The cloud points of various amorphous polyether, polyacrylate, and polysiloxane homopolymers, and a variety of commercially available block copolymers, were measured in CO2 at temperatures from 25 to 65 °C and pressures of ca. 1000−6000 psia. Almost without exception, the solubility of amorphous polymers increases with a decrease in the cohesive energy density, or likewise, the surface tension of the polymer. With this decrease in surface tension, the polymer cohesive energy density becomes closer to that of CO2. Consequently, solubility is governed primarily by polymer−polymer interactions, while polymer−CO2 interactions play a secondary role. The solubility is strongly dependent upon molecular weight for the less CO2-philic polymers. The solubilities of high-molecular-weight poly(fluoroalkoxyphosphazenes) in CO2 were comparable to those of poly(1,1-dihydroperfluorooctylacrylate), one of the most CO2-soluble polymers known.

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.

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