Electrochemical studies on conducting polymers: copolymerization, ion transport, restructuring, overoxidation and reactivation

Qi, Zhigang (1992) Electrochemical studies on conducting polymers: copolymerization, ion transport, restructuring, overoxidation and reactivation. Masters thesis, Memorial University of Newfoundland.

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Abstract

The electrochemical copolymerization of 1-methyl-3-(pyrrol-1-ylmethyl)pyridinium (MPMP⁺) with 3-methylthiophene (MeTh) has been accomplished from a wide compositional range of monomer solutions. An x-ray emission analysis method was developed to determine elemental composition and oxidation levels of such copolymers. A linear relationship was found to exist between the polymerization current density and the composition. The morphologies of the cationic copolymers were studied using scanning electron microscopy with the conclusion that surface roughness increases with increasing MeTh content and film thickness. Dual electrode and AC impedance measurements revealed that both the electronic and ionic conductivities of the copolymers increased with increasing MeTh content. -- When poly(MeThx-MPMP⁺) was studied in aqueous Fe(CN)₆³⁻/⁴⁻, it was found that the anodic to cathodic peak height ratios for Fe(CN)₆³⁻/⁴⁻ were about 1:1 and 0.75:1 at fast and slow scan rates, respectively. This difference was concluded to arise from the competition between HPO₄²⁻ and Fe(CN)₆³⁻/⁴⁻ during the redox process. HPO₄²⁻ is kinetically favoured, while Fe(CN)₆³⁻/⁴⁻ is thermodynamically favoured as the charge compensating counter ion in the copolymer. At fast scan rates, HPO₄²⁻ is the species that moves into and out of the copolymer to balance the excess charge during the redox process. At slow scan rates, Fe(CN)₆³⁻/⁴⁻ transport balances the excess charge during the redox process. It was also found that Fe(CN)₆³⁻/⁴⁻ in the copolymer could not be replaced by HPO₄²⁻ even in aqueous 0.1 M K₂HPO₄, and that cations such as K⁺ and Na⁺ were not involved in the ion transport process. -- Another finding was that the electrochemical response of a Fe(CN)₆³⁻/⁴⁻ saturated copolymer decreased with time. Since x-ray emission analysis revealed that the amount of Fe(CN)₆⁴⁻ in the film did not change during this decrease, it was realized that this decrease resulted from the formation of a strong association between Fe(CN)₆⁴⁻ and the cationic sites in the copolymer. This association limits the mobility of Fe(CN)₆⁴⁻ ions, which in turn leads to a slower charge transport rate between Fe(CN)₆³⁻/⁴⁻ sites. The whole process has been termed restructuring of the copolymer film. The electrochemical activity of restructured copolymers could not be completely restored by C10₄⁻ exchange and resaturated with Fe(CN)₆³⁻/⁴⁻. Restructured films retarded the reentry of Fe(CN)₆³⁻/⁴⁻ into the copolymer. -- Two methods to reactivate poly(3-methylthiophene) films overoxidized in Cl⁺ solution have been discovered. The first is an electrochemical method. The overoxidized polymer was reactivated in Et₄NClO₄/acetonitrile when the potential exceeded 0.9 V. The other method is to oxidize the overoxidized polymer with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The overoxidation process is proposed to occur through a nucleophilic addition between oxidized poly-MeTh and Cl⁺. The reactivation process is believed to involve further oxidation of the overoxidized polymer with the elimination of protons. Oxidation of poly-MeTh in Br⁻ and I⁻ solutions is also discussed.

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