In Situ Conductivity of Poly(3,4-Ethylenedioxythiophene) Electrosynthesized in Aqueous Solutions in the Presence of Larg
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H. J. AHONEN, J. LUKKARI, J. KANKARE Department of Chemistry, University of Turku, FIN-20014 Turku, Finland
ABSTRACT
Thick poly(3,4-ethylenedioxythiophene) films incorporating various anion dopants were anodically deposited from aqueous solutions. In situ AC conductometry was applied during the electropolymerisation and the conductivity values obtained in different electrolyte systems were compared. The results indicate that 3,4-ethylenedioxythiophene can be polymerised also from relatively dilute aqueous solutions containing various types of electrolytes and that the most conducting films are obtained using large organic anions. INTRODUCTION
Organic conducting polymers are widely studied materials in physics, chemistry and materials science due to numerous potential applications of these materials, e.g., in the area of electronics and molecular electronics. Usually, these materials are synthesized chemically or electrochemically from monomer solutions in organic solvents (acetonitrile, propylenecarbonate, dichloroethane, etc.). Contrary to some other monomers like pyrrole and aniline, thiophene is not easily polymerised from aqueous medium. Perhaps the most prohibitive factor is the low solubility of thiophene in water, which leads to a low radical cation concentration during the electrosynthesis. The few radical cations can react instantly and irreversibly with the nucleophilic water molecules instead of each other and, consequently, only soluble oligomers are obtained. The solubility of the thiophene monomer can be enhanced
by a methoxy substitution at the 3-position with some sacrifice of the conductivity of the corresponding polymer due to the introduction of steric effects and loss of the effective conjugation in the polymer backbone. The high oxidation potential of thiophene, which is beyond the electrochemical stability window of water is another limiting factor. On the other hand, for practical, economical and enviromental reasons it would be desirable if high quality materials could be synthesised in a non-toxic and non-expensive medium like water. Indeed, there has been some success in obtaining polythiophene from highly acidic aqueous solutions or methanol/water mixtures, but the properties of the resulting polymers have been inferior to the materials polymerised in organic solvents [1,2]. Due to its high conductivity and electrochemical stability in oxidised state poly(3,4ethylenedioxythiophene) (PEDOT) is presently under intense research in several academic and industrial laboratories. EDOT monomer can be dissolved in water to a sufficiently high concentration (up to 14 mM) and its oxidation potential is relatively low (ca. 1 V vs. SSCE). Furthermore, there have been some recent reports about the improvement of the physicochemical properties of poly(3,4-ethylenedioxythiophene) using sodium dodecylsulfate (SDS) micellar aqueous medium, where regular and well ordered films have been obtained [3]. Molecular order in the film is usually connected to the increased conductivity of the material. Therefor
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