Semi-fluorinated trialkyltin fluorides and fluorinated telechelic ionomersas viscosity-enhancing agents for carbon dioxide

Direct thickeners for dense carbon dioxide were designed and synthesized. E ach thickener contained "CO2-philic" fluorinated groups to impart solubilit y in carbon dioxide and "CO2-phobic" functionalities to promote intermolecu lar associations for viscosity enhancement. Semifluorinated trialkyltin flu orides and fluorinated telechelic ionomers were soluble to at least several weight percent in dense liquid carbon dioxide without the use of a cosolve nt. Increases in solution viscosity at 297 K were measured using falling cy linder viscometry. The viscosity of liquid carbon dioxide was increased by a factor of 2-3 at thickener concentrations of 2-4 wt %. These results demo nstrate that carbon dioxide viscosity enhancement is possible without the n eed for a cosolvent through the design of compounds with the appropriate ba lance of CO2-philic groups for solubility and CO2-phobic associating groups for macromolecular, viscosity-enhancing interactions. Neither compound, ho wever, was as effective as the (29% styrene-71% fluoro-acrylate) copolymer we recently developed. More substantial increases in solution viscosity wer e not attained with the semi-fluorinated trialkyltin fluoride because the f luorinated alkyl chains disrupted the associations that formed viscosity-en hancing, weakly associating, linear polymers. The viscosity increases obtai ned with the telechelic ionomer were also less than expected because of the relatively low molecular weight of the carbon-dioxide-soluble ionomers. Hi gher-molecular-weight ionomers would not be CO2-soluble, however.