Reentrant Stabilization of Grafted Nanoparticles in Polymer Solutions

Polymer chains grafted onto nanoparticles may facilitate the dispersion of such particles in a polymer solution. We explore the optimal strategy for stabilizing polymer-grafted nanoparticles using self-consistent field theory and experiments. The best results are obtained for relatively low grafting densities and for chain lengths of the brush polymer N_B larger than that of the freely floating polymers N_f. When N_f > N_B, one finds a compatibilization gap and re-entrant stabilization: At both very low and very high polymer concentrations particles disperse in the polymer solution, while at intermediate concentrations the particles lose their colloidal stability. At low grafting densities the underlying surface is in contact with the solvent. Particles covered by a bidisperse brush can combine a low grafting outer region with full coverage of the surface by a densely grafted inner layer. Using classical colloid-chemical stabilization criteria the region in the phase diagram for which the particles are expected to mix with a concentrated polymer solution opens up. Now, also upon an increase in the length of the freely dispersed polymers, a re-entrant colloid-chemical stabilization is found for particles on the nanometer length scale: At both short and long polymer chains in solution the particles will not aggregate, whereas at intermediate lengths the colloidal stability is marginal. This multi re-entrant behavior is found from numerical self-consistent field calculations, and these predictions are consistent with corresponding experiments.