Electrosynthesis and properties of conducting polymers derived from aminobenzoic acids and from aminobenzoic acids and aniline

The electrochemical polymerisation of 2-, 3-, and 4-aminobenzoic acids and of mixtures of aniline and aminobenzoic acids in various ratios has been carried out by cyclic potential sweep in sulphuric acid solution. Polymers are formed in all cases and the rate of polymerisation is increased by the addition of aniline. The polymers formed have been characterised voltammetrically and evidence concerning the polymer structure and chemical composition has been obtained from IR spectroscopy and scanning electron microscopy. The results suggest that 2-amino-, 3-amino- and 4-aminobenzoic acid all form short-chain conducting polymers. Films from polymerisation of aniline and aminobenzoic acid mixtures show characteristics of both components and demonstrate the influence of aminobenzoic acids and their oligomers on the electrochemical and morphological properties of polyaniline formed by electropolymerisation.     

Synthesis,characterization and properties of highly conducting organometallic polymers derived from the ethylene tetrathiolate anion

The preparation and electrical conductivity studies of a series of organometallic polymers, (MC₂S₄^x−)_n (M = Ni, Pd, Pt, Au), derived from the ethylene tetrathiolate anion are described. The synthesis of the precursor molecules or molecular complexes (PhCH₂)₄(C₂S₄), (AsPh₄)₂[M(C₃OS₄)₂], (C₃)OS₄Pd(dppe) and (AsPh₄)₂[(C₃OS₄)Cu(C₂S₄)Cu(C₃OS₄)] is also described. From these results the different possible formation mechanisms of the polymers are discussed. The polymers show high compaction powder conductivity at room temperature (from 10⁻⁵ to 5Ω⁻¹ cm⁻¹). [Na_0.31(NiC₂S₄)]_ν, one of the most conductive polymers, exhibits a metal-like temperature dependence of conductivity, as evidenced by voltage shorted compaction (VSC) experiments. These conductivity properties are to be related with the degree of partial oxidation (DPO) observed in the polymers studied.     

Electrochemical polymerization and in situ FTIRS study of conducting polymers obtained from o-aminobenzoic with aniline at platinum electrodes

The electrochemical copolymerization of o-aminobenzoic acid with aniline (ANI) was carried out on platinum electrodes in perchloric acid aqueous medium by cyclic scanning of the potential. Electrochemical homopolymerization of o-aminobenzoic acid was also done. Experimental results showed that the polymers are formed in all cases and the presence of aniline promoted markedly the growth of the rate of copolymerization.The obtained polymer films have been characterized by cyclic voltammetry and in situ Fourier transform infrared (FTIR) spectroscopy. The mechanism of the electrochemical polymerization reaction has been discussed.     

Dielectric properties of thiophene based conducting polymers

Dielectric properties of some conducting polymers were investigated and their electrical properties are compared with each other. Temperature dependencies of their electrical properties are investigated throughout the work. Electrical behaviors of the synthesized systems constitute a good infrastructure for both understanding the intrinsic mechanisms and optimizing new synthesizes. It is found that, it can be possible to attain desired electrical properties by small synthesis modulations in thiophene based polymer materials.     

Electrical and optical properties of molecularly doped conducting polymers

Novel electrical and optical characteristics have been observed in conducting polymers doped with molecular dopants such as fullerenes (C₆₀, C₇₀ etc.), photochromic dyes and another (guest) conducting polymer. Highly effective photo-induced charge transfer results in various effects such as photoluminescence quenching, photoconductivity enhancement, electroluminescence quenching and persistent photoconductivity which have been observed in fullerene-doped conducting polymers. Unique photo-sensitive characteristics have also been found in conducting polymer/ fullerene systems. Photoluminescence and photoconductivity in conducting polymers doped with photochromic dyes have been changed dramatically by photo-induced isomerization of photochromic dyes and memory effects in both dark conductivity and photoconductivity have been observed. We also present here some results on conducting polymer-conducting polymer mixtures and discuss general features of such composites. These characteristics can be explained by the asymmetry of the relative electronic energy states of conducting polymer and molecular dopants on account of self-trapping effects. Superconductivity evolved upon alkali metal doping of C₆₀-conducting polymer composites, and has been confirmed by low-field microwave absorption (LFMA) and SQUID magnetometry measurements. The origin of superconducting phases in this doubly doped conducting polymer is discussed.     

Polyfuran conducting polymers: Synthesis,properties, and applications

In this review, polyfuran (PFu) synthesis methods and the nucleation mechanism; the electrochemical, structural, morphological, and magnetic properties of PFu; thermal behavior; theoretical calculations on PFu, as well as its applications reported to date, have been compiled. Not only PFu homopolymers have been reviewed, but also PFu co-polymers, PFu bipolymers, and PFu composites. The results are listed, discussed, and compared. It is hoped that this assembly of all the relevant data might enhance knowledge about this conducting polymer and lead to new research fields.     

