Formation of crew‐cut aggregates of various morphologies from amphiphilic block copolymers in solution

Abstract

Crew-cut micelle-like aggregates represent a new type of aggregate formed in solution by the self-assembly of highly asymmetric amphiphilic block copolymers. The aggregates are termed crew-cut because the dimensions of the core are bigger than those of the corona. They are prepared by first dissolving copolymers, such as polystyrene-b-poly(acrylic acid), in a common solvent, e.g. dimethylformamide (DMF), and then adding water to the solution to induce aggregation of the polystyrene segments. Since the first description of an experimental study of spherical crew-cut micelles in 1994, over 20 papers have appeared in the literature. One of the most noteworthy phenomena seen in the crew-cut aggregate systems is the existence of multiple morphologies. These include spheres, rods, vesicles, lamellae, large compound vesicles, large compound micelles and several other structures. This paper presents an overview of the phenomenon, and a description of the qualitative principles underlying the formation of crew-cut aggregates of various morphologies in solution. The main topics of this paper are the preparation of crew-cut aggregates, the critical micellization concentration, the aggregation number, the common solvent content in the core, the mobility of the core-forming blocks in the core region, and the relative importance of thermodynamics versus kinetics during micellization as a function of added water content. The formation and morphology of the crew-cuts are also compared with those in other self-assembled systems, i.e. small molecule amphiphiles in solution, block copolymers in bulk and blends of block copolymers and homopolymers. It has been found that the most important factors influencing the morphology of the aggregates are the copolymer composition, the initial copolymer concentration in solution, the added water content, the nature of the common solvent and the type and concentration of added ions (salt, acid or base). The morphology is mainly controlled by a force balance involving three factors, namely the stretching of the polystyrene (PS) blocks in the core, the surface tension between the core and the outside solvent, and the repulsion among the corona chains. The morphogenic effects of the above-mentioned parameters can, in general, be ascribed to their influence on this force balance during the formation of the aggregates. In particular, the length of corona-forming blocks or the addition of ions mainly affects the repulsive interactions of the corona chains; the effect of the water content in solution is related to the change of the surface tension between the PS core and the solvent; and the effect of changing the common solvent from DMF to tetrahydrofuran or to dioxane on the morphology is due mainly to the increased dimensions of the core-forming blocks resulting from the changes of the solvent contents in the core and the aggregation numbers. © 1998 John Wiley & Sons, Ltd.