Phase separation in polystyrene-poly(vinyl methyl ether) blends dilated with compressed carbon dioxide

The phase behavior of polystyrene (PS)/poly(vinyl methyl ether) (PVME) blen ds of various compositions in the presence of compressed carbon dioxide was investigated using in-situ high-pressure fluorescence spectroscopy. Sorpti on of a few weight percent of CO2, which is a poor solvent for both compone nts but selective for PVME, induces phase separation at 40 and 60 degrees C for all compositions studied. These transitions occur as much as 90 degree s C below the coexistence temperatures for the corresponding binary blends at ambient pressure. The Sanchez-Lacombe equation of state qualitatively pr edicts phase separation in the PS/PVME/CO2 system as CO2 pressure is increa sed isothermally. A semiquantitative stability analysis suggests that a dis parity in the free volumes of the polymer components upon dilation with car bon dioxide, rather than enthalpic contributions, is the principal driver f or phase separation in this system.