Cosolvent studies bath the system ethylene/poly(ethylene-co-acrylic acid): Effects of solvent, density, polarity, hydrogen bonding, and copolymer composition

In the industrial high-pressure polymerization process, the manufacture of ethylene-based copolymers like poly(ethylene-co-acrylic acid) must be performed at very high pressures (up to 300 MPa) to avoid demixing due to unfavorable interactions between product (polymer) molecules and molecules of the educt mixture (ethylene and comonomer). Suitable inert cosolvents may help to lower the operating pressure and yet still operate in a single-phase region. In this paper, phase-equilibrium data are presented for mixtures of ethylene and ethylene-co-acrylic acid copolymer with and without the incorporation of various cosolvents in varying amounts. It was found that nonpolar but dense cosolvents like heptane, 2,2,4-trimethylpentane, and decane match the cosolvent potential of a polar but nonassociating substance like AcOEt, especially at the higher temperatures investigated. Strongly polar and associating fluids like EtOH and octanoic acid are superior cosolvents. However, for high concentrations of EtOH and lower temperatures, a shift in phase behavior with increasing demixing pressures has been observed and is attributed to self-association of excess EtOH molecules. This phenomenon is not observed for AcOEt and octanoic acid.