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

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Abstract
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