W.A. Gazotti

All polymeric solid state electrochromic devices

Two examples of all solid state and flexible electrochromic devices assembled under atmospheric conditions are described. The devices were constructed using polypyrrole and polythiophene derivatives as electrochromic materials, deposited on optically transparent plastic electrodes. An elastomer, poly(epichlorohydrin-co-ethylene oxide) containing LiClO4, was used as electrolyte. Results indicated that the systems present high optical contrast in the visible range and fast optical response times. We also discuss some strategies to enhance the optical contrast and optical response time of conducting polymers used in electrochromic devices.

An elastomeric conductor based on polyaniline prepared by mechanical mixing

In this work we describe the conditions of preparation and the electrical, mechanical, thermal and morphological characteristics of a conductive blend prepared by combining the elastomer poly(ethylene-co-propylene-co-diene-monomer) and polyaniline doped with p-toluene sulfonic acid. Polymer mixtures were prepared in a mixer chamber (cam rotors) coupled to a rheometer. A higher mixing temperature improves the homogeneity of the mixtures, as seen by an increase in conductivity and in crosslinking efficiency. In spite of the good homogeneity, the mixtures are imiscible. Therefore, the formation of bonds between the high molar mass chains of polyaniline and the oligomers explain the increase of the conductivity with an increase in polyaniline content. The modulus values also increase by adding polyaniline to the rubber suggesting a reinforcement effect.

Influence of dopant, pH and potential on the spectral changes of poly(o-methoxyaniline): relationship with the redox processes

Abstract Poly( o -methoxyaniline) films doped with functionalized acids show high solubility in conventional organic solvents and a high optical contrast between the reduced and the oxidized forms. They are also more stable towards thermal treatment than when they are doped with HCl. These characteristics make them suitable for use in electrochromic displays. The electrochromic behavior of this soluble derivative of polyaniline was investigated in detail by comparing the curves obtained by the differentiation of absorbance at fixed wavelengths with simultaneously obtained voltammograms. This analysis allows the chromophore species producing color changes to be related to the redox processes taking place during the electrochemical cycling. Molar absorptivities were calculated and the effect of pH on the absorption speetra of the emeraldine form of the polymer is also reported.

Electrochemical, electrochromic and photoelectrochemical behavior of a highly soluble polyaniline derivative: poly(o-methoxyaniline) doped with functionalized organic acids

Poly(o-methoxyaniline) was chemically prepared in the presence of hydrochloric, p-toluene-sulfonic, dodecyl-benzene-sulfonic or dodecyl-sulfuric acids. Films of this polymer were prepared by casting and electrochemical, electrochromic and photoelectrochemical properties were studied. Cyclic voltammetry experiments show well-defined redox waves when the pH of the electrolyte is 1.0. Poly(o-methoxyaniline) films present high optical contrast in the visible range, when the potential is changed from the fully reduced to the fully oxidized state, independently of the dopant acid. The film obtained from the polymer doped with p-toluene-sulfonic acid, however, presents the highest contrast. Photoelectrochemical experiments also showed that this material yields photocurrent when irradiated with monochromatic light (330nm), and that the sign of the photocurrent depends on its oxidation state.

Flexible photoelectrochemical devices based on conducting polymers

In this work, we have investigated the photoelectrochemical behavior of conjugated polymers in flexible solid-state devices exploring some unique features that they exhibit: the possibility of adjusting the band gap energy by employing different structures and improving the light harvesting by the addition of an organic dye. We also report the use of these materials as photoactive materials and counter-electrodes in optical devices, depending on their oxidized or reduced state. The overall conversion efficiency is higher than for other photoelectrochemical devices, which exploit the photoconductivity and photosensitivity of organic materials, and the performance remains constant when the devices are bent; however, it is low in comparison to solid-state devices using inorganic materials.

