Semi-Fluorinated Trialkyltin Fluorides and Fluorinated Telechelic Ionomers as Viscosity-Enhancing Agents for Carbon Dioxide

Abstract

Direct thickeners for dense carbon dioxide were designed and synthesized. Each thickener contained “CO2-philic” fluorinated groups to impart solubility in carbon dioxide and “CO2-phobic” functionalities to promote intermolecular associations for viscosity enhancement. Semi-fluorinated trialkyltin fluorides and fluorinated telechelic ionomers were soluble to at least several weight percent in dense liquid carbon dioxide without the use of a cosolvent. Increases in solution viscosity at 297 K were measured using falling cylinder viscometry. The viscosity of liquid carbon dioxide was increased by a factor of 2−3 at thickener concentrations of 2−4 wt %. These results demonstrate that carbon dioxide viscosity enhancement is possible without the need for a cosolvent through the design of compounds with the appropriate balance of CO2-philic groups for solubility and CO2-phobic associating groups for macromolecular, viscosity-enhancing interactions. Neither compound, however, was as effective as the (29% styrene−71% fluoroacrylate) copolymer we recently developed. More substantial increases in solution viscosity were not attained with the semi-fluorinated trialkyltin fluoride because the fluorinated alkyl chains disrupted the associations that formed viscosity-enhancing, weakly associating, linear polymers. The viscosity increases obtained with the telechelic ionomer were also less than expected because of the relatively low molecular weight of the carbon-dioxide-soluble ionomers. Higher-molecular-weight ionomers would not be CO2-soluble, however.