Effect of UV irradiation on polypyrrole as studied by electron spin resonance

The effect of UV irradiation on polypyrrole (PPy) doped with benzene sulfonate has been investigated by electron spin resonance (ESR). In this paper we examine the changes in the ESR spectrum parameters, i.e., peak-to-peak linewidth, asymmetrical parameter and spin concentration, referring to the change in the PPy conductivity. During UV irradiation, the linewidth of the ESR spectrum increased, probably due to the production of localized spins when the scission of the polymer chains starts to occur. After UV was switched off, the linewidth reverted to its initial value due to the re-unification of the polymer structure. Using high intensity irradiation, the spin concentration gradually decreased, probably due to the rearrangement of the polymer structure. This seemed to induce a decrease in the kink of the polymer, that is, a decrease of both polarons and bipolarons. The decrease of polarons is therefore related to the decrease of the relative spin concentration, and the decrease of bipolarons induces the degradation of the PPy conductivity.