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

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 σ(ω) and dielectric constant ε(ω) as a function of frequency ω, 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 σ 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 σ can also be induced by shearing the fluid at a fixed CNT concentration φ near, but above, the zero shear CNT conductivity percolation threshold φc. We find that the shear-induced conductor-insulator transition has its origin in the shear-rate dependence of φc, which conforms well to a model introduced to describe this effect. Surprisingly, σ of these nonequilibrium materials fully recovers at these elevated temperatures upon cessation of flow. We also find that the frequency dependence of σ(ω) follows a ``universal'' scaling relation observed for many other disordered materials.