MISCIBILITY, DENSITY AND VISCOSITY OF POLY(DIMETHYLSILOXANE) IN SUPERCRITICAL CARBON-DIOXIDE

Phase boundaries, densities and viscosities of solutions of poly(dimethylsiloxane)s (PDMS) in supercritical carbon dioxide have been determined. The demixing pressures of 5 wt% solutions of PDMS of different molecular weights (M(w) = 38 900, M(w)/M(n) = 2.84; M(w) = 93 700, M(w)/M(n) = 2.99; M(w) = 273 500, M(w)/M(n) = 2.29; M(w) = 369 200(1) M(w)/M(n) = 2.19) were determined in the temperature range from 300 to 460 K. At temperatures above 330 K the solutions show typical lower critical solution temperature behaviour and the demixing pressures increase with temperature. Below 330 K, the behaviour is reversed, and demixing pressures show a dramatic increase with decreasing temperature. The phase boundaries and the steep changes in the demixing pressures with temperature are discussed in the framework of the Sanchez-Lacombe model. Evaluation of the demixing pressures along with information on the details of molecular weight distributions obtained by gel permeation chromatography show that the observed demixing pressures are very much influenced by the high-molecular-weight tails of the distributions. Analysis of the density and viscosity data for solutions of PDMS with M(w) = 38 900 at 1, 2 and 5 wt% concentrations show that results can be correlated with a free-volume based relationship of the form eta = A exp[B/(1 - C rho)]. Analysis of the pressure dependence of viscosity at selected temperatures shows that the apparent activation volumes are in the range of 30 to 60 cm(3) mol(-1). Analysis of the temperature dependence at different pressures gives activation energies in the range 7-10 kJ mol(-1).