Surfactant-assisted control of the surface energy and interfacial molecular interactions of polypyrrole

Polypyrrole was chemically synthesized by oxidative polymerization of pyrrole in aqueous solution of ferric sulfate, and an anionic surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT). At low AOT loading, this anion acts essentially as a co-dopant of polypyrrole (PPy) chains, whereas at high loading it not only co-dopes PPy in the bulk of the particles, but acts also as a surfactant as proved by X-ray photoelectron spectroscopy (XPS) and inverse gas chromatography (IGC). The surface energy of the powder specimens was determined by IGC. At 30 °C, the dispersive contribution to the surface energy (γSd) decreased from 62.9 for sulfate-doped polypyrrole to 36.6 mJ/m2 for a polypyrrole with the highest AOT loading. This minimization of the surface energy was found to be beneficial for the dispersion of polypyrrole and the formation of conductive paths in a host polypropylene matrix, as judged from transmission electron micrographs. Moreover, the examination by scanning electron microscopy (SEM) of the fracture surface of polypyrrole-filled polypropylene indicated a better wetting of the AOT-containing polypyrrole by the polypropylene matrix compared to the pure polypyrrole powder particles. The results are interpreted in terms of minimization of the polypropylene–polypyrrole interfacial tension.