Particle growth rates were analyzed for the dispersion polymerization of methyl methacrylate (MMA) in supercritical carbon dioxide at 65 °C stabilized with a poly(dimethyl siloxane)-methyl methacrylate (PDMS-mMA) macromonomer. Although pure CO 2 is a mediocre solvent for PDMS even at 4000 psia, the monomer behaves as a cosolvent to prevent flocculation. As pressure is decreased, the dispersion flocculates sooner, as expected due to the reduced solvent quality of CO2. Final particle size is only mildly dependent on pressure as a result of the solvation from the high monomer concentration during the particle formation stage, however particle coagulation increases with decreasing pressure. There exists both a minimum pressure (∼3000 psia) and stabilizer concentration (∼2 wt % stabilizer/ monomer) below which particles are highly coagulated due to insufficient steric stabilization. Here polymerization rates are reduced due to diffusional restrictions. This threshold pressure and stabilizer concentration are required to change the mechanism from precipitation polymerization to dispersion polymerization, as indicated by product morphology, molecular weight, and molecular weight polydispersity. Final particle size and number density determined from the model of Paine {Macromolecules 1990, 23, 3109} agree with the measured values.

The stabilization and flocculation of emulsions of poly(2-ethylhexyl acrylate) (PEHA) in liquid and supercritical carbon dioxide (SC-CO2) with the homopolymer poly(1,1-dihydroperfluorooctyl acrylate) (PFOA), the diblock copolymer polystyrene-b-PFOA, and the triblock copolymer PFOA-b-poly(vinyl acetate)-b-PFOA were quantified by turbidimetry and measurements of interfacial tension. Upon decreasing the CO2 density, a distinct change in emulsion stability occurs at the critical flocculation density (CFD). Steric stabilization by the homopolymer PFOA is due to a small number of adsorbed segments and a large number of segments in loops and tails as determined by measurements of the PEHA−CO2 interfacial tension. Below the CFD, flocculation is irreversible due to bridging by the high molecular weight PFOA chains. PS-b-PFOA adsorbs much more strongly to the PEHA−CO2 interface than the other two stabilizers. Consequently, it provides the greatest resistance to emulsion flocculation, both above and below the CFD, and the most reversible flocculation. For all stabilizers studied, the CFD correlates very well with the estimated ϑ point for PFOA in bulk CO2.

Dispersion polymerization of methyl methacrylate in supercritical CO2 is studied in situ by turbidimetry at 65 °C from 2000 to 5000 psia for various concentrations of a poly(dimethylsiloxane) monomethacrylate (PDMS−mMA) macromonomer stabilizer. The average particle size, particle number density, and overall surface area are reported vs time during particle formation. Coagulative nucleation and controlled coagulation regions have been identified. They are governed by the amount of stabilizer available relative to the total surface area of the dispersion. Near the end of the controlled coagulation region, which can last tens of minutes, the particle number density approaches the final value. The time in this region is longer than predicted by the model proposed by Paine (Macromolecules 1990, 23, 3190) due to incomplete incorporation of stabilizer, solubility limitations of polymerized stabilizer in the continuous phase, and plasticization of the particles by CO2, which increase particle coagulation. Threshold values of pressure and stabilizer concentration are required to achieve a solvent quality and surface coverage sufficient to prevent uncontrolled coagulation during particle formation.

Alpha calcium sulfate hemihydrate (α-HH) is an important class of cementitious material and exhibits considerable morphology-dependent properties. In the reverse microemulsions of water/n-hexanol/cetyltrimethylammonium bromide (CTAB)/sodium dodecyl sulfonate (SDS), the morphology and aspect ratio of α-HH are successfully controlled by adjusting the mass ratio of CTAB/H(2)O and the concentration of SDS. As the ratio of CTAB/H(2)O is increased from 1.3 to 4.5, the crystal length decreases from 120 to 150 μm to 0.5-1.2 μm with the corresponding aspect ratio reduced sharply from 180 to 250 to 2-7. With increasing SDS concentration, the crystal morphology gradually changes from submicrometer-sized long column to rod, hexagonal plate, and even nanogranule. The preferential adsorption of CTAB on the side facets and SDS on the top facets contributes to the morphology control. This work presents a simple, versatile, highly efficient approach to controlling the morphology of α-HH on a large scale and will offer more opportunities for α-HH multiple applications.

The preparation of stable dispersions of poly(vinyl acetate) or ethylene/vinyl acetate copolymers via dispersion polymerizations in a carbon dioxide continuous phase has been investigated. The effectiveness of stabilizers of various chemical compositions and architectures was compared. Both fluorinated and siloxane-based stabilizers including homopolymers, block copolymers, and reactive macromonomers were employed. Effects of variations in the reaction temperature, pressure, stabilizer composition, stabilizer concentration, and use of cosolvents on the resulting product were investigated. A turbidimetry technique was used successfully to monitor dispersed-phase volume fractions, particle sizes, and number densities during the polymerizations and to give the final particle sizes at the end of the polymerizations. The fluorinated stabilizers gave rise to smaller particles and more stable latexes when compared to the siloxane-based stabilizers.