The genesis of polyaniline nanotubes

Aniline has been oxidized with ammonium peroxydisulfate in 0.4 M acetic acid. Protons are produced in the course of oxidation and the pH decreases as the reaction proceeds. The oxidation had two subsequent phases: (1) the oxidation of the neutral aniline molecules and the initially produced low-molecular weight aniline oligomers at low acidity, followed by (2) the oxidation of the anilinium cation after the acidity became higher. The two phases of oxidation gave different products, aniline oligomers with mixed ortho- and para-coupling of aniline molecules, and polyaniline nanotubes, respectively.The aniline oligomers are produced at first at low acidity, pH > 4, some of them as rod-like crystals. The molecular weight of the oligomers has been assessed by gel-permeation chromatography to be of several thousands. The 2-3 wt.% content of sulfur in deprotonated samples suggests that the oxidation products are partly sulfonated. The oxidation of ortho-coupled anilines combined with intramolecular cyclization produces phenazine units or their blocks, as indicated by FTIR spectra. A high-molecular weight polyaniline is produced at pH < 2. The protonation of the intermediate pernigraniline form of polyaniline is a prerequisite for the polymerization.The nano-sized oligomer crystallites serve as starting templates for the nucleation of PANI nanotubes. Further growth of nanotubes proceeds by the self-organization of the phenazine units or their blocks located at the ends of the PANI chains. Polyaniline nanotubes have a typical outer diameter of 100-200 nm, with a wall thickness of 50-100 nm, an inner diameter of 0-100 nm, and a length extending to several micrometres.     

Polyaniline-silver composites prepared by the oxidation of aniline with mixed oxidants, silver nitrate and ammonium peroxydisulfate: The control of silver content

Aniline was oxidized with mixtures of two oxidants, ammonium peroxydisulfate and silver nitrate, to give polyaniline-silver composites with variable content of silver in the composites. The presence of peroxydisulfate has a marked accelerating effect on the oxidation of aniline with silver nitrate. Oxidations in 1 M methanesulfonic acid produced composites in high yield. The molecular structure of the polyaniline was confirmed by UV-visible and FTIR spectra, and the polymeric character was established by gel-permeation chromatography. The content of silver varied between 0 and 70 wt.%. The silver nanoparticles were smaller than 100 nm. The conductivity of the composites was of the order of units S cm⁻¹. Only at high silver nitrate contents in the reaction mixture, the conductivity of products exceeded 100 S cm⁻¹. The conductivity of the composites sometimes increased after deprotonation of the polyaniline salt to a non-conducting base. Such conductivity behaviour is discussed in terms of the percolation model.     

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.     

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-coated silver nanowires

Two non-conducting chemicals, aniline and silver nitrate, dissolved in formic acid solutions, yielded a composite of two conducting products, polyaniline and silver. As the concentration of formic acid increased, an alternative reaction, the reduction of silver nitrate with formic acid to silver became dominant, and the content of silver in the composites increased. The formation of polyaniline was confirmed by UV–visible, FTIR, and Raman spectroscopies. The typical conductivity of composites was 43 S cm^?1 at 84 wt.% of silver. Silver nanowires coated with polyaniline nanobrushes are produced at low concentrations of formic acid, the granular silver particles covered with polyaniline dominate at high acid concentrations.     

Silver/Polyaniline Nanocomposite for the Electrocatalytic Hydrazine Oxidation

Silver/polyaniline (Ag-PANi) nanocomposites were prepared via in situ reduction of silver in aniline by mild photolysis performed with short wavelength (365 nm) radiation from UV lamp for 12 h. Reduction of the silver in aqueous aniline leads to the formation of silver nanoparticles which in turn catalyze oxidation of aniline into polyaniline. A slightly broadened X-ray diffraction (XRD) pattern suggests small particle which was size consistent with cubic silver nanoparticles. The UV–Vis absorption revealed that the bands at about 400–420 nm due to benzonoid ring of the polyaniline are overlapped and blue-shifted due to the presence of silver nanoparticles in powdered state. Scanning electron microscopy (SEM) of the silver/polyaniline (Ag-PANi) nanocomposite showed a size distribution with nanofibers and granular morphology of silver nanoparticles. Our findings are not only the promising approach for electro-catalytic hydrazine oxidation but also utilized in the other bio-sensing applications.     

