Light-Scattering Study of Diblock Copolymers in Supercritical Carbon Dioxide:  CO2 Density-Induced Micellization Transition

The behavior of polymeric surfactant polyvinyl acetate (PVAC)-b-poly(1,1,2,2-tetrahydroperfluorooctyl acrylate) (PTAN) in supercritical carbon dioxide (CO2) was investigated using static and dynamic light scattering. We observed three regions on the phase diagram of the copolymer in supercritical CO2: (i) two-phase region at low CO2 density; (ii) solutions of spherical micelles at intermediate CO2 densities; (iii) solutions of unimers (individual copolymer chains) at high CO2 densities. The aggregation number (the number of copolymer chains in a micelle) decreases with an increasing density of supercritical CO2 in region (ii). An increase of the CO2 density corresponds to the improvement of solvent quality for both blocks of the copolymer (PVAC and PTAN). The hydrodynamic radius of micelles and unimers was measured using dynamic light scattering in regions (ii) and (iii), respectively. This light-scattering study is the first one reporting a solvent density-induced transition between spherical micelles at lower supercritical CO2 density and unimers at higher CO2 density. The light-scattering technique appears to be a very powerful tool for the analysis of the carbon dioxide density-induced micellization transition. This phenomenon is unique to supercritical fluids and demonstrates a convenient control over the polymer solubility.