A study of dimethylferrocene diffusion and electrode kinetics using microelectrode voltammetry in poly(ethylene glycol) solvent

The heterogeneous electron transfer rate constant (k(s)) of dimethylferrocene (DMFc) was estimated using cyclic voltammetric peak potential separations taken typically in a mixed diffusion geometry regime in a polyelectrolyte, and the diffusion coefficient (D) of DMFc was obtained using a steady-state voltammogram. The heterogeneous electron transfer rate constant and diffusion coefficient are both smaller by about 100-fold in the polymeric solvent than in the monomeric solvent. The results are in agreement with the difference of longitudinal dielectric relaxation time (tau(L)) in the two kinds of solvents, poly(ethylene glycol) (PEG) and CH3CN, indicating that k(s) varies inversely with tau(L); k(s), is proportional to D of DMFc. Both D and k(s) of DMFc in PEG containing different supporting electrolytes and at different temperatures have been estimated. These results show that D and k(s) of DMFc increase with increasing temperature in the polyelectrolyte, whereas they vary only slightly with changing the supporting electrolyte.

Cure processing modeling and cure cycle simulation of epoxy-terminated poly(phenylene ether ketone). V: Estimation of temperature distribution during cure process

A numerical method to estimate temperature distribution during the cure of epoxy-terminated poly(phenylene ether ketone) (E-PEK)-based composite is suggested. The effect of the temperature distribution on the selection of cure cycle is evaluated using a suggested alternation criterion. The effect of varying heating rate and thickness on the temperature distribution, viscosity distribution and distribution of the extent of cure reaction are discussed based on the combination of the here-established temperature distribution model and the previously established curing kinetics model and chemorheological model. It is found that, for a thin composite (<=10mm) and low heating rate (<=2.5K/min), the effect of temperature distribution on cure cycle and on the processing window for pressure application can be neglected. Low heating rate is of benefit to reduce the temperature gradient. The processing window for pressure application becomes narrower with increasing thicknesses of composite sheets. The validity of the temperature distribution model and the modified processing window is evaluated through the characterization of mechanical and physical properties of E-PEK-based composite fabricated according to different temperature distribution conditions.

Studies of ferrocene derivative diffusion and heterogeneous kinetics in polymer electrolyte by using microelectrode voltammetry

The heterogeneous electron transfer rate constants (k(s)) of seven ferrocene derivatives were estimated using cyclic voltammograms under mixed spherical/semi-infinite linear diffusion and steady-state voltammetry at a microdisk electrode in polymer electrolyte. The k(s) and diffusion coefficient (D) are both 100 to 1000-fold smaller in polymer solvent than in monomeric solvents, and the D and k(s) decrease with increasing polymer chain length. The results conform to the difference of viscosity (eta) or relaxation time (tau(L)) for these different solvents. The k(s) and D increase with increasing temperature, and the activation barriers of the electrode reaction are obtained. The influences of the substituting group in the ferrocene ring on k(s) and D are discussed. The k(s) are proportional to the D of the ferrocene derivatives, which indicates that solvent dynamics control the electrode reaction. (C) 1998 Elsevier Science S.A.

An electrochemical study of prussian blue microcrystallines mixed in PEO400 polymer electrolyte by solid-state voltammetry

The electrochemistry of Prussian blue mixed in a polymer medium containing MClO4 (M = Li+, Na+, K+, TBA(+)) as the supporting electrolyte was studied by means of solid-state voltammetry. This approach is new in Prussian blue studies. The behavior of PB in polymer electrolytes is somewhat similar to the well-known behavior for an electrochemically synthesized PB film in aqueous media. Besides, K+, Li+ and Na+ ions can also transport through the crystal of PB because of its zeolitic nature. The transport of TBA(+) ions is possible. Kinetic control lies in the diffusion of cations in and out of the lattice of Prussian blue. Reduction waves of Prussian blue depend on both the size and type of cations. PB is very stable upon electrochemical cycling in polymer electrolytes and air. This system may be used in rechargeable batteries and electrochromic devices.

Cure processing modeling and cure cycle simulation of epoxy-terminated poly(phenylene ether ketone) .3. Determination of the time of pressure application

The curing temperature, pressure, and curing time have significant influence on finished thermosetting composite products. The time of pressure application is one of the most important processing parameters in the manufacture of a thermosetting composite. The determination of the time of pressure application relies on analysis of the viscosity variation of the polymer, associated with curing temperature and curing time. To determine it, the influence of the time of pressure application on the physical properties of epoxy-terminated poly(phenylene ether ketone) (E-PEK)-based continuous carbon fiber composite was studied. It was found that a stepwise temperature cure cycle is more suitable for manufacture of this composite. There are two viscosity valleys, in the case of the E-PEK system, associated with temperature during a stepwise cure cycle. The analysis on the effects of reinforcement fraction and defect content on the composite sheet quality indicates that the width-adjustable second viscosity valley provides a suitable pressing window. The viscosity, ranging from 400 to 1200 Pa . s at the second viscosity valley, is the optimal viscosity range for applying pressure to ensure appropriate resin flow during curing process, which enables one to get a finished composite with optimal fiber volume fraction and low void content. (C) 1997 John Wiley & Sons, Inc.