Introduction of methanol in the formation of polyaniline nanotubes in an acid-free aqueous solution through a self-curling process

This study reports the synthesis of polyaniline (PANI) nanotubes with the introduction of methanol in aqueous solutions. SEM images indicate that well-dispersed PANI nanotubes are produced when the concentration of methanol increases up to 1 M. On the other hand, polymers primarily form irregular agglomerates when only monomers and oxidant are used in the absence of methanol. Transmission electron microscopy (TEM) reveals that the resulting nanotubes have an outer diameter of about 200 nm and an inner diameter of 0-50 nm. Fourier transform infrared (FT-IR) spectra indicate that the intermediate samples obtained at a reaction time of around 60 min have a structure consisting of a head of phenazine-like units and a tail of para-linked aniline units. Ultraviolet-visible (UV-vis) spectra indicate that emeraldine salt polyaniline (ES-PANI) appears in sequence as the reaction time reaches 75 min. At the time interval of 60-75 min, the self-curling behavior of the PANI intermediates first appears at 60 min, and PANI nanotubes can be observed starting at 75 min. This is the first study to observe this self-curling process and propose that it explains the formation of PANI nanotubes. Noteworthily, the nanotubes transform into irregular agglomerates after a de-doping process.     

The electrocatalytic oxidation of ascorbic acid on polyaniline film synthesized in the presence of β-naphthalenesulfonic acid

Polyaniline-β-naphthalenesulfonic acid composite film on platinum electrode surface has been synthesized via the electrochemical polymerization of aniline in the presence of β-naphthalenesulfonic acid (NSA). FT-IR, UV-vis and electrochemical characterization indicate the formation of the doped polyaniline. Further investigations found that the polyaniline (PAN) doped with NSA extended the electroactivity of PAN in neutral and even in alkaline media. The PAN-NSA composite film coated platinum electrodes are shown to be good electrocatalytic surfaces for the oxidation of ascorbic acid (AA) in phosphate buffer solution (PBS) of pH 7.0. The anode peak potential of AA shifts from 0.62 V at bare platinum electrode to 0.34 V at the PAN-NSA composite modified platinum electrode with greatly enhanced current response. A linear calibration graph is obtained over the AA concentration range of 5-60 mM using cyclic voltammetry. The kinetics of the catalytic reaction is investigated using rotating disk electrode (RDE) voltammetry, cyclic voltammetry and chronoamperometry. The results are explained using the theory of electrocatalytic reactions at chemically modified electrodes. The PAN-NSA composite film on the electrode surface shows good reproducibility and stability.     

Characterization of polyaniline/Ag nanocomposites using H2O2 and ultrasound radiation for enhancing rate

Polyaniline/Ag nanocomposites have been synthesized via in situ chemical oxidation polymerization of aniline in silver salt by sonochemical method using H₂O₂ as an external medium. H₂O₂ was used to reduce AgNO₃ to Ag nanoparticles as well as to polymerize aniline to polyaniline in the same pot. The ultrasound radiation as an energy source was applied to facilitate the reaction by reducing the reaction time. Reduction of the silver salt in aqueous aniline leads to the formation of silver nanoparticles which in turn catalyze oxidation of aniline to polyaniline. The research on the structures and properties of the composites showed the individual or aggregated silver nanoparticles are dispersed in the matrix of polyaniline. The composites possess a higher degradation temperature than polyaniline alone, and their electrical conductivity are raised morethan 200 times. The cyclic voltammetry and impedance spectroscopy results showed that the polyaniline/Ag film exhibits considerably higher electroactivity compared with polyaniline film without Ag particles. POLYM. COMPOS., 31:1662–1668, 2010. © 2009 Society of Plastics Engineers     

Ultrafine nano-colloid of polyaniline

We report a synthesis of polyaniline (PANI) suspension of particles with size of about 2-3 nm. This nano-colloid was obtained by aniline oxidative polymerization in dilute and semi-dilute solutions of sodium poly(styrene sulfonate) (PSSNa) with molecular weight equal to 6800 g/mol or higher. The ionic strength of the solution was about 1×10⁻² mol/l, which corresponds to aniline (and, respectively, PANI) concentration lower than 4.6×10⁻² mol/l in 5 M solution of formic acid. To the best of our knowledge, it is the first communication dealing with preparation of particles with a molecular scale dimensions, using a rigid backbone polymer with a very strong intermolecular interactions. Important modification of the electronic properties of such dispersed PANI, compared to those of the well-known bulk PANI, was observed.     

