Mieczysław Łapkowski

Electrochemical synthesis of linear polyaniline in aqueous solutions

Abstract An attempt was undertaken to prepare polyaniline with a fine linear structure in aqueous media. Comparable electrochemical polymerization was carried out in solutions of the following acids: hydrochloric, sulfuric, perchloric and tetrafluoroboric. Spectroelectrochemical measurements have revealed the presence of an intermediate product (nitrenium cation) in the initial stage of aniline polymerization carried out in these media. The polymerization starts after an induction period which depends on the final potential for cyclic voltammetry and the nature of the medium. The number of spins as a function of the applied potential was a criterion to determine the linearity of the polymer chain. In polymers with linear structure (without ortho-bindings and cross-linking effects), polarons formed during the doping process in the presence of double-charged anions undergo a recombination process to form bipolarons. As a result no EPR signal is observed between the two redox systems of polyaniline. This kind of polymer can be formed only by voltammetric polymerization in a 5 M solution of tetrafluoroboric acid. The other media mentioned lead to formation of more or less cross-linked chains. The measurements performed suggest that the mechanism of aniline polymerization in aqueous media is analogous to that in non-aqueous solutions for a eutectic mixture of NH4F; 2.35 HF.

Synthesis of polypyrrole and polythiophene in aqueous solution of Keggin-type structure heteropolyanions

Abstract Immobilization of Keggin-type heteropolyanions (HPA) such as: PMo 12 O 40 3− , SiW 12 O 40 4− , and PW 12 O 40 3− in polypyrrole, poly(3-methyl-thiophene), and polyaniline is performed by electropolymerization of pyrrole (Py), 3-methyl-thiophene (3-MeTh), and aniline (An) in aqueous and nonaqueous solutions in the presence of corresponding heteropolyacids and salts. It was found possible to obtain electroactive films of polypyrrole and polythiophene from aqueous solutions of HPA due to formation of respective HPA-Py and HPA-3-MeTh complexes. Application of acidic environment to the Py polymerization leads to its rapid trimerization and deterioration of electroactive properties of the film obtained. These electrodes were characterized by cyclic voltammetry which provides two one-electron reversible HPA redox systems. They are stable in acidic solutions for poly(3-methylthiophene) and in acidic and neutral solutions for polyaniline.

Electrochemical synthesis of polyaniline/poly(2-acryl-amido-2-methyl-1-propane-sulfonic acid) composite

Abstract Conducting polyaniline/poly(2-acryl-amido-2-methyl-1-propane sulfonic acid) (PANi/PAMPS) composite films were synthesized electrochemically on platinum electrodes using a 10% aqueous PAMPS solution as the electrolyte. The electrochemical properties of the resulting polymer are similar to those observed for polyaniline doped with classical non-polymeric anions. PANi/PAMPS exhibits high cycling stability within the potential range of the first redox couple, but it undergoes degradation when the potential range characteristic of the second redox couple is reached. The degradation occurs independently of the electrolyte medium (HCl or 10% aqueous PAMPS).

Electrochemical investigation of regioregular alkyl substituted oligothiophenes

Abstract Radical cation stabilities and their dimerization reactions have been studied through the series of regioregular oligothiophenes (n=2–6 where n is the number of thiophene rings) with chlorine protected at one α-terminal position. The oligomers with two and three rings undergo one oxidation reaction to produce the radical cations, oligomers with four, five and six rings undergo two stepwise oxidations to produce the radical cations and dications, respectively. The radicals coupled to form dimers in following reactions. Based on double-potential-step data we observed that the dimerization rate decreases as n is increased. This result suggests that the stability of the radical increases with increasing size of its oligothiophene chain.

