The oxidation of aniline with silver nitrate to polyaniline-silver composites

Silver nitrate oxidizes aniline in the solutions of nitric acid to conducting nanofibrillar polyaniline. Nanofibres of 10-20 nm thickness are assembled to brushes. Nanotubes, having cavities of various diameters, and nanorods have also been present in the oxidation products, as well as other morphologies. Metallic silver is obtained as nanoparticles of ∼50 nm size accompanying macroscopic silver flakes. The reaction in 0.4 M nitric acid is slow and takes several weeks to reach 10-15% yield. It is faster in 1 M nitric acid; a high yield, 89% of theory, has been found after two weeks oxidation of 0.8 M aniline. The emeraldine structure of polyaniline has been confirmed by FTIR and UV-vis spectra. The resulting polyaniline-silver composites contain 50-80 wt.% of silver, close to the theoretical expectation of 68.9 wt.% of silver. The highest conductivity was 2250 S cm⁻¹. The yield of a composite is lower when the reaction is carried out in dark, the effect of daylight being less pronounced at higher concentrations of reactants.     

Polyaniline–silver composites prepared by the oxidation of aniline with silver nitrate in solutions of sulfonic acids

Aniline was oxidized with silver nitrate in aqueous solutions of sulfonic acids: camphorsulfonic, methanesulfonic, sulfamic, or toluenesulfonic acids. Polyaniline–silver composites were produced slowly in 4 weeks in good yield, except for the reaction, which took place in sulfamic acid solution, where the yield was low. Polyaniline in the emeraldine form was identified with UV–visible, FTIR, and Raman spectra. Thermogravimetric analysis was used to determine the silver content, which was close to the theoretical prediction of 68.9 wt.%. Transmission electron microscopy demonstrated the presence of silver nanoparticles of ca 50 nm average sizes as the dominating species, and hairy polyaniline nanorods having diameter 150–250 nm accompanied them. The highest conductivity of 880 S cm⁻¹ was found with the composite prepared in methanesulfonic acid solution. Its conductivity decreased with temperature increasing in the 70–315 K range, which is typical of metals such as silver. The conductivity of composites prepared in solutions of other acids was lower and increased with increasing temperature. Such dependence is typical of semiconductors, reflecting the dominating role of polyaniline in the conductivity behaviour. It is proposed that interfaces between the polyaniline matrix and dispersed silver nanoparticles play a dominating role in macroscopic level of conductivity.     

Polyaniline–silver composites prepared by the oxidation of aniline with silver nitrate in acetic acid solutions

The reaction between two non‐conducting chemicals, aniline and silver nitrate, yields a composite of two conducting components, polyaniline and metallic silver. Such conducting polymer composites combine the electrical properties of metals and the materials properties of polymers. In the present study, aniline was oxidized with silver nitrate in solutions of acetic acid; in this context, aniline oligomers are often a major component of the oxidation products. An insoluble precipitate of silver acetate is also present in the samples. The optimization of reaction conditions with respect to aniline and acetic acid concentrations leads to a conductivity of the composite as high as 8000 S cm^?1 at ca 70 wt% (ca 21 vol%) of silver. A sufficient concentration of acetic acid, as well as a time extending to several weeks, has to be provided for the successful polymerization of aniline. Polyaniline is present as nanotubes or nanobrushes composed of thin nanowires. The average size of the silver nanoparticles is 30–50 nm; silver nanowires are also observed. Copyright © 2009 Society of Chemical Industry     

Polyaniline and polypyrrole prepared in the presence of surfactants: a comparative conductivity study

Polyaniline and polypyrrole have been prepared by chemical oxidative polymerization of the corresponding monomers in an aqueous medium containing an anionic surfactant—sodium bis(2-ethylhexyl) sulfosuccinate, dodecylbenzenesulfonic acid and its sodium salt, and sodium dodecyl sulfate. Determination of the yield, elemental composition and density proved, and FTIR spectroscopy confirmed, that the anionic surfactants become incorporated in the conducting polymers. The polymerizations in the presence of a cationic surfactant, tetradecyltrimethylammonium bromide, were carried out for comparison. While the conductivity of polypyrrole became enhanced after the introduction of an anionic surfactant, the changes in the conductivity of polyaniline were marginal. The conductivity changes in both polymers during thermal ageing were measured at 175 °C. The electrical stability of polyaniline was better than that of polypyrrole. The presence of a surfactant improved the stability of conductivity of polypyrrole but reduced the electrical stability of polyaniline.     

