Débora Gonçalves

Electrochemical oxidation of o-aminophenol in aqueous acidic medium : formation of film and soluble products

The electrochemical oxidation of o-aminophenol (OAP) has been studied by cyclic voltammetry on platinum (Pt) and glassy carbon (GC) electrodes. Films formed on Pt exhibited a voltammetric response that is dependent on the final potential applied during the electropolymerization. Although a less electroactive film has been obtained on Pt electrodes cycled at higher potentials, modified GC electrodes showed a well-defined redox response independently of the potential range used. This result is attributed to an activation process of the GC electrode rather than to an electroactive film deposited on the electrode. The deposition process of thin films was also investigated with quartz crystal microbalance experiments. Poly-o-aminophenol (POAP) films were insoluble with an electrochromic response from colorless to brown, and a ladder structure confirmed by infrared spectra. Exhaustive electrolysis of OAP produced a reasonable amount of soluble product, which after solvent extraction, was characterized as 2-aminophenoxazin-3-one (APZ), a product also prepared by chemical oxidation of OAP. A route of electrooxidation of OAP has been proposed based on electrochemical and spectroscopic results.

Electrochromic and redox properties of Langmuir–Blodgett films of ytterbium bisphthalocyanine

Abstract Langmuir–Blodgett (LB) films from pure ytterbium bisphthalocyanine (YbPc 2 ) and mixtures of YbPc 2 and stearic acid have been produced, whose cyclic voltammetric response displays a redox activity strongly dependent on the scan rate and film thickness. Successive redox reactions are accompanied by color variations upon small changes in the electrode potential range which make these films promising for multi-color devices.

Wettability, Shrinkage and Color Changes of Araucaria angustifolia After Heating Treatment

The effects of thermal treatment on the wettability and shrink resistance of Araucaria angustifolia (Parana pine) were studied from 20 to 200 °C. The contact angles of water droplets on untreated and heat-treated samples were measured by the sessile drop method in the grain of heartwood and sapwood cut in the radial, longitudinal, and tangential directions. A significant increase of the contact angles was verified for the samples from room temperature to 120 °C, in particular in the radial and tangential directions; at higher temperatures, the contact angles assumed almost constant values. From 120 to 200 °C, the sapwood of Araucaria angustifolia showed better dimensional stability and lower thermal resistance when compared to the heartwood. Variations of color were also studied by using the CIELab system, which showed to be capable of accurately distinguishing samples treated at different temperatures.

Synthesis and characterization of poly-o-toluidine: Kinetic and structural aspects

Departamento de Fisica e Quimica Faculdade de Engenharia de Ilha Solteira Universidade Estadual Paulista Julio de Mesquila Filho, Av. Brasil, 56, 15385-000 Ilha Solteira - SP

Spectroscopic, electrochemical, and microgravimetric studies on palladium phthalocyanine films

The optical properties of palladium phthalocyanine in dimethylformamide, toluene, acetonitrile, and dichloromethane solutions were studied in the visible range of energy. Palladium phthalocyanine in organic medium presented itself as a mixture of monomeric and dimeric species in equilibrium, and it was only in dichloromethane that the monomeric, non-aggregated form predominated. Palladium phthalocyanine films were prepared by casting, and studied by cyclic voltammetry and spectroelectrochemistry. The films showed an electrochromic response from blue to purple, and stability that depended on the applied potential. Electrochemical quartz crystal microbalance experiments indicated that the film exfoliates into solution after applying high values of final potentials. Raman spectroscopy was used to structurally characterize the palladium phthalocyanine film, which showed a central metal with +2 oxidation state that is not affected even when the film is oxidized.

