Polyaniline, polypyrrole and poly(3,4-ethylenedioxythiophene) as additives of organic coatings to prevent corrosion

The aim of this work is to compare the protection against corrosion imparted by different conducting polymers when these materials are used as anticorrosive additives in the formulation of conventional epoxy paints. Specifically, the polymers employed as anticorrosive additives are polyaniline emeraldine salt, polyaniline emeraldine base, polyaniline emeraldine salt composite with carbon black, polypyrrole composite with carbon black and poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulphonate). Initially, the structural, thermal and mechanical properties of the unmodified epoxy paint as well as the modified coatings, obtained by the addition of 0.3 wt.% of conducting polymers, have been characterized. After this, controlled accelerated corrosion assays in an aggressive solution medium were developed using coated steel panels. Results indicate that the protection against corrosion imparted by the formulations modified by the addition of poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulphonate), polyaniline emeraldine salt and, especially, polyaniline emeraldine base is significantly higher than that of the unmodified paint. In contrast, the use of conducting polymer composite with carbon black reduces the efficacy of the coating. Results indicate that some conducting polymer compositions should be considered as a suitable alternative to replace inorganic anticorrosive pigments currently used in paint formulations.     

聚苯胺在水中的抗菌作用

研究了聚苯胺在水中的抗菌作用及其相应机制。对用掺杂聚苯胺处理后污水的水质分析和研究结果表明,其微生物指标可达到中水水质,但由于掺杂态聚苯胺在水中发生酸溶出效应,造成水体的化学需氧量COD(Chemical oxygen demand)值升高,形成对水的二次污染。当水中菌液浓度为3.0×10~2CFU·mL~(-1)时本征态聚苯胺的抗菌效果达99%,微生物指标达到生活饮用水标准值,同时并不二次污染水体。碘蓝分光光度法分析结果表明,聚苯胺将水中的氧气氧化为活性氧而杀死细菌是聚苯胺在水中抗菌的另一个可能机制。     

The effect of epoxy coating containing emeraldine base and hydrofluoric acid doped polyaniline on the corrosion protection of AZ91D magnesium alloy

Corrosion protection of epoxy coatings containing emeraldine base polyaniline (PANI) or hydrofluoric acid doped PANI on AZ91D magnesium alloy were studied by EIS and Pull-Off Adhesion Test. The results indicated that the addition of emeraldine base PANI or hydrofluoric acid doped PANI could improve the corrosion resistance of epoxy coating. The epoxy coating containing hydrofluoric acid doped PANI had the best performance of the corrosion protection among three systems under investigation. The corrosion product film was analyzed by XPS indicating that PANI changed the chemical structure of the corrosion film. The protective mechanism imparted by PANI was discussed.     

A covalent bond to bromine in HBr-treated polyaniline from X-ray diffraction

When the emeraldine base form of polyaniline is doped by aqueous HBr, the conductivity is increased to 2 (ohm cm)⁻¹ for a compressed pellet. X-ray diffraction results on the conducting polymer together with elemental analysis indicate that a significant portion of the bromine is covalently bound.     

Novel flexible,freestanding and transparent organic/inorganic hybrid materials formed between polyaniline and polyphosphate gel

In this paper we report the synthesis and characterization of new polyaniline/polyphosphate hybrid materials. The synthetic approach involves a single-step procedure in which the polyphosphate inorganic host structure (arising from a sol–gel transition) develops first, providing a restricted environment in which the conducting polymer is entrapped within the gel network. Polyaniline was formed as the green, conducting emeraldine salt form. The samples were characterized by thermogravimetric analysis and UV–vis, Raman, EPR and ³¹P-MAS-NMR spectroscopies. We have synthesized homogeneous, flexible, freestanding and transparent materials in which the conducting polymer presents a coiled conformation typical of primary doped polyaniline. Upon exposition to ammonia vapors the polymer can be converted to the emeraldine base form, which can be reverted by exposition to HCl vapors. Powdered samples present dc conductivities up to 0.2 S cm⁻¹.     

