Block copolymer micelle

Fig. 6. Possible structures of block copolymer micelles. Top Hairy micelle Bottom Crew-cut micelle...

Kwon GS, Naito M, Kataoka K, Yokoyama M, Sakurai Y, Okano T (1994) Block copolymer micelles as vehicles for hydrophobic drugs. Colloids Surf B 2 429-434... 

Nanometer size Pd colloids in block copolymer micelles of polystyrene polyvinylpyridine as catalysts have been used is a novel way by Klingelhofer for Heck reaction of C-C coupling of aryl halides with olefins. 

Kakizawa Y, Kataoka K (2002) Block copolymer micelles for delivery of gene and related compounds. Adv Dmg Deliv Rev 54 203-222... 

Kang N, Perron ME, Prudhomme RE et al (2005) Stereocomplex block copolymer micelles core-shell nanostructures with enhanced stability. Nano Lett 5 315-319... 

Gaucher G, Dufresne MH, Sant VP et al (2005) Block copolymer micelles preparation, characterization and application in drug delivery. J Control Release 109 169-188... 

Lee ES, Shin HJ, Na K et al (2003) Poly(L-histidine)-PEG block copolymer micelles and pH-induced destabilization. J Control Release 90 363-374... 

Colloidal catalysts in alkyne hydrogenation are widely used in conventional solvents, but their reactivity and high efficiency were very attractive for application in scC02. This method, which is based on colloidal catalyst dispersed in scC02, yields products of high purity at very high reactions rates. Bimetallic Pd/Au nanoparticles (Pd exclusively at the surface, while Au forms the cores) embedded in block copolymer micelles of polystyrene-block-poly-4-vinylpyridine... 

The micelle formation is not restricted to solvents for polystyrene but also occurs in very unpolar solvents, where the fluorinated block is expected to dissolve. Comparing the data, we have to consider that the micelle structure is inverted in these cases, i.e., the unpolar polystyrene chain in the core and the very unpolar fluorinated block forming the corona. The micelle size distribution is in the range we regard as typical for block copolymer micelles in the superstrong segregation limit.2,5,6 The size and polydispersity of some of these micelles, measured by DLS, are summarized in Table 10.3. 

Hydrodynamic diameters dh and corresponding Gaussian width c of block copolymer micelles in solvents of decreasing polarity. None means that no micelles could be detected by DLS. 

Abstract This review summarizes recent advances to date in the area of block copolymer micelles and also tries to highlight some new directions in that field. Generalities... 

From a morphological point of view, block copolymer micelles consist of a more or less swollen core resulting from the aggregation of the insoluble blocks surrounded by a corona formed by the soluble blocks, as decribed in Sect. 2.3. Experimental techniques that allow the visualization of the different compartments of block copolymer micelles will be presented in Sect. 2.4. Other techniques allowing micellar MW determination will also be briefly discussed. Micellar dynamics and locking of micellar structures by cross-linking will be commented on in Sects. 2.5 and 2.6, respectively. 

Selected examples of block copolymer micelles in both aqueous and organic media will then be presented in Sects. 3 and 4. Section 4.3 emphasizes stimulus-responsive micellar systems from double-hydrophilic block copolymers. Prediction of the dimensional characteristic features of block copolymer micelles and how it varies with the composition of the copolymers will be shortly outlined in Sect. 5, with a consideration of both the theoretical and experimental approaches. Tuning of micellar morphology and triggering transitions between different morphologies will then be discussed in Sect. 6. 

With the increasing number of publications on block copolymer micelles (a database literature search with these three associated keywords already gives more than 500 references), an exhaustive description of all previous works would not be possible in the framework of the present review. This contribution has rather as its purpose giving a general overview about block copolymer micelles for the nonspecialist and will therefore try to answer such practical questions as how does one prepare block copolymer micelles How does one characterize them What are the different types of structures that can be formed How can we predict them How does one tune the morphology of these micelles These basic questions and the corresponding answers will be illustrated by selected examples. Then, we will focus on the new directions that are currently implemented in this field. 

The research area of block copolymer micelles has been reviewed by other authors including Price [4], Piirma [5], Tuzar and Kratochvil [6], Riess and coworkers [7,8], Webber et al. [9], Alexandridis and Hatton [10], Nace [11], Hamley [2], Alexandridis and Iindman [12], and Xie and Xie [13]. A very complete review on block copolymer micelles was published recently by... 

Generalities about block copolymer micelles have been reviewed by Ham-ley [2] and Riess [14], based on previous works from the 1980s and 1990s. This topic will not be covered in detail, but the basic principle, as well as some important practical issues, will be reviewed. The essential experimental techniques used for block copolymer micelle characterization will also be outlined briefly. 

Although it strongly influences the micellar characteristic features, the method used for the preparation of block copolymer micelles has been very often poorly discussed in the literature. This crucial point was raised in the excellent review of Riess [14]. 

Addition of a selective solvent to molecularly dissolved chains has been used by many research teams to prepare block copolymer micelles. The initial nonselective solvent can be further eliminated by evaporation or can be gradually replaced by the selective solvent via a dialysis process. The stepwise dialysis initially introduced by Tuzar and Kratochvil is now widely used for micelle preparation [6], especially for the formation of aqueous micelles [32],... 

It is important to define clearly the characteristic features of block copolymer micelles. We mentioned above that the insoluble blocks formed a micellar core surrounded by a corona. Depending on the composition of the starting block copolymer, two limiting structures can be drawn (1) starlike micelles with a small core compared to the corona and (2) crew-cut micelles with a large core and highly stretched coronal chains. Both situations are schematically depicted in Fig. 2. 

Fig. 2 Schematic representation of a starlike (a) and a crew-cut (b) micelle. Important structural parameters (Rc and Rm) of block copolymer micelles are indicated in (b)...

Characterization of Block Copolymer Micelles - Experimental Techniques... 

Extensive reviews on experimental techniques suitable for block copolymer micelle characterization have been provided by Tuzar [41], Munk [42], Chu and Zhou [19] Webber [43], Mortensen [44], Zana [45], and Hamley [2]. Moreover, Hamley has systematically listed the different techniques specifically used for different types of block copolymer micelles. 

In the following discussion, we would like to briefly highlight the different methods used for morphological characterization of block copolymer micelles emphasizing advantages and limitations. This will be illustrated by selected examples from our own investigations on various block copolymer micelles. 

The more recently developed cryo-TEM technique has started to be used with increasing frequency for block copolymer micelle characterization in aqueous solution, as illustrated by the reports of Esselink and coworkers [49], Lam et al. [50], and Talmon et al. [51]. It has the advantage that it allows for direct observation of micelles in a glassy water phase and accordingly determines the characteristic dimensions of both the core and swollen corona provided that a sufficient electronic contrast is observed between these two domains. Very recent studies on core-shell structure in block copolymer micelles as visualized by the cryo-TEM technique have been reported by Talmon et al. [52] and Forster and coworkers [53]. In a very recent investigation, cryo-TEM was used to characterize aqueous micelles from metallosupramolecular copolymers (see Sect. 7.5 for further details) containing PS and PEO blocks. The results were compared to the covalent PS-PEO counterpart [54]. Figure 5 shows a typical cryo-TEM picture of both types of micelles. 

Scanning electron microscopy (SEM) techniques have proven to be suitable for the visualization of block copolymer micelles, as illustrated in, e.g., the recent work of Erhardt et al. on Janus micelles (Sect. 7.3) [55]. 

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