| Homopolymers and amphiphilic block copolymers were characterized in a dense (supercritical and liquid) CO2 continuous phase. Small angle neutron and x-ray scattering (SANS and SAXS) have been employed in the determination of the weight average molecular weight (Mw), the radius of gyration (Rg), and the second virial coefficient (A2) of homopolymers dissolved in supercritical CO2. Dilute concentration series of two different molecular weight (Mw = 1.4 × 106 g mol−1 and Mw = 1.1 × 105 g mol−1) samples of poly(1,1-dihydroperfluorooctyl acrylate) (PFOA) were analyzed over a range of temperatures and pressures (40°C < T < 65°C and 340 bar < P < 395 bar). A2 values were found to be positive, indicating that supercritical CO2 is a thermodynamically good solvent for the PFOA over these conditions. Poly(perfluoroether) (Krytox™ 16500) and poly(dimethylsiloxane) (13 kg/mol PDMS) were found to exhibit zero and negative interactions with supercritical CO2 over a similar range of experiments. The self assembly of block copolymer amphiphiles which are interfacially active in carbon dioxide has been demonstrated using both SANS and SAXS. These materials demonstrate the design criteria for molecularly-engineered surfactants which can stabilize and disperse insoluble matter in a CO 2 continuous phase. For example poly(styrene)-b-PFOA copolymers were found to spontaneously self-assemble into polydisperse core-shell type micelles as a result of the disparate solubility characteristics of the different block segments in CO2 (i.e. PFOA is soluble while poly(styrene) is essentially insoluble in CO2 at experimental densities). Poly(styrene)- b-PFOA surfactants were also shown to be capable of emulsifying up to 20 wt% of a CO2-insoluble hydrocarbon into CO2. The effect of density on the association behavior of surfactants in CO2 was explored with poly(vinyl acetate)-b-PFOA. The critical micelle density (CMD) is defined whereby at a given density self-assembled structures will undergo an aggregate to unimer transition. |
