Solvent block-selective

Abstract This article reviews results from our group of the synthesis and characterization of diblock copolymer brushes. Results from the literature are also covered. We report a wide variety of diblock compositions and compare the miscibility of the two blocks with the tendency to rearrange in response to block-selective solvents. Also, we describe the types of polymerization methods that can be utilized to prepare diblock copolymer brushes. We have compared the molecular weight of free polymer and the polymer brush based on results from our laboratory and other research groups we have concluded that the molecular weight of the free polymer and that of degrafted polymer brushes is similar. 

Styrene) (PPFS), poly(heptadecafluorodecyl acrylate) (PHFA), poly(penta-fluoropropyl acrylate) (PPFA), or poly(trifluoroethyl acrylate) (PTFA) [53]. The block at the silicate interface was either PS or PMA. Treatment of the diblock systems with block-selective solvents produced predictable changes in water contact angles except for those diblock brushes based on PHFA. All of these systems were fully characterized by XPS, tensiometry, ellipsometry,... 

Yan X, Liu G, Hu J, Willson CG. Coaggregation of B-C and D-C diblock copolymers with H-bonding C blocks in block-selective solvents. Mactomolecules 2006 39 1906-1912. 

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...

As with phase separation of block copolymers in bulk, the nature and size of the aggregates formed in a block-selective solvent depends on the relative sizes of the two blocks as well as the overall molecular weight of the diblock copolymer. In... 

Fig. 1.6 Formation of spherical micelles from a block copolymer in a block-selective solvent.

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. 

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

In a block-selective solvent, the insoluble block or blocks of a copolymer agglomerate to form nanometer-sized aggregates [5,22,23]. Such aggregates disperse in the solvent and are protected from further agglomeration by the soluble block(s) that form(s) the aggregate corona. 

Aside from block copolymer composition and the quahty of the block-selective solvent, polymer concentration can also affect micellar morphology. For example, the length of cylindrical micelles normally increases with the concentration of diblock copolymers in a block-selective solvent [38-40]. If the aggregates are kinetic products, the morphologies will be affected by sample preparation conditions and history as well. 

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. 

Aside from the coil-coil diblocks and coil-coil-coil ABA triblocks, crystalline-coU poly(ferrocenyldimethylsilane)-foZock-poly(dimethyl silox-ane) or PFS-PDMS diblock copolymers were reported by Raez, Manners, and Winnik [42] to form nanotubes readily in block-selective solvents hexane and n-decane, which solubilized the rubbery PDMS blocks and not the crystalline PFS blocks. 

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... 

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. 

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 two types of nanotubes were joined by a polymer spacer, PAES-l -PS-l7-PAES. It was observed that the different blocks of these assemblies segregated from one another in block-selective solvents, such as DMF or toluene, and also in the solid state. This work represents yet another extension of hierarchical assembly of nano-objects by providing covalent bonds between nano-objects. Presumably, the bonds formed can also be reversible to a specific stimulus to find wider applications. 

Figure 2 Self-assembled structures from block copolymers in a block-selective solvent estimated from the packing parameter or the mass fraction of the hydrophilic block in the copolymer (/phydrophiiic) (Reproduced from Ref. 12. Wiley-VCH, 2009.)...

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