Controlled interconversion of semiconducting and metallic forms of polyaniline nanofibers

Self-assembled polyaniline nanofibers doped with 2-acrylamido-2-methyl-1-propanesulfonic acid were prepared by oxidative polymerization of aniline in the presence of a nonionic surfactant. These nanofibers were dedoped to the semiconducting emeraldine base and then redoped to the metallic emeraldine hydrochloride. It was possible to introduce a different dopant anion from that used in the initial synthesis with no significant changes in fiber morphology or diameter, as observed by scanning electron microscopy (SEM). The method of sample preparation for SEM significantly affected the observed morphology. Deposition from aqueous dispersions resulted primarily in nanofibers that ranged in diameter from 28 to 82 nm (average: 56 nm), whereas drying to solid powder resulted in a less fibrous material. UV–vis–NIR absorbance spectroscopy indicated that the electronic structure of the emeraldine base nanofibers was identical to bulk emeraldine base obtained by conventional synthesis. Estimates from X-ray diffraction data suggested that the fractional crystallinity of emeraldine hydrochloride nanofibers did not differ significantly from the bulk powder.     

Polyaniline,a dynamic block copolymer: key to attaining its intrinsic conductivity?

Photoluminescence studies of spun films of polyaniline base (emeraldine oxidation state) show that films prepared from NMP solution in air having a relative humidity between 43±2 and 57±2% (21°C) exhibit photoluminescence at 401 nm, which persists when the films are held in a dynamic vacuum. Films prepared from NMP solution containing water show similar behavior to those prepared in the above humidity range. When protonated (“doped”) with HCl the above photoluminescence disappears and is replaced by a photoluminescence peak at 467 nm. The reverse behavior occurs on deprotonation with NH₃ vapor, the 467 nm peak disappearing and the 401 nm peak reappearing. These results, together with photoluminescence studies on the phenyl/phenyl end-capped tetramer of aniline in both the emeraldine and leucoemeraldine oxidation states, lead to the conclusion that in solution the emeraldine base is a dynamic block copolymer in which reduced (benzenoid/amine) and oxidized (quinoid/imine) units are constantly interchanging positions via a tautomeric (hydrogen migration) process between nitrogen atoms. This process is promoted by water. The resulting production of long sequences of the reduced and oxidized units, which separate long sequences of the emeraldine base, persist when the NMP solvent is removed, resulting in microphase segregation of these sequences in the solid film. Since only the emeraldine base sequences can be doped by acids, the conductivity of the doped film is less than that of its predicted intrinsic conductivity.     

Electrical conductivity response of polyaniline films to ethanol–water mixtures

Polyaniline emeraldine base was chemically synthesized and converted to polyaniline emeraldine salts through the protonation doping process using HCl and CH₃COOH as the acid dopants. The doped polyanilines were characterized by ultraviolet–visible (UV–VIS) spectroscopy, Fourier transform-infrared (FT-IR) spectroscopy, TGA, GPC. The four-point probe technique was used to evaluate the effect of dopant type and doping molar ratio on the specific conductivity. Weak acid doping, by CH₃COOH, produces films with the specific conductivity which depends solely on the degree of protonation, or the number of charge carriers. On the other hand, stronger acid doping by HCl can induce crystalline domains, a greater electron mobility and hence a greater specific conductivity value. The specific conductivity of the HCl-doped and CH₃COOH-doped polyaniline films responds with positive increments upon exposed to water and ethanol. The interchain H-transfer is suggested to be a common mechanism which increases electron mobility upon exposure to water and ethanol, whereas additional protonation occurs only with the exposure to water. No evidence for ethanol molecules to interact chemically with the doped polyaniline films was found. The film electrical conductivity sensitivity is inversely proportional to ethanol concentration, with a higher sensitivity to concentration found in the film doped with the acid with a lower pK_a value.     

