Counterion-induced processibility of polyaniline: Transport at the metal-insulator boundary

Polyaniline (PANI) films of superior homogeneity can be prepared by using functionalized protonic acids to protonate the polymer and simultaneously induce solubility, in the conducting form, in common organic solvents. The results of measurements of the temperature dependence and magnetic-field dependence of the conductivity of films of PANI complexed with camphor sulfonic acid (CSA) are reported. The intrinsic metallic nature of conducting polyaniline can be observed from the positive temperature coefficient of resistivity in the temperature interval from 180 to 300 K. Signatures of hopping transport are observed below 180 K, indicative of marginal metallic behavior with disorder-induced localization at low temperatures. Resistivity ratios as low as 1:3 (300 to 1.2 K) are found. Analysis of the dependence of the resistivity on temperature and magnetic field indicates that the typical electronic localization length in PANI-CSA films is 100–150 Å. Thus the PANI-CSA is on the metal-insulator (M-I) boundary. A magnetic field (H) shifts mobility edge and causes the system to crossover from disordered metal to insulator: in zero field, ρ(T)∝${\mathit{T}}^{\mathrm{-}\mathrm{\beta }}$ where β=0.36, consistent with transport in the critical region near the M-I boundary, whereas, in H=8 T, ρ(T)=${\mathrm{\rho }}_0$exp[(${\mathit{T}}_0$/T${)}^{1/4}$] indicating variable range hopping between localized states with mean localization length, ${\mathit{L}}_{\mathit{c}}$(H=8 T)≊100–140 Å.