Polyaniline Emeraldine Base Research Articles

Interaction of polyaniline and transition metal salts: formation of macromolecular complexes

Conditions for complex formation of some transition metal salts and emeraldine base of polyaniline were investigated, and the obtained macromolecular complexes have been characterized by the UV-Vis-IR spectroscopy. It was found that complex formation conditions are dependent on both the cation and anion of the inorganic salt used, as well as on the solvent environment. In some cases, formation of the complex occurred in the form of a precipitate directly in the solution. In other cases, no precipitate was observed in the solution, however, the complex was recognized during film casting from the polyaniline - inorganic salt solution. Electronic absorption spectra for both precipitates and films for the majority of samples possessed features characteristic of the protonated, emeraldine salt form of the polymer. However, IR absorption spectra were different from the protonated polyaniline form. Based on the obtained data, the model of the macromolecular polyaniline - transition metal salt complex is proposed.

Nonlinear electrical properties of polyaniline: role of conjugation length

Undoped polyaniline (emeraldine base) exhibits nonlinear effects at high electric field. Conductivity measurements, as a function of a) temperature and b) aging induced disorder, are presented. Current-voltage curves are modeled in a space charge limited current scheme with a field dependent mobility and a non vanishing free carrier density. All the variations of the modeling parameters (cases a and b) are accounted for by models in which transport occurs by hopping between conjugated segments of finite length. For the first time, the relationship between the conjugation length distribution and the mobility behavior is evidenced in conjugated systems.

New Methods for Determining the Molecular Weight of Polyaniline by Size Exclusion Chromatography

Aggregation of polyaniline emeraldine base dissolved in polar aprotic solvents containing lithium chloride salt results in distorted non-Gaussian elution peaks in the size exclusion chromatogram. This results in significant error for the calculation of the weight average and number average molecular weights measured by size exclusion chromatography. We compare the effectiveness of LiCl, LiBr, and LiBF 4 , for limiting polymer aggregation in a high molecular weight sample of polyaniline, and thereby improving chromatographic peak shape. Weight average and number average molecular weight values were most accurately determined for dilute polyaniline in NMP solutions containing LiBF 4 .

Mechanism and life study on polyaniline anti-corrosion coating

Solvent free polyaniline emeraldine base(EB) corrosion protection coating was prepared, employing aliphatic polyamine as solvent of EB as well as hardener of epoxy resin. This coating passed 2000h of salt fog test when the EB loading was about 1 wt%. The interaction between EB and iron indicated that EB acted as a quasi-catalyst to cause the formation of densed iron oxide film in the interface.

Infrared optics applications of thin polyaniline emeraldine base films

Abstract The use of polyaniline emeraldine base films as antireflection coating for near and middle IR optics elements was studied. The optical quality of ZnSe substrates spin-coated with thin PANI EB layers were studied using a Linnik interferometer. The spectral properties of PANI coated ZnSe plates were investigated with FTIR spectrometer. It was shown that PANI coating allows a significant decrease of Fresnel losses in the near and middle IR bands (1.0–6.25 μm). The coating allowed continuous transmission of high power density (up to 3 W/mm 2 ) of IR radiation produced by CO 2 laser. The laser irradiation damage threshold of the PANI EB coating was studied at a wavelength of 1.5 μm. Microhardness of the coated ZnSe is established as satisfactory.

Structure–electrical property study of anisotropic polyaniline films

AbstractThe relationships of the structure and electrical properties of anisotropic HCl‐doped polyaniline (PANI) films cast from N,N′‐dimethylpropylene urea (DMPU) solutions and stretched to different draw ratios were studied. The anisotropic structure of the stretched PANI films was examined by X‐ray diffraction, near‐infrared wave‐guide coupling, and polarized infrared measurements. The PANI emeraldine base (EB) films cast from DMPU solutions had a single‐phase noncrystalline structure, and stretching of the films did not cause crystallization to occur. The transition moment angles of two weakly absorbing infrared bands were determined, and the Hermans' orientation functions for the PANI EB films were calculated. The PANI films were then doped with HCl, and the electrical properties were determined by impedance spectroscopy. With a specially designed test fixture, the in‐plane and through‐plane impedance was obtained. The conductivity along the stretch direction increased with orientation. The in‐plane conductivity was significantly higher than the through‐plane conductivity. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 823–841, 2003

