Enthalpic stabilization of brush-coated particles in a polymer melt

The interaction between spherical particles covered with end-grafted polymers (brushes) immersed in a polymer melt is studied theoretically. It has been known for some time that two densely grafted brushes may attract each other in the presence of a chemically identical polymer melt. In this study we show that this attraction can be eliminated by choosing the melt material to be such that the Flory interaction parameter of the melt chains with the grafted ones is negative (chi < 0). This effect is accompanied by a change in the structure of the brush from a dry brush, where the melt chains do not penetrate deeply into the brush, to a wet brush, where the melt chains penetrate the brush and force the grafted chains to extend into the melt. Scaling arguments are used to describe how the structure of a single brush depends on the Flory interaction parameter, the grafting density, and the indices of polymerization of the grafted and free chains. The density profiles and the interactions between the particles are calculated by solving numerically the self-consistent field equations of the system within the Derjaguin approximation. The effect of van der Waals interactions between brushes is studied by taking into account the particle-brush contrast as well as that of the brush-melt interfaces.