Interactions between polymer-grafted membranes in concentrated solutions of free polymer

Polymers grafted or otherwise immobilized at surfaces are often used to stabilize particle suspensions in situations where nonspecific colloidal forces or specific chemical bonds act to coagulate the particles. Although the motivation is clear, criteria for steric stabilization are not well established for environments that contain large amounts of free polymer which can induce strong attraction through depletion forces. To examine these criteria, we have performed micromechanical tests of adhesion between pairs of large vesicles with membranes made from mixtures of phospholipids and phospholipids grafted with poly(ethylene oxide) PEO chains in aqueous solutions of free (nonadsorbing) PEO chains. Solutions were made with two free polymer fractions (Np ≈ 114 and 444 monomers in average length) over a large range of volume fractions between 0.01 and 0.2. Three graft chain lengths (Ng ≈ 45, 114, and 273 monomers) and several surface densities were used to extend the scale of surface roughness from ∼0.5 nm fo...