Synthesis and study of conductivity behaviour of blended conducting polymer films irradiated with swift heavy ions of silicon

In recent times, blended polymers have shown a lot of promise in terms of easy processability in different shapes and forms. In the present work, polyaniline emeraldine base (PANi-EB) was doped with camphor sulfonic acid (CSA) and combined with the conducting polymer polyfluorene (PF) as well as the insulating polymer polyvinyl chloride (PVC) to synthesize CSA doped PANi–PF and PANi–PVC blended polymers. It is well known that PANi when doped with CSA becomes highly conducting. However, its poor mechanical properties, such as low tensile, compressive, and flexural strength render PANi a non-ideal material to be processed for its various practical applications, such as electromagnetic shielding, anti-corrosion shielding, photolithography and microelectronic devices etc. Thus the search for polymers which are easily processable and are capable of showing high conductivity still continues. PANi–PVC blend was prepared, which showed low conductivity which is limiting factor for certain applications. Therefore, another processable polymer PF was chosen as conducting matrix. Conducting PF can be easily processed into various shapes and forms. Therefore, a blend mixture was prepared by using PANi and PF through the use of CSA as a counter ion which forms a “bridge” between the two polymeric components of the inter-polymer complex. Two blended polymers have been synthesized and investigated for their conductivity behaviour. It was observed that the blended film of CSA doped PANi–PVC showed a room temperature electrical conductivity of 2.8×10 −7 S/cm where as the blended film made by CSA doped PANi with conducting polymer PF showed a room temperature conductivity of 1.3×10 −5 S/cm. Blended films were irradiated with 100 MeV silicon ions with a view to increase their conductivity with a fluence ranging from 10 11 ions to 10 13 per cm 2 from 15 UD Pelletron accelerator at NSC, New Delhi.

Determination of physical parameters of conducting polymers by photothermal spectroscopies

Photothermal spectroscopies, photopyroelectric and photoacoustic, were used to obtain physical parameters of polyaniline–emeraldine base (PANI-EB) and poly(o-methoxyaniline)–emeraldine base (POMA-EB), such as thermic parameters like thermal diffusivity, thermal conductivity, and specific heat. These studies were done for PANI-EB and POMA-EB films in the 400<λ<900 nm wavelength range. The photopyroelectric intensity Vn(f ) and the phase Fn(f ) (f being the chopping frequency) for a given λ of the saturation part of the PPES spectrum signal were independently measured, as well as the intensity Vn(λ) and its phase Fn(λ). Equations of both the intensity and the phase of the PPES signal, taking into account the thermal and the optical characteristics of the PANI and POMA films and the pyroelectric detector, were used to fit the experimental results. It was observed that, in contrast with the strong doping dependence of the electrical conductivity, the thermal parameters of PANI films remained practically unchanged under doping. This apparent discrepancy is explained by the granular metal model of doped PANI.

Effects of confinement on the phase separation in emeraldine base polyaniline castfrom 1-methyl-2-pyrrolidinone studied via dielectric spectroscopy

Measurement in the frequency range 3 mHz–106 Hz of the dielectric characteristics ofemeraldine base polyaniline dissolved in 1-methyl-2-pyrrolidinone (NMP) andcast into bulk free-standing polymer films shows features similar to thosereported by others and which are a result of microphase separation intoreduced and oxidized repeat units. However, upon confinement into thecylindrical pores, of average diameter 20 nm, of a porous membrane suchfeatures of microphase separation do not occur. Strong pinning of the chargecarriers due to interactions of the polymer with pore walls together withconstrained chain packing and a non-uniform rate of evaporation of theNMP solvent from the pores suggests that the microphase separationobserved in the bulk polymer is suppressed. This has the potential ofbeing able to enhance the bulk conductivity after doping by reducingthe internal intrachain disorder introduced by microphase separation.

