Dispersion of Functionalized Carbon Nanotubes in Polystyrene
Cynthia A. Mitchell,† Jeffrey L. Bahr,‡ Sivaram Arepalli,§ James M. Tour,*,‡ and Ramanan Krishnamoorti*,†
Department of Chemical Engineering, University of Houston, Houston, Texas 77204-4004; Department of Chemistry, Department of Mechanical Engineering and Materials Science, and Centerfor Nanoscale Science and Technology, Rice University, MS222, Houston, Texas 77005; and G. B. Tech./ NASA-Johnson Space Center, Houston, Texas 77058 Received June 10, 2002
ABSTRACT: Polystyrene nanocomposites with functionalized single-walled carbon nanotubes (SWNTs), prepared by the in-situ generation and reaction of organic diazonium compounds, were characterized using melt-state linear dynamicviscoelastic measurements. These were contrasted to the properties of polystyrene composites prepared with unfunctionalized SWNTs at similar loadings. The functionalized nanocomposites demonstrated a percolated SWNT network structure at concentrations of 1 vol % SWNT, while the unfunctionalized SWNT-based composites at twice the loading of SWNT exhibited viscoelastic behavior comparable to thatof the unfilled polymer. This formation of the SWNT network structure for the functionalized SWNT-based composites is because of the improved compatibility between the SWNTs and the polymer matrix and the resulting better dispersion of the SWNT.
Introduction Development of single-walled carbon nanotube (SWNT)-based polymer nanocomposites has the potential for the tailoring of unique lightweightmaterials with distinctly superior mechanical, thermal, and electronic properties. The development of such composites has been impeded by the inability to disperse SWNTs, which typically appear as ropes, in the polymer matrix due to the lack of chemical compatibility between the polymers and the SWNTs.1 A cursory summation of important areas for composites with SWNTs would include homogeneity ofdispersion, interfacial compatibility with the matrix, and the exfoliation of SWNT ropes and bundles. Chemical modification of SWNTs, be it covalent or noncovalent, may help address all of these concerns by the attachment of appropriate moieties to the surface of SWNTs. The covalent chemistry of SWNTs has been reviewed,2 and this area is actively being investigated. Using these techniques, one canenvision attachment of moieties that both facilitate dispersion and provide for chemical bonding with the matrix. We demonstrate here the benefit of functionalized SWNTs by examination of melt-state rheology of polystyrene/SWNT composites. We draw inspiration from previous work in layered silicates (smectites or nanoclays), where organic modification, by ion-exchange of the metal cations withorganic cations, renders the pristine hydrophilic silicates to be hydrophobic or organophilic and capable of being intercalated or exfoliated by polymeric matrices.3-5 Two theoretical arguments have been proposed to justify the incorporation of polymers between the silicate layers;5-7 the simpler mean field theory of Vaia and Giannelis7 suggests that the intercalation of polymers results almostentirely due to enthalpic interactions. They demonstrated, with experimental verification,5 that even though the polymer had unfavorable interactions with the silicate, and the tail of the organic surfactant
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modifier had repulsive interactions with both the polymer and the silicate, intercalation was possible by simply ensuring that the polymer/silicate interaction was less unfavorable thanthe surfactant/silicate interaction. However, to obtain a truly exfoliated state, highly favorable interactions between the polymer and the silicate were necessary. In the case of layered-silicate polymer composites, the state of mixing is conveniently probed directly using X-ray diffraction and electron microscopy, because of the layer-layer registry and the large electron density contrast...