Reaction, diffusion and migration in conducting polymer electrodes: analysis of the steady-state amperometric response

The processes of reaction, diffusion and electromigration of a charged substrate within an electronically conducting polymer film deposited on an inert supporting electrode are examined in terms of a quantitative analysis and solution of the pertinent differential equations. An analytical expression for the concentration profiles of substrate within the polymer layer is derived and a theoretical expression for the corresponding steady-state amperometric current response is presented. The transport and kinetics of the substrate are discussed in terms of a dimensionless reaction/diffusion parameter γ and a migration/diffusion parameter β. Received: 13 March 1998 / Accepted: 17 July 1998     

Polycarbazole-based electrochemical transistor

A polycarbazole conducting polymer transistor has been constructed having the dimensions 1 cm × 2 cm × 1 mm. Polycarbazole film used here has a redox potential of 1.30 V. Polymer-coated platinum plates were used as the source and drain. The inter-electrode spacing of the device is typically of the order of 200–500 μm to minimise the internal resistance. The high saturation current region of the transistor in the most positive bias voltage (1.3 V), with negligible hysteresis and greater stability, appears to give a device that is superior to other conducting polymer transistors. Received: 27 May 1997 / Accepted: 17 September 1997     

A model for the propagation of a redox reaction through microcrystals

Chronoamperometry of reversible redox reactions with the insertion of cations into solid particles immobilised at an electrode surface is analysed theoretically using a semiinfinite planar diffusion model. A coupled diffusion of electrons and ions within the crystal lattice is separated in two differential equations. The redox reaction is initiated by the polarisation of the three-phase boundary, where the crystal is in contact with both the electrode and the solution. From this contact line the redox reaction advances on the surface and into the crystal body by the diffusion of ions and conductance of electrons. The effects of the geometry and conductivity of the particles on the current are discussed. Received: 28 December 1996 / Accepted: 17 February 1997     

Electrocatalytic oxidation of some biologically important compounds at conducting polymer electrodes modified by metal complexes

Conducting poly(3-methylthiophene) electrodes were electrochemically prepared. The resulting polymer films were modified with an inorganic complex, ferrocene. The incorporation of the ferrocene/ferrocenium moiety into the polymer film resulted in enhanced charge transfer towards the oxidation of some organic molecules of biological interest. The electrochemical response of the complex-containing polymer electrode was compared to that of the unmodified polymer electrode and that of the substrate. Apparent diffusion coefficients of the redox species were estimated from the cyclic voltammetric data for different biological molecules at the ferrocene-containing polymer electrode. Infra-red spectroscopic measurements for the “as-grown” films revealed the presence of the inorganic complex within the polymer. The modified polymer electrode showed noticeable enhancement for the charge transfer across the film interface and can be used as an electrochemical sensor for biological compounds. Received: 3 June 1997 / Accepted: 7 July 1997     

Electrochemical and chemical modification of poly-o-phenylenediamine redox polymer

Various methods for modification of poly-o-phenylenediamine (PPD) redox polymer with polyaniline (PANI) electron-conducting polymer, ReO ₄ ⁻ and [SiMo₁₂O₄₀]⁴⁻ anions, and palladium clusters are compared. Participation of anions is proven in the processes of PPD synthesis and redox transformations. Formation mechanisms and properties of composites consisting of a redox polymer and electron-conducting polymer and those of two different electron-conducting polymers are compared for example of PPD-PANI and ethylene-dioxythiophene-polybithiophene. A comparison of these systems shows the main change in the potential drop to take place at the metal/polymer interface for a redox polymer and at the polymer/solution interface, for an electron-conducting polymer. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 12, pp. 1409–1416. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The article was translated by the authors.     

Polymer microsphere and polymer-ferrite nanocomposite preparation by precipitation from water-in-oil microemulsions

 A simple method to encapsulate intramicellar solutes (enzymes, nanoparticles) in polymer microspheres is described. The method involves dissolving a polymer in a minimum amount of solvent. The polymer solution is then added to a large volume of a nonsolvent system containing bis(2-ethylhexyl)sulfo-succinate (AOT) reversed micelles. Immediate precipitation of the dissolved polymer takes place. The precipitation results in the formation of stable microparticles ∼0.1–1 μm in diameter. There is a direct correlation between the internal voidage of the particles and the water content of the micelles. Precipitation also leads to encapsulation of intramicellar solutes, resulting in the formation of microspherical composites. Received: 18 February 1997 Accepted: 30 April 1997     