The reaction of polyaniline with iodine

Polyaniline (PANI) (emeraldine) base has been exposed to iodine in an ethanol-water suspension. The conductivity of PANI increased from 10⁻⁹ S cm⁻¹ to 10⁻⁴ S cm⁻¹ already at the molar ratio [I₂]/[PANI] = 1, and a higher content of iodine had only a marginal effect. This is the result of the protonation of PANI base with hydriodic acid, which is a by-product of the oxidation of the emeraldine form of PANI to pernigraniline constitutional units. The reaction is discussed on the basis of FTIR spectra. An alternative reaction, a ring-iodination of PANI, is marginal. Only one iodine atom substitutes a hydrogen atom in about 12 aniline units, even at high iodine concentration, [I₂]/[PANI] = 8. The film of polyaniline base can be used in sensing iodine; after exposure to the iodine vapor, the conductivity of the polyaniline film increased.     

On the molecular properties of polyaniline: A comprehensive theoretical study

A comprehensive study about the molecular and electronic properties of the different forms of polyaniline has been developed using quantum mechanical calculations. Initially the performance of different ab initio and DFT quantum mechanical methods has been evaluated by comparing the results provided for small model compounds containing two repeating units. After this, calculations on the emeraldine base, leucoemeraldine base, pernigraniline base and emeraldine salt (monocationic and dicationic) forms of oligoanilines with n repeating units, where n ranged from 5 to 13, have been performed using the BH&H/6-31G(d) method, which was found to be a very suitable theoretical procedure. Interestingly, calculations indicate that the distribution in blocks of the repeating units containing amine and imine nitrogen is largely preferred for the emeraldine base form. On the other hand, the molecular structure and band gap of the emeraldine base, leucoemeraldine base and pernigraniline base forms have been rationalized according to their differences in the conjugation of the C₆H₄ rings. Calculations on cationic oligoanilines indicate that, when the emeraldine salt form presents a doublet electronic state, the positive charge and the spin density are located in the middle of the chain extending through five consecutive repeating units.     

Engineering detonation nanodiamond - Polyaniline composites by electrochemical routes: Structural features and functional characterizations

Novel detonation nanodiamond (DND) - polyaniline (PANI) composite systems have been prepared by electrochemical polymerization techniques. Thanks to the use of two different electrochemical methods, i.e. cyclic voltammetry and chronoamperometry, it has been possible to emphasize the influence of DND particles on the nucleation mechanism of the conducting polymer. In particular, the presence of DND into the reaction environment has proven to modulate the organization of the aniline oligomers into π-stacked aggregates and to induce the production of one-dimensional nanostructures. Scanning and transmission electron microscopy was used to investigate the morphology of the final composites. The protonated conducting form of the PANI matrix has been evidenced by Raman spectroscopy and the ionic/electronic transport of the PANI-DND systems was tested by means of electrocatalysis measurements toward the iodine/iodide redox couple. Based on the present experimental data, the use of nanodiamond as filler for conducting polymer based nanocomposite represents not only a fascinating challenge for the production of advanced technological applications but it can also be exploited for the controlled growth of polymeric units and for the fabrication of tailored polymeric architectures.     

Impedance characterization of the process of polyaniline first redox transformation after aniline electropolymerization

The paper presents the analysis of the aniline electropolymerization process by a recently developed experimental technique—dynamic electrochemical impedance spectroscopy. The process of the polyaniline first redox transformation was chosen for a detailed investigation. The technique used allowed to obtain the momentary impedance spectra for the potentiodynamic electrochemical reaction. The spectra were further analysed using the proposed electrical equivalent circuit. Thus, we were able to trace the electrochemical properties of the investigated system for potentiodynamic process. Results were in agreement with the expectations and confirmed the electrochemical nature of the analysed process.     

