Anomalous temperature dependence of the electrical conductivity of carbon-poly(methyl methacrylate) composites

Composites of carbon powder and poly(methyl methacrylate) (PMMA) were formed by a low-temperature slurry method and their electrical conductivities measured from 80 to 300 K. These materials exhibit a percolation threshold at approximately 8 vol%, with room temperature conductivities below 0.1 S/cm for carbon concentrations at or above 25 vol%. Temperature-dependent conductivities are described well by a thermally activated tunneling model that includes a change in the tunneling barrier height, area, and/or width above 100 K. Since a similar temperature dependence is observed for composites of other conducting materials in PMMA, we believe this effect is due to a change in the polymer at about 100 K, possibly to the effect of polymer chain reorientation due to methyl group rotation on the polymer backbone. This is the first time such behavior has been reported for conducting composites.