Supercritical carbon dioxide (scCO2) is an environmental friendly solvent for various chemical processes. In many cases scCO2 is unable to replace organic solvents due to the low solubility of most polar and ionic materials in dense CO2. Highly CO2 soluble fluorinated polymers have been successfully designed but they are expensive and environmentally persistent. This project aims developing a non-fluorous compound which dissolves in CO2 and thickens CO2, thereby improving the performance CO2 flooding enhanced oil recovery. An attempt was made to generate CO2-thickeners containing a CO2-phile that promotes dissolution and a CO2-phobe that induces viscosity-enhancing intermolecular associations. The initial research was directed at identifying the most CO2-philic hydrocarbon-based polymer. Subsequently, associating groups would be incorporated. Small sugar acetates are known to be extremely CO2-soluble, but polymeric cellulose triacetate is CO2-insoluble due to its crystallinity. Therefore a high molecular weight, low melting point polymer with per-acetylated monosaccharide side chains, poly (1-O-(vinyloxy)ethyl - 2,3,4,6 - tetra -O - acetyl - β - D - glucopyranoside), P(AcGIcVE) was synthesized. This polymer is second most CO2-soluble hydrocarbon-based polymer and is slightly less CO2-soluble than PVAc.Amorphous poly(lactic acid) has also been shown to be highly CO2 soluble over a broad range of molecular weight. The pressure required for dissolution greatly exceeds that associated with PVAc or P(AcGIcVE), therefore PLA is the third most soluble polymer in CO2. Oligo(vinyl acetate) is a particularly effective CO2-phile. Poly(vinyl acetate), PVAc, remains the most CO2-soluble high molecular weight, non-fluorous polymer that has yet been identified. PVAc was selected as the base polymer for a copolymeric thickener. A pendant phenyl group was selected for viscosity-enhancing intermolecular associations because this mildly CO2-phobic non-polar group was so effectively used for this purpose in a fluorinated CO2 thickener previously designed by our group. Promising results were obtained with poly(vinyl acetate-co-vinyl benzoate5%). The viscosity of CO2 increased by roughly 40% at a copolymer concentration of 1wt% and by 80% at 2wt%, at shear rates of 6200-5080 s-1 at 298 K. Therefore poly (vinyl acetate-co-vinyl benzoate5%) is the first documented non-fluorous CO2 thickener capable of increasing the CO2 viscosity substantially at dilute concentrations of ~1wt%. Unfortunately, the pressure required to dissolve this copolymer in CO2 at 298 K (~65 MPa) greatly exceeds the MMP (Minimum Miscibility Pressure) of CO2 floods at the same temperature (~10 MPa). Because we were not able to identify a hydrocarbon-based polymer more CO2-philic than PVAc, it is doubtful that a non-fluorous, copolymeric thickener capable of dissolving in CO2 at practical CO2 flooding pressure conditions can be identified.The only other non-fluorous polymer known to be more CO2-soluble than PVAc is polydimethyl siloxane PDMS. Therefore, we evaluated three commercially available PDMS polymer with pendant phenyl groups. Two PDMS-based copolymers, poly(phenyl methyl siloxane)10%-co-(dimethylsiloxane) (Mw = 90,000 and 17000) were commercially available from Gelest. Neither was soluble in CO2 and copolymers with lesser degrees of phenyl methyl siloxane were not available. An attempt was also made to design small molecules as thickeners. The first CO2 soluble non-fluorous, acetylated hydrogen-binding compound and the first CO2 soluble non-fluorous dendrimer were synthesized. It was postulated that these compounds would dissolve in CO2 and then form linear macromolecules due to the hydrogen bonding between adjacent molecules. Critical features of these small, self-assembling molecules are the presence of strong and directional hydrogen bonding interactions between carbonyl oxygen and hydrogen in a bis-urea moiety, and the presence of multiple (two or four) highly acetylated "arms" on the periphery of the molecule that promote dissolution in CO2. Although the first non-fluorous, CO2-soluble hydrogen bonding compound (two arms) and hydrogen-bonding dendrimer (four arms) were designed, neither thickened CO2. The hydrogen bonding compound with two arms did form brittle, microfibrillar, free-standing foams upon the removal of the CO2.
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