Aggregation modes of colloids in the presence of block copolymer micelles

Abstract

We investigated the aggregation of colloidal latex induced by the interparticle bridging phenomenon as a function of the surface density of link-forming sites. To perform aggregation experiments on a model system, of reproducible geometry, an unstable colloid was obtained by adsorbing stable spherical diblock copolymer micelles onto latex particles. The conformation of this site, active in the bridging mechanism, did not change either on adsorption or with time, and thus the modified colloid constituted a model to investigate colloid aggregation induced by interparticle polymer bridging. Different aggregation modes were determined in experimental studies of the kinetics of aggregation processes. Dynamic scaling laws were established, which were compared to those determined by computer simulation and theory. By increasing the number of micelles adsorbed onto one latex particle, the following flocculation modes were observed. At very low surface coverage, irreversible reaction-limited processes were found to generate agglomerates having a size frequency function c n ( t) scaling like t − τ with τ equal to 1.50. At a given polymer coating, reversible diffusion-limited aggregation was found to describe the process and a bell-shaped size distribution curve was obtained. At greater surface occupancy, the irreversible reaction-limited aggregation mode was recovered. Due to the presence of adsorbed copolymers, interactions between the colloid/micelle complexes were modified, leading to different reactivity in particle-particle agglomeration. As a result, at each time of the flocculation period, the cluster size distribution c n ( t) scaled like t − τ with a modified exponent τ equal to 1.65. The existence of the unexpected diffusion-limited aggregation process, not evidenced in the presence of the parent flexible polymer, was interpreted in terms of topological constraints governing the collision efficiency in the sticking process. In fact, a minimal surface coverage was found to be necessary to induce this aggregation mode. Above and below this critical domain, the aggregation developed with features characteristic of the classical bridging mechanism induced by water-soluble flexible homopolymers. Diblock copolymers seemed to constitute a new class of very efficient polymeric flocculants.