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A new experimental device is used to monitor in situ the swelling behavior of poly-(dimethylsiloxane) melts in contact with supercritical carbon dioxide. The effects of pressure, temperature, and sample molecular weight on the kinetics and extent of swelling are examined using this experimental technique. The swelling kinetics of all polymer samples exhibit two distinct regimes: an initial region of large swelling in which the diffusion of CO 2 into the polymer follow Fickian behavior and a subsequent region of small volume increase asymptotic to an equilibrium swelling value. Diffusion coefficients of CO2, obtained from the initial swelling kinetics data, are found to be relatively insensitive to pressure, increase with temperature, and decrease with polymer molecular weight with the latter exhibiting a power-law dependence with an exponent of ∼-2. The extent of swelling increases with both pressure and molecular weight but exhibits different trends with temperature depending on system pressure. For pressures below 15 MPa the extent of swelling decreases monotonically with temperature. However, for pressure above this threshold, a maximum in swelling is observed with temperature increments. The maximum with temperature is thought to be a result of large variations in the physical properties of CO 2 near its critical point. The results of equilibrium swelling have been modeled using the Sanchez-Lacombe equation of state and found to be in good agreement with the thermodynamic theory.

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High-pressure rheological behavior of polymer melts containing dissolved carbon dioxide (CO 2 ) at concentrations up to 6 wt % were investigated using a highpressure extrusion slit die rheometer. In particular, the steady shear viscosity of poly(methyl methacrylate), polypropylene, low-density polyethylene, and poly(vinylidene fluoride) with dissolved CO 2 were measured for shear rates ranging from 1 to 500 s Ϫ1 and under pressure conditions up to 30 MPa. The viscosity of all samples revealed a reduction in the presence of CO 2 with its extent dependent on CO 2 concentration, pressure, and the polymer used. Two types of viscoelastic scaling models were developed to predict the effects of both CO 2 concentration and pressure on the viscosity of the polymer melts. The first approach utilized a set of equations analogous to the Williams-Landel-Ferry equation for melts between the glass-transition temperature (T g ) and T g ϩ 100°C, whereas the second approach used equations of the Arrhenius form for melts more than 100°C above T g . The combination of these traditional viscoelastic scaling models with predictions for T g depression by a diluent (Chow model) were used to estimate the observed effects of dissolved CO 2 on polymer melt rheology. In this approach, the only parameters involved are physical properties of the pure polymer melt that are either available in the existing literature or can be measured under atmospheric conditions in the absence of CO 2 . The ability of the proposed scaling models to accurately predict the viscosity of polymer melts with dissolved high-pressure CO 2 were examined for each of the polymer systems.

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.