TRANSPORT IN POLYANILINE NETWORKS NEAR THE PERCOLATION-THRESHOLD

The self-assembled network of conducting polyaniline (PANI), protonate d by camphor sulfonic acid (CSA), in a matrix of insulating polymethyl methacrylate (PMMA) has a remarkably low percolation threshold. The cr itical volume fraction (f) of the PANI-CSA phase segregated in PMMA is inferred from the concentration dependence of the conductivity, f(c) approximate to 0.003 (0.3%). The conductivity at room temperature near the percolation threshold is quite high, 3 x 10(-3) S/cm. Transmissio n-electron micros copy (TEM) results are in agreement with the percola tion threshold inferred from the transport data; the TEM micrographs s how that the connectivity of the PANI-CSA network decreases rapidly fo r f < 0.005. Near room temperature, the positive temperature coefficie nt of resistivity (p), a feature typical of the intrinsic metallic nat ure of PANI-CSA, is retained in the networks. At lower temperatures,; p(T) exhibits a temperature-dependence characteristic of variable rang e-hopping transport, p(T) proportional to exp[(T-0 /T)(gamma)], With t he exponent increasing from gamma = 0.25 to 1 upon decreasing the volu me fraction-of PANI-CSA from f = 1 to f(c). This systematic increase i n gamma results from transport on the fractal structure and to the rel ated superlocalization of the electronic wave functions. Below the per colation threshold, the temperature dependence of the resistivity is l ike that of granular metals with gamma approximate to 0.5, consistent with the morphology and microstructure seen in the TEM micrographs. Th e positive magnetoresistance shows a maximum upon decreasing the volum e fraction of PANI-CSA in agreement with effective-medium theory. Anal ysis pf the magnetoresistance indicates that the localization length n ear the percolation threshold is approximately 25 Angstrom at 4.2 K.