Structure of diblock copolymers in supercritical carbon dioxide and critical micellization pressure

This paper reports a small angle neutron scattering investigation of micell e formation by fluorocarbon-hydrocarbon block copolymers in supercritical C O2(sc-CO2) at 65 degrees C. A sharp unimer-micelle transition is obtained d ue to the tuning of the solvating ability of sc-CO2 by profiling pressure, so that the block copolymer, in a semidilute solution, finds sc-CO2 a good solvent at high pressure and a poor solvent at low pressure. At high pressu re the copolymer is in a monomeric state with a random coil structure. Howe ver, on lowering the pressure, aggregates are formed with a structure simil ar to aqueous micelles with the hydrocarbon segments forming the core and t he fluorocarbon segments forming the corona of the micelle. This unimer-agg regate transition is driven by the gradual elimination of CO2 molecules sol vating the hydrocarbon segments of the polymer. Comparison of these results with related data on the same polymer at different temperatures indicates that the transition is critically related to the density of the solvent. Th is suggests the definition of a critical micellization density, to our know ledge a new concept in colloid chemistry.