Kinetics and mechanism of the rod-to-vesicle transition of block copolymer aggregates in dilute solution

The kinetics and mechanism of the rod-to-vesicle transition in aggregates of polystyrene(310)-b-poly(acrylic acid)sz diblock copolymers in dioxane/water mixtures are explored. The transition is induced by a jump in the water content from a point at which the morphologies are under equilibrium control. The transition mechanism is monitored by transmission electron microscopy (TEM). The transition intermediates are trapped by quenching solution samples to liquid nitrogen temperature and then preserving the aggregate morphologies using the freeze-drying technique. It is shown that the morphological transition goes through a lamellar intermediate state. The corresponding kinetic data are obtained from turbidity measurements. The analysis of the plot of turbidity versus time suggests two consecutive first order steps. The first step appears to be the flattening of short rods in favor of irregularly shaped or circular lamellae, which are the intermediate morphologies observed using TEM. The second step involves the closing of the lamellae to vesicles. Since each of the three species, i.e., rods, lamellae, and vesicles, makes a distinct contribution to the turbidity, the changes in the contribution of each species to the turbidity as a function of time in the transition process can be calculated quantitatively. The kinetic analysis shows that the relaxation times (tau(1) and tau(2)) of the morphological transition are influenced by the initial water content and the polymer concentration. The relaxation times increase with increasing initial water content, while the reciprocals of the relaxation times approach zero when the initial water content reaches the upper boundary of the rod stability region. An increase in the polymer concentration leads to an increase in the relaxation times. The size of the jump in the water content has a little effect on the kinetics.