Direct involvement of acid centers of polyaniline in charge transfer on organic acceptor

Electrochemical behavior of hydroquinone and 4F-hydroquinone has been studied on Pt/polyaniline (PANI) electrodes in 1 M H₂SO₄ solution. Redox transformation of quinone/hydroquinone systems takes place on PANI practically without overpotential and, probably, the two-electron charge transfer processes are realized in such systems. Reaction currents depend on PANI quantities deposited on the Pt electrode and the molecular complex is formed. There exists a great electronic exchange between acid centers of PANI and quinone/hydroquinone molecules, resulting in electronic conjugation of this surface state (adsorbed molecule) with the polymer. Existence of such complexes was confirmed by FT-IR analysis which has been carried out on free-standing polymer films. Based on these findings we have assumed that the charge transfer complexes form along the polymer chain and the acceptor molecules participate in the doping and conductivity processes.     

Protonation and charge transfer in polyaniline: an optical absorption study of the mixed solutions

Interaction of polyaniline (PANI) with a protonating dopant, dodecylbenzenesulfonic acid (DBSA), and an organic electron acceptor, pentadecyltetracyanoquinodimethane (C₁₅TCNQ), is studied in DMSO solutions, using optical absorption spectroscopy. Each of the two agents alone induces an absorption band at 417 nm, attributable to cation-radicals of PANI. Free carrier absorbance in the near-infrared is absent in the case of C₁₅TCNQ. Moreover, the charge-transfer interaction between PANI and C₁₅TCNQ suppresses a transition to the conductive state. When PANI/C₁₅TCNQ mixtures are aged, a new band near 500 nm appears that can be attributed to an adduct of PANI and C₁₅TCNQ.     

Magnetic behavior of polyaniline doped with oxidized ferrocenesulfonic acid

Ferrocenesulfonic acid (FSA) was oxidized by iodine (I₂) and then used as dopant for polyaniline emeraldine base. The resulted polyaniline showed an electrical conductivity of 4.50 × 10⁻² S/cm and a ferromagnetic interaction with a positive Weiss constant of 15 K, the magnetic behavior is attributed to the ferromagnetic coupling between ferrocenium cations in the counter-ions along the polyaniline chain.     

Synthesis and characterization of organo-soluble conducting polyaniline doped with oleic acid

An emulsion polymerization process has been developed for the direct synthesis of the organo-soluble polyaniline (PANI) with oleic acid (OA) as the surfactant and dopant. The polyaniline doped with oleic acid (PANI/OA) powders, prepared by the proposed method, are highly soluble in many organic solvents such as dimethyl sulfoxide (DMSO), dimethyformamide (DMF) and N-methyl,2-pyrrolidinone (NMP). The PANI/OA powders were characterized by elemental analysis (EA), FTIR, UV–vis, TGA and SEM. The conductivities of the PANI/OA powders decreased with the increasing of the amounts of water in the emulsion polymerization.     

EPR and electrical conductivity in microporous polyaniline

We have studied a series of microporous polyaniline (PANI) samples, each of the structure with micro-channels and micro-caves inside (a diameter of 1.5–2.5 μm), formed as a non-periodic network of microrods. Microporous polyaniline is a paramagnetic material and strong EPR signals with the g-factor in the range of 2.0030–2.0032 and the line width ΔB between 1.80 and 6.50 Gs have been measured for samples of various electrical conductivity. The EPR line asymmetry is high (almost 1.5) in the case of PANI samples of the highest electrical conductivity (0.35 S/cm).We found that lithium cations present in the reaction medium can stabilise values of the EPR line asymmetry parameter and g-factor observed for PANI samples prepared in such conditions. Lithium cations are able to influence the intermolecular hydrogen bonding which is essential for the interchain coupling leading to an increase in the EPR line asymmetry and a higher electrical conductivity. Thus, the properties of PANI samples are modified by lithium cations.     

Structure and properties of selenious acid doped polyaniline with varied dopant content

In this study, the effects of dopant content on the structure, especially crystalline structure, and properties of selenious acid doped polyaniline (PAni) were investigated. The structure and properties of PAni were characterized by X-ray diffraction, FTIR spectroscopy, UV–vis spectroscopy, DSC, and TGA. By varying the dopant/monomer molar ratio (D/M), the morphology as well as the crystalline structure of PAni was significantly changed. The presence of water in the crystalline structure of PAni prepared with a low D/M ratio caused a change in the d-spacing of the PAni crystalline structure. The oxidation level and doping degree of PAni were also changed by varying the D/M ratio. The electrical conductivity of PAni increased with increasing D/M ratio, and the thermal stability of PAni in the doped state was about 200 °C for different D/M ratios.     

