In situ doping polymerization of pyrrole with sulfonic acid as a dopant

Effects of sulfonic acid on solubility, electrical and thermal properties as well as morphology of polypyrrole (PPy) prepared by in situ doping polymerization of pyrrole in the presence of sulfonic acid as a dopant have been investigated. It was found that sulfonic acids which have a good solvating ability, e.g. 5-butylnaphthalene sulfonic acid (BNSA), β-naphthalene sulfonic acid (NSA) and p-dodecylbenzene sulfonic acid (DBSA), render PPy soluble in m-cresol, while sulfonic acids, e.g. camphor sulfonic acid (CSA) and p-methylbenzene sulfonic acid (MBSA), cannot. Therefore, it was proposed that a good solvating ability of sulfonic acid induces the solubility of PPy. The UV-Vis and ESR spectra indicated that the charge carriers in PPy doped with NSA, BNSA, CSA, DBSA, p-hydroxylbenzene sulfonic acid (HBSA) and 5-sulfo-isophthalic acid (SIA) are polarons and bipolarons, while only bipolarons serve as charge carriers in PPy doped with alizarin red acid (ARA) and 8-hydroxy-7-iodo-5-quinoline sulfonic acid (QSA). The nature of sulfonic acid also has influence on morphology and thermostability of the resulting PPy. It was found that the polymerization medium significantly influences the solubility and conductivity of PPy. The in situ doping polymerization of pyrrole in organic solvent such as CHCl3, CH3NO2 or THF gave soluble but nonconductive PPy, compared with insoluble but conductive (σ = 18 S/cm) PPy prepared in water.