Self-Assembly Behavior of a Diblock Copolymer of Poly(1,1-dihydroperfluorooctyl acrylate) and Poly(vinyl acetate) in Supercritical Carbon Dioxide

Abstract

High-pressure laser light scattering experiments were performed to study the molecular association behavior of a diblock copolymer of poly(1,1-dihydroperfluoroocty acrylate) and poly(vinyl acetate) in supercritical carbon dioxide. Both pressure-induced and temperature-induce d micellization processes were observed over a pressure range of 90-552 bar and a temperature range of 25-75 °C, respectively. In sequence with increasing pressure at a fixed temperature, five regions appeared: (1) an insoluble solute appeared; (2) a small portion of the copolymer was dissolved to form unimers; (3) around the critical phase separation pressure region, some large aggregates were observed together with unimers; (4) over the critical phase separation pressure, very narrow 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 anomalous large aggregates appeared around the critical micelle pressure. The appearance of the large aggregates can be ascribed to the copolymer composition heterogeneity. Upon lowering the temperature at a fixed pressure, a similar dissolution and association process of the copolymer in CO 2 was observed in terms of the critical phase separation temperature and the critical micelle temperature (CMT), because both increasing pressure and decreasing temperature increase the density of CO 2 and thus improve the solvent quality. The pressure dependence of the CMT with a fixed copolymer concentration, in combination with the pressure dependence of the critical micelle concentration at a fixed temperature, enables us to summarize the results with a mathematical relation among the critical micelle concentration, pressure, and temperature. After knowing either two of them for the copolymer solution in CO 2 , the third critical micellization condition can be predicted. The positive standard enthalpy of micellization (+18.8 kJ/mol) indicates an entropy-driven process.