THEORY OF POLYMER ADSORPTION AND COLLOID STABILIZATION IN SUPERCRITICAL FLUIDS - 2 - COPOLYMER AND END-GRAFTED STABILIZERS

The interaction of two planar surfaces coated with polymeric steric st abilizers is described as a function of polymer conformation with latt ice-fluid self-consistent field theory. For block copolymer stabilizer s, as the solvent density and thus solvent quality are lowered, expans ion of the stabilizer block due to excluded volume interactions associ ated with an increase in adsorption competes with collapse due to a lo ss in stabilizer solubility. In contrast, grafted chains only collapse as solvent density is lowered. The effects of changing anchor and sta bilizer solubility, block lengths, and anchor adsorption energy are ex plored. For two surfaces coated with either adsorbed copolymer or graf ted stabilizers, free energies of interaction are calculated and used to identify the critical flocculation density(CFD). The CFD occurs ver y close to the upper critical solution density (UCSD) for the same mol ecular weight stabilizer tail in bulk solution. This correspondence be tween a surface property (CFD) and a bulk property (UCSD) is in agreem ent with stability experiments in supercritical CO2. Based on the free energy calculations, we identify design criteria for effective end-gr afted and copolymer stabilizers.