Correlation between mobility enhancement and conformational change in polyaniline and its derivatives: Polaron lattice formation

Conformational changes in polyaniline, poly(methylaniline), and poly(methoxyaniline) are studied in perchloric acid and acetonitrile by means of an in situ reflection technique with oxidation level as a parameter. Stretching of polymer chains leading to enhancement of specular reflection at the polymer-coated electrodes is found to be well correlated with the increase in carrier mobility. The concomitant change in mobility and chain conformation observed with the polyaniline family is explained in terms of the formation of polaron lattice at increased oxidation levels.     

Ion, solvent and polymer dynamics in polyaniline conducting polymer films

We describe the application of a new model for the visualization of mobile species (ion and solvent) transfers accompanying redox switching of electroactive films. The system studied was polyaniline, for which the mobile species population changes were determined from redox-driven film mass changes using a thickness shear mode acoustic wave resonator. Acoustic admittance data were used to establish conditions under which the resonator frequency response could be interpreted gravimetrically. Charge and frequency changes accompanying the first redox transition of polyaniline films exposed to aqueous perchloric acid were then used to determine the film's ion and solvent populations. The data are best described by a mechanism in which the early stages of film oxidation are associated with proton transfer (exit) and the latter stages with perchlorate transfer (entry) to satisfy electroneutrality; solvent enters throughout film oxidation and exits throughout film reduction. The alternative of a single ion satisfying electroneutrality throughout would require the rather less likely situation of a non-monotonic solvent flux. Application of the model's diagnostic criteria indicates that the film solvent population is in equilibrium on the timescale of slow scan voltammetry, but shows thermodynamic non-idealities. Hysteresis in the film ion population signals failure of the redox state to maintain equilibrium with the applied potential.     

Comparative study of polyaniline cast films prepared from enzymatically and chemically synthesized polyaniline

Enzymatically and chemically synthesized polyaniline (Pani) were used to prepare free standing cast films from N-methyl-2-pyrrolidinone solutions. The films were redoped by immersion in hydrochloric or p-toluensulfonic acid solutions. Physical and chemical properties were investigated using UV-visible, Fourier transform Infrared and X-ray photoelectron (XPS) spectroscopic techniques. Analysis by XPS revealed changes in the -N/NH- ratio that indicate cross-linking in the surface of both types of films, whereas FT-IR indicates cross-linking in the core of films prepared with enzymatically synthesized Pani. Such changes can be attributed to the catalyzing effect of sulfonic residues and chain defects of the polymer. Consequently, the degree of doping of the films prepared with enzymatically synthesized Pani was lower compared to films prepared with chemically synthesized Pani.     

Layer-by-layer assembly of graphene/polyaniline multilayer films and their application for electrochromic devices

Graphene/polyaniline (PANI) multilayer films were prepared via alternate deposition of negatively charged graphene oxide (GO) and positively charged PANI upon electrostatic interaction, followed by the reduction of their GO components with hydroiodic acid. The thickness of the multilayer film increased linearly with the number of its bilayers and that of each bilayer was measured to be about 3 nm. Cyclic voltammetry studies indicated that these thin composite films were electroactive, and their redox reactions were related to the insertion-extraction of counter ions in PANI layers. Furthermore, the composite films were tested to be promising electrode materials for electrochromic devices even without using the conventional indium tin oxide (ITO) electrodes.     

Morphological characterization and kinetics study of polyaniline film formation by emulsion polymerization

The kinetics and morphology of polyaniline film prepared by using ammonium persulfate (APS) as oxidant and dodecylbenzoyl sulfonic acid (DBSA) as both emulsifier and dopant were studied in this paper. The kinetics of the formation of polyaniline film was determined by using the quartz crystal microbalance (QCM), and the film morphology was characterized by scanning electron microscopy (SEM). The reaction exhibited half-order kinetics with respect to APS concentration and first-order kinetics with respect to aniline concentration. The activation energy is 41.15 kJ/mol, and the growth rate of PANI film increased with increasing temperature and decreased with increasing concentration of DBSA.     

