Insulator—semiconductor composite polyoxyphenylene—polypyrrole: electrochemical synthesis, characterization and chemical sensing properties

A study of the in situ electrochemical preparation of conductive composite films of polypyrrole and non-conducting polyoxyphenylene is presented. Electropolymerizations of pyrrole were carried out in the presence of the phenolic monomers 2-allylphenol (2AP) and 4-hydroxybenzenesulfonic acid sodium salt (4HBS) in an aqueous alkaline solution (ph=9) of tetramethylammoniumhydroxide. The preparation conditions were optimized in order to obtain flat, smooth and pinhole-less electroactive films, which were characterized by measuring their optical (UV—Vis and IR), morphological (scanning electron microscopy) and electrical properties (a.c. conductivity). The optimized polymerization conditions led to the growth of lightly overoxidized films, whose properties depart slightly from the usual wellknown properties of highly electroactive polypyrrole films. Some of the investigated properties resemble those of conducting polypyrrole; however, they showed some differences, mainly in the optical and electrical characteristics due to the preparation of an inert matrix, produced by the simultaneous polymerization of the phenolic compound, 2AP. IR absorption spectroscopy indicates that the second phenolic compound, 4HBS, serves as a dopant for polypyrrole and, therefore, enables the polymerization of a conductive film in alkaline media. The sensing properties of the polypyrrole—polyoxyphenylene (PPy—POP) films towards dimethylmethylphosphonate (DMMP) were investigated by work function measurements and compared with commonly prepared PPy films. Tetrasulfonated metallophthalocyanines (MPcTS, M=Ni, Cu, Fe(II), Co) and 4HBS were used as dopants for PPy. The response time (t90%) of the sensitive layers decreases from 20–40 min for PPy films to 3–5 min for PPy—POP films, which was assumed to be due to the lower packing density of the composite film. The sensitivity of the PPy—POP layers to DMMP lies in the range of 66–76 meV (ppm decade)−1.