Electrochemical and computational studies of electrically conducting polymer coatings

Abstract

A summary of the practical and theoretical (molecular modelling) aspects of conducting polymer research led by the late Dr Sheelagh Campbell at the University of Portsmouth is presented. The wide-range of interest encompasses tailored monomer design, density functional theory calculations, electrodeposition on various substrates, and investigations of polypyrrole as a corrosion inhibitor and as a matrix for release of therapeutic agents.     

SERS,FT-IR and photoluminescence studies on single-walled carbon nanotubes/conducting polymers composites

Surface-enhanced Raman scattering (SERS), Fourier transform infrared (FT-IR) and photoluminescence (PL) spectroscopies were used to investigate composites based on single-walled carbon nanotubes (SWNTs) and different conducting polymers like polyaniline (PANI), poly-paraphenylene vinylene (PPV) and poly 3-hexylthiophene (3-PHT). In the case of SWNTs/PANI, different composites are obtained by adding dispersed SWNTs powder to the polymer solutions and by chemical polymerization of aniline in presence of SWNTs. The difference originates in the irreversible chemical transformation of SWNTs in the polymerization medium. The synthesis medium used for the preparation of PANI transforms SWNTs in fragments of shorter length like closed-shell fullerenes. This explains the similarity of SERS and FT-IR spectra of the composites PANI/SWNTs and PANI/C₆₀, chemically synthesized. Electrochemical polymerization of aniline in an HCl solution on a SWNT film leads to a covalent functionalization of SWNTs with PANI. In this case, Raman scattering data suggest an additional nanotubes roping with PANI as a binding agent. A post treatment with the NH₄OH solution of polymer-functionalized SWNTs involves an internal redox reaction between PANI and carbon nanotubes. As a result, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one. In the case of PPV and 3-PHT, the effect of the concentration of SWNTs on the photoluminescence properties will be described and compared, as probes of interaction.     

Electrochemical studies of self-doped conducting polymers: Verification of the ‘cation-popping’ doping mechanism

Cyclic voltammetry in conjunction with X-ray microprobe analysis has been used to investigate the doping mechanism in covalently-bonded-anion (self-doped) conducting polymers. The polymers investigated are the acid and sodium salt forms of poly 3-(4-butanesulfonate) thiophene (P3-BTSH and P3-BTSNa, respectively) and poly 3-(2-ethane sulphonate) thiophene (P3-ETSNa). These water-soluble polymers, whose synthesis is described in detail, were used (in the form of films cast from solution onto ITO glass or platinum sheets) as working electrodes in cyclic voltammetry experiments in which the proton or Na⁺ concentration of the electrolyte was monitored. The results directly verify the proposed doping mechanism: charge injection into the π-electron system of the conjugated polymer accompanied by proton or sodium ion ejection, without incorporation of electrolyte anion, leaving behind the oppositely charge counterion.     

Electrochemical impedance spectroscopy for studying the degradation of enamel coatings

Vitreous coatings can be used as a means for the corrosion protection of metals and alloys due to their excellent dielectric properties. Their use is of particular interest in the case of highly aggressive media (different pH levels, temperatures up to 600 °C). However, ceramic coatings exposed to aggressive agents and thermal cycles can experience devitrification and spalling problems, with the consequent loss of their protective properties.The present study uses the electrochemical impedance spectroscopy technique to characterize, in a non-destructive way, the appearance of surface defects induced by the effect of temperature and chemical attack on a vitreous enamel deposited on steel. By means of analysis of the impedance diagrams obtained, a relationship is established between the loss of the coating's protective properties and the mechanism by which the corrosion process takes place.     

Electrochemical synthesis and characterization of conducting polymers using room temperature melt as an electrolyte

Aromatic monomers can be polymerised using the chloroaluminate room temperature melt obtained by mixing 1:2 ratio of cetyl pyridinium chloride and anhydrous aluminium chloride miscible in all proportions with organic solvents as an electrolyte. The chloroaluminate (AlCl₄⁻) anion generated in this melt having a tetrahedral symmetry with equal bond lengths and bond angles is the dopant to stabilize macrocation generated near the vicinity of anode to yield better conducting and better ordered electronically conducting free standing polymer film. In this communication, we discuss the polymers derived from benzene and pyrrole and their characterization by various techniques.     

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.     