Poly(ethylene oxide-co-epichlorohydrin)/NaI: a promising polymer electrolyte for photoelectrochemical cells

We have investigated the thermal and ionic conductivity properties of the elastomer poly(ethylene oxide-co-epichlorohydrin) filled with NaI and I2. The reason for using this composition is its potential application as electrolyte in photoelectrochemical cells. This copolymer was characterized as a function of NaI concentration, temperature and relative humidity. According to the data obtained, the Na+ ion interacts with the ethylene oxide repeating units by means of Lewis type acid–base interactions. The empirical Vogel–Tamman–Fulcher equation was used to model the conductivity and temperature relationships, indicating that the conduction occurs in the amorphous phase of the copolymer. The sample with 9.0% (w/w) of NaI presents a conductivity of 1.5×10−5 S cm−1 in a dry atmosphere (30°C, [H2O]<1 ppm) and 2.0×10−4 S cm−1 at 86% relative humidity (22°C).

Polymer electrolytes based on ethylene oxide–epichlorohydrin copolymers

In this work we studied the ionic conductivity for three copolymers of the title co-monomers as a function of LiClO4 content, temperature and ambient relative humidity. We also investigated the interactions between the salt and the co-monomer blocks in the copolymers and its effect on the morphology and thermal properties of the copolymer/salt complexes. Our data indicate that the Li+ ion predominantly interacts with the ethylene oxide repeating units of the copolymers. The copolymer with the highest ionic conductivity was obtained with an ethylene oxide/epichlorohydrin ratio of 84/16 containing 5.5% (w/w) of LiClO4. It showed a conductivity of 4.1×10−5 S cm−1 (30°C, humidity< 1 ppm) and 2.6×10−4 S cm−1 at 84% relative humidity (24°C). The potential stability window of the copolymer/salt complex is 4.0 V, as measured by cyclic voltammetry. For comparison, we also prepared a blend of the corresponding homopolymers containing LiClO4; it showed higher crystallinity and lower ionic conductivity.

Electrochemical impedance spectroscopy studies for chemically prepared poly(o-methoxyaniline) doped with functionalized acids

Poly(o-methoxyaniline) was prepared by the chemical oxidation of the monomer with ammonium peroxydisulfate in the presence of the following acids: hydrochloric, p-toluenesulfonic, dodecylbenzenosulfonic and dodecylsulfuric. The polymers obtained are soluble in dimethylformamide and this enabled the preparation of films by casting. The electrochemical behavior of the polymers prepared by chemical synthesis is very similar to polyaniline prepared by electrochemical methods. Impedance spectroscopy measurements indicated that the acid affects mainly the potential range where the charge transfer resistance is lower. The diffusion coefficients obtained by impedance spectroscopy are higher for poly(o-methoxyaniline) doped with chloridric and p-toluene sulfonic acids than for those doped with dodecylbenzenosulfonic and dodecylsulfuric acids.

Thermal and mechanical behaviour of a conductive elastomeric blend based on a soluble polyaniline derivative

Abstract The electrical, mechanical and thermal behaviour of a conductive polymer blend prepared by combining the elastomer poly(epichlorohydrin- co -ethylene oxide), which presents ionic conductivity when containing LiClO 4 , and a soluble derivative of polyaniline, poly( o -methoxyaniline) doped with p -toluene sulfonic acid, is described in this work. This mixture presents electrical conductivity sufficient for substitution with advantages to the existing conductive elastomers in several applications. Concentrations of the electronic conducting polymer up to 10% (w/w) increase the electrical conductivity of the elastomer by three orders of magnitude, with no changes in its mechanical properties. With 50% (w/w) of poly( o -methoxyaniline), the electrical conductivity of the mixture reaches 10 −3 S cm −1 . Thermal stability of the components are affected by the mixture. DSC and SEM experiments indicate phase separation in the blend.

Thermal and photochemical degradation of dodecylsulfate doped polypyrrole

Abstract The authors report on the changes of the electrical, morphological, chemical and electrochemical properties of polypyrrole dodecylsulfate films aged at 100°C or exposed to a solar simulator light source. The thermal aging induces a loss of conductivity and a gradual decrease of the anodic and cathodic peak currents, as measured by cyclic voltammetry. Irradiation does not substantially alter the properties of the films; the conductivity does not change and the electroactivity is slightly modified. Degradation of the properties is attributed to cross-linking, oxidation and evaporation of low molecular weight oligomers.