Composite polymer particles in supercritical carbon dioxide: Synthesis and characterization

Previous research by DeSimone and others has demonstrated the dispersion polymerizations in supercritical CO₂ of various monomers, including methyl methacrylate, styrene, vinyl acetate, acrylonitrile, 1-vinyl-2-pyrrolidone, glycidyl methacrylate, and hydroxyethyl methacrylate. The focus of this research has been to extend the polymerization technique to the formation of composite polymer particles, which contain two separate polymer phases and are synthesized by sequential dispersion polymerizations in CO₂. By designing composite particles containing polymers with different properties, such as a difference in glass transition temperature or in solubility, such particles find uses as impact modifiers, adhesion enhancers, and drug delivery agents.; As a model system to study the influence of reaction conditions and CO ₂ on the composite particle formation, particles containing polystyrene (PS) and poly(methyl methacrylate) (PMMA) were prepared. Extensive characterization of the morphology, which is the distribution of the two polymer phases within the sub-micron sized polymer particles, was carried out by scanning and transmission electron microscopy. Theoretical calculations predict the formation of a PMMA shell around a PS core or partial engulfment of PS by PMMA. By varying the reaction parameters, especially the monomer-feed ratio, several particle morphologies were obtained experimentally, including half-moon (PMMA half-moon around PS), microdomains of PS within the PMMA, and inverted core-shell (PMMA shell and PS core).; Particles containing PS and a water soluble polymer, poly(vinyl pyrrolidone), or a reactive polymer, such as poly(glycidyl methacrylate) or poly(isocyanatoethyl methacrylate) (PIEM) have also been synthesized. Due to the reactivity of isocyanates with water and alcohol, polyisocyanate particles are difficult to prepare by emulsion or dispersion polymerizations in conventional aqueous or alcoholic media but can be synthesized in CO₂. To demonstrate the reactivity of the particles containing PIEM and a PS, the reaction between the isocyanate functionality and a hydroxy-containing polymer, poly(hydroxyethyl methacrylate) (PHEMA), to form crosslinking polyurethane linkages between the polymer chains, has been monitored by infrared spectroscopy.