Cross-linked micelles

As the quinone stabilizer is consumed, the peroxy radicals initiate the addition chain propagation reactions through the formation of styryl radicals. In dilute solutions, the reaction between styrene and fumarate ester foUows an alternating sequence. However, in concentrated resin solutions, the alternating addition reaction is impeded at the onset of the physical gel. The Hquid resin forms an intractable gel when only 2% of the fumarate unsaturation is cross-linked with styrene. The gel is initiated through small micelles (12) that form the nuclei for the expansion of the cross-linked network. 

Fig. 4. MiceUular gelation mechanism. A shows micelle nuclei, highly cross-linked B, boundary where micelle growth terminates in styrene block polymers.

Other applications are based on the use of solutions of reversed micelles as templates. For example, solutions of reversed micelles have been employed as a matrix to control the porosity of cross-linked polymer resins. The pore size of the polymers was controlled by varying the amounts of water in the AOT-reversed micelles [67]. 

The core of reversed micelles can be transformed to a highly viscous domain (nanogel) by entrapping appropriate species, such as viscous solvents and hydrophilic macromolecules, or by performing in situ appropriate polymerization reactions or intramolecular cross-linking of water-soluble polymer chains [232-234]. 

Solubilization of vinylpyrrolidone, acrylic acid, and A,A -methylene-bis-acrylamide in AOT-reversed micelles allowed the synthesis in situ of a cross-linked polymer with narrow size distribution confined in the micellar domain. These particles displayed high entrapment efficiency of small hydrophilic drugs and have been considered interesting drug delivery systems [239],... 

Nanogels made up of various intramolecularly cross-linked macromolecules have been prepared simply by performing the polymerization of hydrophilic monomers solubilized in the micellar core of reversed micelles, and they represent distinct macromolecular species from those obtained in bulk [191,240]. 

The overall objective of this chapter is to review the fundamental issues involved in the transport of macromolecules in hydrophilic media made of synthetic or naturally occurring uncharged polymers with nanometer-scale pore structure when an electric field is applied. The physical and chemical properties and structural features of hydrophilic polymeric materials will be considered first. Although the emphasis will be on classical polymeric gels, discussion of polymeric solutions and nonclassical gels made of, for example, un-cross-linked macromolecular units such as linear polymers and micelles will also be considered in light of recent interest in these materials for a number of applications... 

The structure of these gel-like systems of micelles is very different from that of conventional electrophoresis media made from chemically and physically cross-linked polymers of polyacrylamide and agarose [75], The absence of chemical or physical cross-links in the Pluronic gel-like phases may allow a larger degree of freedom for macromolecular transport around the obstacles that make up the medium than occurs in conventional electrophoresis media. 

The formation of polymeric capsules can also be achieved by the cross-linking of self-assembled amphiphilic block copolymers [85]. The hydrophobic section of the polymer in an aqueous solution will tend to aggregate on the interior of the micelle, whereas the hydrophilic ends will form the outer shell of the micelle. If the hydrophilic end is appropriately functionalized, it can be cross-linked, giving a polymeric shell. The overarching concept is shown in Figure 5.10. 

Perkin, K.K., Turner, J.L., Wooley, K.L. and Mann, S. (2005) Fabrication of hybrid nanocapsules by calcium phosphate mineralization of shell cross-linked polymer micelles and nanocages. Nano Letters, 5,1457-1461. 

Shell cross-linked micelles, 20 489 Shell cross-linked nanoassemblies,... 

The most abundant milk protein is casein, of which there are several different kinds, usually designated a-, (1-, and K-casein. The different caseins relate to small differences in their amino acid sequences. Casein micelles in milk have diameters less than 300 nm. Disruption of the casein micelles occurs during the preparation of cheese. Lactic acid increases the acidity of the milk until the micelles crosslink and a curd develops. The liquid portion, known as whey, containing water, lactose and some protein, is removed. Addition of the enzyme rennet (chymosin) speeds up the process by hydrolysing a specific peptide bond in K-casein. This opens up the casein and encourages further cross-linking. 

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. 

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... 

An obvious way to stabilize block copolymer micelles consists in the cross-linking of the micellar core or corona. Several strategies have been developed to reach this goal, as briefly illustrated in the following discussion. 

Block copolymer micelles containing PB cores were cross-linked either by UV or fast electron irradiation [79-81]. This was accompanied by a shrinkage of the micelles. 

Liu and coworkers systematically used photo-cross-linking to stabilize micelles containing a PCEMA core [82]. The micellar characteristic features (Z, etc.) were not affected by the cross-linking process, as proven by SLS, DLS, TEM, and SEC. 

Wooley and coworkers have cross-linked the micellar corona and obtained the so-called shell cross-linked knedellike micelles [83,84]. This strategy was further applied to a wide variety of block copolymer micelles. Armes and coworkers have used a similar approach for the preparation of shell cross-linked micelles with hydrophilic core and shell [85]. Many other related examples can be found in the literature. 

Temperature- and pH-sensitive core-shell-corona micelles were also recently reported by Armes et al. Moreover, the shell of these CSC micelles could be selectively cross-linked [275]. 

For some applications, it is desirable to lock the micellar structure by cross-Hnking one of the micellar compartments, as discussed previously in Sect. 2.6. Cross-Hnked core-shell-corona micelles have been prepared and investigated by several groups as illustrated by the work of Wooley and Ma [278], who reported the cross-linking of PS-PMA-PAA micelles in aqueous solution by amidation of the PAA shell. Very recently, Wooley et al. prepared toroidal block copolymer micelles from similar PS-PMA-PAA copolymers dissolved in a mixture of water, THF, and 2,2-(ethylenedioxy)diethylamine [279]. Under optimized conditions, the toroidal phase was the predominant structure of the amphiphilic triblock copolymer (Fig. 19). The collapse of the negatively charged cylindrical micelles into toroids was found to be driven by the divalent 2,2-(ethylenedioxy)diethylamine cation. 

---