Kinetics of Micelle Formation with Change of Micelle Shape in a Dilute Solution of Diblock Copolymers

By means of time-resolved static and dynamic light scattering, association processes of micelles are investigated for polystyrene- block -poly(dimethylsiloxane) in the mixed selective solvent, n- octane/methylcyclohexane, where the micelle can take either a hollow spherical structure or a hollow cylindrical structure, depending on the condition of micelle formation. The following are revealed: The cylindrical micelles are not thermodynamically stable ones but are temporally formed in the course of micelle formation. Micellization from the unimer state or micellar relaxation from one micelle state to another micelle state does not monotonically proceed to reach the destination but passes through the formation of cylindrical micelles having association numbers larger than those of both initial and equilibrium micelles. At lower temperatures and lower concentrations, the cylindrical micelles with larger size and longer time stability are easily formed. The formation of the large cylindrical micelles is the result of a rapid growth of micelles by elongating into the cylindrical shape, which may come from bilayer structure of the micelles.