Shear Orientation in Nematic Carbon Nanotube Dispersions: A Combined NMR Investigation

Carbon nanotubes were dispersed in a sodium dodecylsulfate/decanol/water nematic fluid. The long-term stability of the dispersions; is ensured by the small density gradients existing between nanotubes and the nematic fluid, and by its viscosity, as well. Presumably, surfactant or nematic micelles adsorb onto nanotubes and concur to stabilize them. A Rheo H-2 NMR characterization was performed. It was supported by classical H-2 quadrupole splitting and pulsed field gradient spin echo NMR, allowing to ascertain the diffusive trends therein. The nematic fluid shows uniaxial spectral profiles and marked diffusion anisotropy. No such effects were observed in nanotube-containing nematic dispersions. In addition, the measured water self-diffusion values are substantially lower than the pure nematic fluid. In the absence of shear, dispersed nanotubes do not modify the quadrupole splitting amplitude, but affect the spectral profiles. The reasons for the observed behavior are briefly outlined. In the presence of shear, the spectral modifications are substantial and lead to the onset of isotropic dispersions, after long-time shearing.