No abstract

While ongoing efforts continue to explore the high-pressure phase equilibria 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 blend in the presence of supercritical carbon dioxide (scCO2), a nonselective plasticizing agent. Blends composed of low-molecular-weight polystyrene and polyisoprene have been examined as a function of temperature in scCO2 by visual inspection, small-angle neutron scattering, and spectrophotometry. In the presence of scCO2, the cloud point temperature is depressed by as much as 28 °C, depending on both blend composition and CO2 pressure. Complementary studies performed with nitrogen decouple the plasticization efficacy of CO2 from free-volume compression due to hydrostatic pressure. Existence of a pressure yielding a maximum in CO2-induced cloud point depression is established. These results provide evidence for enhanced polymer miscibility as a result of the plasticizing effectiveness and tunable solubility of scCO2.

Introduction. Blends composed of poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) are of commercial interest due to their synergistic physical properties. These blends marry the chemical/ flame resistance, toughness, and piezoelectric nature of PVDF 1,2 with the modulus, tensile strength, low smoke toxicity, and optical properties of PMMA 3 and likewise exhibit intriguing phase behavior. The intermolecular interactions that arise from the electric moments of the polymers, as well as from hydrogen bonding between the carbonyl oxygen of PMMA and the acidic hydrogens of PVDF, 4,5 are responsible for lower critical solution temperature (LCST) behavior at temperatures well above the normal melting point (T m ) of PVDF. Thus, the blends are completely miscible over a wide temperature window in the melt. They are also partially miscible, existing as amorphous media, over a broad composition range (up to ∼50 wt % PVDF) at temperatures below T m (extending to the solid state at temperatures below the glass transition temperature of the blend, T g,mix ). Under these conditions, the blends can be processed or used as macroscopically homogeneous