Mechanistic studies of the electrodeposition of polythiophenes

Zhao, Ziwei (1993) Mechanistic studies of the electrodeposition of polythiophenes. Masters thesis, Memorial University of Newfoundland.

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Abstract

The electrochemical deposition of poly(3-methylthiophene) and the electrochemical polymerization of 3-bromothiophene in the presence of 2,2'-bithiophene have been studied. Mechanisms have been proposed, based on the electrochemical responses recorded during polymer formation and characterization of the resulting polymer films by cyclic voltammetry, ac impedance spectroscopy, scanning electron microscopy as well as by x-ray emission spectroscopy. -- A rotating Pt disc electrode was applied to probe the potentiostatic deposition of poly(3-methylthiophene). It was observed that poly(3-methylthiophene) films could be formed on both stationary and rotating electrodes even at rotation rates as high as 1600 rpm (revolutions per minute), and that the quantity of polymer deposited decreased with increasing rotation rate. It was therefore concluded that polymer deposition takes place both through the precipitation of oligomeric intermediates from the solution, and via the coupling of monomeric species with polymer chain ends in the polymer film matrix. Chronoamperometric responses recorded during polymerization, as well as the characterization of the resulting polymer films by cyclic voltammetry, ac impedance spectroscopy and scanning electron microscopy, demonstrated that the deposition of oligomeric intermediates from the solution contributes most to polymer formation on a stationary electrode. On the other hand, polymer growth on a rapidly rotating electrode occurs primarily by the incorporation of monomeric species into polymer chains grafted on the electrode surface. Much smoother, more homogeneous films were deposited on rotating electrodes. -- The polymerization of 3-bromothiophene in the presence of 2,2'-bithiophene was investigated by cyclic voltammetry. With a catalytic amount of 2,2'-bithiophene, a significant increase in the polymerization rate, together with a decrease in the potential required for poly(3-bromothiophene) formation, was observed. The polymer coated electrodes from the mixed solution exhibited characteristic electrochemical behaviour in cyclic voltammograms and Br and S peaks in x-ray emission spectroscopy for pure poly(3-bromothiophene). The 2,2'-bithiophene catalyst produces better quality poly(3-bromothiophene) films. A polymerization mechanism involving radical-monomer coupling is supported by these experimental observations. In this mechanism, the 2,2'-bithiophene catalyst is oxidized first, and then couples with 3-bromothiophene to initiate the polymerization.

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