Structural investigation of lignosulfonate doped polyaniline

Lignosulfonate doped polyaniline (LGS-PANI) was prepared in the presence of sulfamic acid as a novel conducting filler. The room temperature conductivity of LGS-PANI increases more than two-fold with LGS at a mass fraction of 11.4 wt% and these conductivities (0.73–5.00 S/cm) exceed the highest reported for polymer acid-doped PANI systems. The conductivity stability and thermal stability are clearly improved at a certain content of LGS in LGS-PANI. These trends are accompanied by significant structural changes as evidenced by X-ray diffraction studies. The material characterization was also carried out by Fourier transform IR spectroscopy, thermogravimetric analysis and scanning electron microscopy.     

Preparation and electrical–magnetic properties of polyaniline doped with ionic ferrocenesulfonic acid

Conducting polyaniline with electrical conductivity of 2.34 × 10⁻¹ S cm⁻¹ was obtained using ferrocenesulfonic acid as dopant. After the ferrocenesulfonic acid was oxidized with FeCl₃, though the electrical conductivity of the doped polyaniline decreased by 1–2 orders of magnitude, the magnetic susceptibility (χ) increased with the increase of the oxidation degree of ferrocenesulfonic acid. EPR spectra showed not only a signal with a g value of around 2, but also a so-called half-field signal with a g value of about 4 even at room temperature. Coexistence of ferromagnetic intrachain interactions and antiferromagnetic interchain interactions in the materials has been suggested.     

The study of electrical and magnetic properties of LiPF6 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 (LiPF₆), PAN-LiPF₆. DC conductivity (σ_dc) and its temperature dependence of PAN-LiPF₆ 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-LiPF₆ 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 LiPF₆ doping process, indicating the formation of polarons. However, the existence of some portions of quinoid rings in PAN-LiPF₆ samples after doping suggests an inhomogeneous doping or a pseudo doping.     

EPR studies of blends of polyaniline with poly(methyl methacrylate-co-glycidyl methacrylate iminodiacetic acid)

A novel conductive blends of polyaniline (PANI) with poly(methyl methacrylate-co-glycidyl methacrylate iminodiacetic acid) (PANI–PMGI) was prepared by in situ dispersion polymerization. The PANI–PMGI blends were characterized by UV–vis, FTIR and electron paramagnetic resonance (EPR) spectra. The structure of the PANI–PMGI blends was similar to emeraldine salt proved by UV–vis and FTIR. The value of ΔH_pp, lineshape, g factor, N_s and A/B ratio of blends were investigated by EPR. The results of EPR indicated that the intermolecular interaction between PANI and PMGI was dependent on the content of PANI and temperature.     

Characterization of polyaniline doped with diphenyl phosphate

UV–Vis spectroscopy and conductivity of the polyaniline (PANI) doped with diphenyl phosphate (DPHP), in comparison with that of HCl-doped polyaniline, show that the decrease in polaron delocalization and structural order results from the steric hindrance imparted by the longer chain counter ion. PANI protonated with DPHP is soluble in common organic solvents such as toluene, chloroform, dimethyl sulfoxide, etc. The conductivity of PANI/DPHP pressed pellet (at 25°C, 1.66 S cm⁻¹) increases with temperature from −40°C (1.18 S cm⁻¹) to +140°C (3.07 S cm⁻¹) due to thermal activation, and decreases with temperature from 140 to 180°C (0.25 S cm⁻¹) due to thermal undoping accompanied by the loss of some polarons. The temperature dependence of spin density is consistent with those of conductivity. After the heating scan, the conductivity (0.52 S cm⁻¹) at room temperature reduces to nearly one-third of the original value.     

Comparison of electronic transport properties of soluble polypyrrole and soluble polyaniline doped with dodecylbenzene-sulfonic acid

The temperature dependences of the conductivity of soluble polypyrrole (PPy) and soluble polyaniline (PANI) films doped with dodecylbenzene-sulfonic acid (DBSA) have been studied. The room temperature conductivities of samples were 1–8 and 95–100 S/cm, with resistivity ratio ρ_r=ρ(20 K)/ρ(300 K)∼10⁴ and ρ_r=ρ(1.4 K)/ρ(300 K)∼32 for PPy-DBSA and PANI-DBSA, respectively. At low temperatures, the temperature dependence follows the σ(T)=σ₀exp[−(T₀/T)^m] law, which has been explained within the framework of granular metals model (m=0.5) for PPy-DBSA and Mott's variable range hopping model (m=0.25) for PANI-DBSA. The magnetoresistance of PANI-DBSA is positive and follows H² and H^1/3 dependencies at low and high magnetic fields, respectively; the magnitude of the magnetoresistance is strongly temperature dependent.     

