The blending of carbon nanotubes (CNT) into polymer matrices leads to intrinsically non-equilibrium materials whose properties can depend strongly on flow history. We have constructed a rheo-dielectric spectrometer that allows for the simultaneous in-situ measurement of both the Alternating Current (AC) electrical conductivity and dielectric constant as a function of frequency, 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. AC measurements of conductivity indicate a conductor-insulator transition in melt-mixed dispersions of multi-wall CNT in polypropylene over a narrow range of CNT concentrations that is well-described by generalized effective medium theory (GEM).A novel conductor-insulator transition in s can also be induced by shearing the fluid at a fixed CNT concentration f near, but above, the zero shear CNT conductivity percolation threshold fc. We find that the shear-induced conductor-insulator transition has its origin in the shear rate dependence of fc, which conforms well to a model introduced to describe this effect. Surprisingly, sconductivity of these non-equilibrium materials fully recovers upon cessation of flow. We also find that the frequency dependence of conductivity follows a universal scaling relation.
Citation: Physical Review B (Condensed Matter and Materials Physics)
Pub Type: Journals
carbon nanotubes, conductivity, percolation theroy, polymer nanocomposites, rheology