Growth instability in diffusion controlled polymerization

We report the observation of a sharp transition in the structure of polypyrrole prepared via diffusion controlled polymerization in a quasi-two dimensional electrochemical cell. As the oxidation potential is increased, the fractal dimension drops sharply from values near 2 (compact structures) to 1 as the growth becomes dendritic. At higher potentials, where the polymerization becomes diffusion limited, a continuum of structures is observed as the dendrites become irregular and the fractal dimension increases to 1.74+-0.01. The mean branch angle was also measured and found to decrease from 90° at the point of dendritic growth, to 45° at high potential.