The molecular weight of polyaniline by light scattering and gel permeation chromatography

Polyaniline base in the emeraldine oxidation state displays strong absorption between 450 and 800 nm which precludes its use at these wavelengths for light scattering (LS) measurements. Therefore, as-synthesized polyaniline in the emeraldine base form was first reduced with hydrazine to polyaniline in the almost colorless leucoemeraldine oxidation state. Absolute weight average and number average molecular weights and polydispersity of the leucoemeraldine base by LS measurements were found to be 25 200, 11 560 and 2.18, respectively. However, the relative weight average molecular weight obtained by gel permeation chromatography of the emeraldine base, used to synthesize the leucoemeraldine base, was found to be ≈52 000 and that of emeraldine base obtained by the air oxidation of the above leucoemeraldine base was ≈36 800. Thus, subjecting emeraldine base to a cycle consisting of reduction with hydrazine followed by air oxidation results in significant chain scission and, hence, in a reduction of molecular weight. It is concluded that a lower limit of the number of ring-nitrogen units per polymer chain in the as-synthesized emeraldine base is 128.     

A magnetic susceptibility study of emeraldine base polyaniline

Paramagnetic spins are found even in carefully synthesized samples of emeraldine base polyaniline. We report here magnetic susceptibility data up to a temperature of 475 K on a number of emeraldine base samples that were synthesized at different oxidant to monomer ratios. All the samples, although electrically insulating, show the presence of Pauli-like susceptibility. A simple albeit physically realistic model involving spin-pairs is shown to lead to such a dependence on temperature. The analysis of the data indicates that polarons in emeraldine base are created in pairs and that the intra-pair interactions encompass a broad distribution of antiferromagnetic exchange couplings.     

Electrochemical impedance spectroscopy investigation of chlorinated rubber‐based coatings containing polyaniline as anticorrosion agent

Corrosion protection of mild steel by a newly developed chlorinated rubber (CR)‐based coating system containing the inherently conductive polymer polyaniline (PAni) as an anticorrosion agent was studied. The synthesis of PAni and preparation of CR‐based paint containing this polymer are described herein. The corrosion behavior of mild steel samples coated with a CR resin, CR/PAni‐EB (emeraldine base), CR/PAni‐ES (emeraldine salt), and CR/DBSA‐doped PAni were investigated in 3.5% NaCl solution. For this purpose, electrochemical impedance spectroscopy and corrosion potential versus time measurements were utilized. It was found that the addition of the two forms of PAni, doped and undoped, to the CR resin increased its corrosion protection efficiency.     

Effective dehydrogenative oxidation of amines and alcohols via radical mechanism by nano-porous polyaniline catalysis

It has been known that doped polyaniline emeraldine salt (PANI-ES) synthesized by self-stabilized dispersion polymerization (SSDP) method has a nano-porous structure. This nano-porous conjugate polymer with a high surface area is effectively used for the dehydrogenative oxidation reaction of benzylamine and benzylalcohol with or without oxygen. The catalytic activity can be controlled by the various oxidation states of PANI. After oxidation, the resulting structure of PANI-ES is transformed into a pernigraniline base (PB). This result suggests the involvement of radical mechanism by the reversible redox properties of conjugated polyaniline.     

Impedance spectroscopy of thin films of emeraldine base polyaniline and its implications for chemical sensing

The conductivity of an emeraldine base polyaniline thin film on an interdigitated electrode in hydrochloric acid solutions of various pHs between 2.25 and 6 was investigated by impedance spectroscopy. In general, protonation induces an emeraldine base (EB) to emeraldine salt (ES) transformation and an associated increase in conductivity only when the pH of the protonating bath is less than 4. However, impedance spectroscopy detects changes in the conductivity behavior that occur prior to the onset of this increase in total conductivity change. These changes can also be measured with AC impedance measurements at a single frequency, which expands the pH range over which emeraldine base polyaniline can be used as a sensor.     

Influence of added hydrogen bonding agents on the chiroptical properties of chiral polyaniline

The chiroptical properties of optically active polyaniline PAn.(?)-HCSA emeraldine salt (ES) products in NMP are stabilised by the presence of potential H-bonding additives such as l-arginine, l-tyrosine, l-alanine, l-valine, and glycine in the precursor emeraldine base/NMP solution prior to acid doping with (R)-(?)-10-camphorsulfonic acid. The stabilisation, i.e. absence of partial solvent-induced deprotonation of the ESs to emeraldine base is attributed to H-bonding interactions between the amino acids and the precursor emeraldine base.     

