E A. Grulke

Orientation of carbon nanotubes in a sheared polymer melt

Optical measurements of the shear response of semidilute dispersions of polymer-dispersed multiwalled carbon nanotubes are presented. For a weakly elastic polymer melt, the data suggest that the semiflexible tubes orient along the direction of flow at low shear stress, with a transition to vorticity alignment above a critical shear stress, σc, corresponding to a critical Deborah number of approximately 0.15. In contrast, data for a highly elastic polymer solution suggest that the tubes orient with the flow field at high shear rates, in the limit of large Deborah number. The measurements are in qualitative agreement with previous experimental and theoretical studies of fiber orientation in elastic fluids under simple shear flow.

Rheo-optical studies of carbon nanotube suspensions.

We use a polarization-modulation technique to investigate the optical anisotropy of multi- and single-wall carbon nanotubes suspended in a variety of solvents under simple shear flow. Measurements of birefringence and dichroism are performed as a function of shear rate, tube concentration, and solvent viscosity. At fixed volume fraction, the anisotropy increases with increasing shear stress due to enhanced flow alignment. At fixed shear stress, the anisotropy increases with volume fraction due to rotational excluded-volume interactions. By considering the rotational diffusivity as a function of nanotube length, diameter, concentration, and solvent viscosity, we demonstrate a leading-order scaling relation for the optical anisotropy in terms of rotary Peclet number Pe. At low Pe, our results are in qualitative agreement with the theoretical predictions of Doi and Edwards. At high Pe, our data suggest that the degree of nanotube alignment scales as Pe16.

Optical measurements of structure and orientation in sheared carbon-nanotube suspensions

We describe an optical metrology for measuring shear-induced structure and orientation in dilute dispersions of multiwalled carbon nanotubes. Small-angle polarized light scattering and optical microscopy are combined in situ to quantify the structural anisotropy of multiwalled carbon nanotubes in semidilute, surfactant-stabilized aqueous suspensions under simple shear flow. Measurements performed as a function of the applied shear rate are used to demonstrate the capabilities and limitations of the experimental technique, which should be suitable for probing the shear response of polymer-nanotube melts and solutions.

Charge transport in melt-dispersed carbon nanotubes

We investigate the effect of interfacial stabilizer on charge transport in polymer-dispersed carbon nanotubes. Despite mechanical contact, samples with dispersant show poor conductivity, which we attribute to a robust interfacial layer between contacted nanotubes. In comparison, results obtained when nanotubes are mechanically mixed into polymer melts without dispersant show much better conductivity. The difference is striking; at comparable loading, neat melt composites have resistivities five orders of magnitude smaller than those containing interfacial stabilizer. Our results highlight a fundamental issue for the engineering of conducting carbon nanotube composites; dispersion stability will typically be achieved at the expense of conductivity.