Onset of fractal growth: Statics and dynamics of diffusion-controlled polymerization

We report experimental studies of the statics and dynamics of diffusion-controlled polymerization. The polymerization involves the random aggregation of a neutral precursor, pyrrole monomer, in a two-dimensional electrochemical cell under a variety of conditions. 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 more irregular and the fractal dimension increases asymptotically to 1.74±{}0.01. The mean branch angle is found to decrease sharply from 90^∘{} in the dendritic regime to approximately 45^∘{} in the diffusion-limited regime. In this regime the width of the active growth zone increases at the same rate as the fractal radius. The spectral dimensionality determined by scale-dependent conductivity measurements is 1.26±{}0.04. .AE