Synthesis and Physical Properties of Highly Sulfonated Polyaniline

Sulfonated polyaniline (SPAN) is a self-doped conducting polymer. It has a high water solubility and a novel pH-dependent DC conductivity that is of interest for fundamental science and also for applications in such areas as rechargeable battery and pH control technologies. We report here the extensive characterization and details of synthesis of a new form of sulfonated polyaniline (LEB-SPAN) which shows novel or significantly improved chemical and physical properties. LEB-SPAN has a high sulfur to nitrogen ratio (S/N) of ∼0.75, 50% larger than that previously reported for EB-SPAN, S/N ∼0.50. This change in composition leads to significant alteration of the properties including an order of magnitude increase in the room temperature DC conductivity to ∼1 S·cm-1, nearly double the solubility in water, and a completely different pH dependence of the oxidation potential (E1/2). For LEB-SPAN the DC conductivity is unaffected by pH over the range 0 ≤ pH ≤ 14, strikingly different from the behavior of both parent polyaniline and EB-SPAN which become insulating for pH ≥3 and ≥7.5, respectively. Temperature-dependent DC conductivity and EPR measurements for LEB-SPAN reveal a lower activation energy for the conductivity and a higher density of states at the Fermi energy as compared with EB-SPAN. The dramatic differences in the pH dependence of the DC conductivity, cyclic voltammetry (CV), FTIR, and UV−vis results for LEB-SPAN and EB-SPAN are shown to be a consequence of the much higher S/N ratio in LEB-SPAN. We propose and describe a novel quasi-random oxidation model for the electrochemical oxidation of polyaniline and its derivatives at the microscopic level. This model quantitatively describes many of the phenomena and physical properties found in the polyanilines including the origin of the defect states and the in situ EPR signal during CV potential scans. Also the statistical nature of this model suggests its general applicability to the oxidation process of other conducting polymers. Computer simulations based on this model are presented and show good agreement with the in situ EPR/CV data reported earlier. In addition, other models are proposed to interpret the reported experimental differences in the pH dependence of E1/2 among LEB-SPAN, EB-SPAN, and its parent polyaniline samples. Mechanisms for the new sulfonation route are proposed.