Evolution of block copolymer micellar size and structure evidenced with cryo electron microscopy

The equilibration of spherical micelles formed by polystyrene-poly(2-vinylpyridine) (PSPVP) diblock copolymers in toluene is studied by means of cryo electron microscopy. The evolution of the micelles in order to reach the equilibrium state involves the transformation of their structure and size and requires a considerable time. The micellar size distribution, which is relatively narrow after dissolution, becomes broader with time. The micellar structure changes with time toward a spherical form with narrow interfacial zone between the core and corona regions. The possible scenario of the equilibration process is discussed. The influence of the dilution on the micelles' evolution toward the equilibrium is investigated. It is found that strong dilution can lead to a decrease of the final size of the micelles. The reasons for this phenomenon are discussed.