Critical micelle density for the self-assembly of block copolymer surfactants in supercritical carbon dioxide

The parameters which influence the self-assembly of molecules in solution i nclude the temperature and solvent quality, and this study illustrates the use of these variables to regulate the degree of association of block copol ymer amphiphiles in highly compressible supercritical carbon dioxide. Small -angle neutron scattering (SANS) has been used to examine the association b ehavior of a block copolymer containing a CO2-phobic moiety, poly(vinyl ace tate), and a CO2-philic block, poly(1,1-dihydroperfluoro-octylacrylate). By adjustment of the density of the medium through pressure and temperature p rofiling, the self-assembly can be reversibly controlled from unimers to co re-shell spherical micelles and this establishes a critical micelle density (CMD), a phenomenon distinctive of highly compressible fluids, such as sup ercritical CO2. Mathematical modeling of the data in terms of core-shell mi celle structures permits a detailed description of the structure and the de gree of swelling (penetration) of the solvent into the different regions of the aggregates throughout this transition.