Year

1998

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

Linear sweep voltammetry - resistometry, UV-vis and Raman spectroscopy, Matrix Assisted Laser Desorption Ionisation mass spectrometry (MALDI-MS) and cyclic voltammetry were employed to study the overoxidation of polypyrrole and substituted polypyrroles. Consideration was given to a range of parameters that affect the degradation of this polymer in an aqueous environment: incorporated counterion, supporting electrolyte solutes and pH, substrate, film thickness, scan rate and applied potential.

From these studies it was observed that the major contributions to ease the overoxidation potential of polypyrrole are the applied potential and supporting electrolyte pH. It was also found that variations in the overoxidation potential corresponded to the ordering of cations and anions in the lyotropic series. The most stable non-substituted polypyrrole films were those containing mono-cationic sulphonates as dopants. However, the substituted polymer poly(3-octylpyrrole) was shown to have by far the highest stability.

Based on Raman spectroscopic studies, a mechanism for the facile overoxidation of poly(3-methylpyrrole-carboxylic acid) was proposed. Further, a chain breaking mechanism for the overoxidation of polypyrrole was supported by MALDI-MS studies. This commenced with the formation of adjacent imine-nitrogens (-N=) at one end of a six pyrrole unit to initiate chain breaking by oxygen attack. At various levels of overoxidation, oxygen-containing and oxygen-free pyrrole dimers, trimers, tetramers, hexamers and heptamers were observed.

From this study it is suggested that realistic anodic limits for practical applications of polypyrrole in an acidic aqueous environment are as low as 0.5 V (vs Ag/AgCl). However, polypyrrole is so susceptible to degradation in basic conditions that it is not possible to suggest any anodic limit. In free-standing two-electrode polypyrrole devices operating in air long term operation can be ensured by limiting the potential difference to ±1.5 V.

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