Spinodal decomposition with varying chain lengths and its application to designing polymer blends

The diffusion equations of spinodal decompositions with unique diffusivities for each species are derived for binary systems and ternary systems. These dynamic equations are linearized to show that the minimum size for growth is independent of diffusivity and is identical to the thermodynamic minimum on phase volume. Increases in chain length will destabilize mixtures and increase quench depth. Numerical simulations were conducted for two-dimensional systems. The considerable influences of chain lengths on morphology represent a competition between smaller diffusivities and larger quench depth when chain length is increased. These influences on several important morphologies in binary and ternary systems are described. The understanding of independent variable chain lengths represents one further step towards the systematical design of polymer blends. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 897–907, 1997     

The oligomeric approach – the electrochemistry of conducting polymers in the light of recent research

Electrochemical investigations on oligomeric model compounds (β-carotenoids) of polyacetylene varying the chain length in the range between 5 and 23 double bonds provide deeper insights into the redox properties of such systems. Furthermore, cyclic voltammetric studies of α,ω-diphenylpolyenes and phenylenevinylenes give clear evidence that the formation of the radical ions is followed by a rapid reversible dimerization between the oligomeric chains. The thermodynamic and kinetic parameters of the chemical reaction are presented. Applying these results to the properties of conducting polymers opens up new perspectives for interpreting charge storage and conductivity. Received: 27 May 1997 / Accepted: 6 October 1997     

Effect of adsorbed polymers on the slow motion of an assemblage of spherical particles relative to a fluid

 The effects of adsorbed polymers on the sedimentation of a homogeneous distribution of colloidal spheres and on the fluid flow through a bed of particles are investigated theoretically. The Reynolds number is assumed to be small, and the surface polymer layers are assumed to be thin with respect to the radius of particles and to the surface-to-surface spacing between neighboring particles. The effects of interaction of the individual particles are taken into explicit account by employing a fundamental cell-model representation which is known to provide good predictions for the motion of a swarm of spheres within a fluid in the absence of adsorbed polymers. To solve the Stokes flow equations within and outside the polymer layer a method of matched asymptotic expansions in a small parameter λ is used, where λ is the ratio of the length scale of the polymer layer to the particle radius. The results for the sedimentation rate and the pressure drop are expressed in terms of an effective hydrodynamic thickness (L) of the polymer layer, which are accurate to O(λ²). When the concentration of particles in a suspension or a bed is increased, L becomes larger, meaning the settling velocity decreases or the pressure drop increases. The O(λ) term for L normalized by its value in the limit λ→0 is found to be independent of the polymer segment distribution, the hydrodynamic inter-actions among the segments, and the volume fraction of the segments. The O(λ²) term for L, however, is a sensitive function of the polymer segment distribution and the volume fraction of the segments. In general, the particle-interaction effects on the motion of polymer-coated particles relative to a fluid can be quite significant. Received: 28 August 1996 Accepted: 23 January 1997     

Carboxylic acid functionalization of nylon 6 by radiation grafting and conversion to zinc salts: Effects on physical properties

 Functionalization of polymers by grafting monomeric species on to the backbone of molecular chains with the use of γ-radiation has been used extensively. In this work methacrylic acid was grafted onto a commercial grade of polycaproamide (Nylon 6) by preirradiating the polymer granules to 15 kGy at a rate of 1.0 kGy per hour and subsequently immersing these in a 10% aqueous solution of methacrylic acid in the presence of small quantities of FeSO₄ as homopolymerization inhibitor. The polymer was subsequently neutralized by mixing it with zinc acetylacetonate in a laboratory scale melt mixing device. The acid-grafting polymer modification resulted in an increase in glass transition temperature, while the addition of zinc acetylacetonate gave rise to two transitions: The lower transition corresponds to a miscible mixture of free polyamide and acid-grafted polymer, both plasticized with undecomposed zinc compound, while the upper transition corresponds to the zinc salt of the acid grafted polyamide. Through rheological measurements it was shown that both the acid-grafted polymer and the derived zinc salt have a branched structure, possibly containing also some crosslinked domains. Large improvements in solvent resistance were observed for both type of polymer modifications. Received: 13 December 1996 Accepted: 10 February 1997     

Interaction of a cationic surfactant to sodium polyphosphates with different degrees of polymerization

