Comparative Study of Solvent Effect on the Electrochemical Deposition of Polythiophene and Polybithiophene
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solvent, electrode surface, temperature and applied electrical bias [3], which are mutually interrelated. As a result, the properties of electrochemically deposited polymer films are often difficult to control and optimize. Amongst the various experimental variables, the solvent is known to have a significant effect on the ECP process. This is because the radical cation intermediates generated during ECP are sensitive to the nucleophilicities of the solvent medium and the electrolyte. Thus far, the effect of the protic solvents on ECP has been the most extensively studied. It is known that such solvents generally lead to oligomer formation and poor quality films [4]. However, for polypyrrole, the best films are deposited from acetonitrile containing a small amount (ca. 1 wt%) of water. The role of water in deposition of polypyrrole remains controversial to date [5]. For the aprotic solvents, e.g. acetonitrile (AN), benzonitrile (BN) and propylene carbonate, the effect of the solvent on the polymerization is not well studied. In an earlier review on the polythiophenes (Pth) [6], it was pointed out that Pth films deposited from AN tend to be powdery whereas those deposited from BN tend to be more compact and robust. However, detailed experimental data was not provided. In this paper, we present a more systematic comparison of the role of the aprotic solvent on the polymerisation of thiophene and bithiophene. It will be demonstrated that the two selected solvents, AN and BN influence the initial stages of film formation on the substrate in significantly different ways. The
191 Mat. Res. Soc. Symp. Proc. Vol. 600 0 2000 Materials Research Society
characterization techniques employed include optical microscopy, tapping mode atomic force microscopy (TMAFM), spectrophotometry and tensile testing. After presenting the experimental data, we compare the solvent properties of AN and BN using data taken from the available literature. EXPERIMENT The Pth and PBth samples used in this study were all deposited in a home made three electrode, single compartment electrochemical cell. Four monomer/solvent mixtures, namely thiophene/acetonitrile, thiophene/benzonitrile, bithiophene/acetonitrile and bithiophene/ benzonitrile were prepared. Samples prepared from these were designated PTh/AN, PTh/BN, PBth/AN and PBth/BN respectively. The purities of the thiophene and bithiophene monomers were 99% and 96% respectively. The AN and BN solvents were of anhydrous or HPLC grade and were used without further purification. The concentration of the monomer was 0. 1M in
all solutions. The supporting electrolyte was 0.1 M tetraethylammonium-tetrafluoroborate TEA-TBF 4 for all solutions. The TEATBF 4 was heated in a vacuum oven at 1000C for 6 hours prior to dissolution. Polymerisation was performed at constant potential using a EC&G 273A potentiostat. The applied potential was 1.4V (vs. Ag) for bithiophene and 2.4V (vs. Ag) for thiophene based solutions. The working electrode was indium tin oxide (ITO) glass pieces with sheet resistance of 150 Q
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