Preparation and Characterization of Microcellular Polystyrene Foams Processed in Supercritical Carbon Dioxide Kelyn A. Arora, Alan J. Lesser,* and Thomas J. McCarthy* Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003 Received December 11, 1997; Revised Manuscript Received May 4, 1998 ABSTRACT: The foaming of polystyrene using supercritical (SC) CO2 has been studied to better understand the microcellular foaming process, as we plan future studies that involve the creation of composite microcellular foams. Rapid decompression of SC CO2-saturated polystyrene at sufficiently high temperatures (above the depressed Tg) yields expanded microcellular foams. The resulting foam structures can be controlled by manipulating processing conditions. Experiments varying the foaming temperature while holding other variables constant show that higher temperatures produce larger cells and reduced densities. Structures range from isotropic cells in samples retaining their initial geometry to highly expanded foams recovered in the shape of the foaming vessel and having oriented, anisotropic cells and limited density reduction. Higher saturation pressures lead to higher nucleation densities and hence smaller cells. Decreasing the rate of depressurization permits a longer period of cell growth and therefore larger cell sizes. Foams having a bimodal distribution of cell sizes can be created by reducing the pressure in two stages. Introduction We have reported a novel method for producing polymer composite materials that involves the super- critical fluid (SCF)sassisted infusion of vinyl monomers into, and subsequent free-radical polymerization within, organic polymer substrates. 1,2 Styrene can be polym- erized within semicrystalline and amorphous polymer substrates to create polymer blends with kinetically trapped morphologies. We have recently begun to use this approach to create composite microcellular foams using SC CO 2 as the solvent for chemical modification reactions and as the foaming agent. Others have demonstrated that CO 2 can be used to foam amorphous materials such as poly(methyl meth- acrylate), polystyrene, polycarbonate, and poly(ethylene terephthalate). 3-9 Typically, a polymer sample is satu- rated with CO 2 (or nitrogen) at room temperature and subsequently removed from the vessel and foamed by heating to a temperature above the normal glass transi- tion (T g ) in a high-temperature bath. Goel and Beck- man 3,4 have reported a different foaming method in which the polymer is saturated with CO 2 at relatively high temperatures and pressures (in the supercritical (SC) regime) followed by rapid depressurization to atmospheric pressure at constant temperature. This method takes advantage of the depression of T g by CO 2 . CO 2 is known to swell and significantly plasticize many amorphous polymers, often reducing the glass transition to temperatures approaching room temperature. 10-14 SCFs offer many desirable properties as both solvents and foaming agents including adjustable solvent strength, 15-17 plasticization, and enhanced diffusion rates. 16,17 Since CO 2 is a gas at ambient conditions, the solvent rapidly dissipates upon release of pressure. A pressure quench from SCF conditions at constant tem- perature also ensures that no vapor/liquid boundary is encountered which can damage the cellular structure of a foam. Finally, CO 2 presents an obvious environ- mental advantage over conventional CFC (chlorofluoro- carbon) foaming agents or organic solvents. We report here the preparation of microcellular polystyrene using SC CO 2 as a foaming agent in a method similar to that described by Goel and Beck- man. 3,4 The initial targets for foamed polymer blends consist of a matrix polymer and an incipient polystyrene phase. The experiments described here were envisioned as single-component control experiments for the blend expansion studies. We found the single-component studies, however, to be quite rich on their own. The cellular structure resulting from this type of foaming procedure can be readily controlled by manipulating such processing parameters as temperature, pressure, depressurization profile, and vessel size. Isotropic foams with uniform cell size can be prepared as well as foams with oriented, anisotropic cells and foams with bimodal distributions of cell sizes. Experimental Section Materials. Polystyrene was purchased from Scientific Polymer Products in the form of pellets and used as received or compression molded at 150 °C into 1 /16-in.-thick plaques. The molecular weight was measured using a Polymer Labo- ratories gel permeation chromatograph with THF as the mobile phase and found to be M n ) 67 000 with a polydisper- sity of 2.3. The reported density was 1.04 g/cm 3 , and infrared spectroscopy showed no measurable impurities. CO2 (Coleman Grade, 99.99%) was purchased from Merriam Graves and used as received. Foam Preparation. Foams were prepared in 316 stainless steel hexagonal high-pressure vessels (10-mL volume) contain- ing glass culture tubes as liners to facilitate removal of the foamed samples. Polystyrene, in the form of pellets or a compression-molded plaque, was weighed and placed in the glass tube inside the steel vessel. The vessel was preheated and filled with SC CO 2 to the desired pressure using a heated high-pressure CO2 manifold. 1 It was then placed in a circulat- ing oil bath to soak at the foaming temperature for a prescribed period of time until saturation conditions were reached. At the end of this period, the vessel was depressurized at the soak temperature following one of two protocols. For rapid depres- surization to atmospheric conditions, a valve was opened, immediately venting the CO 2. For depressurization over a 4614 Macromolecules 1998, 31, 4614-4620 S0024-9297(97)01811-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/19/1998