Electrochemical synthesis and characterisation of polyaniline/poly(2-methoxyaniline-5-sulfonic acid) composites

The potentiodynamic synthesis and subsequent characterisation of a polyaniline (PAn) grown in the presence of a water-soluble conducting polymer, poly(2-methoxyaniline-5-sulfonic acid) (PMAS) is described. The novel polymer is obtained as a water-insoluble film consisting of a PAn backbone with PMAS integrated as the molecular dopant. The electrodeposition of the polyaniline material is enhanced by the presence of the electrically conducting PMAS polyelectrolyte dopant, which functions as a molecular template providing supramolecular pre-ordering as well as simultaneously facilitating charge transport during electrodeposition.     

Optically active fluoro-substituted polyaniline prepared in organic media: The synthesis, chiroptical properties, and comparison with optically active non-substituted polyaniline

Optically active polyaniline (PANI) bearing an electron-withdrawing fluorine substituent, poly(2-fluoroaniline) (F-PANI), was synthesized in organic solvents by using (+)- or (−)-camphorsulfonic acid (CSA) as a chiral dopant, in which 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) was used as an electron acceptor. Formations of the optically active (chiral) F-PANI/(+)- or (−)-CSA, the dedoped F-PANI, and the redoped F-PANI/CSA were confirmed by the FT-IR, UV/vis/near-IR, and/or circular dichroism (CD) spectroscopy. The CD spectra of the chiral F-PANIs/(+)- and (−)-CSA were investigated in various organic solvents and compared with those of non-substituted PANI/(+)- or (−)-CSA. The F-PANI/(+)-CSA and PANI/(+)-CSA interestingly indicated almost mirror-imaged CD spectra in the visible region, although CD absorptions of the F-PANI/CSA were slightly blue shifted compared with those of PANI/CSA. The chiral F-PANI/CSA was unexpectedly confirmed to remain the initial chiral conformation during reversible dedoping/redoping cycles in solution state. The chiroptical properties of F-PANI/(+)-CSA in organic solvents were found to be affected by the solvent, probably due to the solvent effect to the polymer backbone, and the helical conformation was drastically reversed by change of the solvent. Furthermore, the conformation of F-PANI/CSA in m-cresol can be significantly changed when the F-PANI/CSA was dissolved in cosolvent of m-cresol and DMSO at various volume ratios, in which the sign inversions of CD absorption bands were caused when the ratio of DMSO in the m-cresol/DMSO cosolvent exceeded 10%. On the other hand, in the case with PANI/(+)-CSA, the similar sign inversions of CD absorption bands were not observed under the similar conditions, and then the helical conformation of chiral F-PANI/CSA was supposed to be rather flexible compared with that of chiral PANI/CSA, presumably due to relatively weak interaction among F-PANI backbone, CSA, and m-cresol.     

Polyaniline prepared in solutions of phosphoric acid: Powders, thin films, and colloidal dispersions

Polyaniline (PANI) has been prepared by the oxidation of aniline with ammonium peroxydisulfate in the 0-4 M phosphoric acid. The maximum conductivity of PANI, 15.5 S cm⁻¹, was found with PANI prepared in the presence of 1 M phosphoric acid. The mass loss after deprotonation with ammonium hydroxide revealed that relatively large amounts of phosphoric acid were associated with PANI if the polymerization had been carried out at higher acid concentration. This suggests the protonation of both the imine and amine nitrogens in PANI, the increased adsorption of phosphoric acid by PANI, or the presence of polyphosphate counter-ions. The increasing content of phosphoric acid is also reflected in the increase of sample density. FTIR spectra of ammonium salts collected after deprotonation proved that the counter-ions of the sulfate type, resulting from the decomposition of peroxydisulfate, always participated in the protonation of PANI. The proportion of sulfate to phosphate counter-ions was reduced as the concentration of phosphoric acid in the medium increased.Thin PANI films were produced in situ on glass surfaces immersed in the reaction mixture during the polymerization of aniline. Optical absorption has been used to assess their thickness, 70-140 nm, which was found to be virtually independent of the acid concentration. The film conductivity was comparable with the conductivity of the PANI powders produced at the same time. Colloidal dispersions were obtained when the reaction mixture contained poly(N-vinylpyrrolidone). The particle size, 200-260 nm, and polydispersity, determined by dynamic light scattering, were virtually independent of the concentration of phosphoric acid. The films produced on glass during the dispersion polymerization of aniline were thinner, 20-90 nm, compared with those grown in the precipitation polymerization.     