Electrochemical synthesis of polypyrrole-azo dyes composite films

We report an innovative experimental procedure to electrochemically synthesize modified polypyrrole (PPY) films, which consists in polymerizing pyrrole and an azo dye, viz. Brilliant Yellow (BY). The electrochemical polymerization of the composite PPY-BY films was carried out in an aqueous medium containing low concentrations of PY (7 mmol L -1 ) and the same molar ratio of BY. The resulting films were much thicker than those obtained by conventional electrochemical polymerization of unmodified PPY films on ITO substrates under the same experimental conditions. The thickness of the composite films was estimated to be ca 100 nm per each voltammetric cycle as compared to 5 nm from the unmodified PPY films. Furthermore, the voltammetric response of the PPY-BY films shows a stable and electroactive behavior.

Self-assembly of poly(p-phenylene vinylene) using long chain counter-ion: a new process for fabrication of multilayer thin films heterostructures

Layer-by-layer thin films of sulfoniun poly(p-phenylene vinylene) (PPV) precursor have been processed using the long chain dodecylbenzenesulfonate (DBS) anion. Here we adopted the alternative approach consisting of the controlled substitution of the chloride counterion of a water-soluble PPV precursor by DBS. The internal layer structure of these films is therefore likely to be different from conventional layer-by-layer films, since DBS ions are expected to be incorporated into the PPV precursor chain. The main advantage of this novel procedure is that thermal conversion through the elimination of DBS may be realized at considerably lower temperatures (80-100 °C) in a few minutes, with enhanced optical properties. Using this new conversion procedure, the conjugation length can also be varied along the deposition direction by controlling the immersion time or DBS concentration in the film. This opens up a number of new possibilities to realize molecularly band-gap engineered structures such as asymmetric ladder type or V-type structures along the film.

A comparative study of m-cresol treated polyaniline and Langmuir Blodgett films

Polyaniline films prepared by Langmuir-Blodgett technique were characterized by X- Ray diffractometry and Raman spectroscopy. The obtained results were compared with those from secondarily doped samples treated with HCSA and m-cresol. When LB films were doped with HCl vapour, the Raman spectrum obtained is different form that of PANI/HCl samples prepared by chemical or electrochemical polymerization, being very close to spectrum of PANI/CSA/m-cresol samples. In LB films a great increase in the intensity of the band correponding to free carriers indicates the important role played by the extended conformation and conductivity.

Implications of using m-cresol in the Langmuir-Blodgett processing of polyaniline

The use of m-cresol for the Langmuir–Blodgett (LB) processing of polyaniline has been investigated, in particular for probing possible secondary doping processes in LB films. Langmuir monolayers and LB films of polyaniline and polyaniline–cadmium stearate composites have been obtained with the use of camphor sulfonic acid (CSA) both in the presence and absence of m-cresol in the spreading solution. Expanded monolayers with higher molecular areas have been observed upon employing m-cresol. The UV-vis spectral changes for the polyaniline LB films caused by m-cresol were shown to depend both on the preparation conditions and on the doping state of the monolayer. Such effects are likely to originate from changes in molecular conformation since m-cresol is not retained in the transferred film. In addition, processing with m-cresol brought no change to the electrical conductivity nor to the stacking order of cadmium stearate domains in the transferred LB films.

Modeling adsorption processes of poly-p-phenylenevinylene precursor and sodium acid dodecylbenzenesulfonate onto layer-by-layer films using a Langmuir-type metastable equilibrium model.

The adsorption kinetics curves of poly(xylylidene tetrahydrothiophenium chloride) (PTHT), a poly-p-phenylenevinylene (PPV) precursor, and the sodium salt of dodecylbenzene sulfonic acid (DBS), onto (PTHT/DBS)n layer-by-layer (LBL) films were characterized by means of UV-vis spectroscopy. The amount of PTHT/DBS and PTHT adsorbed on each layer was shown to be practically independent of adsorption time. A Langmuir-type metastable equilibrium model was used to adjust the adsorption isotherms data and to estimate adsorption/desorption coefficients ratios, k=kads/kdes, values of 2x10(5) and 4x10(6) for PTHT and PTHT/DBS layers, respectively. The desorption coefficient has been estimated, using literature values for poly(o-methoxyaniline) desorption coefficient, as was found to be in the range of 10(-9) to 10(-6) s(-1), indicating that quasi equilibrium is rapidly attained.