Electropolymerised polyaniline films on AA 7075 alloy and its corrosion protection performance

Polyaniline has been electrodeposited on AA 7075 alloy and its corrosion protection ability has been studied by Tafel and impedance techniques in 1% NaCl. Pure polyaniline film is not found to protect the aluminium alloy due to galvanic interaction of polyaniline and aluminium surface exposed through pinholes and cracks. However, it is found that the corrosion resistance property of the polyaniline film can be substantially increased by post-treatment in cerium salt solution.     

聚苯胺的合成、表征及气敏性能研究

用化学氧化聚合法,以苯胺(An)为单体,过硫酸铵(APS)为氧化剂,控制反应温度,在酸性介质(无机酸和有机酸)中合成聚苯胺(PAn)。用傅里叶红外光谱(FTIR)和紫外可见光光谱(UV-Vis)对聚苯胺掺杂前后结构的变化进行了测试,讨论了酸掺杂对聚合产物结构的影响。结果表明电子的离域使聚苯胺主链结构经质子酸掺杂后形成了共轭结构。常温下,通过聚苯胺的气敏性能测试,得知有机酸掺杂的聚苯胺的气敏性能更好,其中用磺基水杨酸掺杂的聚苯胺对1000ppm氨气的灵敏度最高,达到了14.8580,具有实际应用价值。     

Chemical synthesis of polyaniline in the presence of poly(amidosulfonic acids) with different rigidity of the polymer chain

Conducting polyaniline (PANI) was chemically synthesized in the presence of water-soluble aromatic polyamides containing sulfonic groups: poly-(p,p’-(2,2′-disulfonic acid)-diphenylene-tere-phthalamide) (t-PASA, rigid backbone), poly-(p,p’-(2,2′-disulfonic acid)-diphenylene-iso-phthalamide) (i-PASA, semi-rigid backbone) and their copolymer (co-PASA) with the monomers ratio 1:1, as well as in the presence of flexible-backbone polyacids: poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA) and poly(styrene sulfonic acid) (PSSA). In these conditions the matrix polymerization of aniline results in the formation of water-soluble interpolymer complexes of PANI with the above cited polyacids. The character of spectral changes in the UV, visible and near IR (UV-Vis-NIR) range during the synthesis and the polymerization rates depend strictly on the structure of polyacid matrix. Higher flexibility of the polyacid backbone (PAMPSA, PSSA) contributes to higher relative absorption of localized polarons (750 nm), while in the spectrum of interpolymer complexes with rigid-chain polyacid (t-PASA) the absorption of delocalized polarons (1300-1500 nm) prevails. The complexes with semi-flexible i-PASA and co-PASA exhibit intermediate behavior. The results are interpreted in terms of differences in the inter-chain interactions in the interpolymer complexes of different structure. Fourier transform infrared (FTIR) spectroscopy data support the assumption of the incorporation of the polyacids in the PANI through an interaction between sulfonic groups of the polyacids and nitrogen atoms of PANI. Spectroelectrochemical and electrochemical (cyclic voltammetry) studies of the films cast from the obtained solutions showed that the formation of quinoid units at high oxidation level is retarded in the interpolymer complexes of PANI with rigid- and semi-rigid-chain polyacids. Atomic force microscopy (AFM) and direct current (DC)-conductivity data are also presented.     

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.     

The fabrication and characterization of inkjet-printed polyaniline nanoparticle films

This paper reports on the fabrication and characterization of electrodes modified with conducting polymer nanoparticle films, produced via inkjet printing. The polyaniline nanoparticle formulations were deposited via a desktop inkjet printer onto screen-printed carbon-paste electrodes (SPE), polyethylene terephthalate (PET) and gold-PET and their morphology studied at a range of length scales using profilometry, scanning electron microscopy and atomic force microscopy. The deposited films were found to form continuous polymer films depending upon film thickness, which was in turn dependent on the number of prints performed. The inkjet-printed films exhibited a smooth morphology on the SPEs at the micro-dimensional scale, as a result of the aggradation and coalescing of the nanoparticles upon deposition. The resulting modified electrodes were both conductive and electroactive, possessing good reversible polyaniline electrochemistry. Such a combination of materials and processing offers the potential of producing a range of low cost, solid state devices such as sensors, actuators and electrochromic devices.     