 The binding behavior of dodecylpyridinium chloride to sodium polyphosphate of various degrees of polymerization (DP) was studied by means of a potentiometric titration method using a surfactant-selective electrode in the presence of 10 mM NaCl at 30 °C. Binding isotherms were analyzed by direct calculation of a matrix expressing the partition function. It is found that binding affinity depends prominently on the polymer chainlength when the DP value is less than ca. 35, but becomes nearly independent on DP thereafter. No binding was observed for linear triphosphate or cyclic trimetaphosphate anions. The picture that arises for the binding is that the polymer’s end-effect reduces the apparent cooperativity, while the hydrophobic interaction with neighboring surfactant remains constant because of the short-range nature of the interaction. The so-called end-effect is associated with a superimposition of electrostatic potentials around the polymer rods. Both the matrix method and the Satake–Yang equation were carried out for simulations, and the matrix one shows a better fitting with the experimental data for the short-chain polyelectrolyte. Received: 1 July 1997 Accepted: 21 August 1997     

Amperometric determination of ascorbic acid at a novel ‘self-doped’ polyaniline modified microelectrode

A ‘self-doped’ polyaniline modified microelectrode, prepared by cyclic potential sweep on a microdisk gold electrode from –0.2 to 0.85 V in 0.5 mol/L sulfuric acid containing aniline and o-aminobenzoic acid, has been developed. The copolymerized process and the resulting polymer characteristics were investigated in detail. This composite film indicated a good electrochemical activity in a wide pH range even in basic solution. Meanwhile, the redox couple exhibited an excellent electrocatalytic activity for the oxidation of ascorbic acid. The oxidation overpotential of ascorbic acid was decreased over 200 mV at this modified electrode compared with a bare gold one. Moreover, the effects of film thickness and pH on the catalytic efficiency were further studied. The dependence of catalytic currents on the concentration of ascorbic acid was linear in the range of 1.2 × 10^–5∼ 2.4 × 10^–3 mol/L with a correlation coefficient of 0.996. Also, the determination of ascorbic acid in actual samples was evaluated and the results are satisfactory. Received: 7 October 1997 / Revised: 3 February 1998 / Accepted: 7 February 1998     

Studies of electrochemical behaviour of RuO2-PVC film electrodes: dependence on oxide preparation temperature

Ruthenium dioxide electrodes, prepared on a Pt substrate using coatings of PVC-RuO₂ mixed in THF (designated as RuO₂-PVC film electrode) have been studied for their redox behaviour in 1 M NaOH using variable scan cyclic voltammetry. The various redox transitions in the oxidation state of the central metal ion are characterized using electrochemical parameters such as peak potential, peak current, and surface charge. The effect of oxide preparation temperature, in the range 300–700 °C, on the redox characteristics has also been studied and correlated with the electrochemically active surface area (as measured using small amplitude cyclic voltammetry) and the true surface area (by the BET method). Received: 12 August 1997 / Accepted: 18 October 1999     

Electrochemical redox reactions studied in frozen tetrabutylammonium halide hydrates

The electrochemical redox reactions: Fe(CN)₆ ⁴⁻−e⁻↔ Fe(CN)₆ ³⁻, Ru(NH₃)₆ ³⁺+e⁻↔ Ru (NH₃)₆ ²⁺ and Fc(CH₂OH)₂−e⁻↔ Fc(CH₂OH)₂ ⁺ (Fc–ferrocene) were investigated in tetrabutylammonium halide hydrates at temperatures below and above the electrolyte melting point. They were studied by cyclic voltammetry, potential step chronoamperometry and impedance spectroscopy. Freezing of the electrolyte affects both the shape and position of the cyclic voltammogram on the potential scale. Also the shapes of the current-time relationship and the impedance spectra change at temperatures below the melting point. It has been proposed that this behaviour is caused by slow transport of the reactant and the heterogeneous nature of the electrolyte. The activation energies of reactant transport are about four times larger in frozen electrolytes than those in liquid. It has been concluded that reactant transport is restricted to the intergrain space of the electrolyte. Received: 16 December 1997 / Accepted: 10 February 1998     

Intrinsic redox states of polyaniline studied by high-resolution X-ray photoelectron spectroscopy

