Dispersion and rheology of carbon nanotubes in polymers

We review two generic mechanisms of dispersion of carbon nanotubes in a low-viscosity solvent or high-viscosity polymer, focusing on the neat nanotubes not surface-functionalized in any way. We give estimates of the van der Waals energies involved in nanotube aggregates and examine two main techniques: ultrasonication and shear mixing. For ultrasonic dispersion methods, the local mechanical energy applied to individual tubes is high and bundle separation is assured in the cavitation regime. We analyze and estimate the tube scission during ultrasonic cavitation and predict the characteristic nanotube length L-lim below which scission does not occur. For shear-mixing, our analysis suggests that dispersion is possible in non-parallel bundled nanotube aggregates, in high-viscosity polymers, once a critical mixing time t* is reached. We then examine characteristic features of nanotube-polymer composite rheology and its aging/stability against re-aggregation. We show that at nanotube loading above overlap concentration the tubes form an elastic network in the matrix. Physical junctions of this network are strong and stable enough to provide a rubber-like elastic response with very slow relaxation.