POTENTIAL DEPENDENCE OF THE CONDUCTIVITY OF HIGHLY OXIDIZED POLYTHIOPHENES, POLYPYRROLES, AND POLYANILINE - FINITE WINDOWS OF HIGH CONDUCTIVITY

In situ measurements of the relative conductivity of polythiophenes, polyaniline, and polypyrroles as a function of electrochemical potential reveal that they have finite potential dependent windows of high conductivity. Further, the high conductivity is found where charge is injected or withdrawn from the polymer. The polymers studied, polyaniline and I-VI, are prepared by anodic polymerization onto microelectrode arrays of the appropriate monomer: thiophene (I), 3-methylthiophene (II), 3-phenylthiophene (III), 1-methyl-1′-(3-(thiophene-3-yl)propyl)-4,4′-bipyridinium (IV), 3,4-dimethylpyrrole (V), and N-methylpyrrole (VI). The use of a liquid SO2/electrolyte medium as the electrochemical solvent makes it possible to define the window of high conductivity, because it allows the polymers to be reversibly oxidized to a greater extent than has previously been achieved. The conductivities of I-IV increase by at least 106-109 when they are oxidized from neutral to the potential of maximum conductivity and then decrease by 101-104 when they are further oxidized to the greatest extent possible without irreversible degradation in liquid SO2/electrolyte. Cyclic voltammetry indicates that IV is oxidized by ∼0.3 electron per thiophene repeat unit at the potential of maximum conductivity and by ∼0.5 electron per repeat unit at the most positive potential accessible without irreversible degradation. Visible-near-infrared absorption spectroscopy of II at the most positive potential accessible is similar to that at the potential of maximum conductivity, the spectrum having a broad peak extending into the infrared. For thiophene-based polymers I, II, III, and IV, the maximum conductivities are approximately 10-1, 10, 5 ⨯ 10-2, and 5 ⨯ 10-3 Ω-1 cm-1, respectively, and the widths of the windows of high conductivity are 0.77, 0.98, 0.65, and 0.47 V, respectively. The trend for both properties is II > I > III > IV, consistent with theoretical considerations relating conductivity, band width, and carrier delocalization. Polyaniline undergoes large, reversible, potential dependent changes in conductivity in liquid SO2/electrolyte in the apparent absence of a protonation/deprotonation mechanism. Conductivity increases by at least 108 upon oxidizing polyaniline from neutral to maximally conducting and decreases by at least 108 when polyaniline is further oxidized in the cyclic voltammogram. Polyaniline can be taken to ∼+3.8 V vs SCE in liquid SO2/electrolyte without irreversible degradation. Visible-near-infrared spectroscopy shows that fully oxidized polyaniline absorbs only at high energy with no absorption at the low energy end of the near infrared. Potential dependent windows of high conductivity and cyclic voltammetry for pyrrole-based polymers V and VI are similar to those for thiophene-based polymers I-IV. © 1990, American Chemical Society. All rights reserved.