Density Functional Study of Surface Forces in Athermal Polymer Solutions with Addititve Hard Sphere Interactions. Solvent Effects, Capillary Condensation and Capillary-Induced Surface Transitions.

A density functional theory for polymer solutions is generalized to cases where the monomers have
a different diameter to the solvent. An appropriate free energy functional is obtained by integration
of the generalized Flory equation of state for such systems. This functional predicts that entropic
demixing may occur in polymer solutions in which the solvent particles are smaller than the
monomers. Demixing is promoted not only by a large size disparity, but also by a high pressure as
well as by polymer length. The existence of two separate phases in the bulk solution suggests the
possibility of capillary-induced phase transitions, even when the confining surfaces are hard, but
otherwise inert. We examine such phase transitions and their relation to surface forces and colloidal
stability. The density functional theory also predicts that under certain conditions, layering
transitions will occur at hard and flat surfaces. A transition from a thin to a thick polymer-rich
surface layer may take place as the separation between two surfaces is decreased, and we study the
concomitant change on the surface force. Stable thick phases are predicted even at very large
undersaturations, and they give rise to a profound increase of the range and strength of the surface
force.We furthermore include comparisons with predictions from a model in which the solvent only
enters the description implicitly. Responses of the surface forces to changes in monomer diameter,
solvent diameter, polymer density, and chain length are investigated.