A study of the structure and morphology of poly(p-phenylene) films electrosynthesized in liquid SO2 with different salts at low temperatures

The structure and electrochemical properties of poly(p-phenylene) (PPP) films synthesized by oxidation of benzene in SO2 were investigated by IR, UV, scanning electron microscopy and X-ray diffraction. In the presence of a Lewis acid (SbF5) or boron trifluoride etherate complex ((C2H5)2OBF3), the potential of benzene oxidation is shifted to low values and the lowest potentials are obtained when the interaction between benzene and the acid (π-complex) is the strongest (1.6 and 1.1 V with (C2H5)2OBF3 and SbF5, respectively). As a general rule, the porosity of the PPP films decreases with temperature of synthesis, and their morphology depends on the synthesis conditions. They are amorphous with LiAsF6, but crystalline with other salts such as TBAX (X = BF4−, SbF6−), with SbF5 or(C2H5)2OBF3. A hexagonal structure (a = 11.5 Å, c = 4.2 Å) is found with TBABF4 (crystallite size around 150 and 200 Å), and an orthorhombic system with TBASbF6 (a = 16.50 Å, b = 13 Å, c = 4.12 Å) or with SbF5 (a = 15.03 Å, b = 11.25 Å, c = 4.2 Å). The surface of the films is quite smooth in the presence of (C2H5)2OBF3 or LiAsF6 and very rough with ammonium salts and SbF5. The homogeneity of the films, which is a volume property, is very good with films formed with LiAsF6 or SbF5 at −75 °C and is evidenced by n-doping in THF + TBABF4, which leads to a uniform blue colour on the whole surface. On the contrary, the homogeneity of films formed with (C2H5)2OBF3 is poor, and attested by the appearance of scattered blue stains by n-doping.