Structural properties of some conducting polymers and carbon nanotubes investigated by SERS spectroscopy

In this paper, we present Surface Enhanced Raman Scattering (SERS) experiments performed successfully on some conducting polymers and carbon nanotubes deposited in thin films on rough metallic surfaces. The enhancement mechanism in SERS has a twofold origin: electromagnetic and chemical. The electromagnetic enhancement is, however, the dominant mechanism and consists in the excitation of localized and delocalized surface plasmons (SP). The second enhancing mechanism for SERS, not yet convincingly supported by experimental data, is of chemical origin. This mechanism is due to the increase of the polarizability of the molecules on the metal surface under the action of the incident radiation, leading to the formation of new chemical bonds between the molecules and the metal surface. Unfortunately, the experimental data available to argue the presence of a chemical process are scarce and scattered. In this paper, we try to present some details regarding this aspect. In order to do so, we studied poly 3-hexylthiophene (3-PHT) and polyaniline (PAN) as a function of the type of the rough metallic support (Ag, Au or Cu), the oxidization state and thickness of the polymer layer. We studied also carbon single walled nanotubes. Our experiments reveal the existence of a chemical surface effect. The results obtained for 3-PHT show that SERS spectra depend on the oxidizing properties of the metal surface and on the nature of the solvent. This dependence is explained by the existence of some interfacial reactions that lead to the formation of interface compounds. The SERS measurements reported here reveal an increase of the intensities of the Raman lines, accompanied by a modification of the corresponding intensity ratios, when the degree of doping is increased. We observe for the first time by SERS spectroscopy that doping of 3-PHT with FeCl₃ leads to the appearance of a state of disorder in the structure of the macromolecular chain, as a result of steric hindrance effects. The type of the rough metallic support can modify SERS spectra and such an effect is clearly shown for the polyaniline case. No such dependence on metallic support type is observed on the SERS spectra of carbon nanotubes.     

NMRD of polyaniline-based conducting polymers

Nuclear magnetic resonance dispersion (NMRD) is a technique for measuring NMR relaxation times, which is commonly used to study dynamics in condensed matter. In this paper, we present NMRD profiles, for polyanilines doped with trifluoromethanesulfonic acid (TFSA), recorded as a function of temperature and dopant concentration. This work demonstrates that NMRD can be used to characterise the critical processes that determine the conductivity of the materials. The NMRD profile of the bulk polymer is sensitively dependent on the concentration of the dopant, while the response of the dopant is insensitive to concentration. Complete analysis of the NMRD profiles of these ICPs will give important information, including the rates of spin diffusion and of charge transport between dopant-rich domains.     

Mechanochemical preparation of conducting polymers and oligomers

Three types of conducting polymers – polypyrrole (PPy), polythiophene (PTh), and polyparaphenylene (PPP), – are prepared by environmentally friendly and efficient solvent-free mechanochemical method. The experimental data obtained in the work argue for a high molecular weight for PPy, whereas PTh and PPP are represented by different oligomers. Measurements of conductivity confirm formation of the doped state of PPy, weakly doped state of PTh and non-doped state of PPP. Interrelation between the doping state and degree of polymerization is discussed.     

Interpenetrating networks of two conducting polymers

Interpenetrating networks (IPNs) of two conjugated polymers are prepared by a combination of a chemical oxidation step and a vapour phase polymerisation step on non-conducting surfaces. In this work ferric tosylate was used as the oxidant as it gives very smooth and homogeneous coatings, and because its reaction products can be removed efficiently after the formation of the composite. Several combinations of polymers are demonstrated, and the versatility of the proposed method allows extensions to a wide range of conjugated polymers. The IPNs show optical and electrochemical characteristics, which are sums of the characteristics from the participating conducting polymers.     

Nonlinear optical susceptibilities of conducting polymers

Using the Keldysh non-equilibrium Green's function methods we compute analytic expressions over the full frequency range for the nonlinear optical response functions for a non-interacting model of a conducting polymer. The model has no adjustable parameters. Our results for 3rd harmonic generation are in excellent agreement with experiments on polyacetylene. This constitutes strong evidence that polyacetylene is well described by a model of weakly interacting quasi-particles. Further nonlinear optical effects, which should be experimentally observable, are predicted.     

Studies of photoexcitations in conducting polymers mixed with C60

We have studied photoexcitations in mixed compounds of C₆₀ with derivatives of poly (p-phenylenevinylene) and poly(phenyleneacetylene) using a variety of CW and picosecond (ps) transient spectroscopies. The CW techniques used include photoinduced absorption (PA), photoluminescence (PL) and optically detected PA (ADMR), whereas the ps transient techniques used were time-resolved PA, PL and stimulated emission (SE). Compared with the pristine polymers, all mixed C₆₀ compounds have shown enhancement of PA bands associated with charged photoexcitations. These PA bands have shown a single spin-half ADMR resonance, identified as due to spins on the polymer chains; no spin-half ADMR signal from photogenerated C₆₀⁻ has been observed. In the ps time domain we have observed a pronounced reduction of the transient PL and SE in the mixed compounds. Due to the similarity found between the ps transient and CW PA bands, we have identified the ps photoexcitations in the mixed compounds as polarons (P⁺−P⁻ ) on the polymer chains, which are indirectly photogenerated as a result of exciton dissociation in C₆₀⁻ related defect centers. The ultrafast charge transfer, claimed to exist between the polymer chains and C₆₀ molecules, has not been observed in the ps time domain.     

