M. Nechtschein

The effect of the oxidation conditions on the chemical polymerization of polyaniline

Abstract Chemical polymerization of aniline with four oxidizing agents ((NH4)2S2O8, H2O2, K2Cr2O7 and KIO3) was studied in HCl solutions. It was established that the redox potential of the oxidizing agent is not a dominant parameter. Products of similar quality have been obtained with (NH4)2S2O8, K2Cr2O7 and KIO3. However, KIO3 seems the most convenient, because it yields good quality samples in a wide range of polymerization conditions.

Polarons, bipolarons and charge interactions in polypyrrole: Physical and electrochemical approaches

A quantitative study of polaron and bipolaron formation in polypyrrole upon electrochemical doping is presented. The steady-state spin concentration has been measured as a function of the electrical potential in an in situ ESR-electrochemical experiment and the charge injection has been determined by coulometry and by optics. The data are analysed from both physical and electrochemical points of view. It is shown that the energy for creating a bipolaron is about the same as for creating two polarons. The non-Nernstian behaviour that is observed for the redox process is explained by the introduction of interparticle charge repulsion interactions. The shape of these interactions is specified.

Investigation on the structure of polyaniline: 13C n.m.r. and titration studies

13C n.m.r. studies confirm that reduced polyaniline only consists of benzenoid rings and amine nitrogens. Upon oxidation quinoid structures are formed. The acidic functions have been titrated. One acidic function has been found in reduced polyaniline. In the oxidized form two acidic functions are present. The pK values of these acidic functions have been determined.

Water effects in polyaniline: A new conduction process

Abstract We present n.m.r. and conductivity measurements on polyaniline, which show that upon applying water vapour pressure: (i) protons exchange between the polymer and an adsorbed water phase; (ii) conduction is favoured. The diffusion process of water in a pellet is investigated. Finally, we propose a conduction mechanism based upon electron hopping between localized states, assisted by proton exchange.

Water effects in polyaniline: NMR and transport properties

Abstract Conductivity and thermopower of polyaniline samples have been measured as a function of water vapor pressure. It is observed that the presence of water favours conduction. The water adsorbed in polyaniline has been investigated by NMR. It is shown that protons exchange between the polymer solid phase and the water mobile phase. The proton exchange rate is determined of the order of 103s−1. A mechanism of conduction based upon electron hopping assisted by proton exchange is proposed.

Ageing of PANI: chemical, structural and transport consequences**

Chemical, structural and transport consequences of PANI have been studied for two types of samples: hydrochloric acid protonated PANI powders and camphorsulfonic acid protonated PANI films cast from m-cresol solutions. Different characterization techniques have been applied: elemental analysis, FTIR, XPS, XRD, and conductivity measurements. The combination of these techniques enables us to identify three main degradation processes: dedoping, oxidation/hydrolysis/scission of the chains, and crosslinking. The impact of ageing on transport properties is briefly discussed and a simple model taken into account the kinetics of the conductivity decay is postulated.

Water absorption study in polyaniline

Abstract Water absorption in polyaniline is studied by NMR and weight uptake. It is shown that water molecules are absorbed on the acidic sites of polyaniline. Moreover, evidences are given that fixed water molecules are present even in the dried samples and that they become mobile upon hydration.

Aging studies on polyaniline : conductivity and thermal stability

Abstract The effect of aging on the transport properties of PANI-HCl was studied as a function of the morphology and of the environmental conditions, DC conductivity σdc studies and spin dynamic methods (NMR and ESR) on aged polyaniline point to the fact that a large decay of the conductivity takes place at a macroscopic scale whereas only a slight decrease of the intrachain conductivity σintra is seen at a microscopic level. Thermal studies under air and nitrogen atmospheres of polyaniline doped with new families of dopant show an important enhancement of the thermostability of protonated polyaniline.

Polyaniline: A material still under discussion

Abstract Several studies ( 13 C NMR, vibrational spectroscopy, acidic titration, etc.) have been considered in this coherent description of the structure and the chemical composition of polymers of the polyaniline family. Among the different processes (oxidation, protonation, etc.) which relate the five polymers of this family, three are characterized by insulator-to-conductor transitions. The chemical aspects of these processes are explained in the framework of the preceding descriptions. Then, the variation of the magnetic and transport properties related to these transitions, as well as the models which have been proposed to account for these properties, are discussed. Special attention is paid to the problem of homogeneity of the material, after different types of doping.

On the nature of the conducting state of polyaniline

Abstract The role of disorder in the polyaniline properties is emphasized. Evidences are given against a true metallic state: the presence of two types of acidic functions is not compatible with a regular periodic lattice; the spin dynamics behavior reflects incoherent carrier motion. Pulse and CW ESR data are consistent with high spin concentration clusters (conducting islands), and it is concluded that a conducting island just consists of a single conducting polymer chain.