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-dihydroperflu orooctylacrylate) and poly(vinyl acetate) (PFOA-b-PVAC) in supercritic al 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 c an be observed experimentally, there exist five pressure regions. At v ery low pressures (<148 bar), the PFOA-b-PVAC copolymer is essentially insoluble in pressurized carbon dioxide. At low pressures (148-206 ba r), 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 copolym ers were dissolved to form unimers in solution. In the pressure region of 242-310 bar, micelles with a very narrow size distribution were fo rmed in equilibrium with single polymer chains (or unimers). In the hi gh-pressure region (i.e., 379-552 bar), the micelles were gradually di ssolved to unimers with increasing pressure, while some anomalous larg e aggregates appeared around the critical micelle pressure. The appear ance of the anomalous larger aggregates can be ascribed to the copolym er composition heterogeneity. On the basis of a model of the mass acti on 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) w ere estimated in the normal micellar region. With increasing pressure, the cmc increased, but N decreased. The kinetics of dissolution and o f micelle formation of the copolymer around the phase separation press ure region was also examined.