ELECTROCHEMISTRY OF POLY(PYRROLE CHLORIDE) FILMS - A STUDY OF POLYMERIZATION EFFICIENCY, ION-TRANSPORT DURING REDOX AND DOPING LEVEL ASSAY BY ELECTROCHEMICAL QUARTZ CRYSTAL MICROGRAVIMETRY, PH, AND ION-SELECTIVE ELECTRODE MEASUREMENTS

This study concerns the growth, redox behavior, and analysis of poly(pyrrole chloride) films ranging in thickness from approximately 0.5 to approximately 1.5 mum. Changes in the electrolyte pH from radical-coupling reactions during the oxidation of pyrrole lead to estimates of the polymerization efficiency in 0.1 M KCl which vary from 82% to 92% depending on the charge consumed. These values are compared with the Coulombic efficiency computed earlier in this laboratory using electrochemical quartz crystal microgravimetry (EQCM). The EQCM technique was combined with coulometry and in situ pH and K-ion-selective electrode measurements for reexamining the extent of permselectivity of polypyrrole with respect to the chloride ion in aqueous electrolytes. Chloride ions contribute approximately 75-85% to the transport of the total charge during the redox of polypyrrole in 0.1 M KCl. The residual charge is partitioned between H+ and K+ depending on the initial pH of the electrolyte, which was varied from approximately 1 to approximately 7 in this study. Potassium ion transport was further verified by X-ray photoelectron spectroscopy. The extent of permselectivity is also dependent on the nature of the electrolyte cation, the bulky tetraethylammonium ion showing the most ideal behavior in this regard. Redox cycling of polypyrrole in 0.1 M CsCl and 0.1 M Ha was accompanied by appreciable cation movement especially at potentials below approximately -300 mV (vs Ag/AgCl). Finally, the ion transport during the redox of poly(pyrrole chloride) was found to be intrinsically asymmetric in two related respects: first, the redox charge monitored via chronocoulometry in the two switch directions, i.e., oxidized/reduced, was not identical. Second, the measured redox charge was only 27-35% of that theoretically expected from the polymer doping level for the 0.1 M KCl electrolyte. These data underline the inadequacy of the chronocoulometry/voltammetry procedure for assay of the doping level in conducting polymers such as polypyrrole, especially for rather thick (>a few hundred nanometers) polymer films.