Bistable light scattering effects in a (side chain liquid crystalline polymer)/(low molecular weight liquid crystal) composite and in a ferroelectric liquid crystalline polymer

The electro-optical effects and aggregation states of liquid crystalline polymer (LCP)/ low molecular weight liquid crystal (LC) composite and ferroelectric liquid crystalline copolymer (FLCP) have been investigated. The nematic LCP was observed to be miscible with the nematic LC over wide ranges of concentration and temperature. The binary mixture showed an induced smectic phase in the range of 80/20-20/80 mol%. The electro-optical effects of the LCP/LC composite in an induced smectic phase could be classified into the turbid (light- scattering) and the transparent states upon application of AC and DC electric fields, respectively. The transient scattering mode was obtained by repeated voltage polarity reversal in the chiral smectic C phase of FLCP. The reversible transparent-opaque (light scattering) change was observed in the chiral smectic C state upon application of DC and AC electric fields, respectively. Both transparent and light-scattering states of the LCP/LC composite and the FLCP could be maintained, even after the electric field had been turned off (memory effect). The bistable effects of LCP/LC composite and FLCP are opposite under the same conditions. A novel type of electro-optical effect on light scattering was obtained for liquid crystalline polymer in the smectic states.     

Evidence of a redox equilibrium assisted chain propagation mode for aniline polymerization: in situ spectral investigation in dodecylbenzene sufonic acid based system

Detailed in situ absorption spectral investigation of chemical oxidative polymerization of aniline in dodecylbenzene sufonic acid (DBSA) based system revealed that the chain propagation is assisted by redox equilibrium intermediate (REI) rather than a fully oxidized pernigraniline. The continuous reduction (addition of aniline) and oxidation (by oxidant) of REI, propagate the chain. The polymerization process consists of three prominent stages called induction period (IP), chain propagation and termination. The initial stage of polymerization is realized as IP, the time taken to establish REI. The presence of fully developed polaron band indicates REI has delocalized structure. Invariant spectral features together with tremendous increase in absorbance at propagation phase indicate REI undergoes reductive addition of aniline and oxidation by APS (ammonium persulfate) while maintaining equilibrium structure. The greatly increased absorbance with time at this stage is due to well-known autoacceleration phenomenon observed for aniline polymerization. The increase in redox potential of REI with increase in chain length accelerates the reaction and accounts for autoacceleration. The termination of polymerization would be reduction or oxidation depending on the availability of aniline or oxidant respectively. In a system where the oxidant is the limiting agent, reduction continues until it reaches emeraldine salt that is incapable of further reduction. On the other hand an excess of oxidant, results in the formation of pernigraniline by continuous oxidation.     

Characterization of substituted polyaniline films using Raman spectroscopy: III. Study of a methoxylated polymer: POMA

The electropolymerization of o-anisidine leads to the formation of a polymer which is in the same time strongly oxidized and charged in a large potential range: the poly-o-methoxyaniline (POMA). The POMA Raman features are strongly different from the polyaniline (PANI) ones; in particular a new band, absent from the PANI spectrum, prevails in a large potential range. The nature of the polymer structures obtained in function of the polarization potential is discussed from their Raman spectra. A new polymeric configuration of the oxidized/charged rings is therefore described.     

Electrical and structural properties of polyaniline/cellulose triacetate blend films

Polyaniline (PANI) has been blended with cellulose triacetate (CTA) to obtain both good mechanical properties and good electrical properties. The effects of PANI weight percentage on the optical, structural, morphological and electrical properties in the blend films of polyaniline and cellulose triacetate (PANI/CTA) have been investigated. The phenomenon of percolation was observed in these blend films. It is found that the electrical conductivity of the blend films increases with the increase of polyaniline content up to a value of 10⁻⁴ S cm⁻¹ at 84 weight percentage of PANI. The experimental percolation threshold of the dried blend films is obtained at 9.5 wt% of polyaniline. The values of Mott’s temperature, density of states at the Fermi level [N (E _f)], hopping distance (R _hop), and barrier height (W _hop) for PANI/CTA blend films are calculated. By applying Mott’s theory, it is found that the PANI/CTA blend films obey the three dimensional variable range hopping mechanism.     

