C.P. de Melo

Free-grown polypyrrole thin films as aroma sensors

We describe the use of free-grown thin polypyrrole films as fast response sensors of volatile compounds. Using a variety of dopants, the polymeric films were prepared directly on conducting glass substrates. Ultraviolet-visible and near infrared regions absorbance measurements were used to investigate the deposition process and the doping level of the films. We identify the polymerization time as the fundamental parameter for determining the characteristics of the sensor, since it affects not only the final level of doping but also the kinetics of adsorption and the homogeneity of the films. Electrical measurements confirmed the sensitivity and selectivity of the sensors to the presence of vapors of volatile organic solvents. A fast response time was observed in all cases, although the intensity of the corresponding signal was in general larger for the polar than for the nonpolar compounds. The sensitiveness of the sensors towards simple organic solvents is analyzed as well as their reproducibility and the aging effect upon the specific responses.

Polypyrrole based aroma sensor

The change in conductivity of conducting polymers when exposed to volatile gases can be exploited for the construction of aroma sensors. In this communication we report the preliminary tests of two versions of an aroma sensor based on arrays of doped polypyrrole films. We have used these prototypes for the identification of simple organic vapors and complex aromas. The input signal was the fractional change in the resistance of the films and the data acquisition was performed automatically. The pattern recognition process was implemented by a multivariate analysis and by neural network processing. The discriminating power among different odorants was better than 90%.

Preparation and electrical and dielectric characterization of PVA/PPY blends

Abstract We present the dielectric characterization of conducting polymeric blends composed by conductive polypyrrole (PPY) regions formed inside a matrix of polyvinyl alcohol (PVA) containing ferric chloride (FeCl3). The incorporation of PPY into the PVA matrix results from in situ polymerization after the (PVA+FeCl3) matrix is exposed to the vapor of pyrrole. The physical properties of the samples are dependent on the amount of incorporated PPY as their electrical characteristics can vary from the limit of a highly insulating material to that of a very conducting polymeric film, similar in conductivity to doped electrochemically prepared PPY. We examine the dielectric properties of the blends as a function of the time of exposition to the pyrrole vapor.

Electrical properties of PVA/PPY blends

We present a dielectric study of polymeric blends composed by polypyrrole (PPY) incorporated into a matrix a polyvinyl alcohol (PVA) containing ferric chloride (FeCl 3 ). The PPY conducting regions are formed inside the matrix by in situ polymerization after the (PVA+FeCl 3 ) matrix is exposed to the vapor of the monomer. The mixed dielectric/conducting characteristics of the resulting blend is conveniently characterized by dielectric spectroscopy, and in this manner both the charge transport and charge storing processes can be followed. We have also examined how the temperature variation in the 300-24 K range affects the electrical behavior of the material.

Polypyrrole thin films gas sensors

Thin films of polypyrrole doped with different counter-ions, prepared by in situ polymerization on conducting glass electrodes, are used for the development of gas sensors with fast response time. On exposure to a given vapor, each individual film exhibits a fast change in the fractional difference of its electrical resistivity and with the adopted arrays of 7 sensors it was possible to detect and identify simple organic solvents. The morphology and structure of the films, and therefore the level of response to the volatile compounds, is determined by the structure of the counter-ion used.

Theoretical and experimental investigation of the second hyperpolarizabilities of methyl orange

We describe experimental and theoretical studies of the third-order nonlinear optical coefficients of methyl orange solutions under different pH conditions. A combination of semiempirical and ab initio methods was adopted to investigate the most stable geometrical structures possible for this molecule. The experimental data obtained using the Z-scan technique for the third-order nonlinear optical properties of this compound has allowed the determination of the nonlinear refractive index and nonlinear absorption coefficient under picosecond excitation in the visible (532 nm) spectral region. From those experimental results, the second hyperpolarizability of methyl orange was inferred both for acidic and alkaline solutions. Comparison of these values to the results predicted by semiempirical methods suggests that even at low pH, when the probability of cis-trans isomerization is increased, the trans conformation of the methyl orange molecule should dominate the nonlinear spectra of this compound. The theoretical results were used as an auxiliary tool to identify possible trends on the nonlinear properties changes as a function of the distinct molecular conformations adopted by the methyl orange molecule under different pH conditions.

Dielectric spectroscopy of blends of polyvinylalcohol and polypyrrole

Polymeric blends composed of conducting polypyrrole chains dispersed in a matrix of polyvinylalcohol containing ferric chloride represent a class of materials whose electrical behavior is intermediary between those of insulating and conducting polymers. To investigate the transport and polarization characteristics of these films we examine in this work their dielectric relaxation spectrum in the frequency domain. A relaxation in frequency identified in the resistance–reactance diagram is followed as a function of the relative concentration of the two polymers, as different types of dopants are used to promote the conducting behavior of polypyrrole. We also analyze the change in the relative dc and ac contributions to the total conductivity of the samples as the amount of incorporated polypyrrole is increased.Polymeric blends composed of conducting polypyrrole chains dispersed in a matrix of polyvinylalcohol containing ferric chloride represent a class of materials whose electrical behavior is intermediary between those of insulating and conducting polymers. To investigate the transport and polarization characteristics of these films we examine in this work their dielectric relaxation spectrum in the frequency domain. A relaxation in frequency identified in the resistance–reactance diagram is followed as a function of the relative concentration of the two polymers, as different types of dopants are used to promote the conducting behavior of polypyrrole. We also analyze the change in the relative dc and ac contributions to the total conductivity of the samples as the amount of incorporated polypyrrole is increased.

Comparative study of the electronic structure of conjugated polymers

Abstract We compare the electronic structures of trans-polyacetylene, poly - (p-phenylene), polypyrrole and polythiophene, computed by an Extended Huckel crystalline orbital scheme. Similarities in the valence and conduction densities of states are analyzed in terms of the connectivity of the different systems.

Semiempirical and ab initio investigation of defects in PPV oligomers

We report a theoretical study of the excited states and other electronic properties of para-phenylenevinylene oligomers and related compounds which present conformational defects. Our results reveal the existence of different electronic delocalization patterns for the lowest singlet and triplet structures of these molecules A similar behavior is also observed for the corresponding bond lengths.

Quantum chemistry calculation of resveratrol and related stilbenes

We report a semiempirical investigation of the first excited states and of the spectroscopic properties of resveratrol, a phytoalexin with well-known antioxidative properties, and of structurally related stilbenes. The analysis of the calculated bond length and charge rearrangements resulting from the photoexcitation and of the corresponding theoretical spectra gives us some insight of how chemical modifications of these molecules could affect the possible physiological properties of resveratrol.