A. A. Nekrasov

A comparative voltabsorptometric study of polyaniline films prepared by different methods

Abstract A comparative cyclic voltabsorptometric study was carried out for polyaniline (PAn) films prepared by electrochemical polymerization and vacuum thermal evaporation. The absorption values were measured at different characteristic wavelengths (895, 755, 665, 435, and 325 nm) corresponding to the individual absorption bands separated by us earlier from the spectra using the Alentsev–Fock method. Differential voltabsorptometric curves (dA/dt vs. potential (DCVA)) measured in aqueous HCl were compared with ordinary cyclic voltammetry (CVA). The experiments for different sweep rates were also compared. It was found that the DCVA peak at 895 nm corresponds perfectly to the CVA peak both in the electrosynthesized and the vacuum deposited PAn. The DCVA peak at 435 nm (cation-radicals) precedes the CVA peak (similar to the maximum of the EPR signal), while that at 755 nm (polarons or cation-radical dimers) lags behind the CVA peak. For the vacuum deposited films an intense intermediate DCVA and current peaks (at about 0.6 V vs. Ag/AgCl) are observed. The results are discussed in terms of the heterogeneous structure of PAn; particularly, its specific features in the films prepared using different methods.

Spectroelectrochemical, EPR and conductivity investigations of thin films of vacuum deposited polyaniline

The fractionating vacuum thermal deposition of emeraldine base produces the oligomers of polyaniline dispersed in the polymer organic matrix. The cyclic acid-base treatment of the low temperature fractions results in restoration of their polyaniline-like spectral, electrochemical, electric and paramagnetic properties. This is not the case for the high temperature fractions possibly as a consequence of their more defective structure and higher content of the thermal destruction products.

Electrochromic properties of vacuum-evaporated polyaniline films

Abstract The emeraldine base was deposited on SnO2 glass substrates in vacuum by thermal evaporation in the temperature range 275–325 °C. The as-deposited films differ dramatically from the native polyaniline and are formed by the reduction of oligoanilines with molecular weight about 1500. The cycling acid-base treatment of vacuum thermally deposited films gives rise to formation of polyaniline-like material. The property restoration of sublimed films is apparently determined by the restoration of 3D associates in the film body. Molecular structure defects in vacuum-deposited films are responsible for the lower rates of coloration/decoloration, the lower contribution to imine formation and irreversible injection of anodic charge under the first oxidation/reduction cycle. The lower imine concentration in sublimed polyaniline films results in more coloration/decoloration cycles.

Fractionating vacuum thermal deposition of polyaniline films. Effect of post-deposition acid-base treatment

Abstract Vacuum thermal evaporation of a single portion of chemically synthesized polyaniline (PAN) base powder was performed step-by-step within four temperature intervals in the 240–450 °C range. The vapour was deposited on quartz and SnO2-glass substrates, each fraction being deposited on a separate substrate. The samples obtained were subjected to cyclic contrast acid-base treatment, and UV-Vis spectra of the fractions after each treatment cycle were registered. The spectra of the as-deposited fractions appear to have little in common with conventional PAN spectra. The appearance of absorption bands essential for conventional PAN spectra is found to occur for some of the fractions only after the above treatment. This is apparently due to recombination of PAN oligomer radical fragments, formed in the course of the evaporation, resulting in restoration of PAN macromolecule structure. The recombination may be intensified by conformational changes during the acid-base treatment. Cyclic voltammetry curves for the most PAN-like fractions resemble the ones for conventional PAN.

Effect of pH on the structure of absorption spectra of highly protonated polyaniline analyzed by the Alentsev–Fock method

Abstract This paper presents the results of the investigation of the effect of pH of highly acidic medium (pH 0 and 1) on the structure of the electronic spectra of polyaniline films in the spectral range 300–900 nm. The analysis was carried out using the Alentsev–Fock method, which requires no preliminarily notion about the possible shape and/or number of the individual absorption bands and extensive statistical calculations. Along with the six absorption maximums described earlier by other authors three new individual bands were separated. On the basis of the comparison of our results with the literature data, we are inclined to assign the new band near 530–570 nm (depending on the pH) to the donor–acceptor interaction between the quinoid fragments and the counter anions and the new band at λ max >900 nm to the ‘free carriers’ absorption. We also suppose that the new band near 810–830 nm may be associated with the second absorption band of the cation-radical dimers. The effect of an increase in pH on the structure of polyaniline absorption spectra manifests itself in the increase of the portions of the bands corresponding to different electron transitions in the quinoid fragments, as well as in the decrease of the portions of the bands related to the conductivity of the film (i.e. the bands of the cation-radical dimers (or polarons) and free charge carriers).

Comparative spectroelectrochemical investigation of vacuum evaporated and electrochemically synthesized electrochromic polyaniline films ag

Abstract A spectroelectrochemical study of thin electrochromic polyaniline (PAN) films deposited on SnO 2 glass substrates by a vacuum thermal evaporation method has been carried out. The films obtained were studied by means of spectral, spectroelectrochemical and electrochemical methods, and the results were compared with those obtained for PAN films prepared by electrochemical polymerization. Spectroelectrochemical data (obtained under a potential step) and cyclic voltammetry data showed that the evaporated PAN films did not undergo a second oxidation stage resulting in formation of a quinoid structure. Current vs. time dependences processed according to the Cottrell equation showed that the diffusion coefficient for the evaporated PAN layers was almost a factor of 3 lower than that for the electrochemically deposited films. During cyclic tests in a real electrochromic optical filter the evaporated PAN layers exhibited a stability more than 2.5 times higher than that of the electrochemically deposited films.

