Polyaniline Samples Research Articles

Conducting Polymers and Corrosion: Polyaniline on Steel

Abstract Polyaniline-coated steel panels were studied by electrochemical impedance spectroscopy (EIS) and electrochemical noise methods (ENM). EIS data for the polyaniline sample was obtained for increasing time of immersion. An increase in charge-transfer resistance (Rct) with immersion time was observed. ENM data showed that active electrochemical changes occurred during the early stages of immersion. The mean current from ENM exhibited a rather large oscillatory behavior during early stages of immersion, and the mean potential from ENM displayed a tendency toward positive values. Noise resistance (Rn) values also showed initial oscillatory fluctuations, with values that reflected a poor barrier property for polyaniline coating. Tafel plots showed a lower corrosion rate and a more noble corrosion potential for the polyaniline-coated sample as compared to a bare steel sample. Electrochemical data confirmed that significant interactions between the polyaniline and steel occurred during the first 5 days of...

X-ray absorption spectroscopy of iron-doped conducting polymers

X-ray absorption spectroscopy (XAS) measurements were performed at the Fe K edge to determine the iron local structure in chemically prepared polyaniline (PANI) and polypyrrole (PPy) samples prepared with FeCl 3 as an oxidant. The samples were conditioned at different pHs by an acid–base treatment. In both as synthesized-doped polymers, the observed dispersed Fe atoms were predominantly coordinated to chlorine as Fe(III)Cl x species, where x=6 for PANI and x=4 for PPy samples suggesting that the polymer–counteranion bonding is weaker than the iron–chelate one. For the PANI samples submitted to basic treatment, three different sets of Fe distances were found: five Fe–O at 2.00 Å, three Fe–Fe at approximately 2.80 Å and five Fe–Fe at 3.00 Å and the near-edge spectra showed the presence of octahedrally coordinated Fe +3. These results strongly suggest the presence of small oxide/hydroxide aggregates. Similar data were obtained for PPy treated with NH 4OH.

The study of electrical and magnetic properties of LiPF 6 doped polyaniline

The results of temperature dependent dc conductivity, EPR magnetic susceptibility, and X-ray photoelectron spectroscopy experiments (XPS) are reported for polyaniline (PAN) doped with lithium hexafluorophosphate (LiPF 6), PAN-LiPF 6. DC conductivity ( σ dc) and its temperature dependence of PAN-LiPF 6 samples vary with the average molecular weight (Mw) of polyaniline used and the composition of doping solution. The EPR linewidth and Pauli susceptibility ( χ P) depend on the Mw of PAN used. The σ dc and the χ P of the intermediate Mw PAN-LiPF 6 samples are higher than those of the low Mw PAN samples, indicating more highly conducting state of the intermediate Mw PAN samples. Based on the results of XPS and EPR experiments, some portions of quinoid rings in emeraldine base form of PAN are transformed into benzenoid rings through LiPF 6 doping process, indicating the formation of polarons. However, the existence of some portions of quinoid rings in PAN-LiPF 6 samples after doping suggests an inhomogeneous doping or a pseudo doping.

ZnCl 2-induced changes in the electronic properties of polyaniline—HCl salt and its base

The effects of ZnCl 2 on the electronic properties of polyaniline—HCl salt and its base were studied for different concentrations of ZnCl 2 (0.1, 1.0, 3.0 and 6.0 M) for 1 and 5 days. The polyaniline samples were characterized by conductivity and spectroscopic measurements such as UV-Vis, FT-IR and EPR. ZnCl 2 treatment results in the degradation of the polyaniline salt and, for the polyaniline base, doping occurs depending on the concentration of ZnCl 2. The conversion of polyaniline base to polyaniline salt is less efficient in the case of ZnCl 2 doping when compared to doping with HCl. There is no significant change in the properties of polyaniline salt and base with increase in duration of ZnCl 2 treatment. The poor charge storage capacity and shelf-life of the polyaniline—zinc cell system can be understood from the present study.

