M.-A. De Paoli

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.

Characterization of poly(ethylene terephtalate) after multiple processing cycles

Poly(ethylene terephthalate) (PET) pellets used for production of soft drink bottles were reprocessed in a single-screw extruder and cut into pellets in five processing cycles. The reprocessed pellets were characterized by its rheological, mechanical, and thermal properties. Carboxylic end groups number concentration and melt flow index were also determined. After three reprocessing cycles the mechanical properties and crystallinity degree changed drastically. Moreover, after five cycles we did not observe changes in the onset temperature of mass loss; therefore, the samples were thermally stable. However, an increase in melt flow index and carboxylic end groups concentration indicated a certain degree of mechanochemical degradation.

Ionic exchanges in dodecylbenzenesulfonate doped polypyrrole Part 1. Optical beam deflection studies

Abstract Cyclic voltammetry and optical beam deflection (OBD) techniques in different electrolytes, KCl, KClO 4 , CsCl, NaCl, LiClO 4 and LiCl, were used to study the ionic exchange processes in polypyrrole (PPy) doped with an anionic surfactant, dodecylbenzenesulfonate (DBS − ). Our results show the role played by the cations during the first redox process of the polymer. To compare the OBD curves in each electrolyte, they were normalized with respect to their dependence on the chemical nature of the electrolyte. The use of temporal convolution, proposed by Vieil et al. ( J. Electroanal. Chem., 368 (1994) 183), to relate electrical current and angular optical beam deflection, allows the modeling of the OBD curves and the calculation of apparent diffusion coefficients for each electrolyte and the diffusion coefficient of each ionic species.

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.

Solid state polymeric electrolytes based on poly(epichlorohydrin)

Elastomers constituted of the homopolymer of epichlorohydrin, its copolymer with ethylene oxide and its terpolymer with ethylene oxide and allyl-glycidyl-ether were studied as polymeric electrolytes in polymer/LiClO4 systems. The variation of the ionic conductivity with salt concentration indicated that the copolymer and the terpolymer show the highest conductivities, ca. 10−5 S cm−1 at 30 °C. Also, the fitting of the ionic conductivity versus inverse of temperature curves with the VTF equation indicated a conductivity mechanism similar to poly(ethylene oxide) for the copolymer and the terpolymer. The potential stability window determined by cyclic voltammetry is 3.5 V and the polymers are stable to heating up to 250 °C. These results confirm the feasibility of using the copolymer and the terpolymer systems with LiClO4 as polymeric solid state electrolytes.

Ionic exchanges in dodecylbenzenesulfonate-doped polypyrrole Part II: Electrochemical quartz crystal microbalance study

Abstract Cyclic voltammetry and electrochemical quartz crystal microbalance techniques were combined to study the ionic exchange process in polypyrrole doped with the anionic surfactant, dodecylbenzenesulfonate. A quantitative analysis of the results indicates a doping degree of 20% after electropolymerization. Studies performed in monomer-free solutions show that both anions and cations of the electrolytes play an important role in the redox process. Quantitative calculations also take into account the transfer of neutral species like solvent molecules.

Microwave properties of EPDM/PAni-DBSA blends

Conductive elastomer blends of ethylene-propylene-diene rubber, EPDM, and polyaniline, PAni, doped with dodecylbenzene sulfonic acid, DBSA, were prepared and evaluated as a radar absorbing material, RAM. Reflectivity measurements in the 8-12 GHz frequency range, using the NRL arch method, were done to evaluate the shielding effectivity. The results showed that the dopant and PAni concentrations and also the sample thickness affect these measurements. Good shielding values were obtained (-15 dB) with the 50/50, 40/60 and 20/80 (w/w %) EPDM/PAni blends.

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.

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.

The chemical effects of photo-oxidation on butadiene rubber

The photo-oxidation of butadiene rubber (BR) was studied in detail using films of a commercially available material, containing 33% cis-1,4-, 56% trans-1,4- and 12% 1,2-vinylic units. The spectral data of the products showed that during photo-oxidation the following functional organic groups are formed: hydroperoxides, alcohols, ketones, carboxylic acids and esters. 13C-NMR spectroscopic evidence for the formation of epoxides is presented. By comparison with spectra of model compounds, signals were assigned to ester and carboxylic acids in the allylic position of the oxidized polymer. The presence of ketones and carboxylic acids was also proved by chemical methods. It could be demonstrated that the formation of ketones, carboxylic acids and esters occurs from the photolysis of the OO bond of the hydroperoxides. Kinetic measurements with films containing benzophenone showed a strong increase in the quantum yield for the formation of products in comparison with the pure polymer sample. The opposite effect is observed with films containing the commercially used anti-oxidant, 2,6-di-t-butyl-p-hydroxytoluene. Also, the quantum yield for formation of the oxidation products decreases with increase in the light intensity. This result shows that cross-linking plays an important role in the photo-degradation of BR.