Micelles, cylindrical preparation

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. 

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. 

For polymer chemists it is interesting to know how well-known linear polymers can be linked with dendritic architectures and what the supramolecular consequences of this approach might be. Combination of dendrimers with linear polymers in hybrid linear-dendritic block copolymers has been employed to achieve particular self-assembly effects. Block copolymers with a linear polyethylene oxide block and dendritic polybenzylether block form large micellar structures in solution that depend on the size (i.e., the generation) of the dendritic block [10]. Amphiphilic block copolymers have been prepared by the combination of a linear, apolar polystyrene chain with a polar, hydrophilic poly(propylene imine) dendrimer [11] as well as PEO with Boc-substituted poly-a, -L-lysine dendrimers, respectively [12]. Such block copolymers form large spherical and cylindrical micelles in solution and have been described as superamphi-philes and hydra-amphiphiles , respectively. 

Figure 3.6 TEM image for shell-cross-linked PI320-6-PFS53 cylindrical micelle solid samples prepared by drying a drop of THF solution on a carbon-coated copper grid. THF is a common solvent for both blocks. Scale bar = 250 nm. (From Wang et al.49 Reproduced with permission.)...

The lengths of the cylindrical structures can be increased to over 10 pm by controlling the preparation technique. The cylinders show considerable stability in solution and are unchanged in size after heating to 80 °C. However, ultrasonication of samples of the cylindrical micelles leads to a shortening of their length as indicated by lightscattering and TEM studies. 

Cylindrical and tape-like morphologies have been identified in the case of PI-/ -PFS (PI = polyisoprene) where the PFS block crystallizes. " Water-soluble polyferrocenyldimethylsilane-/ -poly(aminoalkylmethacrylate) co-polymers of narrow polydispersity have also been prepared and cylindrical micelles have been identified. " Block co-polymers generated by transition metal-catalyzed ROP, such as PFS-/ -PDMS-/ -PFS triblock materials, have been shown to self-assemble in hexanes to yield a variety of remarkable architectures that include flower-like assemblies where the... 

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