Supercritical-fluid fractionation of poly(ethylene-co-acrylic acid)

Poly(ethylene-co-acrylic acid) copolymers of 3.1 and 3.9 mol % acrylic acid are fractionated with supercritical propane, butane, and dimethyl ether. The fractionations are performed isothermally using increasing pressure profiling that provides gram-size fractions with molecular-weight distributions of similar to 1.2 to 3.0 as compared to the parent copolymers which have polydispersities near 6.0. It is possible to fractionate these acid copolymers with respect to chemical composition by first using one of the poor quality solvents, propane or butane, that solubilize the nonpolar ethylene-rich oligomers and then using dimethyl ether, a very strong solvent for these acid copolymers, to solubilize the acid-rich oligomers. The performance of the SCF fractionation is directly related to the cloud-point behavior exhibited by the copolymer-solvent mixture.