Copolymer vesicles

Fig. 22 Formation of vesicles with distinct inner (PB) and outer (PS) coronal blocks from a mixture of PS-P4VPQ and PB-PMACs copolymers. Vesicles are stained with Csl in the TEM pictures. TEM picture B is a magnification of the framed area in A. Reprinted with permission from [311]. Copyright (2003) American Chemical Society... 

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. 

The study of liposomes as dmg-delivery agents has been ongoing for decades. The lipids for liposome formation are typically harvested by extraction from egg yolks and soybeans, and a number of recipes exist for generating liposomes of various diameters. Because the shell material in liposomes is not polymeric, we will not discuss them in depth, limiting ourselves only to those aspects that are pertinent to synthetic analogues like the copolymer vesicles of the next section. 

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

Spherical micelles are not the only association structure that is formed by polyelectrolyte block copolymers. With increasing hydrophobic block length there is a tendency to form block copolymer vesicles. A vesicle formed by PB-P2VP.HC1 is shown in the cryo-TEM image in Fig. 14a. The bilayer structure is clearly resolved which shows that block copolymer vesicles are structurally very similar to lipid vesicles. Vesicles can be also imaged by AFM (Fig. 14b) where they exhibit a characteristic outer rim because the interior solution of the vesicle has evaporated during sample preparation leaving a shape that resembles that of an empty football. Vesicles typically have diameters of 100-300 nm and a bilayer thickness of 10-20 nm. 

Fig. 15 Video microscopy snapshots of the rupture and dissolution of block copolymer vesicles (P2VP-PEO) upon addition of acetic acid. Such vesicles may be used for the pH-controlled delivery of drugs [65]...

Fig. 14 Cryo-TEM (a), AFM- (b), and TEM images of polyelectrolyte block copolymer vesicles (PB-P2VP.MeI). The image (c) is taken from a silica-template which was obtained by a sol/gel-process of a concentrated micellar solution [47, 56, 64]...

Block copolymer vesicles, or polymersomes, are of continued interest for their ability to encapsulate aqueous compartments within relatively robust polymer bilayer shells (Fig. 7) [66, 67]. Eisenberg and coworkers were the first to report the formation of block copolymer vesicles from the self-assembly of polystyrene-h-poly(acrylic acid) (PS-h-PAA) block copolymers. They also have described the formation of a wide range of vesicle architectures in solution from the self-assembly of five different block copolymers PS-h-PAA. PS-h-PMMA, PB-h-PAA, polystyrene-h-poly(4-vinyIpyridinium methyl iodide), and polystyrene-h-(4-vinylpyridinium decyl iodide) [68]. Small uniform vesicles, large polydisperse vesicles, entrapped vesicles, hollow concentric vesicles, onions, and vesicles with hollow tubes in the walls have been observed and the formation mechanism discussed. Since vesicles could be prepared with low glass transition polymers such as PB [69, 70] and PPO [71], it has been established than these structures are thermodynamically stable and not trapped by the glassy nature of the hydrophobic part. 

Luo L, Eisenberg A (2002) One-step preparation of block copolymer vesicles with preferentially segregated acidic and basic corona chains. Angew Chem Int Ed Engl 41 1001-1004... 

Brannan AK, Bates FS (2004) ABCA tetrablock copolymer vesicles. Macromolecules 37 8816-8819... 

Tong X, Wang G, Soldera A, Zhao Y. (2005) How can azobenzene block copolymer vesicles be dissociated and reformed by light J Phys Chem B 109 20281-20287... 

Dimova, R., Seifert, U., Pouligny, B., Forster, S., and Dobereiner, H.-G. (2002) Hyperviscous diblock copolymer vesicles. European Physical Journal B, 7, 241-250. 

Beyond the thermodynamic control, the kinetics of chain rearrangement can dramatically influence the phase behavior leading to kinetically trapped structures, which do not necessarily correspond to an absolute free energy minimum of the system. Thus, the formation of block copolymer vesicles, from a kinetic point of view, can be a result of a transition from rod-like aggregates via flat, nonclosed lamellar structures. The kinetics of such transitions has been explored in [8], The transition steps are represented as follows ... 

Fig. 11 Cryo-TEM image of PMOXA-t>-PDMS-t>-PMOXA vesicles prepared by film swelling in water scale bar 200 nm. Reprinted from [187] Kita-Tokarczyk K, Grumelard J, Haefele T, Meier W. Block copolymer vesicles-using concepts from polymer chemistry to mimic biomembranes. Polymer 46 3540, Copyright (2005), with permission from Elsevier...

Features such as mechanical stability, tunable properties, responsiveness to environmental stimuli, ability to encapsulate both hydrophilic and hydrophobic compounds, etc. make polymer membranes excellent candidates for use in medical, pharmaceutical, and environmental fields. Hence, polymer membranes have attracted a considerable attention in recent years. In this section, we review the potential applications of block copolymer vesicles. 

Fig. 16 a-d Schematic representation of polymer nanoreactors, a Cross section of triblock copolymer vesicle, b Polymersome with encapsulated enzyme and membrane-embedded channel protein. In the case described in the text, the substrate entering the vesicle is ampicillin, and the product of the hydrolysis is ampicillinoic acid, c Polymersome with embedded ionophores allowing Ca2+ ions to enter the vesicle ere they react with phosphate ions to form calcium phosphate crystals, d The LamB protein serves as a receptor to the 1 phage virus which can inject its DNA through the channel into the polymersome [259]. Reproduced with permission of The Royal Society of Chemistry... 

Fig. 17 Schematic representation of ion-channel controlled precipitation of calcium phosphate in block copolymer vesicles (middle drawing), and TEM micrographs right drawing) of phosphate-loaded PMOXA-PDMS-PMOXA triblock copolymer giant vesicles after 1 and 24 h of incubation with CaCl2 solution in the presence of the ionophore scale bar. 500 nm [163]. Reproduced with permission of The Royal Society of Chemistry...

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