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.     

Production of ethyl ester from crude palm oil by two-step reaction with a microwave system

The production of ethyl esters of fatty acids from a feed material of crude palm oil (CPO) with a high free fatty acid (FFA) content under microwave assistance has been investigated. Parametric studies have been carried out to investigate the optimum conditions for the esterification process (amount of ethanol, amount of catalyst, reaction time, and microwave power). As a result, a molar ratio of FFA to ethanol of 1:24 with 4% wt./wt. of H₂SO₄/FFA, a microwave power of 70 W, and a reaction time of 60 min have been identified as optimum reaction parameters for the esterification process aided by microwave heating. At the end of the esterification process, the amount of FFA had been reduced from 7.5 wt.% to less than 2 wt.%. Similar results were obtained following conventional heating at 70 °C, but only after a reaction time of 240 min. Transesterification of the esterified palm oil has been accomplished with a molar ratio of CPO to ethanol of 1:4, 1.5 wt.% KOH as a catalyst, a microwave power of 70 W, and a reaction time of 5 min. This two-step esterification and transesterification process provided a yield of 80 wt.% with an ester content of 97.4 wt.%. The final ethyl ester product met with the specifications stipulated by ASTM D6751-02.     

A hybrid feedstock for a very efficient preparation of biodiesel

Biodiesel has been synthesized from karanja, mahua and hybrid {karanja and mahua (50:50 v/v)} feedstocks. A high yield in the range of 95-97% was obtained with all the three feedstocks. Conversion of vegetable oil to fatty acid methyl esters was found to be 98.6%, 95.71% and 94% for karanja, mahua and hybrid feedstocks respectively. The optimized reaction parameters were found to be 6:1 (methanol to oil) molar ratio, H₂SO₄ (1.5% v/v), at 55 ± 0.5 °C for 1 h during acid esterification for the three feedstocks. During alkaline transesterification, a molar ratio of 8:1 (methanol to oil), 0.8 wt.% KOH (wt/wt) at 55 ± 0.5 °C for 1 h was found to be optimum to achieve high yield for karanja oil. For mahua oil and the hybrid feedstock, 6:1 (methanol to oil) molar ratio, 0.75 (w/w) KOH at 55 ± 0.5 °C for 1 h was optimum for alkaline transesterification to obtain a high yield. High yield and conversion from hybrid feedstock during transesterification reaction was an indication that the reaction was not selective for any particular oil. ¹H NMR has been used for the determination of conversion of the feedstock to biodiesel.     

Cyclic polyaniline nanostructures from aqueous/organic interfacial polymerization induced by polyacrylic acid

Cyclic nanostructural polyanilines have been synthesized by means of a simple modified aqueous/organic interfacial polymerization with the aid of polyacrylic acid, and yet only nanofiber nanostructural polyanilines were obtained by conventional interfacial polymerization in the presence of polyacrylic acid. These nanostructures were characterized using TEM, X-ray diffraction and UV-vis. The cyclic polyaniline nanostructures were formed by severe secondary overgrowth through the electrostatic interaction between aniline/oligoaniline and polyacrylic acid chains. The average diameter of the obtained cyclic-structure was ca. 400 nm and its conductivity was 1.06 S/cm.     

Synthesis of radially aligned polyaniline dendrites

Radially aligned polyaniline dendrites with nanotube junctions have been synthesized using tartaric acid as the dopant without the aid of any surfactants. These dendritic nanotubes with 80-400 nm in outer diameter, 30-50 nm in wall thickness, and several micrometers in length can self assemble into urchin-like nanostructures. The geometrical shape of the individual branch is a cone. The nanotube junctions may provide potential applications in nanoelectronic devices. Furthermore, the influences of other organic acid, such as citric acid, oxalic acid, and acrylic acid, on the morphologies on polyaniline nanostructures have been investigated.     

