Natalia Gospodinova

Conducting polymers prepared by oxidative polymerization: polyaniline

Abstract Polyaniline (PANI) is one of the most intensively investigated polymers during the last decade. The establishment of the scientific principles allowing regulation of its properties, determining the potential application areas (alternative energy sources and transformers, media for erasable optical information storage, non-linear optics, membranes, etc.) is an important scientific problem. We have shown for the first time that the behavior of this polymer is subject to the same basic principles as the polymerization process itself. Both the polymerization of aniline and the subsequent transformations of polyaniline have to be regarded as typical redox processes, where the direction and establishment of equilibrium are dependent on the oxidation potentials and concentrations of the reactants (and also on pH of the medium, affecting the values of oxidation potential of the reactants). Such an approach allows us to identify the oxidative polymerization of aniline (and presumably of thiophene and pyrrole) as a new area in cationic polymerization, wherein the conditions of initiation, propagation and termination of the chains can be expressed by means of the electrochemical potential of the system. Furthermore, this allows an elucidation of the key problems related to the main types of transformations of this polymer (the so-called oxidative and non-oxidative doping of polyaniline). It also gave us a reason to suggest a novel original classification of the numerous potential application areas of PANI.

A new route to polyaniline composites

Abstract A novel route to preparation of polyaniline (PANI) composites based on mixing of a solution of host polymer with preliminarily formed colloidal PANI dispersion of very fine particles (lower limit ca 5 nm) is reported. This colloidal PANI dispersion was formed by oxidative polymerization of aniline (ANI) in the presence of dodecylbenzenesulfonic acid in aqueous medium, and can be dried and redispersed in an organic solvent, enabling composites of PANI with water-soluble and water-insoluble polymers to be prepared. Besides its simplicity, this route reveals the possibility of easily controlling the design of composites (particle size and particle size distribution). The polymer composites PANI-poly(vinyl alcohol), thus prepared, combined high electrical conductivity (up to 2–3 S cm −1 ) and transparency (between 20 and 50% at film thickness 50 μm) even in a non-oriented state.

Polyaniline dispersions: preparation of spherical particles and their light-scattering characterization

Abstract Polyaniline dispersions can be prepared by the oxidative polymerization of aniline in the presence of the steric stabilizer, poly(vinyl alcohol). Dispersion particles are spherical and relatively uniform. Dynamic light scattering was used to determine their hydrodynamic radius and static light scattering to estimate mass-average particle mass. From a series of experiments, a typical example of the preparation and characterization of polyaniline dispersion is described.

On the mechanism of oxidative polymerization of aniline

Abstract Oxidative polymerization of aniline in aqueous dispersion stabilized by poly(vinyl alcohol coacetate) was studied in situ by electron absorption spectroscopy. A mechanism is proposed, accounting for the dependence of the main polymerization steps (oxidation and reduction) on both the pH of the medium and the oxidation state of the chain.

Chemical oxidative polymerization of aniline in aqueous medium without added acids

Abstract Chemical oxidative polymerization of aniline in aqueous dispersion stabilized by poly(vinyl alcohol coacetate) in initially neutral and alkaline media was studied by electron absorption spectroscopy and in situ pH measurements. It was shown that polyaniline could be obtained only under conditions when the initial stage of polymerization (formation of oxidized dimer) results in a decrease of pH to values lower than about 2.

Unprecedented Route to Ordered Polyaniline: Direct Synthesis of Highly Crystalline Fibrillar Films with Strong π-π Stacking Alignment.

Films of polyaniline (PANI) featuring about 80% crystallinity and characterised with strong π-π stacking alignment parallel to the film surface have been obtained directly after the original synthesis upon simple drying of the aqueous PANI suspension. A strong anisotropy in the growth of the nano-sized crystals produced during the synthesis results in the formation of micrometer-length fibrils perpendicular to the film surface in the course of water evaporation. The regular intercalation of water molecules between the PANI chains seems to be crucial for their ordering throughout the synthesis and film formation.

Influence of hydrolysis on the chemical polymerization of aniline

The influence of hydrolysis on the process of oxidative polymerization of aniline was evaluated through the effect of polymerization temperature on the yield of polyaniline (PANI) in different pH regions where the polymerization took place. The synthesis parameters were chosen based on the dependence of the polymerization and hydrolysis processes on the pH of the medium, and considering the opposing thermal effects of the two processes (exothermic polymerization and endothermic hydrolysis). The yield of PANI was evaluated from the conductivity of composite films cast from aqueous dispersions of PANI prepared by chemical oxidative polymerization of aniline using poly(vinyl alcohol-co-acetate) as a steric stabilizer. It was found that the influence of hydrolysis is most pronounced in slightly acidic (pH > 4) and strongly acidic (pH < 1.5) media. This imposes the requirement for the polymerization of aniline to be carried out at low temperature (0–5°C) in order for hydrolysis to be suppressed. When the initial stage of polymerization takes place at 4 < pH < 1.5, hydrolysis does not play a significant role and polymerization proceeds with a high yield irrespective of the temperature changes.

Preparation and characterization of aqueous polyaniline dispersions

Abstract Stable aqueous dispersions of polyaniline (PANI) have been prepared using an underivatized poly(vinyl alcohol-co-acetate) (PVAL) as a steric stabilizer. Investigation of aqueous PANI dispersions by scanning electron microscopy and light scattering revealed that oxidative polymerization of aniline in the presence of PVAL results in the formation of spherical particles. Their size does not depend on the concentration of aniline in the polymerizing system but the uniformity of the particles seems to be improved as the aniline concentration is increased at least up to a certain limit. Electron absorption spectroscopy study of PANI dispersions made it possible to follow the changes of PANI on both molecular and supermolecular levels. When the initial concentration of aniline in the polymerizing system was ca 2 wt% (at a concentration of PVAL from 2 to 10 wt%), electrical conductivity of free standing films cast from dispersions reached 10−1S/cm.

Alternative concept of the transition emeraldine base-emeraldine salt

Abstract The transition of the emeraldine base (EB) to the emeraldine salt (ES) form of polyaniline (PANI) is in fact a redox process where ‘EB’ does not represent the emeraldine oxidation state. This redox process itself is an intermediate stage in the transition of insulating pernigraniline to conducting emeraldine. Generally, transition of PANI from insulating (leucoemeraldine, L, or pernigraniline, PN) to conducting (emeraldine, E) state could be represented by the following scheme:

Evidence for Generation of Delocalized Polarons in Conducting Polyaniline: A Raman Scattering Spectroscopy Approach

Abstract Photoexcitation as a novel route for obtaining delocalized polarons in conducting polyaniline is predicted theoretically and confirmed experimentally. Original model experiments and theoretical calculations on protonated emeraldine species with various chain alignments as well as of isolated chains were carried out. In addition, experimental models matching theoretical models of isolated and bulk polyaniline (PANI) chains were synthesized and their absorption and Raman spectra recorded. The detection of the photoexcited states was performed by conventional Raman scattering spectroscopy (generally employed for studying ground states), which is an attractive alternative for generating and characterizing photoexcited delocalized polarons in polyaniline. The occurrence of photoexcited delocalized polaronic species is favored by chain stacking.