Effect of molecular architecture on the phase behavior of fluoroether-functional graft copolymers in supercritical CO 2

Abstract

A series of graft copolymers, poly(methyl methacrylate-co-hydroxyethyl methacrylate-g-poly(perfluoropropylene oxide)), has been synthesized. The backbone, poly(methyl methacrylate-co-hydroxyethyl methacrylate), is poorly soluble in carbon dioxide while the grafted poly(perfluoropropylene oxide) chains are miscible with carbon dioxide at relatively low pressures. We have varied the graft chain length, the graft chain density, and the backbone length of these graft copolymers, and examined the effect of these variables on their phase behavior in carbon dioxide at 40°C. At lower molecular weights, as the concentration of fluoroether groups in the copolymer increases, the cloud-point curves shift to lower pressures, despite the increase in the copolymer molecular weight. At some point, however, the effect of molecular weight overwhelms that of increasing CO 2-philic fraction in the copolymer, and further increases to the fluoroether fraction shift the phase boundary to higher pressure. Thus, there appears to be an optimum extent of grafting which produces materials with the lowest cloud-point curves. Molecular architecture also influences the phase behavior, in that copolymers with many short side chains exhibit phase boundaries at lower pressures than copolymers with the same overall concentration of fluoroether in the form of longer yet fewer side chains. This effect is likely due to a more favorable entropy of mixing and free volume effect in the former case and a better degree of mixing.