Aggregation Kinetics of Coalescing Polymer Colloids

The aggregation behavior of a soft, rubbery colloidal system with a relatively low glass transition temperature, Tg ≈ −20 °C, has been investigated. It is found that the average gyration and hydrodynamic radii, ⟨Rg⟩ and ⟨Rh⟩, measured by light scattering techniques, evolve in time in parallel, without exhibiting the crossover typical of rigid particle aggregation. Cryogenic scanning electron microscopy (cryo-SEM) images reveal sphere-like clusters, indicating that complete coalescence between particles occurs during aggregation. Since coalescence leads to a reduction in the total colloidal surface area, the surfactant adsorption equilibrium, and thus the colloidal stability, change in the course of aggregation. It is found that to simulate the observed kinetic behavior based on the population balance equations, it is necessary to assume that all the clusters are spherical and to account for variations in the colloidal stability of each aggregating particle pair with time. This indicates that, for the given system, the coalescence is very fast, i.e., its time scale is much smaller than that of the aggregation.