Morphological Evolution of Self-Assembled Polyaniline Nanostuctures Obtained by pH-stat Chemical Oxidation

The chemical oxidation of polyaniline is carried out at constant acidity, investigating six separate pH levels (pH = 1, 2, 3, 4, 5, and 6). All prior studies on polyaniline self-assembly used a falling-pH or a temporarily buffered falling-pH method, making this the first application of pH-stat techniques to the study of polymer nanostructure formation. This allows us to isolate with improved precision the various nanostructures formed at different stages of polyaniline self-assembly, which include spheres, flakes and ribbons at neutral pH, fibers and tubes at mildly acidic pH, and grains at highly acidic pH. The evolution of these structures is investigated as a function of both pH and time. This approach results in the observation of two previously unnoticed phenomena: the morphology is found to evolve as a function of time even when pH is kept constant, and polyaniline nanotubes are formed at a constant acidity. Both of these results have important implications for existing self-assembly theories. Fourier transform infrared spectroscopy is used to investigate the polyaniline compositional and structural evolution underlying the visible morphological changes and shows that as pH falls there is increased protonation, decreased orthocoupling, lower phenazine presence, and less sulfonation. The time evolution, on the other hand, shows increased sulfonation and decreased oxidation. Ultraviolet-visible spectroscopy shows a mixture of delocalized and localized polaron behavior for the pH-1-stat to pH-3-stat products, and insulating characteristics for the higher pH structures. This is confirmed by four-point conductivity measurements.