Nanotube networks in polymer nanocomposites: Rheology and electrical conductivity

Single-walled carbon nanotube (SWNT)/poly(methyl methacrylate) (PMMA) nanocomposites were prepared via our coagulation method providing uniform dispersion of the nanotubes in the polymer matrix. Optical microscopy, Raman imaging, and SEM were employed to determine the dispersion of nanotube at different length scales. The linear viscoelastic behavior and electrical conductivity of these nanocomposites were investigated. At low frequencies, G' becomes almost independent of the frequency as nanotube loading increases, suggesting an onset of solidlike behavior in these nanocomposites. By plotting G' vs nanotube loading and fitting with a power law function, the rheological threshold of these nanocomposites is similar to0.12 wt %. This rheological threshold is smaller than the percolation threshold of electrical conductivity, similar to0.39 wt %. This difference in the percolation threshold is understood in terms of the smaller nanotube-nanotube distance required for electrical conductivity as compared to that required to impede polymer mobility. Furthermore, decreased SWNT alignment, improved SWNT dispersion, and/or longer polymer chains increase the elastic response of the nanocomposite, as is consistent with our description of the nanotube network.