J.P. Travers

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.

Multi-scale scanning tunneling microscopy imaging of self-organized regioregular poly(3-hexylthiophene) films

Two-dimensional self-organized poly(3-hexylthiophene) films on highly oriented pyrolytic graphite have been probed at the solid/substrate interface by scanning tunneling microscopy (STM). Structural morphology and typical polymer conformations are visualized and discussed from mesoscopic to nanoscopic scales, including mesoscopic assembly of polycrystals, crystalline monodomain orientations and sizes, grain boundaries, chain folds, and other conformational features. STM estimation of the average chain length is in remarkably good agreement with that derived from size-exclusion chromatography. The multiscale analysis supports a picture where heterogeneities exist at different length scales.

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.

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.

Effect of molecular weight on electronic, electrochemical and spectroelectrochemical properties of poly(3,3″-dioctyl-2,2′∶5′,2″-terthiophene)

Poly(3,3″-dioctyl-2,2′∶5′,2″-terthiophene), obtained from its corresponding monomer by oxidative polymerization with FeCl3, has been fractionated into five fractions of reduced polydispersity, covering the Mn range from 1.50 kDa to 10.50 kDa (measured vs. polystyrene standards). The effect of Mn on spectroscopic, electrochemical, spectroelectrochemical and electrical transport properties has been investigated. Fractions of growing Mn show an increasing bathochromic shift of the band originating from the π–π* transition in the neutral polymer with the appearance of a clear vibrational structure for the two highest molecular fractions. The onset of oxidative doping determined from the cyclic voltammogram shifts towards lower potentials with increasing molecular weight. A similar trend is observed for doping induced near infrared bands, which shift towards lower energies (higher wavelengths) with increasing molecular weight and appear at lower potentials in spectroelectrochemical experiments. Finally, a comparison of the FET mobility in two transistors fabricated under identical conditions from polymer fractions differing in their molecular weight shows that a ca. fourfold increase of Mn (from 2.40 kDa to 10.50 kDa) results in a two orders of magnitude increase in the carriers’ mobility (from μsat = 4 × 10−5 cm2 V−1 s−1 to μsat = 2 × 10−3 cm2 V−1 s−1). The obtained results underline the importance of the control of the macromolecular parameters in the preparation of electronic and electrochemical devices from poly(3,3″-dioctyl-2,2′∶5′,2″-terthiophene).

Spin dynamics study in polyaniline

Abstract Proton spin lattice relaxation time T 1 has been measured in the frequency range 6–340MHz in a set of nine polyaniline samples equilibrated at different pH. With additional ESR data, T 1e and T 2e measured as a function of the frequency, it is concluded that electronic spin motion is dominated by a quasi 1-D diffusion. The diffusion rate along the chain, D is almost independent of the protonation rate y , but the cutoff frequency f c increases suddenly by 3 orders of magnitude accross the insulator-to-conductor transition. This is interpreted in terms of percolation between protonated cluster just consisting in one conducting chain. Conductivity estimated from D agrees with the measured dc conductivity, but the carrier mean free path is too small to be consistent with metallic conductivity.