Because of the recent emphasis on green chemistry, there has been interest in using supercritical carbon dioxide (sc CO2) as a solvent or swelling agent to aid in polymer processing and polymer chemistry (Adamsky and Beckman, 1994; DeSimone et al., 1992; Hayes and McCarthy, 1998; Kung et al., 1998; Mistele et al., 1996; Romack et al., 1995; Watkins and McCarthy, 1995). Supercritical CO2 is a very weak solvent for most polymers (some fluoropolymers and silicones are exceptions); however, it swells most polymers and dissolves many small molecules (Berens and Huvard, 1989). The density of a supercritical fluid (SCF), and thus its solvent strength, is continuously tunable as a function of temperature or pressure up to liquidlike values. This provides the ability to control the degree of swelling in a polymer as well as the partitioning of small-molecule penetrants between a swollen polymer phase and the fluid phase. The low viscosity and zero surface tension of SCFs allows for fast transfer of penetrants into swollen polymers. The lack of vapor/liquid coexistance in SCFs allows the sorption to proceed without the penetrant solution wetting the substrate surface. Since most of the common SCFs are gases at ambient conditions, the removal and recovery of the solvent from the final product is extremely facile. All of these factors aid in a new method we have developed for preparing polymer composites. This method involves the absorption of a supercritical solution of a monomer, initiator, and CO2 into a solid polymer substrate and subsequent thermal polymerization of the monomer to yield a composite system of the two polymers. We have focused on radical polymerization of styrene within various solid semicrystalline polymer substrates (Hayes and McCarthy, 1998; Kung et al., 1998; Watkins and McCarthy, 1995). Table 10.1 lists a number of systems that we have studied to make polymer–polystyrene composites. The method for preparing the polymer blends listed in Table 10.1 involves the soaking of the substrate polymer in a supercritical solution of styrene, a thermal radical initiator, and CO2 at a temperature where the initiator decomposes very slowly (half-lives of hundreds of hours).
