Cross-linked micellar structures

Although micelles are stable in time at fixed conditions, their characteristics depend for a given system on the thermodynamic quality of the solvent and on temperature. For this reason it is impossible to study that system under different conditions, for example in a different solvent, at a different temperature or at various concentrations. The idea of Prochazka and Baloch [227] and of Tuzar et al. [228] to circumvent this problem was to stabilize micellar structures by cross-linking of the micellar core, either by UV or fast electron irradiation. The unimer remaining in the system after cross-linking can easily be removed by fractional precipitation or dialysis. 

Systematic studies on photo-cross-linking block copolymer micelles, with a core of poly(cinnamoylethyl methacrylate) (PCEMA) were published by Liu and co-workers [229]. With PAA as the shell-forming block, these authors could demonstrate by SLS, DLS, TEM and SEC that photo-cross-linking of PCEMA locked in the initial structure of the micelles without any significant change in their aggregation number and size distribution. Corecross-linking of PEO-PMAA micellar systems with Ca ions has recently been described by Kabanov and co-workers [230]. 

The other possibility, at first examined by Wooley and co-workers [231,232] is to crosslink the corona of the micelles. These kinds of nanoparticles are designated by shell cross-linked knedel-Hke (SCK) micelles by these authors. Wooley et al. have applied this concept to a large variety of block copolymers, mainly hydrophobic-hydrophilic copolymers with PAA or quaternized PVP as the water-soluble block, which can be chemically cross-linked in their micellar form. A similar approach has been described by Armes and co-workers [233] for the synthesis of shell cross-linked micelles where core and shell are both hydrophilic. 

A-B-C block and graft copolymers, with three different blocks, have also attracted increasing attention because they display in the solid state a large variety of mesomorphic structures with interesting bulk properties [235,236]. Their synthesis is well documented, however the study of their colloidal properties, and especially their micellization behavior in aqueous medium, has just been started in the last few years. 

In the following, typical examples of these recent studies will be discussed, with the focus on the micellization of water-soluble A-B copolymers with non-linear architectures as well as of A-B-C copolymers having at least one hydrophilic block. 

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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... 

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. 

The process utilizing supramolecular organization involves pore expansion in silicas. A schematic view of such micelles built from the pure surfactant and those involving in addition n-alkane is shown in Figure 4.9. Another example of pore creation provides a cross-linking polymerization of monomers within the surfactant bilayer [30]. As a result vesicle-templated hollow spheres are created. Dendrimers like that shown in Figure 4.10 exhibit some similarity to micellar structures and can host smaller molecules inside themselves [2c]. Divers functionalized dendrimers that are thought to present numerous prospective applications will be presented in Section 7.6. 

These different casein monomers combine with calcium phosphate to form discrete particles on the nano-size scale. The phosphoserines of the caseins are seemingly clustered for the purpose of linking within the micelle to putative calcium phosphate microcrystallites, also known as nanoclusters (Holt, 1992 Home, 1998, 2002, 2003, 2006 Holt et al., 2003 Home et al., 2007). Structural evidence for the existence of such nanoclusters has come from neutron and X-ray scattering (de Kruif and Holt, 2003 Holt et al., 2003 Pignon et al., 2004 Marchin et al., 2007). The presence of nanoclusters allows native casein micelles to be effective natural suppliers of essential calcium salts in the human diet in a readily assimilated functional form. Protein-nanocluster interactions are the central concept of the cross-linking mechanism in Holt s model of casein micellar assembly (Holt et al., 2003 de Kruif and Holt, 2003). Any analogy with conventional soap-like micelles is considered to be... 

At higher shear rates, Watanabe and Kotaka (1983) observed thixotropy, i.e. stress decay increasing as a function of shear rate, in PS-PB diblocks in dibutyl phthalate (DBP), which is a selective solvent for PS.The fact that the flow crossed over from plastic to viscous non-Newtonian on increasing the shear rate indicated the breakdown of the micellar lattice structure, rather than of the individual micelles. This was confirmed by parallel measurements on a cross-linked PB-PS system, where stress decay and recovery were also observed. Thus the... 

Collectively, all of the data obtained on the solution structure with SDSL, sulfhydryl reactivity, and disulfide cross-linking kinetics strongly support the conclusion that the structure of the TM7-H8 sequence investigated is very similar in the crystal and micellar solution state. In solution, the H8 helix is sandwiched between the hydrophobic/aqueous interface on one side and residues 325-328 of the C-terminal tail on the other. 

Peroxidase-catalyzed grafting of polyphenols on lignin was performed by HRP-catalyzed polymerization of />cresol with lignin in the aqueous 1,4-dioxane or reverse micellar system.57 Phenol moiety in lignin was reacted with />cresol to produce a lignin—phenol copolymer with a branched and/or cross-linked structure. The product was highly insoluble in common organic solvents. 

As a result of shell-crosslinking, the one-dimensional micellar structures were locked-in and preserved even if transferred from hexane to a common solvent for both blocks.26,49 Figure 3.6 illustrates a TEM image for PI32o- -PFS53 shell-cross-linked micelles. The TEM sample was prepared from a micellar solution in THF, a common solvent for both PFS and PI blocks. 

Fig. 2 (a-c) Physical polymer-network cross-linking provided by mixed micelles in hydrogels formed via hydrophobic interactions in surfactant solutions. Mixed micelles are formed by aggregation of hydrophobic blocks of per-se hydrophilic polymers and surfactant alkyl tails, (b) Nonionic polymer and ionic surfactant gel system at the state of preparation. For clarity, charges are not shown, (c) Ionic polymer and oppositely charged surfactant gel system after extraction of free micelles, (d) Structure of the hydrophobic monomers used in the micellar polymerization... 

Finally, the structure or network of polymer chains combined with micellar clusters occurring in solution from such associative polymers also results in a modified rheological profile with respect to the viscosity versus shear rate behaviour compared with the previously known Hnear and cross-linked types. This was of particular interest in formulations such as water-based paints and will be covered in more detail in a later section on rheological profiles of acrylic thickeners. 

The validity of the concept given in scheme (d) of Figure 7.8 was demonstrated by Weaver et al. [274] who obtained CSC micellar structures with a ionically cross-linked shell by polyelectrolyte complexation of a PEO-(quaternized PDMAEMA)-PDEAEMA triblock copolymer micellar system with a PEO - poly(styrene sulfonate) diblock copolymer. 

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