Shear-induced conductor-insulator transition in melt-mixed polypropylene-carbon nanotube dispersions

Abstract

The blending of carbon nanotubes (CNTs) into polymer matrices leads to intrinsically nonequilibrium materials whose properties can depend strongly on flow history. We have constructed a rheodielectric spectrometer that allows for the simultaneous in situ measurement of both the electrical conductivity $\ensuremath{\sigma}(\ensuremath{\omega})$ and dielectric constant $\ensuremath{\epsilon}(\ensuremath{\omega})$ as a function of frequency $\ensuremath{\omega}$, as well as basic rheological properties (viscosity, normal stresses), as part of an effort to better characterize how flow alters the properties of these complex fluids. Measurements of $\ensuremath{\sigma}$ indicate a conductor-insulator transition in melt-mixed dispersions of multiwall CNTs in polypropylene over a narrow range of CNT concentrations that is reasonably described by the generalized effective medium theory. A conductor-insulator transition in $\ensuremath{\sigma}$ can also be induced by shearing the fluid at a fixed CNT concentration $\ensuremath{\phi}$ near, but above, the zero shear CNT conductivity percolation threshold ${\ensuremath{\phi}}_{c}$. We find that the shear-induced conductor-insulator transition has its origin in the shear-rate dependence of ${\ensuremath{\phi}}_{c}$, which conforms well to a model introduced to describe this effect. Surprisingly, $\ensuremath{\sigma}$ of these nonequilibrium materials fully recovers at these elevated temperatures upon cessation of flow. We also find that the frequency dependence of $\ensuremath{\sigma}(\ensuremath{\omega})$ follows a ``universal'' scaling relation observed for many other disordered materials.