Electrochemical impedance spectroscopic studies of titanium and its alloys in saline medium

The influence of potential on electrochemical behaviour of pure Ti, Ti‐6Al‐7Nb, and Ti‐6Al‐4V ELI alloy under saline conditions were investigated by electrochemical impedance spectroscopy (EIS). All measurements were carried out in saline solution (0.9% NaCl) at different impressed potentials (corrosion potential (E_corr), 0 mV (SCE) and + 500 mV (SCE)) for 1 h. The experimental results were compared with those obtained by potentiodynamic polarization curves. The corrosion current densities obtained for the titanium alloys showed lower values than for pure Ti, indicating the formation of a stable passive film with time. Electrochemical impedance spectroscopic studies indicated that the resistance of the passive film increased with the impressed potential. The impedance spectra were fitted using a non‐linear least square (NLLS) fitting procedure. The magnitude of the corrosion resistance of titanium alloys under saline solution was compared and results are presented.     

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.     

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.     

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.     

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.     

Optically detected magnetic resonance (ODMR) studies of conducting polymers: an overview

Recent X-band optically detected magnetic resonance (ODMR) studies of films, solutions, and blends of poly(3-alkylthiophenes) and dialkoxy derivatives of poly(para-phenylevinylene) and poly(paraphenyleneacetylene) are reviewed and discussed. All films and blends exhibit three similar (but not identical) photoluminescence(PL)-enhancing features: (i) a strong narrow (10–15 G wide) resonance at g = 2.0025 ± 0.0010; (ii) an 1 kG wide triplet powder pattern around g 2; (iii) a 10–30 G wide peak 30–50 G downfield from hv/(2×2.003ß) (at g 4.07). The narrow resonance is attributed to intrachain ‘distant pair’ polaron recombination. Its spectral dependence, which is similar to the PL spectrum, indicates that the polaron levels are close to the band edges. Features (ii) and (iii) are attributed to intrachain triplet polaronic excitons. Their zero-field splitting parameters are consistent with a triplet size comparable to that of the thiophene and phenylene rings. The dependence of the ODMR features on sample history and the role of metastable and stable native defects is also discussed.     

Mechanistic studies of the electropolymerization of 2,2′-bithiophene and of pyrrole to form conducting polymers

The electropolymerizations of 2,2′-bithiophene and of pyrrole to form conducting polymers were investigated by several electrochemical methods. Cyclic voltammetry indicates that a peak at 1.1 V is associated with the polymer and that the same product is formed from thiophene as from 2,2′-bithiophene. The rate of polymer formation has a zero kinetic order dependence on monomer concentration for polymerization of pyrrole or 2,2′-bithiophene. A dependence on concentration of supporting electrolyte is interpreted as a consequence of anion diffusion and oxidative doping in the polymer. The kinetic results are consistent with an oxidative coupling mechanism for polymer formation. The electrochromic switching of the polymer film was investigated by spectroelectrochemical methods and the kinetics were examined over a range of film thicknesses.     

Electrochemical impedance studies on the electrochemical properties of poly(3-methylthiophene) in aqueous solutions

Electrochemical impedance spectroscopy (EIS) measurements in a wide frequency range were used to study the electrochemical properties of poly(3-methylthiophene) (PMT) films. PMT was synthesized in acetonitril (AC) using BF₄⁻, ClO₄⁻ and CF₃SO₃⁻ as doping anion and Li⁺ as cation. The electrochemical characterization of these films was performed in aqueous solution using the same electrolyte for synthesis. Different parameters as electrolyte resistance, double layer capacitance, pore resistance, fractal dimensions, internal capacitance and charge resistance. SEM observation of PMT films provide strong evidence of the close relationship between type of electrolyte and their morphology, also is in good agreement with ac impedance results interpretation. The results are interpreted on the basis of models developed for intercalation electrodes in relation to the geometry and morphology of the polymer electrode. The results are discussed on the basis of the structural and chemical parameters of the anions and their interaction with the polymer. The discussion shows that the nature of the anion plays a major role on the electrochemical behavior of the polymer.     

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.     

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.     

Electrochemical degradation of polyaniline

The electrochemical degradation of polyaniline was investigated by voltammetry and spectroscopy. The degradation was noticed by a change in the cyclic voltammogram, associated with a decrease in capacity. It occurred at potentials higher than about 0.6 V (versus SCE) in an aqueous solvent and about 0.7 V (versus Ag/Ag⁺) in organic solvents. No degradation was observed below these potentials. The rate of degradation was strongly dependent on the applied potential and on the kind of solvent used. Spectroscopic studies showed that no major chemical modification arose from the degradation.