Polyaniline as Viewed from a Structural Perspective

The local structural ordering of polyaniline (PANI) as emeraldine (i.e., at 50% oxidation) in both its base (EB) and salt (ES) forms has been studied using x-ray diffraction (XRD) techniques and these data reinforce the general belief that this local structure is extremely sensitive to the specific processing history. PANI powders precipitated as ES immediately following synthesis lead to reasonably crystalline materials whereas EB films cast from n-methyl pyrrolidine (NMP) solutions containing hydrogen bonding inhibitors are fully amorphous. Dedoping, by further treatments of ES with NH ₄ 0H solutions, or doping, by immersion of EB in aqueous HF solutions, lead to further changes in the local molecular structure. Many of these structural phases bear a limited resemblance to the previously proposed structures for HCl-doped PANI powder and films [1,2]. The role of water hydration in the structural evolution is also examined and significant structural variations are found to occur in ES at radically different time scales.