Carbon nanotube-based polymer nanocomposites: Fractal network to hierarchical morphology

The dispersion of anisotropic nanoparticles such as single-walled carbon nanotubes in polymeric matrices promises the ability to develop advanced materials with controlled and tailored combinations of properties. However, dispersion of such nanotubes in a polymer matrix is an extremely challenging task due to strong attractive interactions between the nanotubes. The successful dispersion of single-walled carbon nanotubes in poly(ethylene oxide) using an anionic surfactant (lithium dodecyl sulfate) as compatibilizer is reported here. The geometrical percolation threshold (pc, in vol %) of nanotubes, as revealed by melt-state rheological measurements, is found to be at ∼ 0.09 vol % loading, which corresponds to an effective tube anisotropy of ∼ 650. The system shows an even earlier development of the electrical percolation at 0.03 vol % SWNT loading as obtained by electrical conductivity measurements.;In their quiescent state, the nanotubes show hierarchical fractal network (mass fractal dimension ∼ 2.3 +/- 0.2) made of aggregated flocs. Inside the floc, individual or small bundles of nanotubes overlap each other to form a dense mesh. The interfloc interactions provides the stress bearing capacity for these nano composites and are responsible for the unique modulus scaling of these systems (∼(p-pc)delta, 3.0 ≤ delta ≤ 4.5). The interaction is inversely related to the particle dispersion state, which influences the absolute values of the viscoelastic parameters. As a direct consequence of the self-similar fractal network, the linear flow properties display 'time-temperature-composition' superposition. This superposability can be extended for non-linear deformations when the non-linear properties are scaled by the local strain experienced by the elements of the network. More interestingly, under steady shear, these nanocomposites show network-independent behavior. The absolute stress value is a function of the nanotube loading, but the characteristic time scales related to the process are independent of it. For fully grown network in a viscous polymer, cluster dynamics under external shear controls the non-linear behavior of the system.;Significant changes in the melting and crystallization behavior of poly(ethylene oxide) along with a decrease in fractional crystallinity has been observed, in these nanocomposites. The observed changes in the SWNT-based nanocomposites far exceed those observed for an equivalent Li+ ion concentration mixture. The identification of the nature of nanotube-polymer interactions and the nanotube's role during polymer crystallization provide the possibility of developing hierarchical materials with controlled multifunctional properties whose directionality can be easily manipulated. For the case where the nanotubes disturb the formation of polymer crystals, the oriented nanotubes, because of the short inter-tube distances even at low nanotube concentrations, cause a templating of the polymer crystals with the lamellar---normals oriented orthogonal to the nanotube axes. On the other hand for the case where nanotubes nucleate the polymer crystals, a "shish---kebab" structure is realized, with the nanotubes and polymer crystals acting as the shish and kebab, respectively.