Pressure dependence of the resistance in polyaniline and poly( o-toluidine) at room temperature

The electrical resistances of powdered samples of polyaniline and poly( o-toluidine) were measured as a function of pressure using a diamond anvil cell up to 22 GPa at room temperature. Since the primary influence of pressure is sample compression, application of pressure leads to an initial decrease in the resistance (enhanced conductivity) of both samples, indicating greater interchain coherence due to reduced interchain separation. Poly( o-toluidine) is seen to have higher resistance values compared with polyaniline for all pressure values recorded, consistent with the fact that it has greater disorder due to the substitution of the larger methyl group on the polymer backbone. Resistance versus pressure measurements recorded during the loading and unloading cycle show asymmetry, suggesting irreversible structural changes in both samples. The crystallinity in poly( o-toluidine) as deduced from our resistance-pressure data is seen to be slightly less than that for polyaniline.

X-ray photoelectron spectroscopic studies of conducting polyaniline, poly bisphenol A carbonate and electrochemically synthesized composite of the two

X-ray photoelectron studies of electrochemically prepared conducting polyaniline (PAN), polycarbonate (PC) and PAN/PC composite films have been studied. The core level spectra of carbon, oxygen, nitrogen and chlorine have been synthesized using Gaussian distribution functions. The analysis of the first two elements indicates the presence of carbonyl group on the surface of PAN samples. The detailed analysis of nitrogen and chlorine spectra show the presence of NCI bonding apart from usual covalent peaks. No major structural modification in the composite film could be detected. Only the presence of covalent chlorine derived from chloroform used to prepare these films have been observed. From the measurement of electrical conductivity of these films it seems that it is the proportion of ionic chlorine, rather than covalent chlorine to nitrogen that is important in determining the conductivities.

Broadband dielectric spectroscopy of conducting polyaniline

Dielectric spectroscopy can be used to probe both the conduction and polarisation processes of a wide range of materials. The authors focus on the dielectric properties of conducting polymers over the frequency range 1 Hz to 20 GHz. Initial measurements have been made on polyaniline samples at room temperature. This particular conducting polymer is of interest since it is a mixed conductor (i.e. showing both electronic and ionic transport). Analysis of the frequency dependent complex permittivity can be used to indicate free charge conduction, charge hopping or ionic diffusion. Temperature dependent measurements are required to determine the type of hopping mechanism where such a process is observed. Experimental results can be compared to the range of theoretical models that are available from the literature describing these processes (Nalwa, 1997; Mott and Davies, 1979; and Sheng, 1980).

Studies on the generation of polyaniline microstructures using microemulsion polymerization

Molecular self-assembly has the potential to enable the fabrication of technologically useful polymer microstructures [1±8]. As part of our research programme [9±11] to design methods of controlling the microstructural evolution in conjugated polymers, we report here the synthesis conditions for well-de®ned polyaniline (PANI) microstructures using surfactant assemblies (in microemulsions) as structure-directing ` templates''. It is well known [12] that amphiphiles assume different aggregate shapes depending on the microemulsion=micellar composition, expressed through water solubilization factor, w0 and packing parameter, p [13, 14]. We have used this concept recently to demonstrate its ability to improve crystalline order and well-de®ned microstructures [9±11], and now show here the possible mechanism of evolution of PANI microstructures obtained via controlled microemulsion polymerization [10]. Prior to chemical polymerization, aniline was distilled twice. Anionic surfactants, sodium dodecylsulphate (SDS) and sodium bis-2-ethylhexylsulphosuccinate (aerosol OT or AOT) were used as-received from Sigma. PANI was chemically synthesized by mixing two separate parts of water-in-oil (w=o) microemulsion, containing monomer and oxidant, respectively, and microemulsions were formulated taking w0 as a variable for every polymerization reaction to study its in uence on the polymer microstructures. Table I shows four different microemulsion systems using anionic surfactants, which lead to the formation of different microstructures. On examination of Fourier transform infrared (FTIR) spectra of PANI samples, it was found that PANI particles are surfactant-stabilized [11]. We worked on the premise that the formation of these microstructures is the result of adsorption of the cationic ` nascent'' polyaniline chain to the anionic surfactant aggregates. Thus, electrostatic attraction undoubtedly contributes to the adsorptive interaction between the surfactant microstructures and the nascent polymer. We have reported [10] earlier the scanning electron micrographs (SEM) of needles and akes of chemically synthesized PANI powders. Table II shows the in uence of water solubilization factor w0 on the evolution of microstructures. Water solubilization factor or ` water-pool'' ratio w0 is

