Bilayer Morphologies of Self-Assembled Crew-Cut Aggregates of Amphiphilic PS-b-PEO Diblock Copolymers in Solution

Abstract

A wide range of bilayer aggregates, among them tubules, vesicles, large compound vesicles (LCVs), and lamellae, were prepared from various polystyrene- b -poly(ethylene oxide) (PS- b -PEO) diblock copolymers, and studied by transmission electron microscopy (TEM). The preparation method involved copolymer dissolution in DMF at room temperature, followed by the addition of water. In addition, it was found that aggregates of various morphologies can be prepared from an identical block copolymer by changing the solvent from DMF to a water−DMF mixture, by the addition of electrolytes, or by the use of subambient temperatures. All of these methods can be used to facilitate the formation of specific bilayer aggregates. When the preparation method involved copolymer dissolution in water−DMF mixtures, it was found that the morphologies of aggregates under certain conditions also depended on the annealing time. For example, the ratio of tubules to vesicles is related to the annealing time; only tubules appear at long annealing times, while vesicles and tubules coexist at short annealing times. Possible mechanisms for the formation of the bilayers are discussed. Tubules with oscillatory perturbations in their diameters are seen; these might be intermediates in the vesicle to tubule transition. Lamellae are observed frequently in the present system. In addition, bent lamellae have been observed for the first time; they may be intermediates in the lamella to vesicle transition, as suggested by some theories dealing with small molecule amphiphiles. The present system is believed to be the first to yield stable block copolymer amphiphile tubules, as well more complex tubular or vesicular aggregates, such as “plumber's nightmare”, starfishlike vesicles, budding vesicles (i.e. a chain of vesicles), and vesicles consisting of one or more small internalized vesicles within a larger one. Some of the morphologies are biomimetic.