Complexity in charge transport for multiwalled carbon nanotube and poly(methyl methacrylate) composites

We report on studies of the charge transport mechanism of the composites of multiwalled carbon nanotubes (MWCNTs) and poly (methyl methacrylate) (PMMA). The MWCNTs were synthesized by a chemical vapor deposition method using Fe as a catalyst. The dc conductivity (sigma(dc)) and its temperature dependence [sigma(dc)(T)] were measured in a temperature range of 0.5-300 K to study the charge transport mechanism of the composites. The sigma(dc) of composites at room temperature increased as the MWCNT concentration increased, which shows typical percolation behavior with percolation threshold of p(c) at similar to 0.4 wt % of the MWCNTs. The sigma(dc)(T) of the MWCNT-PMMA composites were fitted to the combination of Sheng's fluctuation induced tunneling (FIT) model and the one-dimensional variable range hopping (1D VRH) model. The tunneling mechanism and the 1D VRH transport were attributed to the charge tunneling between MWCNT clusters and the charge hopping through 1D MWCNTs, respectively. Magnetoresistance (MR) of MWCNT-PMMA composites was measured up to 9 T of magnetic field. The results of MR and sigma(dc)(T) showed that the FIT model was dominant in the low temperature region (T less than or similar to 10 K), while the 1D VRH process became dominant as the temperature increased (10 K less than or similar to T less than or similar to 300 K). We observed unusually large negative MR in the composites at the lower temperatures (T < 4 K) due to FIT conduction. We related the parameters specifying charge transport to the percolation structures of MWCNT-PMMA composites based on the results of temperature dependence of sigma(dc) and of the MR.