Multi-wall carbon nanotubes coated with polyaniline

Multi-wall carbon nanotubes (CNT) were coated with protonated polyaniline (PANI) in situ during the polymerization of aniline. The content of CNT in the samples was 0-80 wt%. Uniform coating of CNT with PANI was observed with both scanning and transmission electron microscopy. An improvement in the thermal stability of the PANI in the composites was found by thermogravimetric analysis. FTIR and Raman spectra illustrate the presence of PANI in the composites; no interaction between PANI and CNT could be proved. The conductivity of PANI-coated CNT has been compared with the conductivity of the corresponding mixtures of PANI and CNT. At high CNT contents, it is not important if the PANI coating is protonated or not; the conductivity is similar in both cases, and it is determined by the CNT. Polyaniline reduces the contact resistance between the individual nanotubes. A maximum conductivity of 25.4 S cm⁻¹ has been found with PANI-coated CNT containing 70 wt% CNT. The wettability measurements show that CNT coated with protonated PANI are hydrophilic, the water contact angle being ∼40°, even at 60 wt% CNT in the composite. The specific surface area, determined by nitrogen adsorption, ranges from 20 m² g⁻¹ for protonated PANI to 56 m² g⁻¹ for neat CNT. The pore sizes and volumes have been determined by mercury porosimetry. The density measurements indicate that the compressed PANI-coated CNT are more compact compared with compressed mixtures of PANI and CNT. The relaxation and the growth of dimensions of the samples after the release of compression have been noted.     

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 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.     

Effect of 4-sulfobenzoic acid monopotassium salt on oligoanilines for inducing polyaniline nanostructures

Polyaniline (PANI) nanotubes were chemically synthesized with the introduction of 4-sulfobenzoic acid monopotassium salt (KSBA) into an aqueous solution. As opposed to PANI microplates synthesized in the absence of KSBA, the PANI polymerized with KSBA exhibited tubular morphology, tens of micrometers long, with a total diameter of 130-240 nm, an inner diameter of 10-100 nm, and a wall thickness of 60-70 nm. The formation yield of PANI nanotubes was as high as about 95%, and the electrical conductivity was 4.8 × 10⁻¹ S cm⁻¹. However, the PANI microplates exhibited about 10 times lower electrical conductivity of 3.1 × 10⁻² S cm⁻¹ than PANI nanotubes. The morphologies of the final PANIs were greatly affected by the morphologies of the oligoanilines produced with or without KSBA in the early polymerization stage. In this study, KSBA was introduced to regulate the reactant pH and to control oligoaniline morphology (with KSBA: 1D long nanosheets, without KSBA: 2D microplates). Synthetic time-resolved morphology dynamics revealed that the oligoanilines play a key role as templates for the PANI nanosheets polymerized by anilinium cations. Finally, the nanosheets transform into long PANI nanotubes through conformational changes induced by the protonation of the PANI chains in a highly acidic medium. Interestingly, the final products are a simple mixture of pure PANI nanotubes and oligoaniline complexes composed of oligoanilines and aniline sulfate salts. Thus, the oligoaniline do not grow into the PANI chains but function only as templates for the 1D PANI formation.     

Reprotonation of polyaniline: A route to various conducting polymer materials

Polyaniline (PANI) is one of the most studied conducting polymers. Its properties can be modified by controlling the way of protonation. Polyaniline base was immersed in aqueous solutions of 42 inorganic or organic acids in order to find out, which is able to constitute a salt with the PANI base and what are the properties of products. The conductivity of the reprotonated PANI bases is determined especially by the pH of acid solutions. The highest conductivity, 1.22 S cm⁻¹, was found after reprotonation of PANI base with 50% tetrafluoroboric acid. The reaction with most strong inorganic acids yielded samples with a conductivity of 10⁻¹ S cm⁻¹. Sulfonic acids gave products having conductivity of the order of 10⁻²–10⁻¹ S cm⁻¹. Carboxylic acids were less efficient in protonation, and their ability to produce a conducting polymer depended on increasing the acid concentration. Acids containing an acidic hydroxyl group, like picric acid, also protonated PANI to a good level of conductivity. The lowest conductivity, 1.8 × 10⁻¹⁰ S cm⁻¹, was observed in the absence of any acid. The density of reprotonated PANI varied between 1.19 and 2.06 g cm⁻³, the contact angle between 29° and 102°, and volume change between −14% and +33%, depending of the acid used. The reprotonation of PANI base with various acids offers the opportunity to prepare materials with great variability and versatility in properties.     

