Trapping of intermediate structures of the morphological transition of vesicles to inverted hexagonally packed rods in dilute solutions of PS-b-PEO

The morphological transition from vesicles to inverted hexagonally packed ( hollow) rods was studied for asymmetric diblock copolymers of polystyrene-b -poly(ethylene oxide) (PS-b-PEO) in dilute solutions. The self-assembled ag gregates were prepared by the addition of water to the copolymer solutions in tetrahydrofuran (THF) to induce the aggregation of the PS blocks, and th e morphological transition was induced by an increase in the water content. Many intermediates, such as vesicles with hollow regions in the wall runni ng parallel to the surfaces, as well as various quasihexagonal structures, were trapped. The mechanism of the transition involves a thickening of the vesicle walls accompanied by the formation of the hollow rods in the walls and a decrease in the size of the original water core. Evidence is presente d that the transition proceeds in three steps. In the first step, hollow re gions form in the walls of the vesicles. The second step involves further t hickening of the walls and some alignment of the rods in a hexagonal patter n leading to the formation of quasihexagonal structures. Finally, the fully developed structures of inverted hexagonally packed rods form. This mechan ism, especially in the initial step, is different from those found either i n polystyrene-b-poly(acrylic acid) (PS-b-PAA) diblocks or in small molecule amphiphile systems, both of which involve fusion of vesicles. A comparison of the mechanisms is presented. Because of the ease of trapping of interme diates, the present study of the morphological transition may improve our u nderstanding of the later stages of the process of the biomembrane fusion a nd the lipid lamellar (L-alpha) to inverted hexagonal (H-II) phase transiti on.