Spectroelectrochemical study of vacuum deposited polyaniline films subjected to postdeposition treatment by HNO3

Abstract Thin polyaniline (PAN) films were prepared by evaporation of emeraldine base in vacuum at 275–325°C, followed by deposition of the vapors onto SnO 2 –glass transparent electrodes. With the purpose to restore the original structure of the polymer, the films were subjected to a postdeposition oxidative treatment in HNO 3 according to the following three different procedures: (1) treatment in vapors of 10 M acid, (2) treatment in 2 M HNO 3 aqueous solution, (3) cyclic treatment: in 2 M aqueous HNO 3 , then in 2 M aqueous NH 3 and so on. Electrochemical and spectroelectrochemical characteristics, namely cyclic voltammetry curves at different sweep rates and optical spectra at stationary potentials, were investigated for these films. It was found that the above characteristics of the films oxidized according to the cyclic procedure are most close to those of conventional PAN. However, in the optical spectra recorded at 0.0 V (vs. Ag/AgCl) for all the vacuum deposited films no absorption band in the area longer than 800 nm was observed. The results are discussed in terms of correlation of changes in the spectroelectrochemical characteristics with formation of heterogeneous structure of PAN by means of arising of intermolecular interactions, as well as, with specific features of vacuum the deposited PAN films.

Electrochemical and chemical synthesis of polyaniline on the surface of vacuum deposited polyaniline films

Electrochemical and chemical synthesis of polyaniline (PAN) was performed in an aqueous acidic solution of aniline on the surface of vacuum deposited thin polyaniline films applied on an SnO2 glass electrode. The composite films obtained have a high homogeneity and uniform thickness, essential for vacuum deposited layers. Spectroelectrochemical characteristics of the composite layers prepared by electrochemical synthesis are similar to those of electrodeposited PAN. Composite layers prepared by chemical synthesis have different Spectroelectrochemical characteristics, probably due to the presence of by-products of chemical synthesis. High selectivity of the synthesis was observed on the covered areas of partly sputtered substrates, evidently due to the autocatalytic character of PAN synthesis.

The influence of polyacid nature on poly(3,4-ethylenedioxythiophene) electrosynthesis and its spectroelectrochemical properties

The present study is aimed to elucidate main structural features of polymeric sulfonic acids (the rigidity of main chain, the distance between sulfonic groups on the chain, the hydrophobicity of main chain or side fragments) on the course of 3,4-ethylenedioxythiophene (EDOT) electropolymerization and electronic and chemical structure of the poly(3,4-ethylenedioxythiophene) (PEDOT) films obtained. The films were prepared by electrochemical polymerization in cyclic voltammetry, potentiostatic, and galvanostatic regimes in aqueous solutions of different polyacids in the absence of supporting electrolyte. The effect of the chemical structure of polyacid on the course and rate of PEDOT synthesis was traced by electrochemical and in situ UV-Vis spectroscopic methods. It was shown that the highest rate of EDOT electropolymerization is achieved in the presence of flexible-chain polyacid having hydrophobic fragments (groups) in its structure, followed by hydrophobic rigid-chain polyacids. The lowest rate was observed in the presence of hydrophilic flexible-chain polyacid. The electronic and chemical structure of the PEDOT films obtained was studied by in situ UV-Vis-NIR and Raman spectroelectrochemistry. The films prepared in the presence of rigid-chain polyacids at high anodic potentials demonstrate decreased content of bipolaronic fragments in their structure, while PEDOT complexes with flexible-chain polyacids are very much like conventional polymer prepared in non-aqueous medium. The results are discussed in terms of conformational state (ability to form coils and thus concentrate the monomer) of different polyacids in aqueous solution and hydrophobic interactions between the polyacids and EDOT.

On the nature of influence of polyelectrolyte molecular weight on aniline electropolymerization

Electrodeposition of polyaniline (PANI) complexes with a polymeric sulfonic acid of variable molecular weight has been studied with the purpose to explain specific features of aniline electropolymerization in the presence of polyelectrolytes. Poly-(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA) was taken as a model polyelectrolyte due to the ease of varying its molecular weight during polymerization of the monomer. Depending on the polyacid molecular weight, crucial changes in the shape of charge– and potential–time dependences and cyclic voltammograms have been observed during potentiostatic, galvanostatic, and cyclic voltammetry syntheses of PANI, respectively. The results are considered in terms of the influence of molecular weight on proton condensation ability of the polyacid and, therefore, on the degree of aniline protonation and its local concentration in the vicinity of polyacid macromolecule. The PANI films obtained are characterized by cyclic voltammetry and UV–vis and Raman spectroelectrochemistry, and the influence of the polyacid molecular weight on these properties is discussed.