Janus micelle

Recent progress in novel micellar structures, including micelles containing exotic blocks such as natural or synthetic polypeptides and metal-containing segments, micelles from ABC triblock copolymers, Janus micelles and other noncentrosymmetric micelles, micelles based on interpolyelectrolyte or other noncovalent complexes, and metallosupramolecular micelles, will be discussed in Sect. 7. 

It should, however, be mentioned that the transfer of a bulk-organized system into solution can lead to very interesting structures, as will be demonstrated in Sect. 7.3 in the case of Janus micelles [33]. In this case, a micellar structure is preformed in the bulk, its core is stabilized by cross-linking, and... 

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]. 

Fig. 18 Schematical representation of different types of micelles formed by ABC triblock copolymers. Core-shell-corona micelles with insoluble core and shell (a), core-shell-corona micelles with radially compartmentalized corona (b), and Janus micelles with laterally compartmentalized corona (c)...

The influence of pH on PS-PB-PMAA Janus micelles was also investigated. At high pH, Rh of the supermicelles is larger than for acidic conditions, as a result of the ionization of the PMAA chains leading to their stretching [55]. Janus micelles have also been preliminarily investigated by Saito et al. [295]. 

Fig. 21 Formation of Janus micelles from PS-PB-PMMA copolymers. The copolymers are transformed into PS-PB-PMAA copolymers after hydrolysis of PMMA block. PS coronal blocks collapse in water and supermicelles are formed. A typical SEM picture of supermicelles is shown. Reprinted with permission from [55]. Copyright (2003) American Chemical Society... 

Abstract Polyelectrolyte block copolymers form micelles and vesicles in aqueous solutions. Micelle formation and micellar structure depends on various parameters like block lengths, salt concentration, pH, and solvent quality. The synthesis and properties of more complicated block and micellar architectures such as triblock- and graft copolymers, Janus micelles, and core-shell cylinder brushes are reviewed as well. Investigations reveal details of the interactions of polyelectrolyte layers and electro-steric stabilization forces. 

Keywords Block copolymers ABC triblock copolymers Janus micelles Cylinder brushes Core-shell nanoparticles Graft copolymers Micelles Vesicles Copolyampholytes Polyelectrolyte block copolymers Aggregation... 

The following Sect. 2 deals with the association of diblock copolymers and discusses micelles, vesicles, micellar aggregation and interaction. In Sect. 3 recent work on micelle and vesicle formation of triblock copolymers is presented, whereas in Sect. 4 more complicated architectures such as graft copolymers, cylindrical brushes and Janus micelles are discussed. 

Janus micelles are non-centrosymmetric, surface-compartmentalized nanoparticles, in which a cross-linked core is surrounded by two different corona hemispheres. Their intrinsic amphiphilicity leads to the collapse of one hemisphere in a selective solvent, followed by self-assembly into higher ordered superstructures. Recently, the synthesis of such structures was achieved by crosslinking of the center block of ABC triblock copolymers in the bulk state, using a morphology where the B block forms spheres between lamellae of the A and C blocks [95, 96]. In solution, Janus micelles with polystyrene (PS) and poly(methyl methacrylate) (PMMA) half-coronas around a crosslinked polybutadiene (PB) core aggregate to larger entities with a sharp size distribution, which can be considered as supermicelles (Fig. 20). They coexist with single Janus micelles (unimers) both in THF solution and on silicon and water surfaces [95, 97]. 

Fig. 20 Scheme of the synthesis of a PS-PB-PMMA Janus micelle. Reprinted with permission from ref [95]. Copyright (2001) American Chemical Society... 

Fig. 24 AFFFF-MALS measurements of Janus micelles in water with 1 wt-% NaCl. (—) ...

Finally, it is shown that non-linear amphiphilic structures show different aggregation behavior as compared to block copolymers. Graft copolymers with non-polar backbone polyelectrolyte side chains have a smaller tendency to form micelles than their block copolymer analogs which is attributed to the more facile stabilization of unimers by the sidechains. In contrast, unimolecular micelles are the only possibility for core-shell nanoparticles. Janus micelles, on the other hand, form unique non-centrosymmetrical micelles that have a strong tendency to form centrosymmetrical supermicelles. 

Erhardt R et al (2003) Amphiphilic Janus micelles with polystyrene and poly(methacrylic acid) hemispheres. J Am Chem Soc 125(11) 3260-3267... 

Erhardt R, Boeker A, Zettl H, Kaya H, Pyckhout-Hintzen W, Krausch G, Abetz V, Mueller AHE (2001) Janus micelles. Macromolecules 43(4) 1069-1075... 

Keywords Coacervate Co-assembly Janus Micelle Polyelectrolyte Polymer Scattering Segregation Self-assembly Self-consistent field calculations... 

In this contribution, we describe our recent experimental and theoretical findings on complex coacervate core micelles. We have investigated the co-assembly of several types of oppositely charged ionic-hydrophilic block copolymers into mixed micelles. In particular, we have focused on chain mixing/segregation in the micellar corona as a function of monomer type and (the ratio between the) chain length of the polymer blocks in the corona. Our aim has been to employ co-assembly in such systems as a route towards formation of reversible Janus micelles. These are micelles with a corona that exhibits two distinguishable sides (hemispheres in the case... 

As we are interested in reversible Janus micelles, i.e. non-centrosymmetric nanoparticles with compartmentalised shells (Fig. 1), complex coacervate core micelles are a rather natural choice. As described in the previous section, electrostatic interaction is a rather weak driving force as compared to hydrophobic interaction. C3Ms may thus form under full thermodynamic control. Although ABC triblock copolymers in selective solvents (poor solvent for B good solvent for both A and C) may also yield Janus micelles, they most frequently aggregate into micelles with a quenched rather than a dynamic nature, such that the aggregation number is fixed and no reversible association/dissociation is observed (on experimental time scales). 

Now that we have established the requirements for the formation of reversible Janus micelles, we turn our attention to the choice of ionic-hydrophilic block copolymers. The ionic blocks have to be oppositely charged to ensure co-assembly in aqueous solutions, whereas the neutral blocks have to be water-soluble. Furthermore, the unlike water-soluble polymer blocks need to segregate, not mix within the micellar corona. Since the classical works of Flory and Huggins, extended by Scott to describe binary polymer solutions [58], it is well known that two unlike polymers... 

Fig. 1 Representation of a prolate ellipsoidal (cigar-like) Janus micelle with an oblate ellipsoidal (disc-like) core. The complex coacervate core is depicted in grey, while the corona is depicted in blue (ethylene oxide monomers) and green (acryl amide monomers)...

Fig. 4 Representation of the various types of internal organisation that may occur in micelles formed through co-assembly of charged block copolymers. Depicted are micelles with a corona consisting of two different neutral monomers (shown in blue and green) that are (a) mixed, (b) laterally segregated, (c) radially segregated, and (d) laterally and radially segregated. Both cen-trosymmetric micelles (a, c) and non-centrosymmetiic micelles (b, d) are depicted. The micelle with a laterally segregated corona is generally referred to as a Janus micelle (b). The micelle depicted in (c) is most commonly known as onion-like or core-shell-corona micelle. The micelle with a laterally and radially segregated corona (d) is usually called a patched micelle. Note that in the case of (c) and (d) the blue and the red/black chains may also switch position...

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