MTL Annual Report

Figure 1: Controlled-morphology polymer nanocomposites: (a) Image of 1% aligned-CNT forest, 1% aligned-polymer nanocomposite (A-PNCs) and pure epoxy samples, (b) SEM image of 1% A-PNCs with an inset schematic of the direction of CNT alignment.

Carbon nanotube (CNT) composites are promising new materials for structural applications thanks to their mechanical and multifunctional properties. We have undertaken a significant experimentally-based program to understand both microstructures of aligned-CNT nanocomposites and nano-engineered advanced composite macrostructures hybridized with aligned CNTs. Aligned nanocomposites are fabricated by mechanical densification and polymer wetting of aligned CNT forests ^[1]. Polymer wetting is driven by capillary forces that arise upon contact of the polymer with the nanostructured CNT forest ^[2] [3], the rate of which depends on properties of the CNT forest (e.g., volume fraction) and the polymer (viscosity, contact angle, etc.). Here the polymer is unmodified aerospace-grade epoxy. CNT forests are grown to mm-heights on 1-cm² Si substrates using a modified chemical-vapor-deposition process. Following growth, the forests are released from the substrate and can be handled and infiltrated. The volume fraction of the as-grown CNT forests is about 1%; however, the distance between the CNTs (and thus the volume fraction of the forest) can be varied by applying a compressive force along the two axes of the plane of the forest to give volume fractions of CNTs exceeding 20% (Figure 1a). Variable-volume fraction-aligned CNT nanocomposites were characterized using optical scanning electron (SEM) and transmission electron (TEM) microscopy to analyze dispersion and alignment of CNTs as well as overall morphology. Physical property testing is underway.

Figure 2: Aligned-CNT nano-engineered composite macro-scale architectures: nanostitching (top) and fuzzy fiber (bottom).

Nano-engineered composite macrostructures hybridized with aligned CNTs are prepared by placing long (>20 μm) aligned CNTs at the interface of advanced composite plies as reinforcement in the through-thickness axis of the laminate (Figure 2). Three fabrication routes were developed: transplantation of CNT forests onto pre-impregnated plies ^[4] (the “nano-stitch” method), placement of detached CNT forests between two fabrics followed by subsequent infusion of matrix, and in situ growth of aligned CNTs onto the surface of ceramic fibers followed by infusion or hand-layup ^[5] [6]. Aligned CNTs are observed at the composite ply interfaces and give rise to significant improvement in interlaminar strength, toughness, and electrical properties. Interestingly, toughness improvement has demonstrated a favorable nano-scale size effect ^[7]. Analysis of the multifunctional properties and nanoscale interactions between the constituents in both the nanocomposites and hybrid macrostructures is underway. A new route to fabricate these materials continuously is being developed.

1. B.L. Wardle, D.S. Saito, E.J. Garcia, A.J. Hart, and R. Guzman de Villoria, “Fabrication and Characterization of Ultra-High Volume Fraction Aligned Carbon-Nanotube-Polymer Composites,” Advanced Materials, vol. 20, pp. 2707-2714, 2008. [↩]
2. H. Cebeci, R. Guzman de Villoria, A. John Hart, and B.L. Wardle, “Multifunctional properties of high volume fraction aligned carbon  nanotube polymer composites with controlled morphology,” Composites Science and Technology vol. 69, no 15-16, pp. 2649-2656, 2009. [↩]
3. E.J. García, A.J. Hart, B.L. Wardle, and A.H. Slocum, “Fabrication and Nanocompression Testing of Aligned Carbon-Nanotube-Polymer Nanocomposites,” Advanced Materials, vol. 19, pp. 2151-2156, 2007. [↩]
4. E.J. Garcia, B.L. Wardle, and A.J. Hart, “Joining Prepreg Composite Interfaces with Aligned Carbon Nanotubes,” Composites Part A vol. 39, pp. 1065-1070, 2008. [↩]
5. N. Yamamoto, A.J. Hart, S.S. Wicks, E.J. Garcia, B.L. Wardle, and A.H. Slocum, “High-yield atmospheric-pressure growth of aligned carbon nanotubes on ceramic fibers for multifunctional enhancement of structural composites”, Carbon, vol. 47, pp. 551-560, 2009. [↩]
6. S.S. Wicks, R. Guzmán de Villoria, and B.L. Wardle, “Interlaminar and Intralaminar Reinforcement of Composite Laminates with Aligned Carbon Nanotubes,” Composites Science and Technology, 70, pp. 20–28, 2010. [↩]
7. J. Blanco, E.J. Garcia, R. Guzman de Villoria, and B.L. Wardle, “Limiting Mechanisms in Mode I Interlaminar Toughness of Composites Reinforced with Aligned Carbon Nanotubes,” Journal of Composite Materials, vol. 43, no. 8, pp. 825-841, 2009. [↩]