Tuning the morphologies and electrical properties of azobenzene-4,4′-dicarboxylate-doped polypyrrole via ultraviolet light irradiation and medium pH alteration

In this work, the azobenzene group-containing carboxylic acid salt azobenzene-4,4′-dicarboxylate disodium salt (ADDS) was selected as the dopant for the controlled synthesis of nanostructured polypyrrole (PPy). By controlling the cis-trans configuration of ADDS (via ultraviolet light irradiation) as well as the concentration of divalent ADDS anion (via the medium pH alteration), a series of ADDS-doped PPy hydrogels with different morphologies and electrical properties were prepared accordingly. Under weak alkaline conditions (pH = 8.0), the morphology of the resulting PPy changes from nanofiber (nanobelt) to nanoflake and then to granular structure when the exposure time of ADDS aqueous solution to the UV-light irradiation was extended step by step from 0 to 15 min. This phenomenon is mainly due to the gradual decrease of the trans isomers in ADDS aqueous solution. Under visible light conditions, the morphology of the resulting PPy transforms from nanofiber (nanobelt) to granular structure when the pH of ADDS aqueous solution was varied from pH 8.0 to pH 3.0. This phenomenon is mainly due to the gradual decrease of the divalent doping anions (all trans isomers). Since trans ADDS isomers are co-planar linear molecules, and under weakly alkaline conditions, ADDS exists mainly in the form of divalent anion, the conductivity of PPy doped with all-trans ADDS (10.4 S cm-1) is therefore higher than that of PPy doped with UV-light irradiated ADDS (some trans isomers are transformed to the corresponding cis isomers) (1.0 S cm-1). It can be concluded that the morphologies and the electrical properties of ADDS-doped polypyrrole conducting hydrogels can be tuned just by varying the UV irradiation time and the medium pH, which results in the change of the steric configuration and the existing form of ADDS. The present new strategy is helpful for the controlled synthesis of conducting polymer hydrogels.