Wadia Dhaoui

Influence of dopant on dielectric properties of polyaniline weakly doped with dichloro and trichloroacetic acids

We report the results of dielectric measurements over the frequency range10 Hz–1 MHz and the temperature range 150–300 K on polyaniline subjected to doping with dichloroacetic acid (DCA) and trichloroacetic acid (TCA) with various doping levels (6.1%, 8.2%, 13.3% and 4.1%, 6.15%, 13.5%, respectively). Conductivity is increased when the doping level or temperature is increased and samples doped with TCA are more conductive than those doped with DCA. A high frequency relaxation peak is observed in the loss factor curves attributed to the motion of charge carriers in the bulk polymer. A second loss peak appears in the low frequency range when we use the dielectric modulus representation and is attributed to electrode polarization. Both relaxations are well fitted by the Havriliak–Negami function, and the fitting parameters are determined. The characteristic relaxation frequency is described by the Arrhenius law. The activation energy for both relaxations is decreased by increasing the doping level and it is lower in the case of TCA doping acid.

Comparative studies of the structure, morphology and electrical conductivity of polyaniline weakly doped with chlorocarboxylic acids

We report the results of studies on two series of polyaniline (PANI), doped with dichloroacetic (DCA) and trichloroacetic (TCA) acids, respectively, at various doping rates and obtained by the in situ polymerization method. Samples were characterized by x-ray diffraction, scanning electron microscopy and conductivity measurements. The direct current (dc) and alternating current (ac) electrical conductivities of PANI salts have been investigated in the temperature range 100–310 K and frequency range 7–106 Hz. The results of this study indicate better chain ordering and higher conductivity for PANI doped with TCA. The dc conductivity of all samples is suitably fitted to Motts three-dimensional variable-range hopping (VRH) model. Different Mott parameters such as characteristic temperature T0, density of states at the Fermi level (N(EF)), average hopping energy (W) and the average hopping distance (R) have been evaluated. The dependence of such values on the dopant acid used is discussed. At high frequencies, the ac conductivity follows the power law σac(ω,T) = A(T)ωs(T,ω), which is characteristic for charge transport in disordered materials by hopping or tunnelling processes. The observed increase in the frequency exponent s with temperature suggests that the small-polaron tunnelling model best describes the dominant ac conduction mechanism. A direct correlation between conductivity, structure and morphology was obtained in our systems.

Investigation of Spectral Properties, Thermal Stability, and Morphology in Polyaniline Doped with Chlorocarboxylic Acids

Abstract The morphology of polyaniline (PANI), granular or tubular, depends on the acidity conditions during the reaction rather than on the chemical nature of the acid. In reference to acetic acid in which media nanotubes had been observed, dichloroacetic acid (DCA) and trichloroacetic acid (TCA) were used to synthesize polyaniline by the oxidative polymerization technique. Characterization by Fourier-transform infrared (FTIR), UV-visible, and electron paramagnetic (EPR) spectroscopy techniques indicated that the polymers are in the protonated emeraldine form and suggest that the protonation of PANI in media containing such acids was achieved with the help of sulfuric acid produced during the reaction from ammonium peroxydisulfate. The conductivity of polyaniline prepared in DCA was 0.04 S cm−1 and 0.5 S cm−1 in TCA. The thermal stability of polyaniline salts was tested by thermogravimetric analysis and a three-step decomposition pattern was observed. Wide-angle X-ray diffraction (WAXD) study showed enhanced crystallinity in polyaniline prepared in TCA. The morphology is discussed in relation to the formation of polyaniline nanotubes reported in the literature.