Phase diagrams and applications of amphiphilic block copolymers in aqueous solutions

Conventional amphiphilic block copolymers are macromolecules consisting of at least a hydrophilic segment covalently attached to a hydrophobic segment. They are unique and versatile building blocks in supramolecular polymer chemistry, both for the generation of highly organized, self-assembled structures and for the structural control of material interfaces. In the absence of solvents, the phase-behavior of block-copolymers is sparingly described. In aqueous solutions block copolymers self-assemble into nanoscopic objects. These structures are gaining more and more attention for technical formulations. Self-assembly is simply induced by dissolving dry powder of amphiphilic copolymers in water. This self-assembly process, however, cannot be described solely based on bulk phase behavior and very few experimental phase diagrams of block copolymer/water mixtures have been reported. In order to control, predict and eventually achieve a comprehensive understanding of this process and in particular the formation of vesicles, worm/rod-like micelles and spherical micelles it is important to systematically investigate the phase behavior of block copolymers across the whole water concentration range. We therefore undertook investigations of the self-assembly of two block copolymers. Preliminary encapsulation studies reveal their potential for applications relevant for drug delivery.
The concentration profile of the identified amphiphilic block-copolymers diluted in aqueous solution was examined across the whole concentration range with four poly(ethylene oxide)-block-poly(γ-methyl-ε-caprolactone) (PEO-b-PMCL) block copolymers keeping the hydrophilic block length constant but varying the hydrophobic lengths. Bulk polymers did not display any ordered morphology. With increasing water concentrations the polymers underwent transitions from lamellar phases to packed vesicles and eventually all polymers self-assembled into vesicles in dilute aqueous solutions. At high concentration the largest polymer organized further into an inverse hexagonal phase prior to self-assembly into a lamellar phase. The smallest block-copolymer also self-aggregated into rod-like micelles and exhibited a hexagonal phase between the packing of the rod-like micelles/vesicles and the lamellar phase.
The concentration profile of phases assembled by amphiphilic block-copolymers diluted in aqueous solution was examined as well across the whole concentration range with three poly(isobutylene)-block-poly(ethylene oxide) block copolymers having the same hydrophobic block length but varying in their hydrophilic lengths. In contrast to PEO-b-PMCL these polymers self-aggregated depending on the hydrophilic length into micelles, worm-like micelles and for the shortest polymer into vesicles. Thus, increasing the polymer concentration lead to a different phase behavior. The largest polymer which solely formed micelles, underwent transitions from packed micelles with a few packed worm-like micelles to a hexagonal phase and finally into a lamellar phase, in contrary to the middle length polymer, which self-aggregated into worm-like micelles. The best to our knowledge, it is the first time that an experimental phase diagram is built such that it displays the formation of worm/rod-like micelles. With the composition studied in here, the worms, with increasing polymer concentration, pack and undergo afterwards into a hexagonal and finally a lamellar phase has been observed. For the shortest polymer the phase behavior was similar to that of vesicles self-assembled by PEO-b-PMCL. In contrast, the pure polymer formed an inverse hexagonal phase.
It was eventually shown that PEO23-PMCl32 can be used to encapsulate both small and large molecules. By adjusting the pH towards acidic conditions the responsiveness of the vesicles could be triggered to release the encapsulated material.