Raman spectroscopy of conductive polypyrroles

Abstract

Recent studies of conductive polymers have focused on the need to produce air-stable processable materials. Since the discovery that conductive polypyrrole films could be prepared electrochemically [1], this polymer has attracted interest because of its much greater stability in air compared with polyacetylene [2]. Recently it has been shown that film morphology and mechanical properties are strongly influenced by the choice of electrolysis conditions. Films with thicknesses of 1 mm or more with properties ranging from rubbery to tough elastomeric can now be obtained with conductivities in the range 20 100 S/cm [3 5]. Factors that influence film properties are solvent, counter-ion and preparation temperature. It is clear that the differences observed at the macroscopic level reflect differences in polymer structure at the microscopic level. The ideal polypyrrole chain structure is a planar array of monomer units joined by ~ (2 5) bonds. This is not the only possible bonding sequence; other links also occur, as shown by the greater crystallinity that results when these links are blocked by the use of fi fl substituted monomers [6]. The elastomeric nature of the polymer films also suggests that they contain crosslinks. It has not yet proved possible to determine directly the concentration of these different defect structures in polypyrrole films by a non-destructive technique. In principle, Raman spectroscopy provides a method for characterization of polypyrrole films, since it has proved to be a powerful method for the study of other conjugated polymers [7]. To date there have been very few reports of Raman spectra for polypyrrole, particularly in the oxidized, conducting state [5]. The broad absorption bands and the rough sample surfaces result in dense black samples, which give weak Raman spectra. We have studied a number of samples prepared using different electrode materials and obtained well-resolved Raman spectra from the surface adjacent to the electrode. The spectra show well-resolved bands in the range 800 to 1600 cm 1. Differences in band positions and frequencies show a strong influence of the electrode material and growth temperature on polymer structure. As yet, a complete assignment of the observed spectra has not been possible. Model calculations and comparison of other polymer properties are being used to aid interpretation.