SOLUBILITY AND DEMIXING OF POLYETHYLENE IN SUPERCRITICAL BINARY FLUID MIXTURES - CARBON-DIOXIDE CYCLOHEXANE, CARBON-DIOXIDE TOLUENE, CARBON-DIOXIDE PENTANE

Solubility of polyethylene in three different supercritical binary solvent systems-carbon dioxide-cyclohexane, carbon dioxide-toluene, and carbon dioxide-pentane-have been studied. Solvent compositions that lead to complete dissolution at pressures below 70 MPa have been identified. Demixing pressures have been determined for a range of polymer concentrations at temperatures up to 200-degrees-C. It is shown that the behavior of the solutions depends strongly on the fluid composition. In the composition ranges studied, solutions in cyclohexane-carbon dioxide mixtures were found to show lower critical solution temperature (LCST) behavior. It is found that the behavior of solutions in carbon dioxide-toluene and carbon dioxide-pentane mixtures shifts from upper critical solution temperature (UCST) to LCST with increasing toluene or pentane content.