Water-borne conductive polyaniline doped by acidic phosphate ester containing polysilsesquioxane precursor

Acidic phosphate ester (APES) bearing polysilsesquioxane precursor was prepared employing the ring-opening reaction of (3-glycidoxypropyl)trimethoxysilane (GPTMS) with acidic phosphate monoester carrying long hydrophilic tail. Doped with APES, polyaniline emeraldine base (PANI) was successfully dissolved or dispersed in water, and a free-standing film with electrical conductivity as high as 0.05 S/cm was available. The obtained film showed water-resistance due to the confinement from inorganic networks of amorphous polysilsesquioxane characterized by ²⁹Si-MAS NMR spectrum. When the conductive film was soaked in water for 14 days, its electrical conductivity showed a little decline at the first stage of 6 days, and a considerably long platform existed at the following stage. However, for the film without polysilsesquioxane network, the electrical conductivity showed a sharp decline, and the film was broken into pieces for only 1 day soaking. Covalently linked with inorganic network of polysilsesquioxane by POC bond, thermal stability of the conductive polyaniline was also improved compared with that of the conductive hybrid film from physical blending.     

The fractal structures in highly sulfonated polyaniline

Using transmission electron microscope (TEM), the fractal structures have been found both in the colloid aggregates and in the colloidal particles of highly sulfonated polyaniline. In the present paper, the concept and mathematics model of fractal have been introduced into conjugated polymer. The colloid aggregates appear as an open dendriform structure with fractal dimension 1.43 and its growing process can be simulated by the Diffusion Limited Cluster Aggregate (DLCA) model. The simulated pattern by computer and the theoretic fractal dimension are consistent with the experimental results. Since the colloidal particle is generated in the evenness field formed by the dispersion media, the screening effect has been weakened and the colloidal particle is a spherical structure with fractal dimension 1.86, which is denser than colloid aggregates. The Random Rain (RR) model can be applied here and the simulation pattern and fractal dimension are coincident with the experiment results. So the system can be accounted as multi-fractals structure.     

Anticorrosion pigments with a shell of doped polyaniline

It has been determined that a dopant acid affects the dispersion composition of the prepared coreshell pigments. Anticorrosion properties of the pigments with the polyaniline (PAN) shell have been investigated. Pigments with a PAN shell doped by phosphoric acid possess the highest anticorrosion properties. The dependence of the electrical conductivity on the substrate-to-PAN ratio has been determined. Anticorrosion properties of polymer coatings containing the pigments with PAN shell have been studied.     

Magnetic properties of polyaniline doped with FeCl3

Magnetic properties of polyaniline (PANI) doped with FeCl₃ were investigated as a function of temperature and magnetic field. The temperature dependence of the magnetic susceptibility (χ) of PANI doped with FeCl₃ exhibits dominant paramagnetic component (well described by Curie–Weiss law with a negative Curie–Weiss temperature Θ) accompanied by a small temperature-independent paramagnetic contribution. Magnetization measured as the function of magnetic field shows smaller saturation effect than expected for non-interacting iron ions. These two observations suggest weak antiferromagnetic (AF) interactions between Fe ions.     

Electrochemical behavior of mild steel coated by polyaniline doped with organic sulfonic acids

A comparison of electrochemical behavior was made of coatings of pure undoped PANI cast from solution and this coating after doping as a film on the substrate. These were also compared to coatings of pure PANI/sulfonic acids adducts formed from the outset in the doped state through casting from xylene or chloroform solutions. The results obtained allowed to assume that the formation of protective iron oxide layers occurs mainly during the coating of pure undoped PANI (emeraldine base). During the doping stage a modification of these layers obviously proceeds leading to a change of the corrosion currents of the whole coating system. As a consequence, the formation of pure doped PANI coatings by casting doped PANI from solutions cannot give as good of results as does the two stage process.     

Corrosion inhibition of aluminum alloy in chloride mediums by undoped and doped forms of polyaniline

Corrosion inhibition of Al 3003 alloy by pure undoped PANI (emeraldine base) and PANI doped with p-toluene-sulfonic, camphorsulfonic and dodecylbenzenesulfonic acids was investigated. Corrosion resisting properties of PANI coatings with an artificially created hole defect were evaluated by electrochemical impedance spectroscopy (EIS) in aqueous 3.5% NaCl and 0.1 N HCl solutions. The highest corrosion inhibition factor was obtained for undoped PANI being equal to 12 and 4.4 in neutral and acidic media, respectively. The results indicated that corrosion protection of the bare aluminum alloy surface resulting from a defect of the PANI coating is in line with an increase of the thickness of the oxide layer protecting the aluminum alloy surface. The efficiency of corrosion protection of mild steel and aluminum alloy by polyaniline coatings was compared.