Block copolymer vesicles

Abstract

Amphiphilic block copolymers have the ability to assemble into multiple morphologies in solution. Depending on the length of the hydrophilic block, the morphology can vary from spherical micelles, rods, and vesicles to large compound micelles (LCMs). Vesicle formation is favored upon an increase in total molecular weight of the block copolymer, that is, an increasing bending modulus (K). Owing to the polymeric character of this type of vesicle (also called polymersomes), they possess remarkable properties. The diffusion of (polymeric) amphiphiles in these vesicles is very low compared to liposomes and for high-molecular-weight chain entanglements even lead to reptation-type motions, which make it possible to trap near-equilibrium and metastable morphologies. Additionally, in contrast to liposomes, membrane thicknesses can exceed 200 nm. As a consequence, this increased membrane thickness, in combination with the conformational freedom of the polymer chains, leads to a much lower permeability for water of block copolymer vesicles compared to liposomes. The enhanced toughness and reduced permeability of polymersomes makes them, therefore, very suitable as stable nanocontainers, which can be used, for example, as reactors or drug delivery vehicles.Self-assembly of amphiphilic block copolymers in solution has been a topic of active research for more than 30 years. The most commonly observed morphology in these systems is the star-micelle. "Star" refers to the fundamental core-corona structure, which consists of a small core and a large corona. These star-micelles can be divided into regular and reversed micelles, which are formed in polar and apolar solvents, respectively.Over the past few years, the ability of highly asymmetric, amphiphilic block copolymers to assemble into aggregates of multiple morphologies in solution has attracted much attention. For these "crew-cut" aggregates, a term proposed by Halperin et al. [1], the longer block forms the core of the aggregate, while the corona is composed of the short segment. Manipulation of the relative block lengths and environmental parameters, such as solvent composition, the presence of additives, and temperature, has resulted in a variety of morphologies, including spheres, rods, vesicles, lamellae, tubules, large compound micelles (LCMs), large compound vesicles (LCVs), and hexagonally packed hollow hoops (HHHs).Several of these block copolymer morphologies are classified as vesicles because they all have hollow-spherical structures containing walls composed of bilayers of polymer molecules. The field of block copolymer vesicles (polymersomes) has only recently been explored. The earliest reports on polymersomes focused on vesicles prepared from bulk copolymer systems [2] and block copolymer/homo-

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Quantitative membrane loading of polymer vesicles

“…[5][6][7] More recently, polymersomes (50 nm-50 mm diameter synthetic polymer vesicles) have been formed from a number of amphiphiles, offering a rich diversity in chemical and material properties. [8][9][10][11][12] Through synthetic chemistry, rational control of block copolymer compositions is readily achieved and has enabled the generation of polymer vesicles that are substantially more stable than their phospholipid-based counterparts; polymersomes have been further designed to systematically degrade over longer timescales. 13 Dependent upon the molecular weight of their component amphiphiles, polymersomes also possess a membrane core thickness that can vary between 5-50 nm, affording enormous mechanical strength that makes them 5-50 times tougher than liposomes, even when in the fluid state.…”

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