Temperature dependence of the resistance of self-assembled polyaniline nanotubes doped with 2-acrylamido-2-methyl-1-propanesulfonic acid

Hollow polyaniline (PANi) nanotubes doped with 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) have been synthesized and their electrical resistance measured as a function of temperature. The average length of the nanotubes was in the range 2–5 μm and the average diameter was in the range 200–400 nm. Longer reaction times did not affect the morphology of the nanotubes. The resistance of AMPSA doped nanotubes were measured as a function of temperature and compared to emeraldine base PANi doped with AMPSA in the solid state. Dissolving the PANi nanotubes in dichloroacetic acid leads to better polymer chain ordering.     

Self-assembled necklace-like polyaniline nanochains from elliptical nanoparticles

High quality necklace-like polyaniline nanochains assembled by elliptical nanoparticles have been synthesized in chitosan aqueous solution by a facile dispersion polymerization method. The synthetic procedure is very simple and easy to scale up. The necklace-like nanochains coated by a layer of chitosan are typical doped polyaniline in its emeraldine salt form, which is easy to form stable polyaniline dispersion in water. Open-circuit potential measurements show that the rate of polymerization of aniline has a substantial decrease due to the steric effects of chitosan. The synthetic parameters, such as reaction times, and the concentrations of chitosan, aniline, and dopant acid, have profound influences on the sizes and morphologies of polyaniline nanostructures. A new self-assembly process of elliptical nanoparticles has been proposed for the formation of necklace-like polyaniline nanochains with fluctuant diameters, which is different from that of other one-dimensional polyaniline nanostructures including nanofibers and nanotubes.     

A novel material based on polyaniline doped with [Cs][In(dmit)2], (cesium) [bis(1,3-dithiole-2-thione-4,5-dithiolato)indium (III)]

A novel conducting hybrid material based on polyaniline doped with a monoanion derived from [cesium][bis(1,3-dithiole-2-thione-4,5-dithiolato)indium(III)], ([Cs][In(dmit)₂]), is reported. Fourier transform infrared (FTIR), UV–vis and X-ray photoelectron spectroscopies showed that the [Cs][In(dmit)₂] acts as the doping agent for the emeraldine base (insulating form of the polyaniline). This new material presents lower resistivity than the starting materials, and forms a flexible, self-supporting film with good homogeneity, which is an important feature for the development of new devices to be useful in the electroelectronic industries.     

Suitability of emeraldine base polyaniline-PVA composite film for carbon dioxide sensing

A CO₂ sensor based on a composite thin film of emeraldine base polyaniline and poly (vinyl alcohol) cast from N-methyl pyrrolidone on an interdigitated electrode was characterized using impedance spectroscopy. The response of the sensor was slow and smaller in magnitude as compared to reports in the literature for similar sensors. Materials characterization indicated that the desired emeraldine base phase was not present after heat treatment. In addition, the mechanism for carbon dioxide detection previously reported in the literature cannot explain the CO₂ sensitivity, because the pH established between CO₂ and carbonic acid in the water dissolved in the poly (vinyl alcohol) matrix is not sufficient to induce a significant change in the conductivity of the emeraldine base polyaniline.     

Preparation of Se-doped polyaniline emeraldine base films

Our letter focuses on the preparation of polyaniline emeraldine base films doped with selenium. PANI EB and Se were dissolved in dimethylsulfoxide. Thin films of selenium-doped PANI EB were obtained by spin-coating of Si and ZnSe substrates with a PANI EB+Se solution in DMSO. We studied the distribution of the selenium in the PANI EB films using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The IR spectra of these thin films were obtained with an FT-IR spectrometer.     

Micellar chemical polymerization of aniline

Chemical oxidative polymerization of aniline was studied in an aqueous sodium dodecylsulfate (SDS) micellar system. The polymerization proceeded quickly to result in a homogeneous emeraldine-colored dispersion of polyaniline. Absorption spectra of polyaniline dispersion at various pH values showed that the transition from emeraldine salt to emeraldine base form occurred at about pH 7–8 in this anionic micellar system.