Change of mechanical and electrical properties of polypyrrole films with dopant concentration and oxidative aging

The process of loss of mechanical and electrical properties of polypyrrole (Ppy) was studied for up to 12 months of aging in air at room temperature. Tensile strength of films highly doped with p-toluene sulphonate was lower than that of lightly doped ones. This difference is attributed to the presence of stress concentrations on the nodular surfaces of the films with high dopant concentrations. Highly doped films were electrically more stable, with a lower rate of loss of conductivity with time, compared with lightly doped ones. The conductivity of the latter initially decreased at a different rate than that observed after a prolonged time, exhibiting two regions in the aging behavior. In contrast, highly doped PPy films followed first-order kinetics with a very small loss of conductivity over the aging period. The rate of decline of conductivity was dependent on aging time, dopant type, and concentration, suggesting a complex mechanism of conductivity decay. Examination of FT-IR spectra of aged Ppy films revealed an increase in intensity of an α,β-unsaturated conjugated carbonyl peak, which may be correlated with the loss in conductivity.