Characterization of silver and silver/nickel composite particles prepared by spray pyrolysis

Pure silver and silver/nickel composite particles were prepared by spray pyrolysis of aqueous solutions of AgNO₃, and mixed salts of AgNO₃ and Ni(NO₃)₂·6H₂O, respectively. In the case of pure silver, reduction to metallic silver and subsequent sintering to highly spherical and dense particles took place immediately and almost simultaneously once favorable conditions for the former were imposed, irrespective of the nature of the carrier gas. For the composite particles, the high rates of reduction and sintering of the silver were still maintained, while crystallization of the silver, and the reduction and sintering of the nickel were considerably retarded, compared to the spray pyrolysis of each pure salt. Once the counterpart salt was added, the size of the composite particles increased compared to that of each pure metallic particles, but it was little affected by the furnace set temperature, the residence time and the molar ratio of the two precursor salts. Within single particles, an increase in either the temperature or the residence time caused segregation—silver in the shells and nickel in the cores—and improved the particles' surface smoothness and sphericity accordingly.     

Electroless recovery of silver by inherently conducting polymer powders, membranes and composite materials

A wide variety of inherently conducting polymers, including polypyrroles and polyanilines, were shown to reduce silver ion in aqueous solution, demonstrating that these materials may form the basis of a novel approach to silver recovery. The effect of varying the polymer, dopant, and underlying substrate (reticulated vitreous carbon or fabric) was investigated, as was the effect of changing the pH of the solution containing silver. Silver recovery was found to proceed more readily from solutions with near neutral pH than from solutions with strongly acidic pH, and to occur selectively from solutions containing both silver and iron. Scanning electron microscopy (SEM) showed deposits that formed on polymeric materials after prolonged exposure to concentrated silver nitrate solutions were partially crystalline in nature. XPS spectra provided support for a mechanism of silver recovery involving a redox reaction between the polymer and silver ions leading to the formation of silver metal.     

Sustainable conducting polymer composites: study of mechanical and tribological properties of natural fiber reinforced PVA composites with carbon nanofillers

ABSTRACT

Ball milled jute fiber (JF) was added to Polyvinyl Alcohol (PVA)/20 wt.% multi-layer graphene (MLG) composites in various proportions (0, 5, 10, 15 and 20 wt.%) to prepare sustainable and biodegradable conducting polymer composites. Also, PVA/17.5wt.%MLG/2.5wt.%MWCNT/20wt.% JF composite was prepared for comparison purpose. A dynamic mechanical analysis of the composites was conducted to analyze their viscoelastic nature. The electrical conductivity of the composites was measured to study their suitability for various applications. Jute reinforcement increased the electrical conductivity of PVA/MLG nanocomposites. The PVA/20wt.%JF/17.5wt.%MLG/2.5wt.%MWCNT hybrid composite had the highest electrical conductivity of 3.64 × 10^?4 S/cm among all the composites prepared. Multilayered structures of the hybrid composite films were made by hot-pressing, and their effectiveness in electromagnetic interference shielding was tested. The shielding effectiveness of the composites decreased with jute addition. The wear resistance of PVA/MLG/JF composites increased with an increase in the jute content up to an optimum value of 10 wt.%, and then it started deteriorating.     

Anticorrosion performances of epoxy coatings modified with polyaniline: A comparison between the emeraldine base and salt forms

The protection against corrosion imparted by epoxy paints modified by the addition of polyaniline emeraldine base (0.3% w/w) and Zn₃(PO₄)₂ (10% w/w) has been investigated and compared. For this purpose, accelerated assays through corrosion cycles, which simulate the marine conditions, have been performed using a home-made robotized equipment. Results were compared with those recently obtained for a formulation constituted by the same epoxy paint modified with polyaniline emeraldine salt (0.3% w/w) [E. Armelin, R. Pla, F. Liesa, X. Ramis, J.I. Iribarren, C. Alemán, Corr. Sci. 50 (2008) 721.]. A detailed analysis of the results using the ASTM standard method D-1654-79 allows to conclude that the coating modified with polyaniline emeraldine base performs better than both the unmodified coating and the coatings modified with the inorganic corrosion inhibitors. This behaviour should be attributed to a mechanism based on the ability of polyaniline emeraldine base to store charge. On the other hand, as the epoxy paint modified with the latter polymer provides better results than the formulation containing Zn₃(PO₄)₂, we conclude that inorganic corrosion inhibitors, which may have detrimental effects on both the environment and the human health, can be replaced by a small concentration of environmentally friendly organic polymers.     

