Micelle Formation of BAB Triblock Copolymers in Solvents That Preferentially Dissolve the A Block

This study concerns the micellization of BAB triblock copolymers in solvents that preferentially dissolve the middle block. Spherical micelles in these systems would consist of a core containing B blocks, surrounded by loops of A blocks. The existence of this microstructure has not yet been established. In fact, the absence of micelles in these systems has been reported by a number of workers. Previous theoretical work indicates that the entropic penalty associated with the looping of the coronal block is substantial and precludes the possibility of micelle formation in these systems. Quasi-elastic light scattering measurements on solutions of poly(vinylpyridine)-polystyrene-poly(vinylpyridine) triblock copolymers in toluene, which is a selective solvent for the polystyrene block, indicate that these systems can form micelles. Furthermore, the diffusional characteristics of these micelles are similar to those of micelles formed by poly(vinylpyridine)-polystyrene diblock copolymers in toluene, implying a similarity in the overall architecture of the aggregate. It is found that the molecular weight of the middle block (polystyrene) plays a crucial role in determining the size and concentration of the micelles. Calculations presented here demonstrate that the previous analysis had overestimated the entropy of loop formation of the coronal block, and that micelle formation in these systems is indeed possible. However, the theory is only in partial agreement with experimental observations.