Charge transport and optical and magnetic properties of polyaniline synthesized with use of organic acids

Results of charge-transport, optical, and magnetic measurements of polyaniline (PAN) synthesized in the presence of paratoluenesulphonic acid (PTSA) and sulphosalicylic acid (SSA) are reported. As in the cases of HCl- and H2SO4-doped PAN, protonation leads to an increase in the electrical conductivity. dc conductivity and thermopower measurements down to about 50 K suggest that conduction takes place in an inhomogeneous medium consisting of ''metallic'' polymer particles or fibers embedded in an insulating matrix. While the optical-absorption spectrum resembles that of HCl-doped PAN, there are significant differences such as the persistence of the π-π* transition at about 3.8 eV, the absence of free-carrier absorption in [PAN-(SSA)x], and the coexistence of a redshifted band at 2 eV along with a band at 1.3 eV for all protonation levels in [PAN-(PTSA)x] and the persistence of the 2-eV band for all protonation levels in [PAN-(SSA)x]. dc-magnetic-susceptibility (χ) data is fitted to an equation of the form χspin=χ0+C/T. The temperature-independent susceptibility (χ0), however, decreases by one order of magnitude with increasing x, unlike the case of HCl- and H2SO4-doped PAN. For x=0.5, χ0 is comparable to the value obtained in HCl-doped PAN. However, no monotonic variation in C was observed. X-band ESR spectra are resolved into Gaussian and Lorentzian line shapes at both 300 and 77 K, while ESR absorption is absent at 300 K in the case of electrochemically synthesized PAN. This also suggests the coexistence of protonated and unprotonated regions and the possible formation of bipolarons as well as polarons at different stages of oxidation and protonation. © 1993 The American Physical Society.