Thermodynamics of Poly(dimethylsiloxane)/Poly(ethylmethylsiloxane) (PDMS/PEMS) Blends in the Presence of High-Pressure CO<INF>2</INF>

Processing polymer blends in the presence of high-pressure carbon dioxide (CO<INF>2</INF>) affords numerous advantages over organic solvents and is becoming a commercially viable and environmentally responsible alternative in the development of new multicomponent materials. A prerequisite to such processing is a fundamental understanding of how high-pressure CO<INF>2</INF> influences the phase behavior of polymer blends. In this work, we use high-pressure spectrophotometry to measure the cloud point (T<INF>cp</INF>) of poly(dimethylsiloxane)/poly(ethylmethylsiloxane) (PDMS/PEMS) blends as a function of CO<INF>2</INF> pressure (P) in the vapor phase. Results obtained here at different blend compositions indicate that values of T<INF>cp</INF> for this upper critical solution temperature (UCST) blend (i) generally increase with increasing pressure and (ii) collapse onto a master curve of ΔT<INF>cp</INF>(P) for pressures up to about 35 MPa. These data are analyzed by the Sanchez−Lacombe equation of state to ascertain the temperature dependence of an effective interaction parameter (χ) in terms of A + B/T, where A and B are both pressure-sensitive. We also compare our results with previously reported data to delineate the role of hydrostatic pressure on the phase behavior of these blends.