Micelles, formation from diblock copolymers

Figure 11.8 Formation of ordered nanoparticles of metal from diblock copolymer micelles, (a) Diblock copolymer (b) metal salt partition to centres of the polymer micelles (c) deposition of micelles at a surface (d) micelle removal and reduction of oxide to metal, (e) AFM image of carbon nanotubes and cobalt catalyst nanoparticles after growth (height scale, 5 nm scan size, lxl pm). [Part (e) reproduced from Ref. 47].

A-B diblock copolymers can be used to form micelles that allow the preparation of thin, coherent films of well-developed stmcture [73-79]. For example, both poly (styrene)-block (fo)-poly(ethylene oxide) and poly(styrene)-b-poly(2-vinylpyridine) have been identified as useful combinations for generating well-ordered compartments in which metal nanoparticies can be fabricated in a variety of ways. The formation of diblock copolymers from polymer A and polymer B, ultimately producing nanospheres that can be transferred to films of highly ordered micelles, is illustrated in Figure 4.30a. The formation of metal nanoparticies inside the micelles is shown schematically in Figure 4.30b. 

Figure 4.30 (a) Formation of diblock copolymers from polymer A and polymer B, resulting in micelles (b) Formation of metal nanoparticles inside the micelles. 

The last reported diblock copolymer family that formed tubular aggregates in block-selective solvents was poly(phenylquinoline)-fc/ock-polystyrene or PPQ-PS, where PPQ was a rigid-rod block [47]. Such tubes are not discussed further for the following reasons First, the tubes had diameters of several micrometers and were not nanotubes. Second, the formation mechanism and chain packing in such tubes were not well understood at all. While Halperin [48] has developed a scaling theory for micelle formation from rod-coil diblock copolymers with the rod block forming the core, the theory did not apply to the PPQ-PS system as the block-selective solvents used were good for the rod PPQ block rather than the coil PS blocL... 

Multiscale surface structures have been directly obtained by simple spin-coating from diblock copolymer/homopolymer blends. For instance. Park et al. [89] prepared films from PS-b-P2VP/PMMA blends. These films were prepared from a selective solvent either for PS or PMMA. The pure block copolymers (BCP) form nanometer-sized micelles as a consequence of the microphase separation due to the incompatibility between the constituent blocks (Fig. 6.13(a)). However, blending the BCP with a homopolymer induces macrophase separation between the BCP micelles and the PMMA and the formation of isolated PMMA micrometer-size domains (Fig. 6.13(b)-(e)). Other groups including Jeong et al. [50] or Ibarboure et al. [42] also used homopolymer/BCP blends to fabricate multiscale ordered surfaces. Jeong and coworkers used P(S-b-MMA)/PMMA blends with variable composition. 

Arimura H, Ohya Y, Ouchi T (2005) Formation of core-shell type biodegradable polymeric micelles from amphiphilic poly(aspartic acid)-Wock-polylactide diblock copolymer. Biomacromolecules 6 720-725... 

Nolte and coworkers reported on the formation of micelles with a helical superstructure from AB amphiphilic diblock copolymers prepared from an amine-end-capped polystyrene that was used as the initiator for the polymerization of various dipeptide-derived isocyanides (Fig. 16). 

By covalent linkage of different types of molecules it is possible to obtain materials with novel properties that are different from those of the parent compounds. Examples of such materials are block-copolymers, soaps, or lipids which can self-assemble into periodic geometries with long-range order. Due to their amphiphilic character, these molecules tend to micellize and to phase-separate on the nanometer scale. By this self-assembly process the fabrication of new na-noscopic devices is possible, such as the micellization of diblock-co-polymers for the organization of nanometer-sized particles of metals or semiconductors [72 - 74]. The micelle formation is a dynamic process, which depends on a number of factors like solvent, temperature, and concentration. Synthesis of micelles which are independent of all of these factors via appropriately functionalized dendrimers which form unimolecular micelles is a straightforward strategy. In... 

Since they act as surfactants, copolymers are added in only small amounts, typically from a thousandth parts to a few hundredth parts. Theoretically, Leibler [30] showed that only 2% of a diblock copolymer may thermodynamically stabilize an 80%/20% incompatible blend with an optimum morphology (submicronic droplets). However, in practice kinetic control and micelle formation interfere in this best-case scenario. To a some extent, compatibilization increases with copolymer concentration [8,31,32], Beyond a critical concentration (critical micellar concentration cmc) little or no improvement is observed (moreover, for high amounts, the copolymer can act as a plasticizer). Copolymer molecular weight influence is similar to that of the concentration effect. For example, in a PS/PDMS system [8,31,32], when the copolymer molecular weight increases, domain size decreases to a certain extent. Hu et al. [31] correlated their experimental results with theoretical prediction of the Leibler s brush theory [30]. Leibler distinguishes two regimes to characterize the behaviour of the copolymer at the interface... 

The literature is abundant in describing the formation of spherical micelles from block copolymer self-assembly. From the classical diblock copolymer AB, two types of micelles could be prepared as a function of their structural parameters (Fig. 5). 

Diblock copolymer poly(methyl methacrylate)-W0c -poly(2-hydroxyethyl methacrylate) PMMA- -PHEMA was used as the template for the self-assembly of palladium nanoparticles.98 Thin films of the metal-free block copolymer were obtained by dip coating from different solvents. The copolymer existed in the ordinary form and inverted micelles when it was dissolved in 1,4-dioxane and methanol, respectively. This was attributed to the hydrophilic properties of the PHEMA block. 2-Methoxyethanol, which is a common solvent to both blocks, was also used in the coating. Films obtained from 1,4-dioxane and methanol solutions showed the formation of spherical domains that were arranged in a 2-D hexagonal lattice, while the film obtained from 2-methoxyethanol showed a stripe pattern. Their AFM topography images are shown in Figure 5.19. The block copolymer thin films were then exposed to... 

Micelle Formation via Complexation. Micellization of molecu-larly soluble block copolymers due to interaction with metal compounds was observed in both organic media and water, if one of the two blocks (for a diblock copolymer) is inert while the other is able to form complexes with metal compounds. Micellization of Pd-, Pt-, and Rh-containing polymers derived from PS-fc-PB with a short PB block was first reported in 1998 [48], The crosslinks formed due to complexes between metal atoms and PB blocks of different macromolecules were shown to cause micellization. By contrast, Fe carbonyl... 

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