H2 sorption in HCl-treated polyaniline and polypyrrole

Hydrogen sorption in conducting polymers was investigated in order to determine their potential as hydrogen storage media. The conducting polymers, polyaniline and polypyrrole, were treated with an acid, which resulted in an exceptionally high hydrogen sorption, 6 and 8 wt% at room temperature and under 9.3 MPa. Both the molecular effect and electrical effects by the conducting polymers appear to play an important role in hydrogen sorption. This paper presents the preliminary results of hydrogen sorption in a conducting polymer along with its characterization by XRD, scanning electron microscopy, TGA, and conductivity measurement using a four-probe method. A possible mechanism for the extraordinarily high hydrogen storage is suggested.     

Synthesis of graphene/vitamin C template-controlled polyaniline nanotubes composite for high performance supercapacitor electrode

A graphene nanosheet/polyaniline nanotube (GPNT) composite is prepared for the first time by in-situ chemical oxidative polymerization of aniline using vitamin C as a structure directing agent. The vitamin C molecules lead to the synthesis of polyaniline (PANI) nanotubes through the development of rod-like assembly by H-bonding in an aqueous medium. The initially synthesized graphene oxide/polyaniline nanotubes composite is reduced to graphene using hydrazine monohydrate followed by re-oxidation and protonation of the PANI to produce the GPNT nanocomposite. This novel composite showed a high specific capacitance of 534.37 F/g and an excellent energy density of 74.27 Wh/kg at a constant current of 0.5 mA. Besides, the GPNT composite exhibited excellent cycle life with 91.4% specific capacitance retained after 500 charge-discharge cycles. The excellent performance is due to the synergistic combination of graphene which provides good electrical conductivity and mechanical stability, and PANI nanofiber which deals with good redox activity.     

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.     

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.     

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.     

Enrichment of chromium-content in passive layers on stainless steel coated with polyaniline

Enrichment of chromium-content in passive layers on a 304 stainless steel (SS) by coating polyaniline (PANI) has been confirmed by means of electrochemical and X-ray photoelectron spectroscope (XPS) techniques and thus prepared oxide films demonstrated to be highly stable in aggressive environment. PANI films are deposited on SS from a sulfuric acid solution with aniline monomer. The PANI-coated SS is found to maintain the passive state in a deaerated 1 M H₂SO₄ at 45 °C for several weeks. The passive state of SS remains in the same solution for several days even after the PANI films were completely removed. The passive layers underlying the PANI are certified to have higher chromium-content compared with the air-formed and anodically-formed oxide films on SS. It is suggested that PANI films enhance the passivation of SS by increasing the Cr₂O₃ content in the passive layer and by preventing the chromic oxide from directly interacting with aggressive solution.     

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.     

Characterization of viscoelastic properties of composite films involving polyaniline and carbon nanotubes

The deposition and properties under electrochemical potential control of composite films of polyaniline and single walled carbon nanotubes were studied using high frequency acoustic wave measurements at quartz crystal resonators. Acoustic admittance spectra were used to provide shear moduli for the growing films as functions of thickness and dopant anion (sulfate, nitrate or perchlorate). The electrochemical and viscoelastic data were compared with those obtained in a parallel set of experiments in the absence of carbon nanotubes. The presence of carbon nanotubes in the deposition solution enhances the rate of polyaniline deposition by a factor of 3-5, according to the dopant. In all media, the shear modulus components increased with film thickness and in any given medium both the storage and the loss moduli were greater in the presence of carbon nanotubes. Since SEM images do not reveal the presence of the carbon nanotubes directly, they are deduced to interact sufficiently strongly with the polymer as to become fully encapsulated. A mean field model is adapted to provide predictive capability at a simplistic level for composite film viscoelastic properties in terms of the mechanical properties of the constituents. When applied to the experimental data, it suggests that the level of carbon nanotube incorporation is modest, despite its significant effect on film deposition and properties.     

