Nucleation of Polyaniline Microspheres and Interconnected Nanostructures from Strongly Coupled Micelle-like Systems

A mechanistic study on the nucleation of aggregates exhibiting a weak intermolecular coupling and high molecular mobility for major components, exampled by an entity of aniline and salicylic acid in the preparation of polyaniline microspheres (PANI-NS) and interconnected structures (PANI-NC), is explored by in situ H-1 NMR experiments. Three different procedures, namely, hydration of the aniline salicylic acid (SA) entity, removal of the extra charges to the surroundings, and sphere-to-rod transitions, afford the smooth nucleation of products in characteristic morphologies. At the beginning, water plays a fundamental role in attenuating the high chemical potential system by hydrating both the aniline SA entity and the in situ formed protons in the reaction, and removing the latter to bulk water when the chemical potential increment from the in situ produced proton is at a low initially. The driving force for this process is the increased intermolecular distances between aniline and SA induced by the electrostatic repulsions between positively charged protons in the entity, which paves the pathway for water in bulk to diffuse into the system. When a large amount of protons have been released in the reaction, the high chemical potential can be lowered down by repulsing both large and small sized positive charges to the external surroundings through electrostatic interactions or a sphere-to-rod structural transition initiated by continuously formed oligomers sheathed at the exterior of the spheres, which affords the formation of PANI-NS and PANI-NC, respectively. The competition of the two depends on the relative amplitude between the releasing rate of the protons and the mechanical strength of the aniline-SA entity in the reaction. Our work demonstrates that in situ dynamic NMR experiments such as measurements of NOE and spin- lattice relaxation times, and line shape analysis, provide new perspective powers for resolving the formation profile and more importantly the driving forces for each procedure at the molecular scale.