Cluster-cluster aggregation in two-monomer systems

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Abstract
The authors have investigated the structure and growth kinetics of clusters formed in dilute solutions from two kinds of polyfunctional monomers. This was done by computer simulations within the framework of the cluster-cluster aggregation model with mass-independent cluster diffusion constants. The systems considered were of the type A2+Bf and Af+Bf, where A2, Af and Bf are different monomers. The monomer functionality f is equal to the coordination number of the lattice used in the simulations. The results of simulations obtained at different relative monomer concentrations are described and several novel effects are reported. Large clusters always have a fractal-like geometry but their effective fractal dimensionality, D, appears to depend on the composition of the mixture. Only in a restricted composition range does the aggregation process lead to a single large cluster. There, the cluster size distribution function Ns(t) can be described by the scaling form s-2f(s/tz). Outside this range, aggregation results in small clusters (oligomers) whose structure is governed by 'geometric selection rules'. Only certain non-consecutive oligomer weights are observed. The two boundary points of the interval in which kinetic scaling holds are shown to possess the properties of tricritical points.