Self-assembly in Block-selective Solvents

Based on the studies of organic block copolymers (see Chapter 1, Section 1.2.5), polyferrocenylsilane block copolymers can be expected to self-assemble in solvents which are selective for one of the blocks. In the case of organic block copolymers, most studies have led to the identification of spherical structures, where the less 

Cylindrical and tape-like morphologies have been identified in the case of Pl-b-PFS (PI=polyisoprene) where the PFS block crystallizes [154]. Water-soluble poly-ferrocenyldimethylsilane-f)-poly(ammoalkylmethacrylate) copolymers of narrow 

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Block Copolymer Self-Assembly in Block-Selective Solvents... 

There have been only few reports on nanotube formation from the self-assembly in block-selective solvents of copolymers consisting of only coiled blocks. Since nature abhors vacuum, the spontaneous formation of tubular structures from block copolymers in bulk has not been reported and is probably impossible. While the direct preparation of block copolymer nanotubes by self-assembly has been so far difficult, it has been relatively easy to prepare cylindrical nanoaggregates or micelles from ABC triblock copolymers in selective solvents for A only. In such aggregates or micelles, the A block comprises the corona and the C and B blocks comprise the core/shell cylinders. In bulk at the right triblock copolymer composition, the different blocks of an ABC triblock copolymer segregate predictably into C and B core/shell cylinders dispersed in the A matrix [50,51], if the interfacial tension between the A and C blocks are comparable to that between the A and B blocks and that... 

Block copolymer nanotubes can be prepared either directly from block copolymer self-assembly in block-selective solvents or from the chemical processing of ABC triblock copolymer nanofibers. There has been only one report on the formation of self-assembled nanotubes from coil-coil AB diblocks in block selective solvents, and it occurred for a sample with a very low weight fraction of the soluble block. Nanotubes were formed from coil-coil-coil ABA triblock copolymers at much higher weight fractions for the soluble A blocks. Still, lower soluble block weight fractions were required for nanotube than for vesicle formation. It remains to be seen if these trends can be generalized to other block copolymers containing purely coil blocks. 

Figure 18a,b displays SFM images of SV films that have been prepared from chloroform and from toluene solutions, respectively. The mixed pattern of featureless areas and round-shaped stripes in Fig. 18a can be identified as in-plane lamella and perpendicular-oriented lamellae, respectively. The microstructure prepared from toluene solutions (Fig. 18b) is attributed to P2VP micelles surrounded by the PS shell. The micelle morphology is a result of the SV self-assembly in a selective solvent [119], We have made use of this morphological difference to study the microstructure response to solvent uptake by block copolymer films. 

We start with a brief reminder of the theory of self-assembly in a selective solvent of non-ionic amphiphilic diblock copolymers. Here, the focus is on polymorphism of the emerging copolymer nanoaggregates as a function of the intramolecular hy-drophilic/hydrophobic balance. We then proceed with a discussion of the structure of micelles formed by block copolymers with strongly dissociating PE blocks in salt-free and salt-added solutions. Subsequently, we analyze the responsive behavior of nanoaggregates formed by copolymers with pH-sensitive PE blocks. The predictions of the analytical models are systematically complemented by the results of a molecularly detailed self-consistent field (SCF) theory. Finally, the theoretical predictions are compared to the experimental data that exist to date. 

As already described in the previous sections, the classical approach for preparation of polymeric nano-size aggregates from amphiphilic block copolymers by self-assembly in a selective solvent involves several steps. The first step is synthesis of an amphiphilic copolymer with a narrow molar mass distribution, followed by purification and characterization. Then, the self-assembly is performed by adding a non-solvent of one block into dilute copolymer solution or by direct dissolution of the copolymer in a selective solvent. Usually, the copolymer concentration varies between 1 and 10 g (0.1-1 wt%). 

