Correlation between One-Directional Helical Growth of Polyaniline and Its Optical Activity

A theoretical model for nanofiber growth of polyaniline (PANI) is proposed in consideration of the classical nucleation/growth theory and the structure of electric double layers, and then the correlation between the fibrillar growth of PANI and its optical activity is investigated in the present work. The formation of initial PANI nanofibers is accompanied by formation of electric double layers around the nanofibers due to the positive charges induced by the protonation and/or partial oxidation of PANI chains. The formed electric double layers may protect the initial nanofibers from heterogeneous nucleation but allow their elongation at their two ends. In the presence of optically active camphor sulfonic acid ((+)-CSA), the initial nanoseeds can elongate helically, resulting in highly optically active PANI. PANI products have been prepared under different conditions, followed by the measurements of their circular dichroism spectra. Being consistent with theoretical prediction, the experimental results have confirmed that the conditions favoring the formation of nanofibers would also yield a higher optical activity. Consequently, PANI nanofibers with high chirality are easily prepared under conditions of low aniline concentrations, at higher CSA concentrations, at room temperature, without stirring, and with UV light illumination.