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

This paper reports a small angle neutron scattering investigation of micelle formation by fluorocarbon-hydrocarbon block copolymers in supercritical CO2(sc-CO2) at 65 degrees C. A sharp unimer-micelle transition is obtained due 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 pressure the copolymer is in a monomeric state with a random coil structure. However, on lowering the pressure, aggregates are formed with a structure similar to aqueous micelles with the hydrocarbon segments forming the core and the fluorocarbon segments forming the corona of the micelle. This unimer-aggregate transition is driven by the gradual elimination of CO2 molecules solvating 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. This suggests the definition of a critical micellization density, to our knowledge a new concept in colloid chemistry.