(52g) Volumetric Properties of Cyclohexane + CO2 Mixtures at High Pressures

Processing polymers at high pressures in mixtures of organic solvents with CO₂ is of high interest for the production of particles, fibers, and foams. Of particular interest is the ability to tune the solvent power of these mixtures with pressure. In this study the thermophysical properties of mixtures of cyclohexane and CO₂ were explored at pressures up to 350 bar. Volumetric properties were determined for mixtures containing 0, 10, 20, 30, 40, and 50 % by mass CO₂ and at 40, 60, 80, 100, and 120 ^oC at pressures ranging from 50 to 350 bar.

Measurements were conducted in a special high pressure view-cell which utilizes dual movable pistons, the positions of which are monitored by computer-interfaced Linear Variable Differential Transformers (LVDT) which allow real-time recording of the cellâ??s internal volume. Since the initial mass loading of the cell is known, the real-time volume data generated during a pressure scan allows the generation of continuous density profiles of the fluid mixture inside the view-cell. The density data are then modeled with lattice-fluid models such as the Sanchez-Lacombe equation of state and the derived thermodynamic properties such as the isothermal compressibility, isobaric expansivity, internal pressure, and the solubility parameters of the mixtures are evaluated. The density data that have been generated were found to be within 1% of the NIST values for pure cyclohexane. For mixtures of cyclohexane and carbon dioxide, the present measurements extend the data base beyond what is available in the open literature which is limited to 70 bar. We are exploring these mixtures as pressure-tunable processing solvents for polyolefin-based copolymers.