The morphology of block copolymer micelles in supercritical carbon dioxide by small-angle neutron and X-ray scattering

Above its critical point, carbon dioxide forms a supercritical fluid which promises to be an environmentally responsible replacement for the organic solvents traditionally used in polymerizations. Many lipophilic polymers such as polystyrene (PS) are insol- uble in CO2, though polymerizations may be accomplished via the use of PS-fluoropolymer stabilizers, which act as emulsifying agents. Small-angle neutron and X-ray scattering have been used to show that these molecules form micelles with a CO2-phobic PS core and a CO2-philic fluoropolymer corona. When the PS block was fixed in length and the fluorinated corona block was varied, the number of block copolymer molecules per micelle (six to seven) remained constant. Thus, the coronal block molecular weight exerts negligible influence on the aggregation number, in accordance with the theoretical predictions of Halperin, Tirrell & Lodge [Adv. Polym. Sci. (1992), 100, 31-46]. These observations are relevant to understanding the mechanisms of micellization and solubilization in supercritical fluids.