Q A1

Figure 5. d YJ d H(Q) for 3.7k/16.6k polystyrene-PFOA block copolymer in supercritical CO2as a function of concentration (C) of added polystyrene oligomer

Figure 6. d X/ d Q) for 6% (w/v) polyvinyl acetate-b-PFOA block copolymers in CO2. At high pressures (275 bar), the scattering arises from single molecules; as the pressure is lowered, micelles form below a critical CO2 pressure.

Figure 6. d X/ d Q) for 6% (w/v) polyvinyl acetate-b-PFOA block copolymers in CO2. At high pressures (275 bar), the scattering arises from single molecules; as the pressure is lowered, micelles form below a critical CO2 pressure.

has the effect ofbreaking apart a collection of aggregates of relatively low polydispersity (65 OC), into a collection of smaller aggregates of higher polydispersity. This suggests the existence of a critical micellar density (CMD), which corresponds to the density of the solvent at which the micellar aggregates disappear. Recent experiments [3] on poly(vinyl acetate)-b-PFOA diblock copolymers demonstrate this new phenomenon (Figure 6). The SANS cross section increases by over an order of magnitude as the CO2 pressure decreases, and thus signals the transition from unimers to micellar aggregates.

The self-assembly of molecules in condensed phases is a ubiquitous phenomenon found in many biological structures, as well as in systems of interest to colloid and surface science. Supercritical CO2 affects unique perturbations over associative block copolymer systems, as the solvent strength is easily tunable with density and the system may be driven from an aggregated to a dispersed state simply by changing the pressure or temperature to control the solubility. The CMD represents a novel phenomenon, into which neutron scattering provides detailed structural insight. While it is analogous in some respects to other transitions (e.g. changing solvent properties by adding a co-solvent), we believe that the beauty of the CMD is that it is reversible with exquisite control and it is possible to design precisely where this transition will occur. The ability to create and disperse micells, coupled with the fact that CO2 - phobic materials can be solubilized within the aggregates [3,6,20] makes the CMD an effect that may be of technological importance in the development of environmentally benign processes.

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