Optical and electrical characterization of a conducting polypyrrole composite prepared by in situ electropolymerization

A study of the optical and electrical properties of a conducting polypyrrole-polyoxyphenylene composite, PPy-POP, prepared by in situ electropolymerization is presented. Electropolymerization was performed potentiostatically in a solution of pH 9 which contained the monomers pyrrole, allylphenol and sodium 4-hydroxybenzenesulfonate (4HBS), by applying a potential of 1.25 V vs. SCE. The films obtained were characterized optically by UV/VIS and IR spectroscopy and electrically by measurements of the temperature dependence of the ac and dc conductivity. FTIR measurements indicated that the polymer blend obtained consists of PPy and the insulating polymer poly-2-allyloxyphenylene (POP), whereas the third monomer, 4HBS, is incorporated into the PPy-POP film as dopant for the conducting PPy. Furthermore, optical characterizations show a light degree of overoxidation of PPy in the PPy-POP composite. In the UV/VIS spectra, the formation of both polaron and bipolaron electronic states of the band structure of PPy can be seen, but the IR spectra demonstrate the transition of the PPy structure from a conducting quinoid to benzoid type with increasing polymerization potential. This is accompanied by the introduction of a carbonyl group into the PPy backbone and a reduction of the conjugation length of the polymer chain, which has a strong influence on the conductivity of the polymer composite. Despite this overoxidation process, the PPy-POP film retains a conductive character which allows the growth of thick films. The temperature dependence of the ac and dc conductivity of PPy-POP was investigated. The total ac conductivity, σ(tot)(ω), in the frequency range 10²-10⁵ Hz, changes by approximately four orders of magnitude in the range from 77 to 300 K, showing a sub-linear dispersive behavior. The temperature dependence of the dc conductivity of such a polymer composite can be described by Mott's variable range hopping (VRH) model according to σ = σ₀ exp[-(T₀/T)(γ)], with y = 1/2, which corresponds to a pseudo-one-dimensional conduction process.