Direct Thickeners for Carbon Dioxide

Abstract Direct thickeners for dense carbon dioxide were designed and synthesized. Each thickener contained highly CO2-soluble "CO2-philic" fluorinated groups, while "CO2-phobic" functionalities promoted viscosity-enhancing intermolecular associations. Semifluorinated trialkyltin fluorides, fluorinated telechelic ionomers, fluoroacrylate-styrene copolymers and sulfonated fluoroacrylate-styrene copolymers, were soluble to at least several weight percent in dense carbon dioxide without the use of a cosolvent. Increases in solution viscosity were measured using falling cylinder viscometry. The most promising CO2 thickener was a styrene - heptadecafluorodecylacrylate bulk-polymerized random copolymer. The optimal composition of the copolymer was about 30 mol% styrene - 70% fluoroacrylate. For example, a 29% styrene-71%fluoroacrylate copolymer increased the viscosity of liquid CO2 by a factor of 250 at a concentration of 5 wt% and a shear rate of 20 s−1 at 297K and 34 MPa. A 0.2 wt% copolymer - 99.8 wt% CO2 solution exhibited a viscosity twice that of neat CO2 at shear rates of about 3000 s−1. Greater proportions of styrene in the thickener led to decreased CO2 solubility and solution viscosity, while lower proportions of styrene resulted in greater CO2 solubility but decreased solution viscosity. Solution viscosity increased with increasing copolymer concentration, increasing pressure and decreasing shear rate.