Polymerization of substituted aniline and characterization of the polymers obtained

Chemical polymerization of m-toluidine, o-chloroaniline and nitroaniline has been studied, and characterization of the resulting polymers performed by DSC, X-ray, i.r., XPS and conductivity measurements. It is found that the substituted group of aniline affects not only the polymerization reaction, but also the properties of the polymers obtained. The polymerization conditions, such as the pH value of the polymerization solution, have a certain influence on the polymer yield and its conductivity. The solubility of polymers of substituted aniline (SPA_n) is similar to that of polyaniline. The results on X-ray diffraction of SPA_n reveal that the substituted group has some effect on the regularity of the polymers. DSC measurements show the existence of an endothermic peak as a disordering of the regular segments. The spin concentration of SPA_n increases with increasing conductivity of the polymers to a certain degree. On the basis of the XPS and doping reaction of SPA_n, a possible structure of the conducting polymer is proposed.     

Inhibition of corrosion of steels with the exploitation of conducting polymers

Corrosion of steels causes a loss of about 121 billion dollars annually and half the iron produced at present is used for the replacement of the existing deteriorated structures. Corrosion, being an electrochemical phenomenon, can be tackled through the use of electrochemistry and conducting polymers. Stainless steels can be passivated by applying a potential of about 0.1 V versus SCE and a current of about 100 μA/ cm². These can be provided by a conducting polymer, emeraldine, which can be applied chemically on the surface of the stainless steel. Pre-treatments with some chelating agents can inhibit the corrosion of stainless steel from 0.1 M HCl for about 1 month.     

X-ray absorption spectroscopy of iron-doped conducting polymers

X-ray absorption spectroscopy (XAS) measurements were performed at the Fe K edge to determine the iron local structure in chemically prepared polyaniline (PANI) and polypyrrole (PPy) samples prepared with FeCl₃ as an oxidant. The samples were conditioned at different pHs by an acid–base treatment. In both as synthesized-doped polymers, the observed dispersed Fe atoms were predominantly coordinated to chlorine as Fe(III)Cl_x species, where x=6 for PANI and x=4 for PPy samples suggesting that the polymer–counteranion bonding is weaker than the iron–chelate one. For the PANI samples submitted to basic treatment, three different sets of Fe distances were found: five Fe–O at 2.00 Å, three Fe–Fe at approximately 2.80 Å and five Fe–Fe at 3.00 Å and the near-edge spectra showed the presence of octahedrally coordinated Fe⁺³. These results strongly suggest the presence of small oxide/hydroxide aggregates. Similar data were obtained for PPy treated with NH₄OH.     

Spectroscopic and thermal properties of the copolymer of aniline with dithiodianiline

Electrochemical copolymerization of aniline (ANI) with dithiodianiline (DTDA), an aniline derivative containing –S–S– links was carried out in 2 M H₂SO₄ using cyclic voltammetry. The cyclicvoltamogram (CV) of the deposited copolymer was recorded in monomer free background electrolyte. A close comparison of the CV characteristics of the films deposited during polymerization of mixture of ANI and DTDA clearly the copolymer deposition. The UV–VIS spectroscopy, spectroelectrochemical studies, thermal analysis and FTIR spectroscopic analysis were used to identify the differences between the copolymer and PANI in the optical, thermal properties and structure. The copolymer was found to have a branched structure in comparison with PANI. The copolymer exhibit different colors upon changing applied potentials.     

Definition of exciton binding energy for conducting polymers

The exciton binding energy ɛ_b for a conducting polymer can only be defined unambiguously in the long chain limit. In that limit it can be taken as the difference between the bandgap or the ‘single particle bandgap’ — the former if the exciton dissociates into a free electron and hole, the latter if it dissociates into a pair of oppositely charged polarons — and the optical absorption edge.     

Electrochemical behaviour and some characteristics of conducting polymers issued from the autoxidation of some bromopyrroles

2- and 3-bromopyrrole, and 2,5-, 2,3- and 3,4-dibromopyrrole were prepared and found to undergo a self-polymerization reaction leading to conducting polymers (called autopolymers). This reaction occurs as soon as they are pure or in concentrated solution in a neutral solvent. All these autopolymers, and especially that derived from 3-bromopyrrole, present most of the features of the classical polypyrroles. In particular, they are electroactive in common electrolytes provided a carbon paste electrode is used. A comparative study of these materials and classical polypyrroles is reported based on the physical and electrochemical characteristics of both polymers. A general structure is also proposed for the autopolymers. In addition, this unusual method of synthesis allows the autopolymers to be included in various substrates by condensation in situ. In this way composites with conductivity up to 10⁻³ S are obtainable.     

A molecular viewpoint of lattice dynamics and spectra of conducting polymers

The well known features of vibrational spectra of pristine and doped polyacetylene are reinterpreted extending standard concepts of molecular dynamics. In particular, dipersion of Raman frequencies with the exciting wavelength, activation of new bands by doping or photoinduction and the appearance of the so-called S-modes may receive an explanation.