A comparison between physical properties of carbon black-polymer and carbon nanotubes-polymer composites

Multiple-walled carbon nanotube (CNT)-and carbon black (CB)-polymer composites have been fabricated by mechanical mixing with different loadings, the polymeric matrix being identical between the two series of samples. The main focus of this work is to report measurements of physical properties of these mixtures in ambient conditions and to discuss the origin of similarities and differences among them according the kind of carbonaceous filler. The uniform dispersion of the carbonaceous phase in the dielectric matrix was probed by high-resolution transmission electron microscopy. The good dispersibility of the filler particle is also reflected in the much lower conduction threshold observed for CNT-containing samples than in the CB composites. This is likely due to the high aspect ratio of the CNTs. Mechanical properties show that the storage modulus of the two kinds of samples is close to the modulus value of the neat styrene-butadiene rubber (SBR), independent of filler content over a wide range of compositions (phi<0.1) encompassing the percolation threshold. Microwave measurements show that the real part of the effective permittivity exhibits a flat frequency response, with the exception of the sample containing 30 vol % CB for which an inverse-power law is observed revealing a behavior that has been seen for many random heterogeneous soft materials. No resonant dielectric absorption is evidenced within the frequency range explored and for the filler concentrations investigated. The results were also compared with analytical effective (mean-field) models. The symmetric Bruggeman model is in very good agreement with the microwave effective permittivity once account is taken of the depolarization factor which is close to the value found for a three-dimensional (3D) random dispersion of monodisperse spherical conductive inclusions within a dielectric matrix. By combining microwave frequency-domain spectroscopy with uniaxial tension, we obtain the effective permittivity as a function of the elongation ratio. Our results indicate that the effective permittivity spectrum of the CNT-polymer samples and their CB-based counterparts is not very sensitive to the applied stress in the range of elongation ratios explored. For the sample containing 30 vol % CB, the relative variation in the effective permittivity as a function of the elongation ratio is well described by the Gaussian molecular network model. The experimentally determined mechanical and microwave properties of these nanocomposites is related to the change in the mesostructure, formed by the heterogeneous 3D interconnected network of polymer and of aggregates (or agglomerates) of filler particles, as the composite is stretched. The results of this study provide another insight and opportunities to the comprehension of multifunctional materials, including novel nanoelectronic components, and carbon-based systems. (C) 2010 American Institute of Physics. [doi:10.1063/1.3486491]