Polyaniline synthesis catalysed by glucose oxidase

A novel method for synthesis of polyaniline (PANI) in aqueous media based on application of oxidizing-enzyme glucose oxidase (GOx) is reported. Hydrogen peroxide was produced during catalytic reaction of oxidizing-enzyme glucose oxidase from Penicillium vitale and initiated the polymerization of aniline. The increase in optical absorbance in the range of 340-700 nm was exploited for the monitoring of PANI polymerization process. The role of GOx in the formation of PANI, influence of the initial concentrations of GOx, and glucose and aniline monomer on the aniline polymerization rate was studied. The study of pH influence on polymerization rate showed that PANI polymerization was occurring in a broad pH range from the pH 2.0 to 9.0. Optimal polymerization/oligomerization temperature was found to be at 37 °C, which is also optimal for GOx-catalysed enzymatic reaction. After 10 days of continuous GOx-catalysed polymerization PANI appeared as colloid-microparticles visible by an optical microscope.     

Potential controlled electrochemical assembly of chiral polyaniline with enhanced stereochemical selectivity

Chiral polyaniline with absolute sterochemical selectivity in the order of magnitude 10⁻² is electrochemically assembled onto various electrodes in the presence of chiral inducing agent (1S)-(+)-10-camphorsulfonic acid ((S)-(+)-CSA) or (1R)-(−)-10-camphorsulfonic acid ((R)-(−)-CSA). The polymerizing potential dependence of the absolute stereochemical selectivity of the resultant chiral polyaniline is investigated in detail. The one-step (low potential at 0.7 V) and two-step (potential higher than that of aniline oxidation potential of the electrode applied first in the short period of time and then low potential at 0.7 V applied in the relatively long time) potential control strategies are developed for the different conducting substrates in order to produce chiral polyaniline with enhanced absolute stereochemical selectivity. The possible mechanism on polymerizing potential dependence of chiral polyaniline with enhanced absolute stereochemical selectivity is discussed. The structure and morphology of the resultant polyaniline films are investigated by scanning electron microscopy (SEM). The strong mirror images of the circular dichroism (CD) spectra for the (R)-(−)-CSA and the (S)-(+)-CSA doped polyaniline films suggest that the obtained chiral polyaniline films are enantioselective.     

Surface modification of silver microparticles with 4-thioaniline

Silver microparticles have been prepared by chemical reduction method onto a rotating copper disc. It was found that in the presence of polyvinylpyrrolidone (PVP) in the initial AgNO₃ aqueous solution the radius of the Ag particles decreased from 1.18 to 0.48 μm and the formation of dendritic clusters with the fractal dimension of 1.61 occurred. The two-stage modification of the surface of synthesized silver particles with bifunctional 4-thioaniline (TAn) and further grafting of polyaniline (PAn) chains to the end amino-group of formed self-assembled surface layer were carried out. The modified surfaces were studied by scanning electron microscopy and X-ray diffraction methods, the physico-chemical properties of the formed products have been investigated. The obtained results indicate that the highest thermal stability is displayed by the silver particles produced in the presence of PVP, while the sample with grafted polyaniline exhibits the lowest thermal stability among the investigated samples. The reactivity of TAn has been examined by cyclic voltammetric method. In contrast to aniline, the electrochemical oxidation of TAn did not result in the formation of an electroconductive film on Pt electrode. It was found that oxidative condensation of aniline is suppresses by the TAn: when the molar ratio of aniline/TAn is 2:1 the formation of an electrochemically active product is stopped. The phenomenon of inhibition of the oxidative polymerization of aniline with TAn is related to an impossibility of growing of polymeric chains in the result of the “head-to-tail” type of bonding of the thioaniline links.     

Corrosion protection coating containing polyaniline glass flake composite for steel

Corrosion protection of steel by glass flake (GF) containing coatings is widely used in marine atmosphere. Even though, the coatings containing glass flake are highly corrosion resistant, their performance is decreased due to the presence of pinholes and coating defects. It is well established that polyaniline containing coating is able to protect the pinhole defects in the coatings due to passivating ability of polyaniline. Hence a study has been made on the corrosion protection ability of steel using polyaniline-glass flake composite containing coating with 10% loading of glass flake in epoxy binder. The polyaniline glass flake composite (PGFC) was synthesized by chemical oxidation of aniline by ammonium persulphate in presence of glass flake. The corrosion protection ability of GF and PGFC containing coating on steel was found out by salt spray test and EIS test in 3% NaCl. In both the tests, the resistance value of the PGFC containing coating has remained at 10⁸-10⁹ Ω cm² where as for the GF containing coating, the resistance values decreased to 10⁵ Ω cm². The enhanced corrosion protection ability of the PGFC containing coating is due to the passivation ability of the polyaniline present in the coating.