Cure processing modeling and cure cycle simulation of epoxy-terminated poly(phenylene ether ketone) .4. Cure cycle simulation

Epoxy-terminated poly(phenylene ether ketone) (E-PEK) developed in this Institute is a candidate matrix resin for polymer composites as structural materials. Cure cycles for this reaction system were simulated according to the previously established processing model. It is found that for the E-PEK system, the curing process is best completed by a stepwise cure cycle comprising two isothermal processes at different temperatures, T-1 and T-2. The cure cycles over a wide range of processing parameters simulated, based on the established processing model, indicate that the processing window is width-adjustable. Analysis of the mechanical properties of the composite sheets showed that the simulated cure cycles are acceptable and reliable. (C) 1997 John Wiley & Sons, Inc.

Cure processing modeling and cure cycle simulation of epoxy-terminated poly(phenylene ether ketone) .1. DSC characterization of curing reaction

The curing reaction process of epoxy-terminated poly(phenylene ether ketone) (E-PEK) with 4,4'-diaminodiphenyl sulfone (DDS) and hexahydrophthalic acid anhydride (Nadic) as curing agents was investigated using isothermal differential scanning calorimetry (IDSC) and nonisothermal differential scanning calorimetry (DDSC) techniques. It was found that the curing reactions of E-PEK/DDS and E-PEK/Nadic are nth-order reactions but not autoaccelerating. The experimental results revealed that the curing reaction kinetics parameters measured from IDSC and DDSC are not equivalent. This means that, in the curing reaction kinetics model for our E-PEK system, both isothermal and nonisothermal reaction kinetics parameters are needed to describe isothermal and nonisothermal curing processes, The isothermal and nonisothermal curing processes were successfully simulated using this model. A new extrapolation method was suggested. On the basis of this method the maximum extent of the curing reaction (A(ult)) that is able to reach a certain temperature can be predicted. The A(ult) for the E-PEK system estimated by the new method agrees well with the results obtained from another procedure reported in the literature. (C) 1997 John Wiley & Sons, Inc.

Cure processing modeling and cure cycle simulation of epoxy-terminated poly(phenylene ether ketone) .2. Chemorheological modeling

Chemorheology and corresponding models for an epoxy-terminated poly(phenylene ether ketone) (E-PEK) and 4,4'-diaminodiphenyl sulfone (DDS) system were investigated using a differential scanning calorimeter (DSC) and a cone-and-plate rheometer. For this system, the reported four-parameter chemorheological model and modified WLF chemorheological model can only be used in an isothermal or nonisothermal process, respectively. In order to predict the resin viscosity variation during a stepwise temperature cure cycle actually used, a new model based on the combination of the four-parameter model and the modified WLF model was developed. The combined model can predict the resin viscosity variation during a stepwise temperature cure cycle more accurately than the above two models. In order to simplify the establishment of this model, a new five-parameter chemorheological model was then developed. The parameters in this five-parameter model can be determined through very few rheology and DSC experiments. This model is practicable to describe the resin viscosity variation for isothermal, nonisothermal, or stepwise temperature cure cycles accurately. The five-parameter chemorheological model has also successfully been used in the E-PEK systems with two other curing agents, i.e., the diamine curing agent with the addition of a boron trifluride monoethylamine (BF3-MEA) accelerator and an anhydride curing agent (hexahydrophthalic acid anhydride). (C) 1997 John Wiley & Sons, Inc.

Cyclic voltammetry of dye-modified supported bilayer lipid membranes (s-BLMs)

The voltammetric behaviour of dye-modified supported bilayer lipid membranes is investigated. (C) 1997 Elsevier Science S.A.

