Fibrillar Growth in Polyaniline

Chemical oxidative polymerization of aniline in dilute aqueous acids using an ammonium peroxydisulfate oxidant proceeds via the intermediacy of large, colorless aggregates that act as ‘seeds’ in orchestrating the overall morphology of the product polyaniline. Observed for the first time by continuously monitoring the polymerization by static and dynamic light-scattering (LS) measurements, these aggregates are formed during the induction period just prior to the onset of polymerization and are believed to be aggregates of the anilinium cation and the peroxydisulfate anion. Depending on the reaction conditions, these aggregates can be spherical or rodlike with the latter leading to polyaniline with a bulk nanofibrillar morphology. These findings expand the role of our recently described ‘nanofiber-seeding’ method in which small quantities (seeds) of insoluble materials with nanofibrillar morphology seeds added just prior to the onset of polymerization help orchestrate bulk fibrillar polymer growth. Even conventional polyaniline nanofiber synthesis could be viewed as a ‘seeded-polymerization’ system, only this time the seeds are aggregates formed in situ during the early stages of the reaction. The intermediacy of these aggregates provides an important synthetic vector that can be leveraged to control nanoscale morphology in these systems. There are several synthetic approaches to the chemical synthesis of polyaniline nanofibers, for example, interfacial polymerization, nanofiber seeding, oligomer-assisted polymerization, surfactant-assisted polymerization, and nontemplate polymerization. The polyaniline system is particularly prone to fibrillar polymer growth compared to other conducting-polymer systems such as polypyrrole, poly(3,4-ethylenedioxythiophene), and polythiophene; for example, it has recently been reported that nanofibers are obtained by carrying out the reaction under dilute reaction conditions in the absence of templates. In one report it was shown that under relatively dilute conditions, while nanofibers are obtained when the oxidant peroxydisulfate is added all at once, only granular polyaniline is obtained when it is added dropwise. The rationale offered was that when the oxidant is added dropwise, secondary polymerization processes begin to disrupt the primary process, namely, fibrillar polymer growth that is favored when the oxidant is added all at once. However, in these reports, including those from our group, there has not been a clear mechanistic rationale for nanofiber formation in the polyaniline system, and, particularly, reasons why fibrillar polymer growth is intrinsically favored in the polyaniline system over other conducting-polymer systems are lacking. It is also important to note that in spite of the diversity in the synthetic approaches outlined above, the polyaniline nanofibers have strikingly similar morphology, that is, they generally precipitate from the reaction mixture as a mat composed of 30–60 nm diameter nanofibers. We believe this points to a common underlying mechanism. In our earlier reports on the synthesis of polyaniline nanofibers, we raised the possible role of the soluble aniline dimer intermediate, N-phenylphenylenediamine, in directing fibrillar polymer growth. This was based on an early study on electrochemically synthesized polyaniline where addition of small amounts of aniline dimer resulted in fibrillar polyaniline. While the role of the aniline dimer was unclear at that time, we believe that the process by which the dimer is formed, that is, processes governing the induction period of the reaction, could be important in the evolution of the bulk polymer morphology.