When block copolymers (in which “blocks” of dissimilar polymers are connected in a single chain) are placed in solution, they can behave like surfactants if the two blocks of the polymer have differing affinities for the solvent and thus form micelles, spheres, cylinders, and vesicles.
Luo and Eisenberg studied a system of polystyrene-b-poly(acrylic acid) (PS-b-PAA) in a mixed solvent, where vesicles form with a hydrophobic PS corona sandwiched between hydrophilic PAA inner and outer shells. The size of the vesicles is thermodynamically (and reversibly) controlled by solvent composition. Increasing the water content raises the interfacial energy and increases the vesicle size. For small vesicles, segregation is observed between the long PAA segments that migrate to the outside of the vesicle and the short blocks that remain inside. Small changes in solvent composition lead to rapid and dramatic changes in the equilibrium vesicle size, which indicates that there is a constant flux in the vesicle sizes, possibly through fusion and fission mechanisms.—MSL