Nanotube networks in polymer nanocomposites: Electrical and thermal properties, rheology, and flammability

The superior electrical and thermal conductivity, mechanical properties, and high aspect ratio of single wall carbon nanotube (SWNT) have made them potential fillers for polymer nanocomposites. In this dissertation, we have developed new fabrication methods for SWNT/polymer nanocomposites and explored the relationship between the properties and structures of these materials.; A coagulation method was developed to prepare SWNT/poly(methyl methacrylate) (PMMA) nanocomposites, providing uniform nanotube dispersion. Optical microscopy, Raman imaging, SEM, and AFM were employed to determine the nanotube dispersion at different length scales, from 100 mum to 1 nm. The resulting SWNT/PMMA nanocomposites exhibit improvement in elastic modulus and thermal stability. The addition of SWNT also significantly improves flammability of the polymer matrix, and nitrogen gasification forms a self-supporting nanotube network covering the whole nanocomposite.; Both nanotube concentration and alignment significantly influence electrical conductivity of the SWNT/PMMA nanocomposites; the electrical conductivity exhibits percolation behavior with increasing nanotube loading or isotropy due to the formation of an electrically conductive nanotube network. The thresholds for alignment percolation shift to more anisotropic as the loading increases. Near the concentration threshold, the highest conductivity, sigmamax , occurs in slightly anisotropic nanocomposites. These experimental results were further supported by two-dimensional simulations.; The SWNT/PMMA nanocomposites show rich rheology; they exhibit solid-like non-terminal behavior at low frequencies due to the formation of a hydrodynamic nanotube network. A rheological threshold was found to be significantly smaller than the conductivity threshold. We understand this difference in terms of the smaller nanotube-nanotube distance required for electrical conduction, as compared to impeding polymer motion. The filler dispersion, alignment, size, and shape and the molecular weight of polymer matrix have strong influence on the rheological behavior of the nanocomposites.; We compared thermal conductivities with electrical conductivities of these nanocomposites. The electrically conductive nanocomposites have the same thermal conductivities as pure PMMA, indicating that the electrically conductive nanotube network is not thermally conductive. A large interfacial thermal resistance between the nanotubes and the polymer matrix is the cause for the poor performance in thermal conductivity. To avoid the interfacial thermal resistance, an infiltration method was used to make composites with more enhanced thermal conductivity.