Processing-Induced Changes in the Local Structure of Amorphous Polyaniline by Radial Distribution Function Analysis of X-ray Scattering Data

The local molecular structure of amorphous polyaniline (PANI) film in the emeraldine oxidation state has been studied using radial distribution function analysis of X-ray scattering data in combination with model calculations. As-cast emeraldine base (EB) films exhibit an interchain ordering that is significantly poorer than that for the intrachain structure. Protonic acid (HBr or HCl) doping of these films, to yield the electrically conductive emeraldine salt (ES) form of PANI, leads to significant changes in both the intrachain and interchain structure of the polymer chains. Structural changes in the intrachain ordering are found to be essentially reversible on undoping since, after all of the dopant is removed from ES films by immersion in a strong base solution, the original as-cast EB intrachain features become more pronounced. In contrast, evidence of irreversible changes in the interchain packing is observed during this doping/undoping process following the treatment with base. These changes in the EB local molecular ordering may be intimately coupled to the electronic properties intrinsic to conducting polymers and they occur in such a way that subsequent exposure to protonic acids initiates a highly site-specific redoping of the polymer host. These findings shed light on the doping-induced variations of the physical properties in conducting polymers such as charge and mass transport through polyaniline films.