Control of polyaniline electroactivity by ion size exclusion

By using an electrolytic salt solution of two anionic species differing by their ionic radius or their molecular size, it is possible to accurately control the concentration of anionic doping agent without modification of the electrolyte conductivity. The conducting polymer film is electrosynthesized on the electrode in a solution containing small ions. Once de-doped, the film keeps the memory of the size of the doping anion and can be only re-doped by comparable or smaller anions. After the synthesis, the film is transferred to another electrolyte containing heteropolyanions ensuring the conductivity and a variable concentration of small ions acting as dopants. Measurement of the electroactivity of the polyaniline film is made by linear sweep voltammetry in classical electrochemical conditions.

Effect of the nature of the electrolyte on the properties of unpaired spins in polyaniline

Abstract An extended study has been carried out to find the relation between the number of spins, the applied potential and the charge of the doping anion in the case of polyaniline electropolymerized in 5 M tetrafluoroboric acid. It was found that in the presence of acids which supply double-charged anions, a fast recombination of polarons to bipolarons occurs. This is in contrast to single-charged anions which stabilize polarons. Three types of spins were detected in the study: (i) localized spins, formed at the beginning of the oxidation process; (ii) mobile spins existing in the presence of single-charged anions; (iii) spins localized mostly at structural defects.

Microgravimetric and laser beam deflection studies of redox reactions in polyantline at various concentrations of doping anions

Abstract Quartz crystal microbalance and laser beam deflection techniques were used to study the role of ion exchange in electrochemical oxidation and reduction of polyaniline films synthesized in various acids. Mass changes and beam deflection angles were measured during voltammetric cycling of polyaniline in aqueous solutions containing H 4 SiW 12 O 40 and various amounts of HCl or HClO 4 . The obtained results confirm the initial hypothesis that due to their large size heteropolyanions cannot dope the bulk of the polymer. An increase in the doping anion concentration leads to significant changes of mass and a deflection angle associated with redox reactions in polyaniline. Doping anions as well as protons are exchanged between the polymer and the supporting electrolyte. The contribution of protons and anions depends not only on the concentration but also on the conditions under which the film was synthesized.

Electrochemical properties of 4-(2-pyridylazo)-resorcinol (PAR) film deposited on a platinum electrode

Abstract A new kind of modified electrode prepared by electropolymerization of 4-(2-pyridylazo)-resorcinol (PAR) on to platinum and other electrode materials has been obtained. Some interesting properties of this electrode—relatively high electrochemical stability and good resistance to weather and water, aqueous solutions of various salts and organic solvent—have been found. The modified electrode has been found to possess specific electrochemical properties, i.e. it causes decay of electrode reactions of a certain group of organic compounds and considerable change in electrode reactions of copper and cobalt ions and their complexes.

Synthesis of conducting polymers doped with heteropolyanions of Dawson-type structure

Abstract Immobilization of Dawson-type heteropolyanions (HPA) such as: P 2 W 18 O 62 6− in poly(3-methyl-thiophene), and polyaniline is performed by electropolymerization of 3-methyl-thiophene (3-MeTh) and aniline (An) in nonaqueous solutions in the presence of corresponding heteropolyacids and salts. It was found possible to obtain electroactive films of polyaniline and polythiophene from acetonitrile solutions of HPA due to formation of respective HPA-An and HPA-3-MeTh complexes. These electrodes were characterized by cyclic voltammetry which provides three one-electron reversible HPA redox systems. They are stable in acidic solutions for poly(3-methylthiophene) and in acidic and neutral solutions for polyaniline.

A New Route to Light Emitting Organic Materials Based on Triazine Derivatives

Novel light-emitting organic materials comprising conjugated oligomers chemically attached via a flexible spacer to an electron–or hole-conducting core were synthesized by Stille cross-coupling methodology and designed for tunable charge injection and optical properties. The hyperbranched structures have been characterized by 1H NMR, 13C NMR, and UV-VIS spectroscopy. The obtained structures show good stability in common organic solvents such as CHCl3, toluene, and CH2Cl2 and exhibit excellent thermal stability. A preliminary electroluminescence properties were also investigated. This class of materials will help to balance charge injection and transport and to spread out the charge recombination zone, thereby significantly improving the device efficiency and lifetime of unpolarized and polarized OLEDs.