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 stabilizers is described as a function of polymer conformation with lattice-fluid self-consistent field theory. For block copolymer stabilizers, as the solvent density and thus solvent quality are lowered, expansion of the stabilizer block due to excluded volume interactions associated with an increase in adsorption competes with collapse due to a loss in stabilizer solubility. In contrast, grafted chains only collapse as solvent density is lowered. The effects of changing anchor and stabilizer solubility, block lengths, and anchor adsorption energy are explored. For two surfaces coated with either adsorbed copolymer or grafted stabilizers, free energies of interaction are calculated and used to identify the critical flocculation density(CFD). The CFD occurs very close to the upper critical solution density (UCSD) for the same molecular weight stabilizer tail in bulk solution. This correspondence between a surface property (CFD) and a bulk property (UCSD) is in agreement with stability experiments in supercritical CO2. Based on the free energy calculations, we identify design criteria for effective end-grafted and copolymer stabilizers.