Oxidative and polymeric acid doping of polyaniline and related Donnan phenomena

The main purpose of this study was: (A) To attempt to determine the oxidation state of polyaniline as synthesized by the conventional method, (B) To determine factors affecting the degree of doping (protonation) of polyaniline by the monomeric acid, HCl, (C) To investigate the doping of polyaniline using two novel approaches, namely: (1) self-protonic-acid doping by using polymeric acids as dopants and (2) chemical oxidative doping, (D) To study gas separation properties of polyaniline membranes. The most significant results and conclusions are: (A) Volumetric analysis (employing TiCl$\sb3)$ of polyaniline prepared by conventional procedures show that the polymer obtained is in a significantly higher oxidation state than the desired, assumed emeraldine oxidation state. When prepared in the absence of air (argon atmosphere) it is obtained in an oxidation state significantly lower than the emeraldine oxidation state. These observations suggest that some of the studies on polyaniline in which it was assumed to be in the emeraldine oxidation state may need re-interpretation. (B) The degree of doping (as measured by conductivity) by protonation of polyaniline (emeraldine base) depends not only on the pH of the dopant solution but also on the presence or absence of neutral salts such as KCl. Dissolved KCl (0.5M) increased the conductivity of the (dry) powder by $\sim$10$\sp9$ at pH $\sim$ 5, consistent with Donnan Equilibrium effects. (C.1) The use of a polymeric acid such as polyvinylsulfonic acid results in protonic acid doping up to pH $\sim$ 9, at which value the HCl-doped polyaniline is completely undoped and is an insulator, consistent with Donnan Equilibrium effects. (C.2) Chemical oxidative doping (by Cl$\sb2)$ of polyaniline (leucoemeraldine base) has been demonstrated for the first time to give the same highly conducting polymer as that obtained by the non-oxidative (protonic acid) doping of emeraldine base. (D) Excellent separation of O$\sb2/$N$\sb2$ and H$\sb2/$N$\sb2$ gas mixtures by polyaniline membranes as reported by Reiss and Kaner (1990) has been confirmed