.; Chou, T. W. J. Appl. Phys. 2002, 91, 6034–6037. (4) Veedu, V. P.; Cao, A.; Li, X.; Ma, K.; Soldano, C.; Kar, S.; Ajayan, P. M.; Ghasemi-Nejhad, M. N. Nat. Mater. 2006, 5, 457–462. (5) Fan, Z. H.; Hsiao, K. T.; Advani, S. G. Carbon 2004, 42, 871–876. (6) Rutkofsky, M.; Ait-Haddou, H.; Folaron, R.; Criscuolo, L. “Zyvex Application Note 9715: Using Zyvex’s NanoSolve Additive with Carbon Nanotubes for Enhanced Epoxy Composites”, Zyvex. (7) Wu, Z.; Pittman, J. C. U.; Gardner, S. D. Carbon 1995, 33, 597–605. (8) Ramanathan, T.; Bismarck, A.; Schulz, E.; Subramanian, K. Compos. Sci. Technol. 2001, 61, 599–605. (9) Deng, S. Q.; Ye, L.; Mai, Y. W. AdV. Compos. Mater. 1998, 7, 169– 182. (10) Rabotnov, J. N.; Perov, B. V.; Lutsau, V. G.; Ssorina, T. G.; Stepanitshev, E. I. Carbon FiberssTheir Place in Modern Technology; The Plastics Institute: London, 1974; pp 65–70. (11) Downs, W. B.; Baker, R. T. K. J. Mater. Res. 1995, 10, 625–633. 1862 Chem. Mater. 2008, 20, 1862–1869 10.1021/cm702782j CCC: $40.75  2008 American Chemical Society Published on Web 02/09/2008