Thermodynamic size control of block copolymer vesicles in solution

Poly(styrene)-b-poly(acrylic acid) (PS-b-PAA) diblock copolymer vesicles ar e thermodynamically stable in solutions of dioxane/THF/H2O or DMF/THF/H2O. The vesicle sizes can be changed reversibly by changing the solvent composi tion, especially the water content. The size change with water content is p rimarily driven by the interfacial energy contribution to the free energy, such that with increasing water content, as the interfacial energy increase s, the system minimizes the total interfacial area by increasing vesicle si zes. For low vesicle sizes, the interfacial area is relatively strongly siz e-dependent, which also induces a narrow size distribution. By contrast, fo r large vesicle sizes, the interfacial area is very weakly size dependent, and a wide size distribution is observed. In a previous study, it had been shown that the segregation of hydrophilic PAA blocks is responsible for the thermodynamic stabilization of vesicles, with the long PAA chains preferen tially segregated to the outside, and the short chains to the inside. In th e present study, fluorescence quenching experiments have shown that this se gregation is size dependent. The larger the vesicles, the lower the degree of segregation becomes. Furthermore, the extent of segregation is reversibl e with changing vesicles size. In addition, the mechanism of reequilibratio n of vesicles is explored, and is shown to involve fusion and fission. Duri ng these processes, the PAA block chains can diffuse through a PS wall so t hat a corona chain segregation equilibrium can be reestablished.