Transport in polyaniline networks near the percolation threshold

The self-assembled network of conducting polyaniline (PANI), protonated by camphor sulfonic acid (CSA), in a matrix of insulating polymethylmethacrylate (PMMA) has a remarkably low percolation threshold. The critical volume fraction (f) of the PANI-CSA phase segregated in PMMA is inferred from the concentration dependence of the conductivity, ${\mathit{f}}_{\mathit{c}}$≊0.003 (0.3%). The conductivity at room temperature near the percolation threshold is quite high, 3×${10}^{\mathrm{-}3}$ S/cm. Transmission-electron microscopy (TEM) results are in agreement with the percolation threshold inferred from the transport data; the TEM micrographs show that the connectivity of the PANI-CSA network decreases rapidly for f<0.005. Near room temperature, the positive temperature coefficient of resistivity (ρ), a feature typical of the intrinsic metallic nature of PANI-CSA, is retained in the networks. At lower temperatures, ρ(T) exhibits a temperature-dependence characteristic of variable range-hopping transport, ρ(T)∝exp[(${\mathit{T}}_0$/T${)}^{\gamma }$], with the exponent increasing from γ=0.25 to 1 upon decreasing the volume fraction of PANI-CSA from f=1 to ${\mathit{f}}_{\mathit{c}}$. This systematic increase in γ results from transport on the fractal structure and to the related superlocalization of the electronic wave functions. Below the percolation threshold, the temperature dependence of the resistivity is like that of granular metals with γ≊0.5, consistent with the morphology and microstructure seen in the TEM micrographs. The positive magnetoresistance shows a maximum upon decreasing the volume fraction of PANI-CSA in agreement with effective-medium theory. Analysis of the magnetoresistance indicates that the localization length near the percolation threshold is approximately 25 Å at 4.2 K.