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Electronic and structural effects on the electrochemistry of polypyrrole Warren, Mya

Abstract

The electrochemical properties of polypyrrole are influenced by a complex interconnection of electronic, structural, and chemical properties of the polymer matrix. These properties were investigated individually in polypyrrole doped with the hexafluorophosphate ion in order to create a complete model of polypyrrole electrochemistry. The electronic conductivity of polypyrrole was investigated through D C conductivity measurements of films in varying oxidation states. The well-known doping induced metal-insulator transition was observed at an electrode potential of -0.1V versus standard calomel electrode (SCE). X-ray diffraction and macroscopic actuation measurements revealed that a structural change begins at the same potential, possibly due to a loss of pi-stacking in the polymer crystals. Electrochemical impedance spectroscopy was performed over a range of oxidation states, and showed that the impedance goes through a sudden transition at -0.1V as well, which is attributed to changes in ionic conductivity. The non-linear dependence of the electronic and ionic conductivity of polypyrrole with oxidation state were included in a finite difference time domain model which simulated potentiostatic charging and discharging of the polymer matrix. The modelv was able to explain the moving phase front phenomenon during charging of polypyrrole that has been observed in the literature. The front is a shock wave that occurs whenever the polymer is stepped from a state of low to high ionic or electronic conductivity. It also presents an explanation for the redox peaks observed in cyclic voltammograms that does not rely on the Nernst equation, and is therefore more consistent with the electronic nature of polypyrrole.

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