Kinetic analysis of the thermal decomposition of polyaniline

Polyaniline (PANI) in its emeraldine base (insulating) and salt (conducting) form was investigated by thermogravimetry (TGA) in inert and oxidative atmosphere. Typical two and three step decomposition profiles were obtained. On the basis of isoconversional analysis by the method of Friedman and Ozawa–Flynn–Wall it was found that for all the PANI samples the value of the activation energy ( E) is not constant during the degradation and therefore cannot be applied for the description of the overall decomposition process. Further kinetic studies revealed that the model function, describing the mechanism of decomposition, is either of a n-th order reaction, or of Jander's diffusion type. This leads to the conclusion that the degradation mechanism strongly depends on the experimental conditions and structure of the PANI form. The obtained kinetic parameters in the form of E, the pre-exponential factor A and the model function f( α) allow a prediction of the polymer's thermal behaviour in an extrapolated range of the degree of conversion ( α), time ( t) and temperature ( T).

Inelastic neutron scattering studies of polyanilines and partially deuterated analogues

Inelastic neutron scattering spectra have been measured from 16 to 4000 cm −1 for various polyaniline samples at 30 K: the emeraldine-base and the emeraldine-salt, the ring-deuterated analogues and their hydrated forms. The spectra of the totally hydrogenated samples are dominated by bands due to protons bound to the aromatic rings. The spectrum of the ring-deuterated base reveals that most of the remaining protons are not bound to nitrogen atoms. There is a continuum of intensity due to the recoil of free particles with mass 1 amu. In the emeraldine-salt additional protons (H +) are trapped in very shallow potential-wells with dissociation threshold ≈ 300 cm −1. At energy transfer greater than this threshold, these protons are free to recoil. The spectra of the hydrated samples reveal that free entities are not trapped by water molecules. These new dynamics are tentatively related to the electronic structure and conductivity of these polymers. These are supposed to be determined by the position of the electronic state of “H 0” entities relatively to the half-filled π-band of the metal-like form of the pemigraniline-base. In the emeraldine-base electrons transferred from “H 0” entities to the conduction band give an insulator with a totally-filled band and a gas of H + entities. In the emeraldine-salt the electronic state of “H 0” entities is lowered. The structure is that of a metal-like half-filled band with a gas of “H 0” entities and weakly bound protons.

Coherent and dissipative dc transport in quasi-one-dimensional systems of coupled polyaniline chains

We present numerical studies on the conductance of coupled polyaniline chains. Our investigation is based on a tight-binding Hamiltonian, which comprises the description of the single polymer chains as well as interchain interactions. Phase-breaking processes are included in the Hamiltonian via imaginary self-energy corrections within the Green's-function formalism describing the system. The variation of these self-energies allows one to describe the transport over the full range in between the coherent and dissipative regimes. In the coherent limit we observe a transition from exponential to power-law localization upon increasing the size of the cross section of the quasi-one-dimensional systems. This behavior is all the more pronounced, the smaller the disorder in the specimen is. Upon introducing dissipation into the system we can identify metallic, insulating, and critical samples of polyaniline. These observations are consistent with recent experimental results, which characterize polyaniline-camphor sulfonic acid as a system at the metal-insulator transition.

Structural aspect of metal-insulator transition in doped conducting polymers

Partially crystalline structure of doped polypyrrole and polyaniline is investigated by the X-ray diffraction experiment and compared with the results from transport measurements. The increase of crystallinity along the direction of polymer chain in pyrrole-hexafluorophosphate (PPy-PF 6) film indicates that intrachain disorder plays a critical role in transport in the metallic regime of the metal-insulator ( M-I) transition. The X-ray structure of doped polyaniline sample in the insulating regime close to the M-I transition boundary suggests that the temperature dependence of electrical resistivity is related to the geometry of aligned polymer chains in crystalline region. In the far insulating regime, no clear relation to transport data is observed in the microscopic crystalline structure possibly due to a strong disorder from large scale inhomogeneity

