In block copolymers

Molecular architecture modifies the phase behavior of block copolymers. In block copolymers, macrophase separation is prevented by the connectivity of the polymer chains. The transition from a homogeneous melt to a heteroge-... 

Ceresa, R. J. "Synthesis and Characterization of Natural Rubber Block"and "Graft Copolymers in Block and Graft Copolymerization" Ceresa, R. J., Ed. John Wiley New York, 1973 Chap, 3. 

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. 

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. 

Pavlopoulou E, Anastasiadis SH, latrou H, Moshakou M, Hadjichristidis N, Portale G, Bras W (2009) The micellization of miktoarm star Snin copolymers in block copolymer/homopolymer blends. Macromolecules 42 5285-5295... 

Figure 24.4 Possible structures for single-chain micelles of an ABABA pentablock copolymer in block-selective solvents for the B blocks. From left to right are a triple-beaded chain, a double-beaded loop, a tethered balloon, and a butterfly. The beads correspond to the A blocks, while the loops correspond to the B blocks.

A nanombe diblock structure can be viewed as a macroscopic counterpart of a diblock copolymer. In block-selective solvents, a multiblock nanotube sample was shown to undergo hierarchical assembly to form supermiceUes (Yan et ai, 2004). Aside from coupled nanostructures, ordinary micelles have also been used to perform double", or hierarchical, assembly in order to yield hierarchical structures (Hu et al., 2008a). In these cases. Tier I assembly is performed using block copolymers to form micelles. At the Tier II level, the micelles are further assembled into hierarchical structures. 

Marechal E (2000) Block copolymers with polyethers as flexible blocks Future trends in block copolymers, in Block Copolymers (Eds. Balta Calleja F J and Roslianec Z) Marcel Dekker, pp. 29-62 541-572. 

In block copolymers [8, 30], long segments of different homopolymers are covalently bonded to each otlier. A large part of syntliesized compounds are di-block copolymers, which consist only of two blocks, one of monomers A and one of monomers B. Tri- and multi-block assemblies of two types of homopolymer segments can be prepared. Systems witli tliree types of blocks are also of interest, since in ternary systems the mechanical properties and tire material functionality may be tuned separately. 

Wanka G, Floffman FI and Ulbrict W 1990 The aggregation behavior of poly-(oxyethylene)-poly(oxypropylene)-poly-(oxyethylene)-block copolymers in aqueous solutions Colloid Polym. Sc/. 268 101-17... 

The desired form in homopolymers is the isotactic arrangement (at least 93% is required to give the desired properties). Copolymers have a random arrangement. In block copolymers a secondary reactor is used where active polymer chains can further polymerize to produce segments that use ethylene monomer. 

G-5—G-9 Aromatic Modified Aliphatic Petroleum Resins. Compatibihty with base polymers is an essential aspect of hydrocarbon resins in whatever appHcation they are used. As an example, piperylene—2-methyl-2-butene based resins are substantially inadequate in enhancing the tack of 1,3-butadiene—styrene based random and block copolymers in pressure sensitive adhesive appHcations. The copolymerization of a-methylstyrene with piperylenes effectively enhances the tack properties of styrene—butadiene copolymers and styrene—isoprene copolymers in adhesive appHcations (40,41). Introduction of aromaticity into hydrocarbon resins serves to increase the solubiHty parameter of resins, resulting in improved compatibiHty with base polymers. However, the nature of the aromatic monomer also serves as a handle for molecular weight and softening point control. 

Acrylamide copolymers designed to reduce undesired amide group hydrolysis, increase thermal stability, and improve solubility in saline media have been studied for EOR appHcations (121—128). These polymers stiH tend to be shear sensitive. Most copolymers evaluated for EOR have been random copolymers. However, block copolymers of acrylamide and AMPS also have utiHty (129). 

Moreover, commercially available triblock copolymers designed to be thermoplastic elastomers, not compatihilizers, are often used in Heu of the more appealing diblock materials. Since the mid-1980s, the generation of block or graft copolymers in situ during blend preparation (158,168—176), called reactive compatibilization, has emerged as an alternative approach and has received considerable commercial attention. 

Fig. 44. Thermal mechanical behavior of a styrene—butadiene—styrene block copolymer in nitrogen at —180 to 150°C (280).

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Certain block copolymers have also found appHcation as surfactants (88). Eor example, AB or ABA block copolymers in which one block is hydrophilic and one block is hydrophobic have proven useful for emulsifying aqueous and non-aqueous substances and for wetting the surface of materials. Examples of such surfactants are the poly(propylene oxide- /oi / -ethylene oxide) materials, known as Pluronics (BASC Wyandotte Co.). 

R. P. Quirk, D. J. Kiniiing, and L. J. Fetters, in G. AHen, ed.. Block Copolymers, in Comprehensive Polymer Science The Synthesis, Characterisation, Reactions... 

Finally, block copolymers have been made in a two-step process. First a mixture of chloroprene and -xylenebis-Ai,Ar-diethyldithiocarbamate is photopolymerized to form a dithiocarbamate terminated polymer which is then photopolymerized with styrene to give the block copolymer. The block copolymer has the expected morphology, spheres of polystyrene domains in a polychloroprene matrix (46). 

In principle it is possible to extend the method to produce block copolymers in which each of the blocks is monodisperse but the problems of avoiding impurities become formidable. Nevertheless, narrow size distributions, if not monodisperse ones, are achievable. 

More recently Fina Chemicals have introduced linear SBS materials (Finaclear) in which the butadiene is present both in block form and in a mixed butadiene-styrene block. Thus comparing typical materials with a total styrene content of about 75% by weight, the amount of rubbery segment in the total molecule is somewhat higher. As a result it is claimed that when blended with polystyrene the linear block copolymers give polymers with a higher impact strength but without loss of clarity. 

Tough transparent sheet may be produced by blending standard polystyrene with block copolymer in an extruder in the ratios 80 20 to 20 80, depending on the application of the products subsequently thermoformed from the sheet. For example, sheet for thermoforming an egg tray will not require the same level of impact strength as that required for jam jars. 

In Chapters 3 and 11 reference was made to thermoplastic elastomers of the triblock type. The most well known consist of a block of butadiene units joined at each end to a block of styrene units. At room temperature the styrene blocks congregate into glassy domains which act effectively to link the butadiene segments into a rubbery network. Above the Tg of the polystyrene these domains disappear and the polymer begins to flow like a thermoplastic. Because of the relatively low Tg of the short polystyrene blocks such rubbers have very limited heat resistance. Whilst in principle it may be possible to use end-blocks with a higher Tg an alternative approach is to use a block copolymer in which one of the blocks is capable of crystallisation and with a well above room temperature. Using what may be considered to be an extension of the chemical technology of poly(ethylene terephthalate) this approach has led to the availability of thermoplastic polyester elastomers (Hytrel—Du Pont Amitel—Akzo). 

With a typical of 25 000-30000 the molecular size is low compared wjth most conventional covalently cross-linked elastomers. With such rubbers values of about 100000 are desirable so that the effects of a significant amount of non-load-bearing chain ends do not occur. Such a problem does not arise in block copolymers terminated by hard segments. 

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