Aging effects in polypyrrole probed by positron annihilation

Positron annihilation spectroscopy and conductivity measurements have been used to probe aging-induced changes in para-hydroxybenzenesulfonate doped p olypyrrole. Polypyrrole was synthesized by a chemical oxidation process and aged in air at 110 degrees C and 140 degrees C up to 250 days. Positron an nihilation experiments reveal modifications of the electronic structure acc ompanied by a dramatic loss of conductivity as a function of the aging time and temperature. The electron-positron momentum distribution at the 511 ke V annihilation peak exhibits a strong shift towards higher momentum annihil ation 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.