High-Pressure Viscosity and Density of Polymer Solutions at the Critical Polymer Concentration in Near-Critical and Supercritical Fluids

This paper reports on the density and viscosity of solutions of poly(dimethylsiloxane) (PDMS; M w = 93 700) in supercritical carbon dioxide and of polyethylene (PE; M w = 121 000) in n -pentane. The measurements have been carried out at critical polymer concentrations, which are 5.5 wt % for a solution of PDMS in CO 2 and 5.75 wt % for a solution of PE in n -pentane. For the PDMS + CO 2 system, the measurements were conducted at 55, 70, 85, and 100 °C in the pressure range from 30 to 50 MPa. For the PE + n -pentane system, the measurements were conducted at 140 and 150 °C in the pressure range from 10 to 50 MPa using a falling-cylinder-type viscometer. At these temperatures and pressures, the viscosities were observed to be in the range from 0.14 to 0.22 mPa·s for the PDMS + CO 2 system and from 2.3 to 4.6 mPa·s for the PE + n -pentane system. The flow activation energies and activation volumes in the PDMS + CO 2 system were about 7 kJ/mol and 55 cm 3 /mol, respectively, compared to about 18 kJ/mol and 70 cm 3 /mol for the PE + n -pentane system. The close-packed volumes suggested from density correlations were found to be around 0.33 cm 3 /g for the PDMS system and 0.48 cm 3 /g for the PE system. Comparison of the viscosity data with the data on the kinetics of pressure-induced phase separation confirms that the slower kinetics in the PE + n -pentane system stems from the higher viscosity in this solution compared to the PDMS + CO 2 system, despite the similarity in the molecular weight of the polymer and the critical polymer concentrations.