SELF-ASSEMBLY BEHAVIOR OF A DIBLOCK COPOLYMER OF POLY(1,1-DIHYDROPERFLUOROOCTYL ACRYLATE) AND POLY(VINYL ACETATE) IN SUPERCRITICAL CARBON-DIOXIDE

High-pressure laser light scattering experiments were performed to stu dy the molecular association behavior of a diblock copolymer of poly(1 ,1-dihydroperfluorooctyl acrylate) and poly(vinyl acetate) in supercri tical carbon dioxide. Both pressure-induced and temperature-induced mi cellization processes were observed over a pressure range of 90-552 ba r and a temperature range of 25-75 degrees C, respectively. In sequenc e with increasing pressure at a fixed temperature, five regions appear ed: (1) an insoluble solute appeared; (2) a small portion of the copol ymer was dissolved to form unimers; (3) around the critical phase sepa ration pressure region, some large aggregates were observed together w ith unimers; (4) over the critical phase separation pressure, very nar row size-distributed micelles in equilibrium with unimers were formed in the solution; (5) with a further increase in pressure, the micelles were gradually dissolved to form unimers; in the meantime, some anoma lous large aggregates appeared around the critical micelle pressure. T he appearance of the large aggregates can be ascribed to the copolymer composition heterogeneity. Upon lowering the temperature at a fixed p ressure, a similar dissolution and association process of the copolyme r in CO2 was observed in terms of the critical phase separation temper ature and the critical micelle temperature (CMT), because both increas ing pressure and decreasing temperature increase the density of CO2 an d thus improve the solvent quality. The pressure dependence of the CMT with a fixed copolymer concentration, in combination with the pressur e dependence of the critical micelle concentration at a fixed temperat ure, enables us to summarize the results with a mathematical relation among the critical micelle concentration, pressure, and temperature. A fter knowing either two of them for the copolymer solution in CO2, the third critical micellization condition can be predicted. The positive standard enthalpy of micellization (+18.8 kJ/mol) indicates an entrop y-driven process.