Conducting properties of polyaniline blends

We report on our dc conductivity measurements of mixed samples of polyaniline in the conducting form blended with polyaniline in the insulating form and made conducting subsequent to processing. Emeraldine base polyaniline (PANEB) dissolved in N-methylpyrrolidinone (NMP) and emeraldine base polyaniline doped with camphor sulfonic acid (CSA) but dissolved in NMP [and also in dimethyl sulfoxide (DMSO)] were mixed in various ratios according to the symbolic notation (PANCSA-NMP)1−x(PANEB-NMP)x and (PANCSA-DMSO)1−x(PANEB-NMP)x, respectively. Films obtained were then equilibrated in 1 M HCl for a period of 40–60 h and vacuum dried overnight prior to performing conductivity measurements. It is shown that the room temperature conductivity increases with increasing values of x.

Cyclic voltammetry of KI at polyaniline-filmed Pt electrodes Part I: Formation of polyaniline-iodine charge transfer complexes

Cyclic voltammetry of KI at high concentrations was studied on Pt and polyaniline-modified Pt electrodes in 0.1 m HClO4 solution. The oxidation of iodide to iodine proceeded at a polyaniline-filmed Pt electrode with a mechanism similar to that at a bare Pt electrode. New peaks appeared in the cyclic voltammograms on the polyaniline-filmed Pt electrodes, depending on the experimental conditions, due to interactions of iodine species with the polyaniline chains. XPS analysis of polyaniline samples doped with iodine during, or after, the synthesis of polyaniline provided further evidence of the existence of polyaniline-iodine charge transfer complexes.

Vibrational Properties of PolyanilineIsotope Effects

The effect of polyaniline deuteration on the Raman spectra of this polymer has been studied in detail. Four types of samples have been prepared: nondeuterated polyaniline, polyaniline deuterated selectively on the ring, polyaniline deuterated selectively on nitrogen, and polyaniline deuterated both on the ring and on the nitrogen. Selective deuteration allowed for experimental verification of the Raman modes assignment proposed for different types of polyaniline. In particular, the contribution of C−H bending to C−C stretchings in benzoid and quinoid rings of the polyaniline chain has been confirmed. In addition, selective deuteration enabled us to identify, for the first time in polyaniline, the band associated with N−H bending deformations. Raman spectroelectrochemical investigations of the four above-mentioned types of polyaniline samples gave evidence that in protonated polyemeraldine two types of structures coexist, namely, semiquinone radical cation (polaronic lattice) and quinoid dication (bipolaronic). The vibrations of the former are in resonance with the energy of the blue (457.9 nm) excitation line, whereas the vibrations of the latter undergo resonance enhancement if the red (676.4 nm) line is used. The calculated and experimentally observed Raman modes for both polaronic and bipolaron structures are in very good agreement in all four cases of differently deuterated polyaniline.

Transport studies in H3PO4-doped polyaniline.

An experimental study of the temperature and frequency dependence (in the audio and rf and microwave regions) of the conductivity of ${\mathrm{H}}_{3}$${\mathrm{PO}}_{4}$-doped polyaniline samples is presented. The dopant concentration of the samples varies between 0.09M and 3.60M. For all the samples studied, the temperature dependence of the dc conductivity is consistent with the expression for Mott's variable-range hopping. On the other hand, in the temperature range 88 \ensuremath{\le}T\ensuremath{\le}298 K and in the audio frequency range 20 Hz \ensuremath{\le}f\ensuremath{\le}1 MHz, we remark that the ac conductivity (${\mathrm{\ensuremath{\sigma}}}_{\mathrm{ac}}$) does not vary much with frequency. From our wide-band measurement in the microwave region, however, we find that, for the 0.09M-doped sample, ${\mathrm{\ensuremath{\sigma}}}_{\mathrm{ac}}$ varies as ${\mathrm{\ensuremath{\omega}}}^{\mathit{s}}$ with s\ensuremath{\simeq}0.6; using Mott's model for the hopping conduction in the ac regime, we find that this value of s yields a reasonable estimation for the hopping probability: ${\ensuremath{\nu}}_{\mathit{p}\mathit{h}}$\ensuremath{\sim}6.92\ifmmode\times\else\texttimes\fi{}${10}^{13}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$.

