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 w hich promises to be an environmentally responsible replacement for the organic solvents traditionally used in polymerizations. Many lipophil ic polymers such as polystyrene (PS) are insol- uble in CO2, though po lymerizations may be accomplished via the use of PS-fluoropolymer stab ilizers, which act as emulsifying agents. Small-angle neutron and X-ra y 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 s even) remained constant. Thus, the coronal block molecular weight exer ts 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 understan ding the mechanisms of micellization and solubilization in supercritic al fluids.