AB block copolymer micelles

Kwon GS, Naito M, Yokoyama M, Okano T, Sakurai Y, Kataoka K. Physical entrapment of adriamycin in AB block copolymer micelles. Pharm Res 1995 12 192-195. 

In this section, we would like to briefly describe the micellization behavior of hybrid AB copolymers, leading to remarkable structures with new potential applications. By hybrid AB block copolymers we mean copolymers in which at least one of the constituent blocks is not a classical poly-... 

The vast majority of block copolymer micelles has been constructed from AB diblock copolymers. However, ABC triblock copolymers have attracted a great deal of interest due to the huge number of different morphologies that have been observed so far in bulk and because the introduction of a third block may introduce interesting new functionalities. Although many investigations have... 

Macrophase separation after microphase separation has been observed in an AB block copolymer/homopolymer C blend (Hashimoto et al 1995). Blends of a PS-PB starblock copolymer (75wt% PS) and PVME homopolymer were prepared by solvent casting. Binary blends of PS and PVME exhibit a lower critical solution temperature (LCST), i.e. they demix at high temperatures. The initial structure of a 50% mixture of a PS-PB diblock and PVME shown in Fig. 6.20(a) consists of worm-like micelles. Heating led to macrophase separation as evident... 

Pharmaceutical research on polymeric micelles has mainly focused on two kinds of block copolymers, namely, AB block copolymers or diblock copolymers and ABA or BAB block copolymers known as triblock copolymers (Bader et al., 1984 Yokoyama et al., 1990,1991 Kwon and Okano, 1996, 1999 Kwon, 1998, 2003 Alakahov and Kabanov, 1998). The most common hydrophilic block (A) of the block copolymers is polyethylene oxide (PEO). This polymer is highly hydrated through hydrogen bonding and sterically stabilizes surfaces of the polymeric micelles in aqueous systems. 

Kwon, G., M. Naito, M. Yokoyama, T. Okano, Y. Sakurai and K. Kataoka. 1993. Micelles based on ab block copolymers of polyethylene oxide) and poly(-benzyl-asparttte)gmuir9 945-949. 

Kwon, G., Naito, M., Yokoyama, M., et al. Micelles based on AB block copolymers of poly(ethylene oxide) and poly(benzyl-aspartat). Langmuir 9 945—949. 1993. 

Fig. 5 Schematic representation of AB diblock copolymer micelles in a selective solvent of the A block, Rc core radius, L shell (corona) thickness. Adapted from Riess [29]. Copyright 2003, with permission from Elsevier...

It is well known that block copolymers in a selective solvent (a good solvent for one block but a non-solvent for the other) form a micellar structure through the association of the insoluble segments. In contrast with micelles formed from low molecular weight surfactants, block copolymer micelles dissociate slowly to free polymeric chains. They have a greater capacity for solubilizing aromatic molecules and express lower CMCs. The AB block copolymers are considered useful vehicles for hydrophobic drugs. 

Fig. 3. Formation of a micelle (aggregation number 2=8) of AB-block copolymers...

Fig. 5.26. A Mnear (AB) block copolymer with preferential solvation of the B block (top), an inverted micelle (middle), and a swollen lamellar morphology (bottom). The magnified view at the bottom shows the chain trajectories in the interphase near the A-B interface in the fluid microphase-separated morphology.

Whitmore and Noolandi [101] derived the structural parameters of monodis-persed AB diblock copolymer micelles within an A homopol5mier by minimizing a simple free energy functional. The CMC was calculated and shown to be dominated by an exponential dependence on (x is the Hory-Huggins interaction parameter and Nb the degree of polymerization of the B block of the copolymer). The importance of diblock copolymer composition was emphasized as well. The CMC was calculated as ... 

Mixed micellar solutions of enantiomeric ABA or BAB block copolymers exhibited very different gelation behavior and crystal structure. As the third system, the micellar solutions of AB block copolymers, PLLA-PEG and PDLAPEG, were examined for the hydrogel formation. The AB system is similar to ABA in that the B-blocks form the corona shape in micelles as illustrated in Figure 1, while having a similarity with BAB because the mobility of the core A-blocks is similar to each other. 

Surface activity in 1,4-polyisoprene-polyacetylene, AB, block copolymer solutions was to be expected from the amphiphilic properties of such a diblock system with one moiety so insoluble because of the strong polyacetylene-polyacetylene attractive interactions. The present experiments allow access, for the first time, to some of the thermodynamic parameters of these interactions and give a structural model for the surface excess above and below the critical micelle concentration. This has been identified as about 10" moles/L for the lelated polymer 1,4-polyisoprene-polyacetylene (MW 8000 520) in toluene at 20 C using the drop weight method to determine surface tension. From ca. 10" molar to molar the surface tension drops by about 3.5% to a constant value of ca. 28.4 dyne cm at concentrations above 10 3 molar (ca. 1% w/w). Referring to Figure 3 we see that it is above ca 1% that a broad peak develops in the solution/solvent reflectivity ratio for 0.15 < k / < 0.25. The area per... 

Generally, the number of the shell chains in a microsphere ranges from a few hundred to a few thousand. The range of the diameter of the core is from 10-100 nm. Such a core-shell structure is very similar to the (AB)n type star block copolymers, which have many arms and spherical polymer micelles of the block or graft copolymers formed in selective solvents that are good for the corona sequence and bad for the core sequence. In fact, many theoretical investigations of the chain con-... 

AB diblock copolymers in the presence of a selective surface can form an adsorbed layer, which is a planar form of aggregation or self-assembly. This is very useful in the manipulation of the surface properties of solid surfaces, especially those that are employed in liquid media. Several situations have been studied both theoretically and experimentally, among them the case of a selective surface but a nonselective solvent [75] which results in swelling of both the anchor and the buoy layers. However, we concentrate on the situation most closely related to the micelle conditions just discussed, namely, adsorption from a selective solvent. Our theoretical discussion is adapted and abbreviated from that of Marques et al. [76], who considered many features not discussed here. They began their analysis from the grand canonical free energy of a block copolymer layer in equilibrium with a reservoir containing soluble block copolymer at chemical potential peK. They also considered the possible effects of micellization in solution on the adsorption process [61]. We assume in this presentation that the anchor layer is in a solvent-free, melt state above Tg. The anchor layer is assumed to be thin and smooth, with a sharp interface between it and the solvent swollen buoy layer. 

Polymer micelles are nanometer sized (usually several tens of nanometers) self-assembled particles having a hydrophobic core and hydrophilic outer shell composed of amphiphilic AB- or ABA-type block copolymers, and are utilized as drug delivery vehicles. The first polymer micelle-type drug delivery vehicle was made of PEG-b-poly(aspartic acid) (PEG-b-PAsp), immobilizing the hydro-phobic anticancer drugDXR [188-191]. After this achievement by Kataoka et al., a great amount of research on polymer micelles has been carried out, and there are several reviews available on the subject [192-194]. 

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