| Block copolymers are macromolecules composed of two or more sections that have distinctly different solvent preferences, for example, one likes nonpolar solvents while the other does not. As a result, such schizophrenic macromolecules tend to self assemble to protect the reluctant block from an objectionable solvent, which is exploited to make nanoparticles, for example drug-delivery and gene-delivery nanoparticles. In this work, we have explored the pressure-sensitive micellization of model copolymers in subcritical and supercritical solvents, such as propane, using transmitted- and scattered-light probes. The micellar region of the high-pressure polystyrene-block -polydiene copolymer solutions in compressible solvents expands to higher pressures and temperatures upon increasing copolymer concentration, molecular weight, and styrene/diene block ratio. The reason for a growing micellar region is that the micellization pressure strongly increases with increasing copolymer concentration, molecular weight, and styrene/diene block ratio, while the micellar cloud pressure (the pressure at which the micelles aggregate and precipitate from solution) is mostly insensitive to these variables. In other words, neither the block size nor the block ratio nor the copolymer concentration seems to affect the copolymer separation from solution in the micellar region. |