Polyaniline emeraldine base in N‐methyl‐2‐pyrrolidinone containing secondary amine additives: A rheological investigation of solutions

AbstractFrom a rheological study of emeraldine base (EB)/N‐methyl‐2‐pyrrolidinone (NMP)/2‐methyl‐aziridine (2MA) solutions, a correlation between the solution concentration and solution viscosity was found. We investigated the rheokinetic mechanism of the EB dissolution process and determined the reaction rate, activation energy, equilibrium constant, and Gibbs free energy (ΔGo) for the complexation between 2MA and EB tetrameric molecules ({EB}). The low rate constant (∼3.0 × 10−4 mol−2 L2 min−1 at 298 K) indicates that the process of EB/NMP/2MA solution formation is slow. The {EB} and 2MA molecules need approximately 76 kJ/mol energy to form the complexes, and this implies that stable bonds may need to be broken before the complexes can form. Therefore, increasing the temperature can accelerate solution formation. The equilibrium constant increases with temperature, and this indicates that EB · 2MA complexation is endothermic. A positive value of ΔGo (5.26 kJ/mol) indicates that EB · 2MA complexation is a thermodynamically unfavorable reaction; therefore, the concentrated EB/NMP/2MA solutions eventually gel. Furthermore, we find that the activation energy of EB/NMP viscous flow is 80 kJ/mol, which is about 3–4 times the energy of NH hydrogen bonding. This suggests that at least three hydrogen bonds can form between two {EB} molecules, which might be responsible for the poor solubility of EB in organic solvents. The effects of the temperature, EB concentration, and 2MA:{EB} molar ratio on the gelation process have also been investigated. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2702–2713, 2002

Anion exchange nature of emeraldine base (EB) polyaniline (PAn) and a revisit of the EB formula

The anion exchange nature of emeraldine base (EB), which is totally unexpected from the conventional EB formula, is reported here. Based on the ion exchange experiments of this work as well as published ESCA data, EB is suggested to be a mixture of undoped emeraldine with doped emeraldine, in which the counterion is hydroxide ion. This is in contrast to the commonly held concept that identifies EB as undoped emeraldine. Accordingly, a modified EB formula is proposed.

Pauli-like magnetic susceptibility in emeraldine base polyaniline

Magnetic susceptibility measurements on emeraldine base (EB) polyaniline are found to exhibit a Pauli-like behavior up to a temperature of 475 K. 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 EB are created in pairs and that the intra-pair interactions encompass a broad distribution of antiferromagnetic exchange couplings. The results on EB are further shown to have essential implications for a correct interpretation of the magnetic susceptibility data in doped conducting polymers.

Spectral and electrochemical studies on blends of polyaniline and cellulose esters

AbstractBlends of chemically prepared polyaniline emeraldine base (PANi) with cellulose esters were studied as films by UV–visible spectroscopy and cyclic voltammetry. The cellulose esters used were acetate, propionate, acetate butyrate, and acetate hydrogen phthalate. Films were prepared by casting from N‐methylpyrrolidone or formic acid, and the effect of doping by acids on their spectral and electrochemical properties was studied. Similar behavior was observed with the acetate, propionate, or acetate butyrate, with spectral changes on adding acid due to protonation of the PANi. In agreement with previous studies, kinetic measurements on PANi in a cellulose acetate matrix shows a relatively slow spectral change on protonation. In contrast, with cellulose acetate hydrogen phthalate (CAHP), no changes were observed on adding acid, and it is suggested that the hydrogen phthalate group acts as proton donor. This was mirrored by the cyclic voltammetry behavior in hydrochloric acid solution. Electrochemical studies on films of PANi/CAHP blends in different relative proportions in sulfuric acid solution show a marked dependence on the solvent used for casting, with higher currents and better electrical conductivity being observed in films prepared from N‐methylpyrrolidone. This is shown to be due to the presence of PANi particles in the films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2182–2188, 2002

Physical Stabilization or Chemical Degradation of Concentrated Solutions of Polyaniline Emeraldine Base Containing Secondary Amine Additives. Volume 35, Number 13, June 18, 2002, pp 5304−5313.

Physical Stabilization or Chemical Degradation of Concentrated Solutions of Polyaniline Emeraldine Base Containing Secondary Amine Additives. Volume 35, Number 13, June 18, 2002, pp 5304−5313.

Effect of Elevated Temperature on the Reactivity and Structure of Polyaniline

The structure and oxidation state of emeraldine base polyaniline (PANI) in N-methyl- pyrrolidinone solutions that have been heated to 120-190 °C have been characterized by solid-state NMR, UV/vis, FTIR, and GPC. The spectroscopic studies reveal that the polymer is completely converted to its fully reduced leucoemeraldine form after heating at 175-190 °C for only 2 h, and bubbling oxygen gas through the solutions after heating results in partial reoxidation of the polymer. Cross-links, consisting of phenazine structures, have been identified in PANI films cast from heated NMP solutions by 15N interrupted decoupling solid-state NMR. After heating, the polymer is partially cross-linked with the cross-link density increasing with annealing temperature, and the non-cross-linked regions are the reoxidizable portions of the polymer. A mechanism by which the cross-linking reaction of PANI leads to the polymer reduction is discussed.

