Laser light scattering study of pressure-induced micellization of a diblock copolymer of poly(1,1-dihydroperfluorooctylacrylate) and poly(vinyl acetate) in supercritical carbon dioxide

Laser light scattering has been employed to study the pressure-induced associating behavior of a diblock copolymer of poly(1,1-dihydroperfluorooctylacrylate) and poly(vinyl acetate) (PFOA-b-PVAC) in supercritical carbon dioxide, a pressure-dependent selective good solvent for the PFOA block. Over a pressure range of 90-552 bar at 65 degrees C, as can be observed experimentally, there exist five pressure regions. At very low pressures (<148 bar), the PFOA-b-PVAC copolymer is essentially insoluble in pressurized carbon dioxide. At low pressures (148-206 bar), a small portion of the copolymer is dissolved to form unimers with a constant average hydrodynamic radius [R-h] of about 2.2 nm. Around a phase separation pressure of similar to 225 bar, most of the copolymers were dissolved to form unimers in solution. In the pressure region of 242-310 bar, micelles with a very narrow size distribution were formed in equilibrium with single polymer chains (or unimers). In the high-pressure region (i.e., 379-552 bar), the micelles were gradually dissolved to unimers with increasing pressure, while some anomalous large aggregates appeared around the critical micelle pressure. The appearance of the anomalous larger aggregates can be ascribed to the copolymer composition heterogeneity. On the basis of a model of the mass action law of micelle formation and the bimodal size distribution obtained by the CONTIN analysis, quantitative information such as the critical micelle concentration (cmc), and the average association number (N) were estimated in the normal micellar region. With increasing pressure, the cmc increased, but N decreased. The kinetics of dissolution and of micelle formation of the copolymer around the phase separation pressure region was also examined.