Fabrication and electrochemical characterization of polyaniline nanorods modified with sulfonated carbon nanotubes for supercapacitor applications

The novel composites of sulfonated multi-walled carbon nanotubes (sMWCNTs) modified polyaniline (PANI) nanorods (PANI/sMWCNTs) were synthesized successfully by in situ oxidative polymerization method in the HClO₄ solution. FTIR and Raman spectra revealed the presence of π–π interaction between the PANI and the sulfonated carbon nanotubes and the formation of charge transfer composites. It was found that the specific capacitance of the PANI/sMWCNT composites was markedly influenced by their morphological structure and the content of PANI which was coated onto the sMWCNT. The specific capacitance of the PANI/sMWCNT composite exhibited a maximum value of 515.2 F g⁻¹ at the 76.4 wt% PANI. The charge–discharge tests showed the PANI/sMWCNT composites possessed a good cycling stability (below 10% capacity loss after 1000 cycles) compared to PANI nanorods.     

Film mechanical resonance phenomenon during electrochemical deposition of polyaniline

Crystal admittance measurements during potentiodynamic electrodeposition from aqueous HCl of polyaniline films on a thickness shear mode resonator show the phenomenon of film resonance. This special situation arises when the combination of materials properties (film shear modulus and density) and sample nature (film thickness) is such that the acoustic phase shift across the film, φ = π/2 radians. Admittance spectra in the acoustically thick (but pre-film resonance) region were fitted to a standard Butterworth van Dyke (BvD) electrical equivalent circuit (with series L, C, and R elements) to yield film storage and loss shear moduli. From these values, it was possible to calculate φ of the growing film and thereby track the onset of film resonance. As φ approached π/2 radians, the admittance peak rapidly shifted to higher frequencies and changed dramatically in amplitude. An electrical equivalent circuit with parallel L, C, and R elements was used to model film resonance data; this simplified model ignores the contribution of the bathing electrolyte. Agreement of film shear moduli from both models, and of resonant film thickness from the parallel element equivalent circuit model and coulometric extrapolation of pre-resonant responses, supports the basic tenets of the film resonance model. At film resonance, the cyclic voltammetric response changed dramatically; it is speculated that this is a consequence of the unusual polymer dynamics at film resonance.     

Determination of the specific capacitance of conducting polymer/nanotubes composite electrodes using different cell configurations

Composite materials containing 20 wt.% of multiwalled carbon nanotubes (MWNTs) and 80 wt.% of chemically formed conducting polymers (ECP) as polyaniline (PANI) and polypyrrole (PPy) have been prepared and used for supercapacitor electrodes. The well conducting properties of MWNTs and their available mesoporosity allow a good charge propagation in the composites. Moreover, due to the good resiliency of MWNTs, an excellent stability of the supercapacitor electrodes is observed. It has been shown that the capacitance values for the composites strongly depend on the cell construction. In the case of three electrode cells, extremely high values can be found from 250 to 1100 F/g, however in the two electrode cell much smaller specific capacitance values of 190 F/g for PPy/MWNTs and 360 F/g for PANI/MWNTs have been measured. It highlights the fact that only two-electrode cells allow a good estimation of materials performance in electrochemical capacitors. The applied voltage was found to be the key factor influencing the specific capacitance of nanocomposites. For operating each electrode in its optimal potential range, asymmetric capacitors have been built with PPy/MWNTs as negative and PANI/MWNTs as positive electrodes giving capacitance values of 320 F/g per electrode material.     

The effect of anion identity on the viscoelastic properties of polyaniline films during electrochemical film deposition and redox cycling

Acoustic admittance measurements at thickness shear mode resonators were used to determine shear moduli for polyaniline films during their potentiodynamic electrodeposition and subsequent redox cycling in aqueous background electrolyte. Data were acquired for films doped with perchlorate, sulphate or chloride anion. For all media, film shear moduli increased progressively with film thickness, from values consistent with a diffuse fluid-like layer to values typical of a viscoelastic material. At any given thickness, both the storage and loss moduli were largest in perchlorate medium; values in chloride and sulphate media were similar to each other, but smaller than in perchlorate. These measures of polymer dynamics are consistent with a previous classification of polyaniline film behaviour, in which perchlorate-doped films are viewed as compact while chloride- and sulphate-doped films are viewed as more open. In monomer-free background electrolyte solution, both film shear modulus components for all anions increased modestly upon film oxidation. Despite some hysteresis on the timescale of slow scan voltammetry, these variations were chemically reversible. Based on measurements involving deposition from chloride medium and transfer to sulphate medium, film shear moduli respond promptly to changes in dopant identity; this is consistent with rapid redox-driven exchange of anions with the bathing electrolyte.     

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.