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.

Control of the block copolymer morphology in templated epoxy thermosets

“…As the curing of the resin progresses, the unstable, distorted spherical micelles adhesively collide with neighboring micelles and form a structure similar to a string of interconnected spheres reported by Discher and others. 39,45,46 From this collection of interconnected spheres, the more stable wormlike micelle structure forms. The high magnification TEM images of the uncured Sample 8, shown in Figure 7, support this method of wormlike micelle formation.…”

Self‐Assembled Block Copolymer Aggregates: From Micelles to Vesicles and their Biological Applications

“…The type of structure formed is due to the inherent curvature of the molecule, which can be estimated through calculation of its dimensionless packing parameter, p. Figure 2. Transmission electron micrographs of: a) 'hamburger' micelles, [12] b) helical micelles, [23] c) bilayer tubules, [29] and d) mixtures of polymer vesicles and 'octopi' structures. [18] (Reproduced with permission from ref.…”

“…[18] (Reproduced with permission from ref. [12,18,23,29] membrane permeability, with the addition of protein channels [57] and membrane plasticization [58,59] greatly increasing trans-membrane diffusion. Biological membranes are much more complex and sophisticated than the simple vesicle structures reported here, since the former contain many types of proteins and cellular recognition elements.…”

“…[24][25][26][27] Spherical vesicles are not the only possible bilayer-type structures generated through self-assembly. Bilayer tubules have been generated from direct dissolution of copolymers in solvent mixtures followed by dialysis [28,29] ( Figure 2c) and also in pure organic solvents, [30] with tubes up to centimetres in length being obtained. [31] Unfortunately, there are few reports that describe the formation of colloidally stable, self-assembled tubes in purely aqueous solution, since the energetically favoured fate of a copolymer bilayer under these conditions is the spherical vesicle.…”

Synthesis, Thermal Behavior, and Aggregation in Aqueous Solution of Poly(methyl Methacrylate)-B-Poly(2-Hydroxyethyl Methacrylate)

“…The core is formed by the hydrophobic segments, whereas the hydrophilic segments form the shell which provides solubility in water and stability to the aggregates. A number of hydrophilic polymers have been used but poly(ethylene glycol) PEO is the most common due to its unique properties, and many studies on PEO-based copolymers have been reported [10][11][12] . Recently, poly(2-hydroxyethyl methacrylate) (PHEMA)) has attracted great interest because this polymer exhibits excellent biocompatibility 13 and good blood compatibility 14 .…”