Magnetoresistance 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 (fc) of PANI-CSA phase segregated in PMMA is fc ∼ 0.003 (0.3%). The conductivity at room temperature near the percolation threshold is quite high, 3 × 10−3 S/cm. The systematic increase in the exponent of the temperature dependence of conductivity (γ) results from transport on the fractal structure and to the related superlocalization of the electronic wavefunctions with systematic variations in the fractal dimensionality upon dilution of the PANI-CSA. Below the percolation threshold, the temperature dependence of the resistivity is like that of granular metals with γ = 0.5. The positive magnetoresistance (MR) shows a maximum upon decreasing the volume fraction of PANI-CSA in agreement with effective medium theory. Analysis of the MR indicates that the localization length near the percolation threshold is approximately 25 Å at 4.2 K.