Enhanced miscibility of low-molecular-weight polystyrene polyisoprene blends in supercritical CO2

While ongoing efforts continue to explore the high-pressure phase equilibri a of polymer blends, few studies have attempted to address the impact of a supercritical (sc) fluid on such equilibria. In this work, we report on the phase behavior of an upper critical solution temperature (UCST) polymer bl end in the presence of supercritical, carbon dioxide (scCO(2)), a nonselect ive plasticizing agent. Blends composed of low-molecular-weight polystyrene and polyisoprene have been examined as a function of temperature in scCO(2 ) by visual inspection, small-angle neutron scattering, and spectrophotomet ry. In the presence of scCO(2), the cloud point temperature is depressed by as much as 28 degrees C, depending on both blend composition and CO2 press ure. Complementary studies performed with nitrogen decouple the plasticizat ion efficacy of CO2 from free-volume compression due to hydrostatic pressur e. Existence of a pressure yielding a maximum in CO2-induced cloud point de pression is established. These results provide evidence for enhanced polyme r miscibility as a result of the plasticizing effectiveness and tunable sol ubility of scCO(2).