Effect of block and graft copolymers on the stability of colloidal silica

We use dynamic light scattering to measure the time-dependent increase of the average hydrodynamic radius of colloidal silica particles in the presence of salt. This increase appears to be linear in time up to twice the radius of unaggregated particles. The method is a very useful tool in monitoring the stabilizing effect of adsorbed polymers. Four different series of diblock and graft copolymers were used to stabilize an aqueous silica dispersion against aggregation by salt. For two series of nonselectively adsorbing diblock copolymers with relatively low adsorbed amounts, we found a clear correlation between the adsorbed amount and the stabilization: a higher adsorbed amount leads to slower aggregation and thus provides a better stabilization. This is rationalized in terms of retarded bridge formation. Excellent steric stabilization was obtained with a series of amphiphilic diblock copolymers and with two graft copolymers. All these polymers produce adsorbed layers with a clear "anchor-buoy" structure, either due to selective solvency (amphiphilic diblocks) or selective adsorption (graft copolymers). Such layers cannot form bridges so that the layers are purely repulsive. (C) 1999 Academic Press.