Nanostructure of Solid Precipitates Obtained by Expansion of Polystyrene-block-Polybutadiene Solutions in Near Critical Propane: Block Ratio and Micellar Solution Effects.

In contrast to incompressible liquid solutions, compressible near-critical solutions of block copolymers allow for controlling rapid structure transformations with pressure alone. For example, when dissolved in near-critical propane, polystyrene-block-polybutadiene can form a random molecular solution at high pressures, a micellar solution at moderate pressures, and a solvent-free precipitate at low pressures. In contrast to the unstructured virgin copolymer, such a propane-treated precipitate rapidly self assembles toward structures characteristic of equilibrated block copolymers, such as lamellae, spheres or cylinders, which depend on the block ratio rather than on the decompression rate or temperature, at least within the rate and temperature ranges investigated in this work. At lower temperatures, however, say below 40 °C, glass transition of the styrene-butadiene diblocks can inhibit independent structure formation while crystallization of their hydrogenated-butadiene analogs can preserve the micellar-solution structure.