NMR and Raman spectroscopic characterization of single walled carbon nanotube composites of polybutadiene
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Frank J. Owens Department of Physics, Hunter College of CUNY, New York, New York 10065; and Armament Research, Development, and Engineering Center, Picatinny, New Jersey 07806 (Received 24 November 2008; accepted 5 May 2009)
Significant shifts of the frequency of the Raman spectra of the tangential mode of single walled carbon nanotubes (SWNTs) and fluorinated tubes (FSWNTs) in composites of polybutadiene (PB) were observed relative to the pristine SWNTs indicative of the interaction between the polymer and the SWNTs. Proton NMR line width measurements demonstrate partial suppression of polymer segmental motion for both types of nanotube composites and spin-lattice relaxation results indicate that short time-scale localized motions are also affected by SWNT inclusion, more so for FSWNTs. Hardness measurements as a function of wt% SWNTs and FSWNTs in the polymer show larger enhancements of hardness in the composite with the fluorinated tubes. I. INTRODUCTION
Because single walled carbon nanotubes (SWNTs) have the largest aspect ratio of any known fiber and have tensile strength on the order of 45 GPa, they have the potential to be the ultimate reinforcing material. Enhancements of hardness and tensile strength have been observed in SWNT/polymer composites.1 However, in most cases the increases have been considerably less than predicted. Typical enhancements range from 23–78% for polymers containing 5% by weight of SWNTs. For example a composite of poly(p-phenylene benzobisoxazole) containing 5% SWNTs shows a 23% increase in tensile strength compared to the undoped polymer, while a similar composition of an SWNT poly (vinyl alcohol) composite showed a 78% increase.2,3 Possible reasons for the less than optimum enhancements of strength are poor dispersion of the tubes, lack of alignment, and the existence of bundles of tubes. It is likely that the bundling of the tubes is the most serious obstacle, firstly because the aspect ratio is reduced, and secondly the weak van der Waals forces between the tubes in the bundles means the tubes can easily slip under stress, resulting in a shear modulus comparable to graphite. It has been shown that linking neighboring tubes in the bundles may result in a significant increase in the loading modulus by eliminating sliding between the nanotubes.4 This suggests that side walled functionalized
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0292 J. Mater. Res., Vol. 24, No. 7, Jul 2009
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tubes may be more effective in increasing the mechanical strength of composites. It has been shown that side wall fluorinated nanotubes can produce a significant increase in hardness compared to unfunctionalized single walled tubes in polyacrylonitrile and polyisobutylene.5,6 However, the generality of this result and its applicability to other polymer systems need to be examined. Enhancement of strength also requires interaction between the single walled nanotubes and the polymer. An
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