Size-dependent electrocatalytic reduction of nitrite at nanostructured films of hollow polyaniline spheres and polyaniline–polystyrene core–shells

Nanostructured films of the conducting polymer polyaniline (PANI) were prepared by its potentiostatic polymerization in the presence of thin polystyrene (PS) nanoparticle templates. Two sizes of PS nanoparticles were used (50 and 100 nm) and electron microscopic analysis showed that core–shell composite films of PANI (PANI–PS₅₀ and PANI–PS₁₀₀) and, following removal of the PS, hollow structures (PANI₅₀ and PANI₁₀₀) were formed due to the growth mechanism of PANI around the PS templates. The electrochemical behaviour of the PANI-modified electrodes was studied by cyclic voltammetry, chronocoulometry and amperometry in the presence of nitrite and it was found that nitrite reduction was enhanced by the nanostructuring of the films. The electrocatalysis was dependent on the size of the template, being more pronounced at 50 nm than at 100 nm. The hollow PANI₅₀ film was also a better catalyst than PANI–PS₅₀, whereas for structures based on 100 nm templates, the composite film was better. Such behaviour could be explained in terms of larger surface area and surface concentration of PANI₅₀ and PANI–PS₅₀ films on the electrodes and by higher differential capacitance of those films.     

Light scattering and viscoelasticity study of poly(vinyl alcohol)-borax aqueous solutions and gels

Poly(vinyl alcohol)-borate (PVA-borate) aqueous solutions properties with PVA concentrations ranging from 2 to 60 g/L and borax concentrations of 0.0 and 0.2 M were investigated at room temperature using static and dynamic light scattering (SLS and DLS), and dynamic viscoelasticity measurements. Light scattering and viscoelasticity data revealed that all the PVA-borate aqueous systems, except those with [PVA]≥40 g/L and [borax]=0.2 M, behaved as solutions. For PVA-borate aqueous systems with [PVA]≥40 g/L and [borax]=0.2 M, light scattering data revealed that these systems behaved like gels, but viscoelasticity data showed that these systems were in flow states. The experimental data suggest that PVA-borate aqueous systems with [PVA]≥40 g/L and [borax]=0.2 M are thermoreversible gels with finite equilibrium life time of thermoreversible borate-PVA di-diol crosslinks. The thermoreversible crosslinks can be observed by the non-perturbing light scattering technique but not by the pertubing rheometric method. These results indicate the advantage of light scattering relative to rheometers for studying the physical or reversible crosslink gels.     

Evaluation of the integral methods for the kinetic study of thermally stimulated processes in polymer science

This paper reports on the accuracy of the integral methods used for the kinetic analysis of degradation and crystallization of polymers. Integral methods are preferred by many authors over the differential ones because often the experimental data obtained, such as thermal degradation studied by thermogravimetry, are integral and the differentiation of the integral data usually produces an unwilling increase of the noise. A problem of the integral methods is the fact that Arrhenius integral function does not have an exact analytical solution. Thus, several approximated equations have been proposed in literature. Some of these approximations lead to a linear relation between the logarithm of g(α) and a predetermined function of T, in such a way that the activation energy can be determined from the slope of the plot of ln g(α) versus the predetermined T function. The most popular approximations to the Arrhenius integral in polymer science are those of van Krevelen et al., Horowitz and Metzger, and Coats and Redfern. Although these three approaches where proposed 50 years ago, they are extensively used nowadays and several hundreds of citations to the original papers can be found in recent polymer science publications. Despite their popularity, there are cast doubts on the accuracy of these approximations, because they provide significant deviations in the determination of the actual values of the Arrhenius integral when used for simulating α-T plots. Nevertheless, a comprehensive study of the systematic errors in the activation energy calculated from these integral methods is still missing. In this paper a comparative study of the accuracy of the different integral methods is performed. The calculated errors are tested with simulated and experimental results.     

