Vesicle block copolymers

Variation of block lengths further allows one to control the shape of the micelles. Block copolymers with large soluble B-blocks form spherical micelles, whereas cylindrical micelles and vesicles result from block copolymers with continuously smaller soluble blocks. Cylindrical micelles, e.g., of poly(butadi-ene-h-ethyleneoxide) (Fig. 5B) may have lengths of several micrometers [27]. Block copolymer vesicles were observed with diameters from 100 nm up to several micrometers. Compared to lipid vesicles, block copolymer vesicles are mechanically and thermodynamically much more stable [28, 29] and are well suited as templates. Figure 5C shows vesicles of poly(2-vinylpyridine-h-ethylene oxide) (P2VP-PEO) with diameters of more than 10 pm (giant vesicles) [30]. 

Stability. Because of their increased toughness and bending elastisticity, block copolymer vesicles are more stable compared to lipid vesicles. Block copolymer vesicles have been reported to be stable over several years with no changes in size or size distribution (156). 

Recently, unique vesicle-forming (spherical bUayers that offer a hydrophilic reservoir, suitable for incorporation of water-soluble molecules, as well as hydrophobic wall that protects the loaded molecules from the external solution) setf-assembUng peptide-based amphiphilic block copolymers that mimic biological membranes have attracted great interest as polymersomes or functional polymersomes due to their new and promising applications in dmg delivery and artificial cells [ 122]. However, in all the cases the block copolymers formed are chemically dispersed and are often contaminated with homopolymer. 

The vesicles made from lipid bilayers are analogous to polymersomes, which are vesicles formed from high molecular weight amphiphilic block copolymers [94—96], Unlike the micelles discussed earlier from the similar copolymer components, the presence of bilayer walls formed from the aggregation of hydrophobic domains provides new properties. They can be designed to respond, for example, by opening or by disassembly, to external stimuli such as pH, heat, light, and redox processes [97]. This makes them usable as scaffolds for cascade reactions, even those with combinations of enzymes [98, 99]. 

Instead of the familiar sequence of morphologies, a broad multiphase window centred at relatively high concentrations (ca. 50-70% block copolymer) truncates the ordered lamellar regime. At higher epoxy concentrations wormlike micelles and eventually vesicles at the lowest compositions are observed. Worm-like micelles are found over a broad composition range (Fig. 67). This morphology is rare in block copolymer/homopolymer blends [202] but is commonly encountered in the case of surfactant solutions [203] and mixtures of block copolymers with water and other low molecular weight diluents [204,205]. 

Goltner, C. G. Berton, B. Kramer, E. Antonietti, M. 1999. Nanoporous silicas by casting the aggregates of amphiphilic block copolymers The transition from cylinders to lamellae and vesicles. Adv. Mater. 11 395-398. 

Fig. 14 TEM micrographs showing a vesicles, b, c micellar fibers, and d superhelices from a PEO-PMPS block copolymer (the chemical structure of this copolymer is represented below the micrographs). Reprinted with permission from [238]. Copyright (2001) American Chemical Society...

Block copolymer/low-MW-molecule complexes were also examined in organic solvents, as recently exemplified by the works of Jiang and coworkers [319,320]. These authors investigated mixtures of PS-P4VP copolymers with various low-MW molecules including perfluorooctanoic acid and formic acid. Such molecules are expected to form hydrogen-bonded complexes with 4VP units in organic solvents such as chloroform. This further resulted in the formation of vesicles. 

Figure 6.5 Illustrations of nanoscale spherical assemblies resulting from block copolymer phase separation in solution are shown, along with the chemical compositions that have been employed to generate each of the nanostructures (a) core crosslinked polymer micelles (b) shell crosslinked polymer micelles (SCKs) with glassy cores (c) SCKs with fluid cores (d) SCKs with crystalline cores (e) nanocages, produced from removal of the core of SCKs (f) SCKs with the crosslinked shell shielded from solution by an additional layer of surface-attached linear polymer chains (g) crosslinked vesicles (h) shaved hollow nanospheres produced from cleavage of the internally and externally attached linear polymer chains from the structure of (g)...

Vesicle-based capsules The shell is self-assembled into a hydrophobic shell or bilayer surrounded inside and outside by solution. These materials are formed using either small amphiphiles or amphiphilic block copolymer vesicles. 

Vesicles for use as materials can be divided into two categories naturally occurring vesicles, or liposomes, which are composed of natural amphiphiles, usually phospholipids and polymer vesicles, which are generally composed of block copolymers. 

Soo PL, Eisenberg A. Preparation of block copolymer vesicles in solution. J Polym Sci B Polym Phys 2004 42 923-938. 

Sha et al. applied the commercially available dual initiator ATRP-4 for the chemoenzymatic synthesis of block copolymers. In a first series of publications, the group reported the successful synthesis of a block copolymer comprising PCL and polystyrene (PS) blocks [31, 32]. This concept was then further applied for the chemoenzymatic synthesis of amphiphilic block copolymers by macroinitiation of glycidyl methacrylate (GMA) from the ATRP functional PCL [33]. This procedure yielded well-defined block copolymers, which formed micelles in aqueous solution. Sha et al. were also the first to apply the dual enzyme/ATRP initiator concept to an enzymatic polycondensation of 10-hydroxydecanoic acid [34]. This concept was then extended to the ATRP of GMA and the formation of vesicles from the corresponding block copolymer [35]. 

Figure 5.10 Self-organization of di-block copolymers. Block copolymers can form spherical and cylindrical micelles, vesicles, spheres with face-centered cubic (fee) and body-centered cubic (bcc) packing, hexagonally packed cylinders (hex) minimal surfaces (gyroid, F surface, and P surface), simple lamellae and modulated and perforated lamellae. (Adapted from Bucknall and Anderson, 2003.)...

Block copolymers that contain segments with different characteristics, such as hydrophobic and hydrophilic blocks, can form phase-separated solid systems, or they can be employed to form micelles or vesicles in aqueous media. Considering that the lengths of each polymer block can be controlled, the opportunities for property variations are almost infinite through such systems. 

Keywords Block copolymers Director Hydrodynamics Layer normal Layered systems Liquid crystals Macroscopic behavior Multilamellar vesicles Onions Shear flow Smectic A Smectic cylinders Undulations... 

Vriezema, D. M., Hoogboom, J., Velonia, K., et al, Vesicles and polymerized vesicles from thiophene-containing rod-coil block copolymers. Angew. Chem., Int. Ed. 2003, 42, 772-776. 

FIGURE 7.34. Top structure of PEO-PMPS block copolymer. Bottom transmission electron micrographs showing the formation of (a) vesicles, (b, c), micellar fibers, and (d) superhelices from this block copolymer. Reproduced with permission from the American Chemical Society. 

FIGURE 7.36. Schematic model of a vesicle with a corona of segregated polymer chains, as formed in a mixture of oppositely charge block copolymers. Reproduced with permission from the American Chemical Society (2003). 

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