Micelle hybridization

The dynamics of micellar systems is a very important concern that is relevant at several levels. In this respect, the dynamics required for the establishment of the unimer-micelle equilibrium is one aspect. The so-called problem of micelle hybridization that deals with the exchange rate of unimers between different micelles is closely related. Finally, the chain dynamics of polymer blocks in either the core or the corona is another concern that can, however, be linked to some extent to the first two mentioned concepts. These different aspects have been scarcely studied and reviewed by Tuzar and Kratochvil [6,41], and... 

Tian et al. used sedimentation velocity measurements to study the rate of micelle hybridization when two micellar solutions containing micelles made up from two different di-or triblock copolymers of st3rrene and methacrylic acid are mixed. Dioxane/water mixtures were used as solvents. In contrast to results reported in References 71 and 72, the exchange kinetics was found to depend veiy strongly on the composition, molecular weight, and architecture of the copolymer and also on the quality of the solvent (thermod3mamic control). 

More recently Frechet and Gitsov [130] used a similar approach as above and synthesized a novel series of dendritic copolymers derived from a central penta-erythritol core unit. These hybrid star molecules behaved as unimolecular micelles with different core-shell conformational-structures as a response to the polarity of the solvent used. 

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. 

Time-resolved in situ Small Angle Neutron Scattering (SANS) investigations have provided direct experimental evidence for the initial steps in the formation of the SBA-15 mesoporous material, prepared using the non-ionic tri-block copolymer Pluronic 123 and TEOS as silica precursor. Upon time, three steps take place during the cooperative self-assembly of the Pluronic micelles and the silica species. First, the hydrolysis of TEOS is completed, without modifications of the Pluronic spherical micelles. Then, when silica species begin to interact with the micelles, a transformation from spherical to cylindrical micelles takes place before the precipitation of the ordered SBA-15 material. Lastly, the precipitation occurs and hybrid cylindrical micelles assemble into the two-dimensional hexagonal structure of SBA-15. 

In this paper, we report about a recent in situ SANS study of the synthesis of the SBA-15 materials [5]. Our results demonstrate that SANS (using D20 instead of H20 as a solvent) is a powerful tool for the determination of size and shape of hybrid inorganic-organic micelles during the first synthesis step when the precursor species undergo hydrolysis and form an inorganic framework around the copolymer micelles. 

SAXS studies are sensitive mainly to the inorganic species [6,7] and offer then a complementary vision of the hybrid micelles with respect to SANS. A comparison between SAXS and SANS is given in part 3. 

Figure 2 Evolution of the neutron scattering intensities with time. Only spherical micelles of P123 block copolymer are present in the synthesis mixture within the first few minutes of the reaction (300 s), during the hydrolysis of the silica precursor. Then, hybrid organic-inorganic cylindrical micelles are detected (300-1400 s). The SBA-15 hexagonal phase is formed when the precipitation occurs, after 1400 s.

The reason why the hybrid micelles evolve from sphere to cylinder is not yet completely understood, but it results from the fact that when silica species are adsorbed onto the surface of the micelles, the average curvature of the micelles is decreasing [9], Polymerisation of silica species by condensation leads to precipitation of the ordered hexagonal mesoporous material. 

Most examples discussed so far made use of amorphous inorganic supports or sol-gel processed hybrid polymers. Highly disperse materials have recently become accessible via standard processes and, as a result, materials with various controlled particle size, pore diameter are now available. Micelle-templated synthesis of inorganic materials leads to mesoporous materials such as MCM-41, MCM-48, MSU, and these have been extensively used as solid supports for catalysis [52]. Modifications of the polarity of the material can increase the reactivity of the embedded centre, or can decrease its susceptibility to deactivation. In rare cases, enhanced stereo- or even... 

Exchange of unimers between two different types of block copolymer micelles has often been referred to as hybridization. This situation is more complex than for the case described above because thermodynamic parameters now come into play in addition to the kinetic ones. A typical example of such hybridization is related to the mixing of micelles formed by two different copolymers of the same chemical nature but with different composition and/or length for the constituent blocks. Tuzar et al. [41] studied the mixing of PS-PMAA micelles with different sizes in water-dioxane mixtures by sedimentation velocity measurements. These authors concluded that the different chains were mixing with time, the driving force being to reach the maximum entropy. 

In this section, we would like to briefly describe the micellization behavior of hybrid AB copolymers, leading to remarkable structures with new potential applications. By hybrid AB block copolymers we mean copolymers in which at least one of the constituent blocks is not a classical poly-... 

Fig. 17 Formation of rodlike micelles with diameter of 25 nm from self-assembly of a protein (lipase enzyme)-polymer (PS) hybrid in THF. Reprinted with permission from [264]. Copyright (2003) American Chemical Society... 

Micellar electrokinetic chromatography is a hybrid of electrophoresis and chromatography. Introduced by Terabe in 1984 (9), MEKC is one of the most widely used CE modes. Ionic micelles, formed from surfactants added... 

Jeong JH, Park TG. Novel polymer-DNA hybrid polymeric micelles composed of hydrophobic poly(DL-lactic acid-co-glycolic acid) and hydrophiUc oligonucleotides. Bioconjugate Chem 2001 12 917-923. 

---