Orientation dynamics in multiwalled carbon nanotube dispersionsunder shear flow

摘要

We report studies of the orientation state of multiwalled carbon nanotubes (MWNTs) dispersions insteady and transient shear flows. Uncured epoxy was used as a viscous Newtonian suspendingmedium and samples were prepared from "aligned" MWNTs using methods previously reported [S. S. Rahatekar et al., J. Rheol. 50, 599 (2006)]. Orientation measurements were performed in boththe flow-gradient (1-2) and flow-vorticity (1-3) plane of simple shear flow using in situ x-ray scattering techniques. Steady state measurements in the 1-2 plane indicate that the MWNT orientation is shear rate dependent, with the MWNTs orienting closer to the flow direction at highershear rates. During steady shear, anisotropy was measured to be higher in the 1-2 plane than in the1-3 plane, demonstrating that the nanotube orientation state is not unaxially symmetric in shear. It is hypothesized that the steady state MWNT orientation is governed primarily by a rate-dependentstate of nanotube aggregation/disaggregation, which was separately characterized by opticalmicroscopy of the same samples under shear. High flux synchrotron radiation allowed fortime-resolved structural studies in transient flows. A partial relaxation of flow-induced anisotropy was observed following flow cessation, despite the very small rotational diffusivity estimated forthese nanotubes. Long transients are observed in step-down experiments, as the orientation state changes in response to the slow tube aggregation process.