Probe beam deflection combined with cyclic voltammetry studies of C-60-modified electrode film

C-60 films, prepared by solution casting, were studied by means of in situ probe beam deflection (PBD) combined with cyclic voltammetry (CV). PBD is a powerful technique for investigation of phenomena at the electrode/electrolyte interface in acetonitrile with quaternary ammonium and alkali metal salts as supporting electrolytes. In tetra-n-butylammonium (TBA(+)) salt solution, a stable CV can be obtained during the first two reduction/reoxidation waves. On reduction, injection of cations to maintain charge balance and dissolution of small amount of C-60(-) (TEA(+)) and/or C-60(2-) (TBA(+))(2) are detected. During the reoxidation process ejection of cations and injection of anions occur simultaneously, especially for the second reoxidation wave. In the case where TBABr is the supporting electrolyte, the accompanied behavior is more complicated than in TBABF(4), TBAClO(4), and TBAPF(6) solutions. A small pair of prewaves in CV are proposed due to oxidation/reduction of C-60 domains but not dissolution/redeposition of C-60 film. Extending the potential scan range to the third reduction wave, no apparent corresponding reoxidation wave is related to the third reduction wave, the electroactivity of the film disappears rapidly and dissolution of C-60 film is observed. In tetraethylammonium (TEA(+)) and NAClO(4) solutions, the electrochemistry of the C-60 films is unstable, and potential scans lead to dissolution of flaking of the film.

Study on ferrocene derivatives diffusion dynamics in polymer electrolyte by solid-state voltammetry

The diffusion rates of seven ferrocene derivatives have been estimated in polyelectrolyte PEG . LiClO4 by using non-steady-state chronoamperometry. The D-app of ferrocene derivatives increases with temperature, and the dependency of D-app on temperature obeys the Arrhenius equation. The D-app of ferrocene derivatives decreases with increasing size of electroactive species. The Delta D-app values of D-T>Tm and D-T T-m in the polyelectrolyte. On the other hand, the diffusion behaviour of ferrocene derivatives is qualitatively analyzed by using cyclic voltammetry. Copyright (C) 1996 Elsevier Science Ltd

VOLTAMMETRY OF CYTOCHROME-C ENTRAPPED IN HYDROGEL MEMBRANE ON GRAPHITE ELECTRODE

The voltammetric behavior of cytochrome c entrapped in hydrogel membranes at paraffin wax-impregnated spectroscopic graphite electrodes (WISGE) was studied in this paper. A pair of well-defined peaks appeared at +70 mV (vs. Ag/AgCl). Beside these two peaks, another pair of peaks emerged at around +225 mV. Further investigations suggested that at least three states of cytochrome c existed in the membranes due to the special structure of the hydrogel. The native conformation of cytochrome c molecules was stabilized by the hydrophilic environment that was formed by the hydroxyl structure of the membranes and facilitated the cytochrome c electron transfer reaction at +70 mV. The molecules directly adsorbed on the surface of the graphite electrode were responsible for the redox peaks at around +225 mV. Whether the adsorption peaks were detectable or not was related to the thickness of membranes and the pre-retaining time before the formation of membranes.

SOLID-STATE VOLTAMMETRY OF ELECTROACTIVE SOLUTES IN POLYMER SOLVENTS (PEO) AT ULTRAMICROELECTRODES

This paper describes the electrochemical oxidation and reduction of electroactive solutes which are dissolved in and diffusing through the polymer electrolyte solvent, poly(ethylene oxide) (PEO). The characteristics of electrochemical reactions in polymeric solutions are discussed, including how rigid solvent environments affect mass transport rates, and the transport phenomenon of electroactive species in PEO with bathing gases is explained by using the voltammetric theory of ultramicroelectrodes. The possibility that the microelectrode coated with PEO film can be used as a gas sensor has been discussed.

IMPEDANCE STUDY OF THE INTERFACES BETWEEN LITHIUM, POLYANILINE, LITHIUM-DOPED MNO2 AND MODIFIED POLY(ETHYLENE OXIDE) ELECTROLYTE

Impedance study was carried out for the interfaces between lithium, polyaniline (PAn), lithium-doped MnO2 and modified poly(ethylene oxide) (PEO) electrolyte under various' conditions. The interfacial charge-transfer resistances R(ct) on PEO/PAn, R(ct) on PEO/LiMn2O4 increase with depth-of-discharge and decrease after the charge of the cell containing modified PEO as electrolyte. The charge-transfer resistance R(ct) on PEO/PAn is higher than R(ct) on PEO/LiMn2O4 under the same condition, since inserted species and mechanism are different for both cases. In the case of PAn, an additional charge-transfer resistance might be related to the electronic conductivity change in discharge/charge potential range, as it was evident from a voltammetry curve. With increasing cycle numbers, the charge-transfer resistance increases gradually. The impedance results also have shown that at low frequency the diffusion control is dominant in the process of the charge and discharge of Li/PEO/PAn or Li/PEO/LiMn2O4 cell. The diffusion coefficients have been calculated from impedance data.