Electropreparation of polyaniline in the presence of anhydrous cuprous ions in acetonitrile

Abstract The effect of anhydrous cuprous ions on the preparation and properties of polyaniline was investigated. It was observed that the presence of cuprous ions accelerates the formation of polyaniline films. The dry conductivity of the film was also improved. The polyaniline samples of 10 S cm −1 conductivity were obtained when Cu 2+ and H + ions are electrogenerated concurrently during the electro-oxidation of aniline. A mechanism related to the role of cuprous ions which involves chemical and electrochemical oxidation steps is proposed.

Evolution of the conducting state of polyaniline from localized to mesoscopic metallic to intrinsic metallic regimes

The results of microwave dielectric constant ( ϵ mw ) and dc conductivity ( σ dc ) of doped polyaniline (PAN) samples in different charge delocation regimes are reported. With camphor culfonic acid doped PAN prepared in m-cresol solvent (PAN-CSA ( m-cresol)) respresenting the most delocalized material, a negative ϵ mw is measured indicating an intrinsic metallic state. The Drude model at low frequency is used to estimate a relatively small plasma frequency ω p ⋍ 0.015 eV and an anomalously long scattering time τ ⋍ 1.2 × 10 −11 s. This τ implies the importance of phonon back scattering for the most delocalized electrons as expected for the open Fermi surface of a highly anisotropic metal. Depending on the crystallinity, there are two different charge localization regimes in PAN samples: uncoupled metallic islands and coupled “mesoscopic” metallic state. The three-dimensional nature of metallic state is demonstrated by the correlation of low temperature dielectric response with the parallel and perpendicular crystalline domain lengths obtained from x-ray data. The charge localization to delocalization transition dependence on the polymer processing will be discussed. We present the temperature dependent nonmetal-metal transition for some preparations of PAN-CSA ( m-cresol). The intrinsic conductivity of metallic PAN is estimated to be ∼ 10 7 S/cm.

Electric relaxation in chemically synthesized polyaniline: study using electric modulus formalism

A method of dielectric spectroscopy was used to determine the electric properties of chemically synthesized polyaniline (PANI) with special emphasis on the influence of acid and water doping levels on the polymer properties. The dependence of the dielectric loss factor (tan δ) on the temperature (in the range from − 100 to + 100°C) and the frequencies of the ac voltage ( F = 0.1–30 000 Hz; amplitude of the voltage 1 V) were measured. They were subsequently transformed into the temperature and frequency dependences of a complex electric modulus, M∗. Tan δ( F, T) and M( F, T)) presentations were both used in the analysis of the electric properties of PANI samples to compare their informative possibilities. At least two modes of the polymer response to varying temperature and frequency were resolved. Each mode corresponds to a certain area in an electric map of the system (log F versus 1/ T plane) with their positions displaced towards higher frequencies and lower temperature with increases of both acid and water concentration in PANI. This effect is interpreted in terms of a unified mechanism of interaction of water and acid molecules with polymer backbone.

Temperature effect on conducting polyaniline salts: Thermal and spectral studies

AbstractFive different polyaniline salts have been prepared by chemical polymerization of aniline in aqueous solution of different acids. Polyaniline samples have been heat treated at four different temperatures (150, 200, 275, and 375°C) and characterized by electron paramagnetic, electronic absorption, and infrared spectral measurements. Thermal stabilities of the chemically synthesized polyaniline salts have been studied by thermal analysis and spectral methods. Polyaniline salts undergo a three‐step weight‐loss process in the heating cycle. The first step (up to 110°C) corresponds to the loss of water molecules from the polymer chain. In the second step (110–275°C), a small amount of acid escapes as volatile gas, and after 275°C the polymer undergoes oxidative thermal degradation in the third step. It was found that thermal stability of polyaniline salts depends on the counteranion used and the polymer is apparently stable up to 250°C. No structural changes have taken place up to 200°C and this has been confirmed from infrared and electronic absorption spectra. No definite correlation exists between conductivity and spin concentration. © 1994 John Wiley & Sons, Inc.