Figure 1 depicts structures of nanotubes that have so far been derived from block copolymer self-assembly. While the nanotubes are drawn as being rigid and straight, they, in reality, can bend or contain kinks. The top scheme depicts a nanotube formed from either an AB diblock copolymer [15,16] or an ABA triblock copolymer [17], where the gray B block forms a dense intermediate shell and the dark A block or A blocks stretch into the solvent phase from both the inner and outer surfaces of the gray tubular sheU. Such tubes have been prepared so far from the direct self-assembly or tubular micelle formation of a few block copolymers in block-selective solvents, which solubilize only the dark A block or blocks. Nanotubes with structures depicted in the middle and bottom schemes have been prepared from precursory ABC triblock copolymer nanofibers, which consist of an A corona, a cross-linked intermediate B shell, and a C core [18] A fully empty tubular core was ob-... 

Overall, reports on preparation of nanotubes from block copolymers have been rare, and there have been no reports on practical applications of such structures. For this, the emphasis of this chapter will be on the fundamental aspects of these materials. In Sect. 2, nanotube or tubular micelle formation from the direct self-assembly of block copolymers in block-selective solvents will be reviewed. Section 3 will be mainly on nanotubes derived from the chemical processing of cross-linked triblock copolymer nanofibers. Example nanotube preparations will be given, dilute solution properties of the nanotubes will be discussed, and the different reaction patterns of the nanotubes will be examined. Concluding remarks will be made in Sect. 4. 

Despite many reports on nanotube formation from phospholipids, glycoUpids, peptidic amphiphiles, and other small-molecule surfactants and theoretical studies of this subject [14], reports on nanotube formation from the self-assembly of block copolymers are rare. There have been no theoretical treatments examining their formation or properties. It is not even known if block copolymer nanotubes are thermodynamically stable entities or kineti-cally controlled association products when formed in block-selective solvents. 

The self-assembly of crystalline-coil and rod-coil diblock copolymers in block-selective solvents presented quite some surprises. Crystalline-coil diblocks formed tubular nanoaggregates in block-selective solvents for the coil blocks at coil to crystalUne block repeat unit number ratios substantially larger than 1, e.g., 12 and 18 for the PFS-PDMS diblock copolymers. This made the block copolymer nanotubes much easier to access. It again remains to seen if such a trend can be generalized to other diblock copolymers. Thus, much remains to be done to establish the best experimental conditions for formation of self-assembled nanotubes. Theories need to be developed to understand the formation and property of self-assembled block copolymer nanotubes. 

The latter example leads us to still another characteristic feature of supramolecular concepts i.e., their hierarchical assemblies allow functional materials. Functionality is regarded as one of the hallmarks of supramolecular science [13] as even in classical chemistry and self-organization, complexes, salts, etc. are formed due to chemical matching. One can point out that the borderline between classical chemistry and supramolecular chemistry is not strict. For example, in general block copolymers dissolved in block-selective solvents might not be regarded as supramolecular chemistry, whereas some of the more specific further developments [18, 82] clearly allow supramolecular functional materials, as will be shown in Section 3. 

This chapter focuses on polyferrocenylsilanes (PFSs) where iron and silicon are in the main chain. Subsequently, PFS block copolymers will be reviewed. These materials represent an area of rapidly growing interest as a result of their self-assembly into phase-separated metal-rich nanodomain structures in thin films and micelles in block-selective solvents. The resulting nanostructured materials have a wealth of potential applications and recent breakthroughs in this area are discussed. The subject matter of the chapter is divided up into subsections covering PFS homopolymer and block copolymer synthesis, solution and solid-state self-assembly and applications of the latter, which have been extensively developed by ourselves and our collaborators and also by other research groups. 

The production of a mixture was unfortunate. However, the presence of the p-(PtBA)2(PM-MA)2 contaminant should not stop the self-assembly of p-(PtB A)(PMMA)2 in the solid state, or in block-selective solvents. It will only shift the phase diagrams of p-(PtBA)(PMMA)2 somewhat. As far as the preparation of materials from the self-assembly of miktoarm copolymers is concerned, the presence of this contaminant is therefore of no consequence. 

Linse P (2000) Modelling of self-assembly of block copolymers in selective solvent. In Alexandridis P, Lindman B (eds) Amphiphilic block copolymers self assembly and applications. Elsevier, Amsterdam... 

Figure 1. Schematic of self-assembly assisted polypolymerization (SAAP) of triblock copolymers in a selective solvent for the synthesis of long multiblock copolymers with a controllable chain sequence and block length. [3 5]...