Study of epoxy and alkyd coatings modified with emeraldine base form of polyaniline

In this work we examine the ability of the emeraldine base form of polyaniline to impart protection against corrosion when it is used as additive of commercial paints. For this purpose, three paints, which are used as primers in marine environments, were checked: two epoxy coatings that differ in the presence or absence of inorganic anticorrosive pigment (zinc) and one alkyd coating. In a first stage, the rheological, structural, thermal and mechanical properties of the three coatings were characterized using viscosity measurements, infrared spectroscopy, thermogravimetric analyses and stress–strain assays, respectively. Furthermore, we observed that the resulting properties were not altered by the addition of a low concentration of polyaniline (0.3%, w/w). Accelerated corrosion tests were performed to compare the degree of protection of both the modified and unmodified paints. The polyaniline did not affect to the protective properties of the epoxy without inorganic anticorrosive pigment nor the alkyd formulations. In opposition, the polyaniline added to the epoxy paint with inorganic anticorrosive pigment induced the formation of a zinc oxide layer, which promoted the corrosion attack.     

In situ investigation of corrosion localised at the buried interface between metal and conducting polymer based composite coatings

Conducting polymers are controversially discussed for application in corrosion protection. In our earlier work we presented a hypothesis for explaining why sometimes very good performance is observed and sometimes fast and disastrous coating break-down, especially in the presence of defects in the coating. By avoiding continuous macroscopic networks of conducting polymer in composite coatings the latter can be prevented, opening up the application of conducting polymers for secure performance in intelligent corrosion protection. In this paper we add further proof. However, at the interface between coating and metal, corrosion induced by galvanic coupling between conducting polymer and the metal still remains a problem, even in the absence of defects in the coating. Now we found that for some metals this corrosion at the buried interface can be studied in situ by Scanning Kelvin Probe microscopy, providing the means to better develop strategies for counter acting this problem.     

Electrosynthesis of PAni/PPy coatings doped by phosphotungstate on mild steel and their corrosion resistances

Polyaniline/polypyrrole (PAni/PPy), polyaniline-phosphotungstate/polypyrrole (PAni-PW₁₂/PPy) and PAni/PPy-PW₁₂ have been successfully electrodeposited on mild steel (MS) by cyclic voltammetry in aqueous oxalic acid solutions. It was found that the incorporation of PW₁₂ enhanced the corrosion resistance of PAni/PPy coating. Moreover, in comparison to PAni-PW₁₂/PPy, PAni/PPy-PW₁₂ coating exhibited better corrosion resistance for mild steel. After immersion of 36 h in 0.1 M HCl, for instance, the polarization resistance of PAni/PPy-PW₁₂ coating reached 1695 Ω cm², more than those of both PAni/PPy and PAni-PW₁₂/PPy.     

The effect of platinum and silver deposits in the photocatalytic oxidation of resorcinol

The effects of platinum (Pt) and silver (Ag) metallisation in the photocatalytic oxidation of resorcinol at pH 3 ± 0.5 have been investigated. The photocatalytic degradation of resorcinol was significantly improved by Pt/TiO₂, while the presence of Ag/TiO₂ enhanced the initial photocatalytic degradation rate of resorcinol slightly. Likewise, the photocatalytic mineralisation of resorcinol continued to be enhanced by Pt/TiO₂, but it was retarded when Ag/TiO₂ was used.

The function of Pt and Ag deposits on the surface of TiO₂ has been found to be markedly influenced by the interaction of resorcinol and its degradation products with the metal deposits. The presence of Pt or Ag on the surface of TiO₂ altered the distribution of degradation products of resorcinol as well as the production of photoactive species for the photocatalytic oxidation of resorcinol. The X-ray photoelectron spectroscopy (XPS), zeta potential and transmission electron microscopy (TEM) analyses have indicated that the contrasting effect of Pt and Ag deposits were governed by the oxidation states and the catalytic property of metal deposits. In addition to that, it has been found that the roles of metal deposits are specific and should not be generalised.