Synthesis, Characterization, and Modeling of Nanotube Materials with Variable Stiffness Tethers
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Synthesis, Characterization, and Modeling of Nanotube Materials with Variable Stiffness Tethers S. J. V. Frankland1, M. N. Herzog2, G. M. Odegard1, T. S. Gates3, C. C. Fay3 1 National Institute of Aerospace, Hampton, VA 2 National Research Council, Hampton, VA 3 NASA Langley Research Center, Hampton, VA ABSTRACT Synthesis, mechanical testing, and modeling have been performed for a carbon nanotube material in which the nanotubes are functionalized with variable stiffness tethers (VST) capable of cross-linking the nanotubes. Tests using nanoindentation indicated a six-fold enhancement in the storage modulus when comparing the base material (the cross-linking agent with no nanotubes) to the composite (functionalized nanotube material) that contained 5.3 wt% of nanotubes. To understand how crosslinking the nanotubes may further alter the stiffness, a model of the system was constructed using nanotubes crosslinked with the VST. The model predicted that for a composite with 5 wt% nanotubes at random orientations, crosslinked with the VST, the bulk Young’s modulus was reduced to 30% that of the non-crosslinked equivalent. INTRODUCTION Nanostructured composites are of importance to the aerospace industry as candidate materials for future light-weight structural components. Interest continues to center on the suitability of carbon nanotube based composites for structural applications. However, the theoretical improvements to mechanical performance have been difficult to realize because of limited ability to disperse carbon nanotubes in polymeric materials, uncontrolled nanotube alignment in the composites, and weak polymer-carbon nanotube interfaces within the material. Crosslinked nanotube materials provide some potential solutions to these problems in that the nanotube/crosslinker interface should be strengthened by the presence of the additional covalent linkages and that the crosslinking agent would intercalate between the nanotubes resulting in better dispersion. Previous work has proposed the synthesis of crosslinked nanotube materials with short rigid tethers and has used multi-scale model to address the effect of crosslinking the carbon nanotubes on the constitutive properties [1]. In related work, others have fabricated polyacrylonitrile (PAN) fibers reinforced with carbon nanotubes that demonstrated tensile modulus increases of up to 100 % at 10 wt% single-walled carbon nanotube (SWNT) content [2] and 30 % increases in modulus at 5 wt% multi-walled nanotube content [3]. In the SWNT/PAN fibers, the nanotubes also align well and are partially exfoliated [2]. In the present work, the influence of crosslinking is explored with a multi-scale model. The materials of interest consist of SWNT crosslinked with a long variable stiffness tether (VST). The VST crosslinking agent consists of rigid and flexible sections which are capable of forming covalent linkages to and packing between the SWNT. The objective of the work is to demonstrate the effect of nanotube functionalization on the constitutive properties of the
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