Block copolymer micelles cores

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

This approach was first described in [64, 65]. When metal nanoparticles are located in the block copolymer micelle cores [64] or in microgels [65] and these polymeric systems are used as templates for silica casting, both pore size and... 

Three classes of polymers are amenable to study by fluorescence techniques. The first class of materials is that consisting of extrinsic fluorescent molecules incorporated into the polymer solution. This class has applications in the study of solubilization of small molecules in block copolymer micelle cores (6) but is not of interest for configurational studies such as the present work. 

Figure 4.8. Schematic image of metallation of the block copolymer micelle cores.

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. 

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 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. 

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. 

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. 

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. 

In previous sections, much emphasis has been put on block copolymer micelles with a spherical morphology. It was shown in Sect. 5 that the characteristic sizes of both the spherical core and corona of block copolymer micelles can be precisely adjusted by essentially controlling the chemical nature and the degree of polymerization of the constituent blocks. For several applications of block copolymers micelles including, e.g., micellar templating... 

Block copolymer micelles in which the core-forming polymer blocks are able to crystallize are relatively similar to rod-coil copolymers. A significant part of these crystalline-core micelles is actually resulting from the self-assembly of rod-coil block copolymers. 

Micelles of type (1) were the first investigated examples of ABC triblock copolymer micelles. These micelles are generally characterized by the so-called onion, three-layer, or core-shell-corona structures, i.e., the first insoluble A block forms the micellar core, the second insoluble B block is wrapped around the core, and the third soluble C block extends in the solution to form the micellar corona (Fig. 18). To the best of our knowledge, there are no known examples of ABC block copolymer micelles with A and C insoluble blocks and a B soluble block. 

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. 

Polymeric micelles with selected chemistries and molecular architecture of block copolymers, such as PIPAAm-CigHgs, PIPAAm-PSt, PIPAAm-PBMA, and PIPAAm-PLA micelles, showed the same LCST and the same thermoreponsive phase transition kinetics as those for PIPAAm irrespective of the hydrophobic segment incorporation. This confirms two points (a) that hydroxyl groups or amino goups of PIPAAm termini completely react with the hydrophobic segment end groups and (b) that the block copolymers form core-shell micellar structures with hydrophobic iimer cores completely isolated from the aqueous phase. 

A simple scaling model of block copolymer micelles was derived by de Gennes (1978). He obtained scaling relations assuming uniformly stretched chains for the core radius, RB, of micelles with association number p.This model can be viewed as a development of the Alexander de Gennes theory (Alexander 1977 de Gennes 1976,1980) for polymer brushes at a planar interface, where the density profile normal to the interface is a step function. In the limit of short coronal (A) chains (crew-cut micelles) de Gennes (1978) predicted... 

It is worth emphasizing that all scaling theories (due to de Gennes, Daoud and Cotton, Zhulina and Birshtein, and Halperin) for block copolymer micelles with a small core and large corona predict that the association number and core radius are independent of the coronal chain length. 

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