Variation in carbon nanotube polymer composite conductivity from the effects of processing, dispersion, aging and sample size

We investigated a number of parameters which can affect the apparent electrical conductivity of a polymer/ carbon nanotube composite. Focusing on the stress input generated by a dispersion technique, the effect of shear-mixing and ultrasonication on the morphology of carbon nanotube is examined. Using these two processing techniques, we fabricated composite films of multi-wall carbon nanotubes isotropically dispersed in a crosslinked polydimethylsiloxane matrix. The resulting electrical properties were characterized by AC impedance and DC current-voltage characterization across the film thickness using a parallel-plate geometry, and by surface resistivity using a 4-probe technique, also including the aging effects. We observed a large variation in conductivity depending on the electrical probing distance or the sample size. The presence of localized, dense conductive networks with length scales comparable to the film thickness (or the sensing probe separation) is suggested to account for the observed variation.