Coagulation is frequently applied to polymer particles e.g. as a separation step after emulsion polymerization. The resulting cluster structure greatly influences the properties of the resulting material. Universal cluster structure has been found for aggregates of rigid particle systems [1]. In contrast, soft particle clusters may undergo densification after aggregation due to polymer chain diffusion. The extent of this sintering or coalescence process is governed by surface properties of the primary particles.

In order to investigate this phenomenon the aggregation kinetics of industrial elastomer latices has been studied within the industrially relevant aggregation regime of diffusion limited cluster aggregation (DLCA) in quiescent fluid. The aggregation extent was followed by particle sizing via dynamic light scattering (DLS). Simulation of the aggregation using the kinetic approach (population balances) allows an interpretation of the light scattering result in terms of cluster structure, which is described by the mass fractal dimension – the key parameter for fitting the average hydrodynamic radius resulting from simulation to experimental DLS data. Additionally the cluster structure has been probed by cryogenic electron microscopy. Our findings indicate that cluster structures evolving during soft particle aggregation strongly depend on the surface properties of the elastomer particles with respect to the latex stability. Bare elastomer particles, stabilized with surfactants (mobile charges) only, allow formation of dense, sphere-like clusters due to very strong sintering of primary particles. On the other hand sintering is practically absent if the particles carry a sufficiently high number of fixed charges. In agreement, latex particles that are stabilized by both fixed charges and surfactants form clusters of intermediate sintering extent.

References: [1] M.Y. Lin, H.M. Lindsay, D.A. Weitz, R.C. Ball, R. Klein, P. Meakin, Nature 339 (1989) 360-362.