Dan, N. and M. Tirrell. 1993. Self-assembly of block copolymers with strongly charged and a hydrophobic block in a selective, polar solvent. Micelles and adsorbed la ifasromolecule 6 4310—4315. 

Nagarajan, R. and K. Ganesh. 1989. Block copolymer self-assembly in selective solvents theory of solubilization in spherical micellefc/lacromolecule 2 4312-4325. 

It should be noted that the development of such polymer systems is stimulated by existing experimental works. In particular, the experimental methods of preparation of nanometer-sized hollow-sphere structures have been suggested [58-63] because of their possible usage for encapsulation of molecules or colloidal particles. The preparation of hollow-sphere structures, generally, is based on self-assembling properties of block copolymers in a selective solvent, i.e., on the formation of polymer micelles with a nanometersized diameter. Further cross-finking of the shell of the micelle and photodegradation [64] of the core part produce nanometer-sized hollow cross-linked micelles. 

Adsorption of block copolymers onto a surface is another pathway for surface functionalization. Block copolymers in solution of selective solvent afford the possibility to both self-assemble and adsorb onto a surface. The adsorption behavior is governed mostly by the interaction between the polymers and the solvent, but also by the size and the conformation of the polymer chains and by the interfacial contact energy of the polymer chains with the substrate [115-119], Indeed, in a selective solvent, one of the blocks is in a good solvent it swells and does not adsorb to the surface while the other block, which is in a poor solvent, will adsorb strongly to the surface to minimize its contact with the solvent. There have been a considerable number of studies dedicated to the adsorption of block copolymers to flat or curved surfaces, including adsorption of poly(/cr/-butylstyrcnc)-ft/od -sodium poly(styrenesulfonate) onto silica surfaces [120], polystyrene-Woc -poly(acrylic acid) onto weak polyelectrolyte multilayer surfaces [121], polyethylene-Wocfc-poly(ethylene oxide) on alkanethiol-patterned gold surfaces [122], or poly(ethylene oxide)-Woc -poly(lactide) onto colloidal polystyrene particles [123],... 

Janus micelles are non-centrosymmetric, surface-compartmentalized nanoparticles, in which a cross-linked core is surrounded by two different corona hemispheres. Their intrinsic amphiphilicity leads to the collapse of one hemisphere in a selective solvent, followed by self-assembly into higher ordered superstructures. Recently, the synthesis of such structures was achieved by crosslinking of the center block of ABC triblock copolymers in the bulk state, using a morphology where the B block forms spheres between lamellae of the A and C blocks [95, 96]. In solution, Janus micelles with polystyrene (PS) and poly(methyl methacrylate) (PMMA) half-coronas around a crosslinked polybutadiene (PB) core aggregate to larger entities with a sharp size distribution, which can be considered as supermicelles (Fig. 20). They coexist with single Janus micelles (unimers) both in THF solution and on silicon and water surfaces [95, 97]. 

Block copolymers self-assemble to form nanoscale organized structures in a selective solvent. The most common structures are spheres, with the insoluble core surrounded by a solvent-swollen corona. In some instances, disk- or worm-like micelles form, and are of particular interest, since the control of their association can lead to a broad range of new applications [1,2]. An important subset of block copolymer micelles are those which contain metal atoms, through covalent attachment or by complexa-tion [3], These structures are interesting because they take advantage of the intrinsic properties of their components, such as the mechanical properties of the polymer micelles and the optical and magnetic characteristics of the metal atoms. Moreover, the assembly permits the control of the uniformity in size and shape of the nanoparticles, and it stabilizes them. 

ABC triblock copolymers also undergo self-assembly in selective solvents or in bulk leading to more complex morphologies [19]. Over the past 10 years, approximately ten additional structures have been discovered. Block segregation pattern complexity increases further for tetra- and pentablocks, thus making the number of morphologies almost infinite. 

Fig. 3 Various self-assembled structures formed by amphiphilic block copolymers in a block-selective solvent. The type of structure formed is due to the inherent curvature of the molecule, which can be estimated through calculation of its dimensionless packing parameter, p [28]. Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission...

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