High-resolution X-ray photoelectron spectroscopic (XPS) measurements of the various intrinsic redox states of polyaniline (PANI), using a monochromatized Al—Kα source, were carried out. The presence of the imine, amine and positively charged nitrogen species corresponding to a particular intrinsic redox state and protonation level of the polymer was resolved quantitatively and unambiguously. The result confirmed the peak assignments of former XPS core-level studies using the lower resolution non-monochromatized Mg—Kα X-ray source. Thus, the high-resolution XPS using a monochromatized Al—Kα X-ray source is a truly unique tool for the convenient and quantitative analysis of the various intrinsic redox states of PANI. Received: 16 May 2000/Accepted: 29 August 2000     

Enhanced recovery of chlorophenols from surface waters using polymer based extraction cartridges

 The effectiveness of a new polymer based solid phase extraction cartridge (SDB 1) to recover phenolic compounds from aqueous samples has been evaluated by comparison of performance against conventional silica based cartridges. The polymer based material is found to be much more retentive, requiring a larger volume of solvent to achieve maximum recovery. Recovery efficiency is enhanced if the recovery solvent (methanol) is acidified to 0.1% with trifluoroacetic acid. The optimised extraction procedure has been applied to surface water samples and yields quantitative recovery at the 10 ng ml^-1 level of all nine phenols studied. Received: 26 November 1996/Revised: 21 March 1997/Accepted: 30 March 1997     

Spectroelectrochemical identity of Prussian blue films in various electrolytes: comparison of time-derivative voltabsorptometric responses with conventional cyclic voltammetry

Using Prussian blue (PB) electrodeposited on gold-covered foil as a model system, we have demonstrated the usefulness of the time-derivative measurements of absorbance versus potential (linear potential-scan voltabsorptometry) for spectroelectrochemical characterization of thin electrochromic films. The time-derivative signals were monitored for PB at 680 and 420 nm in potassium, sodium and lithium electrolytes. Information obtained from cyclic voltabsorptometry is equivalent or complementary to that from conventional cyclic voltammetry. In the case of PB films investigated in lithium electrolyte, the voltabsorptometric time-derivative peaks are better defined than the respective voltammetric peaks. The combination of voltabsorptometry with voltammetry enables molar absorptivity and/or film loading to be determined. Also, concentration changes of differently colored mixed-valence redox centers can be monitored as a function of applied potential. Received: 16 January 1997 / Accepted: 11 March 1997     

A model for the coupled transport of ions and electrons in redox conductive microcrystals

Coupled diffusion of ions and electrons in microcrystals of insertion compounds immobilized at an electrode surface is theoretically analysed by a lattice-gas model without interactions. The transport in the direction perpendicular to the electrode surface depends on Wagner's factor for electrons, while the transport parallel to the electrode depends on this factor for ions. The iso-concentration profiles may depend on the orientation of the particle on the electrode surface. Chronoamperometric responses of volume and surface redox reactions are calculated. Received: 5 June 1998 / Accepted: 22 August 1998     

Estimation of surface morphology of composite polymer particles prepared by the stepwise heterocoagulation method with ζ-potential measurement

 Core-shell composite polymer particle was prepared by the stepwise heterocoagulation of cationic small polymer particles (SPs) onto an anionic large polymer particle (LP), following heat treatment at temperatures which were higher than glass transition temperature (T _g=18 °C) of SP. At pH 9 ζ-potential of the hetero-coagulated particle (HP) was negative, but it changed to positive by the heat treatment and increased with the treatment time and finally attained to that of SP. The treatment time to attain the ζ-potential of SP became short by elevating the treatment temperature. This indicates that during the heat treatment, SP continued to melt on the surface of LP and finally formed a continuous shell. Received: 3 September 1997 Accepted: 29 October 1997     

Chemical speciation code CHEMSPEC and its applications

The adsorption and migration behavior of a radionuclide in geological media heavily depends on its chemical forms in a given chemical environment. In order to predict the temporal and spatial distribution of radionuclides around a disposal site when its canister is damaged, it is necessary to develop coupled chemical speciation-solute transport models and relevant software. For that reason, we wrote a new chemical speciation program CHEMSPEC. In this paper, the principles and structure of CHEMSPEC are briefly described, and the strategy and algorithms that were used in this code are interpreted in some detail, such as the measures adopted to prevent divergence in iteratively solving the mass balance equations, the “predictor-corrector” algorithm for calculation of the number and quantities of solid species formed, and the alternate use of “freezing” and “defreezing” oxidation states in handling of co-existent redox and precipitation equilibria. Four examples are given to illustrate CHEMSPEC’s features and capabilities.