Conducting polymer gel: formation of a novel semi-IPN from polyaniline and crosslinked poly(2-acrylamido-2-methyl propanesulphonicacid)

In the present article, a simple two-step technique for synthesis of electrically conducting hydrogel is described. The synthesized hydrogel is originally a semi-interpenetrating polymer network (IPN) in which conducting polyaniline (linear) is entrapped within a crosslinked polyelectrolyte gel viz. poly(2-acrylamido-2-methyl propane sulphonic acid) (PAMPS). A conventional photo-polymerization technique was followed for synthesis of the PAMPS gel while aniline is in situ polymerized within the gel, giving rise to the desired semi-IPN. For comparison, a neutral gel based semi-IPN PAn-PAAm (polyaniline-polyacrylamide) was also prepared following the similar route. PAn imparts appreciable electrical conductivity to both PAMPS and PAAm based gels without hampering their existing properties that indicates the success of the work. Synthesis of the gel was studied in detail and different experimental conditions were optimized. Different physical properties of the gel viz. its degree of swelling, electrical conductivity, mechanical strength, etc were also studied. The resultant semi-IPN or the composite hydrogel was found to possess appreciable electrical conductivity, good swellability and mechanical strength. The electro-driven volume contraction of the composite gel was found to occur at sufficiently lower voltage compared to the pure PAMPS gel, which indicates its promising application aspect in fabrication of chemomechanical devices.     

Electrochemical synthesis of polyaniline in the presence of poly(amidosulfonic acid)s with different rigidity of polymer backbone and characterization of the films obtained

We have studied electrochemical matrix polymerization of aniline in the presence of poly(amidosulfonic acid)s of different nature: poly(2-acrylamido-2-methyl-1-propanosulfonic acid) (PAMPSA, flexible backbone); poly(p,p′-(2,2′-disulfoacid)-diphenylene-iso-phthalamid) (i-PASA, semi-rigid backbone); poly(p,p′-(2,2′-disulfoacid)-diphelylene-tere-phthalamid) (t-PASA, rigid backbone). Also, we have investigated spectral and electrochemical properties of the films obtained, as well as their surface morphology. The matrix polymerization results in the formation of interpolymer complexes of polyaniline (PANI) and the above-cited polyacids. The acceleration of aniline electropolymerization in the presence of poly(amidosulfonic acid)s was observed due to association of aniline molecules to sulfonic groups of the polyacid and higher local concentration of protons near the polyacid backbone. The rigid-chain polyacids interfere with the normal course of the electropolymerization, which manifests itself in the changes of the shape of time dependences of absorbance and charge. Cyclic voltammetry and spectroelectrochemical experiments showed that the formation of interpolymer complex with rigid-chain polyacids distorts spectroelectrochemical characteristics of PANI. This evidently results from steric hindrances in the formation of quinoid units.     

Electrochromic features of hybrid films composed of polyaniline and metal hexacyanoferrate

Hybrid organic/inorganic films, composed of polyaniline (PANI) matrix and Prussian blue-like nickel hexacyanoferrate redox centers, showed reversible electrochromic behavior in acidic potassium salt electrolytes. The system's coloration properties were assessed from various spectroelectrochemical measurements including voltabsorptometry that involved monitoring of the time-derivative signal of absorbance at 700 and 410 nm as a function of linearly scanned potential. Gold-covered foil was used as a conductive, optically transparent, substrate onto which the composite film was electrodeposited by potential cycling in the mixture for modification consisting of aniline monomer, Ni²⁺, Fe(CN)₆³⁻ and electrolyte containing K⁺ and H⁺ ions. An important feature of hybrid (composite) material was that its electrochromic properties were dominated by color changes occurring in the PANI component. Coloration originating from nickel hexacyanoferrate barely affected the system's electrochromic characteristics. But the cyanometallate redox centers distributed in the PANI matrix behaved reversibly as expected for a system capable of fast charge transport.