Aging effects in polypyrrole probed by positron annihilation

Abstact on annihilation spectroscopy and conductivity measurements have been used to probe aging-induced changes in parahydroxybenzenesulfonate doped polypyrrole. Polypyrrole was synthesized by a chemical oxidation process and aged in air at 110 °C and 140 °C up to 250 days. Positron annihilation experiments reveal modifications of the electronic structure accompanied by a dramatic loss of conductivity as a function of the aging time and temperature. The electron-positron momentum distribution at the 511 keV annihilation peak exibits a strong shift towards higher momentum annihilation events upon aging. The experimental results are explained in terms of positron trapping in the vicinity of negative dopants whose concentration is reduced after aging.