The use of supercritical carbon dioxide as a processing solvent for the physical processing of polymeric materials is reviewed. Fundamental properties of CO2/polymer systems are discussed with an emphasis on available data and measurement techniques, the development of theory or models for a particular property, and an evaluation of the current state of understanding for that property. Applications such as impregnation, particle formation, foaming, blending, and injection molding are described in detail including practical operating information for selected topics. The review concludes with some forward-looking discussion on the future of CO2 in polymer processing.

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drop of the suspension on a TEM grid, and letting the solvent evaporate slowly in a fume hood. XRD patterns were recorded on powder samples using a Philips PW1710 diffractometer (Cu Ka radiation, k = 1.54056 ) at a scanning rate of 0.02 s ±1 for 2h in the range of 10 to 70. UV-vis spectra were measured using a diode array spectrophotometer (Hewlett Packard 8452 A, Palo Alto, CA) with a resolution of 2 nm. Photoluminescence spectra were recorded using a luminescence spectrophotometer (Perkin Elmer LS-50B, Norwalk, CT) with pulsed high pressure xenon source. Polymer±Clay Nanocomposite Foams Prepared Using Carbon Dioxide** By Changchun Zeng, Xiangmin Han, L. James Lee,* Kurt W. Koelling, and David L. TomaskoPolymeric foams (or porous polymeric materials) are used in many applications because of their excellent strength-toweight ratio, good thermal and sound insulation properties, flexibility of generating desired morphologies to meet specific applications, materials savings, etc.[1] Foams with nanometersized voids are under investigation for potential applications as the next generation materials of low dielectric constants.[2]However, compared to bulk polymers, foams have reduced mechanical strength and lower dimensional and thermal stability. Recently developed microcellular foams provide improved mechanical properties over conventional foams,

Intercalated and exfoliated polystyrene/nano-clay composites were prepared by mechanical blending and in sihr polymerization respectively. The composites were then foamed by using CO, as the foaming agent in an extrusion foaming process.The resulting foam structure is compared with that of pure polystyrene and polystyrene/talc composite. At a screw rotation speed of 10 rpm and a die temperature of ZOO' C, the addition of a small amount (i.e., 5 wt%) of intercalated nano-clay greatly reduces cell size from 25.3 to 1 1.1 pm and increases cell density from 2.7 x lo7 to 2.8 x 108 cells/cm3. Once exfoliated, the nanocomposite exhibits the highest cell density (1.5 X lo9 cells/cm3) and smallest cell size (4.9 pm) at the same particle concentration. Compared with polystyrene foams, the nanocomposite foams exhibit higher tensile modulus, improved fire retardance, and better barrier property. Combining nanocomposites and the extrusion foaming process provides a new technique for the design and control of cell structure in microcellular foams.

The continuous production of polystyrene microcellular foams with supercritical CO2 was achieved on a two‐stage single‐screw extruder. Simulations related to the foaming process were accomplished by modeling the phase equilibria with the Sanchez‐Lacombe equation of state and combining the equations of motion, the energy balance, and the Carreau viscosity model to characterize the flow field and pressure distribution in the die. The position of nucleation in the die was determined from the simulation results via a computational fluid dynamics code (FLUENT). Experimental parameters were selected according to the Tg and phase equilibria. The effects of CO2 concentration and die pressure are explored. Below the solubility limit, higher CO2 concentrations lead to smaller cell size and greater cell density. With an increase of die pressure, the cell size decreases and the cell density increases.