Thermodynamic size control of block copolymer vesicles in solution

Poly(styrene)-b-poly(acrylic acid) (PS-b-PAA) diblock copolymer vesicles are thermodynamically stable in solutions of dioxane/THF/H2O or DMF/THF/H2O. The vesicle sizes can be changed reversibly by changing the solvent composition, especially the water content. The size change with water content is primarily driven by the interfacial energy contribution to the free energy, such that with increasing water content, as the interfacial energy increases, the system minimizes the total interfacial area by increasing vesicle sizes. For low vesicle sizes, the interfacial area is relatively strongly size-dependent, which also induces a narrow size distribution. By contrast, for 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 preferentially segregated to the outside, and the short chains to the inside. In the present study, fluorescence quenching experiments have shown that this segregation is size dependent. The larger the vesicles, the lower the degree of segregation becomes. Furthermore, the extent of segregation is reversible with changing vesicles size. In addition, the mechanism of reequilibration of vesicles is explored, and is shown to involve fusion and fission. During these processes, the PAA block chains can diffuse through a PS wall so that a corona chain segregation equilibrium can be reestablished.