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 pKa value.

Polyaniline/lignin blends: FTIR, MEV and electrochemical characterization

Abstract Blends of polyaniline (emeraldine base) and Eucalyptus grandis kraft lignin were prepared by casting method. The maximum amount of lignin that could be used for blending was 36% (w/w); beyond that, fractile films were produced. The IR spectra of the blends indicated that interactions occurred between polyaniline and lignin. Cyclic voltammetry measurements showed peaks that were readily attributed to the oxidation/reduction of polyaniline and a new oxi-reduction peak due to oxidation/reduction of sites created during interaction of two polymers. Scanning electronic microscopy showed that all blends were homogeneous.

Application of Solid-State NMR To Characterize the Interaction of Gel Inhibitors with Emeraldine Base Polyaniline

The extent and types of changes occurring to polyaniline (PANI) films cast from solutions containing a gel inhibitor were characterized by solid-state NMR (SSNMR). Two secondary amine gel inhibitors, 2-methylaziridine (2-MA) and pyrrolidine (Py), were studied and observed to have very different reactivities with the polymer. 13C and 15N SSNMR data show that pyrrolidine adds to the quinoid ring of ∼50% of the repeat units, converting these units from the emeraldine base form to the leucoemeraldine form. The presence of the pendant amine group also disrupts chain packing, increasing the amount of kilohertz regime chain motion in the solid state as determined from the magnitude of T1ρ(H). In contrast, 2-MA reacted only slightly with PANI, addition to ∼5% of the repeat units, but 0.7 2-MA/repeat unit was physically trapped in the PANI film. The greater reactivity of Py (pKb = 3) relative to 2-MA (pKb = 6) is related to the amine's nucleophilicity and orbital arrangement.

Polyaniline emeraldine base in N-methyl-2-pyrrolidinone containing secondary amine additives: B. Characterization of solutions and thin films

Secondary amines are added to emeraldine base (EB)/ N -methyl-2-pyrrolidinone (NMP) solutions to stabilize the EB solution. However, amines can interact with the EB molecule either by physical interaction, such as with the hydrogen bond (H-bond), or by chemical reaction. Several amines with different basicity and geometry were investigated. The UV–VIS–NIR, FT-IR, GPC, and TGA results reveal that secondary amines, due to their different basicity and molecular size, can chemically degrade EB in two different ways. Secondary amines with strong basicity and small size, such as azetidine (AZ) and pyrrolidine (PY), attack both the quinoid ring and the NC site in the EB backbone immediately. The attacked EB not only loses its conjugated structure due to ring substitution, but also loses its molecular weight because the chain breaks down. Secondary amines with moderate or strong basicity and large geometry, such as heptamethyleneimine (HPMI), reduce most of the quinoid ring to form a ring-substituted material. The final product loses its conjugated structure, but its molecular weight does not appreciably change. However, amines with weak basicity, such as 2-methyl-aziridine (2MA), form H-bonds with EB but do not appreciably degrade the EB structure.

Physical Stabilization or Chemical Degradation of Concentrated Solutions of Polyaniline Emeraldine Base Containing Secondary Amine Additives

In this study, we systematically investigated 39 secondary amines as additives in concentrated EB/NMP solutions for gelation and degradation. We discovered that when both the width (defined as the longest distance between two hydrogens in the plane perpendicular to the NH bond of the amine) and depth (defined as the longest distance between two atoms in a plane perpendicular to the width) of the amines are <4.53 Å and their pKa is >7.7, the amines significantly extend the gelation times of 20 mass % EB/NMP solutions for more than 12 h. However, some of these amines also significantly degrade the polymer. Amines with small width and depth and strong basicity, such as azetidine and pyrrolidine, can significantly destroy the EB structures. This was evidenced by order-of-magnitude decreases in doped film conductivity, by significantly changed UV−vis spectra, and by significantly reduced molecular weights of the aged EB solutions as measured by gel permeation chromatography (GPC). However, when both the width and depth of amines are >4.53 Å, these amines neither prolong gelation time nor appreciably degrade EB.