Three-block copolymers

Pedemonte E., Alfonso G.C., Dondero G., De C.F., and Araimo L. Correlation between morphology and stress strain properties of three block copolymers. 2. The hardening effect of the second deformation. Polymer, 18, 191, 1977. 

SBS is a three-block copolymer of styrene and butadiene. Incompatibility of PS and poly-butadiene (BR) causes segregation of the chain parts a two-phase system results in which PS and BR each have their own individuality and their own Tg. The fact that the highest Tg is lower than the Tg of normal PS, is a result of the relatively low molar mass of the PS chain ends. In general the effect of M on Tg is insignificant Tg= - K/ M, but when K is, e.g. 10 and M = 10,000, Tg can be 10°... 

The most important hydrocarbon copolymers are styrene-butadiene rubbers (SBR) produced by free-radical emulsion or anionic polymerization. Anionic polymerization allows the manufacture of styrene-butadiene and styrene-isoprene three-block copolymers. 

Recently, a new class of inhibitors (nonionic polymer surfactants) was identified as promising agents for drug formulations. These compounds are two- or three-block copolymers arranged in a linear ABA or AB structure. The A block is a hydrophilic polyethylene oxide) chain. The B block can be a hydrophobic lipid (in copolymers BRIJs, MYRJs, Tritons, Tweens, and Chremophor) or a poly(propylene oxide) chain (in copolymers Pluronics [BASF Corp., N.J., USA] and CRL-1606). Pluronic block copolymers with various numbers of hydrophilic EO (,n) and hydrophobic PO (in) units are characterized by distinct hydrophilic-lipophilic balance (HLB). Due to their amphiphilic character these copolymers display surfactant properties including ability to interact with hydrophobic surfaces and biological membranes. In aqueous solutions with concentrations above the CMC, these copolymers self-assemble into micelles. 

Figure 3.5 shows the organized mesoporous structure of one of these materials. The wide variety of organic molecules with self-assembling properties allows for the synthesis of a myriad of solids with controlled porosity. Cationic surfactants, like the hexadecyltrimethylammonium used to synthesize MCM-41 [26], or three-block copolymers (hydrophilic-hydrophobic-hydrophilic) are two good examples. 

The situation is quite different with block copolymers. As an example we again take a copolymer of styrene and butadiene, but now as a three-block copolymer, SBS. The incompatibility of polystyrene and polybutadiene now results in a phase separation, which is enabled by the circumstance that the blocks can live their own life . The polystyrene chain ends clog together into PS domains, which lie embedded in a polybutadiene matrix. These glassy domains act as physical cross-links, so that the polymer has the nature of a thermoplastic rubber. The glass-rubber transitions of PS and BR both remain present in between these two temperatures the polymer is in a, somewhat stiffened, rubbery condition (see Figure 3.8). This behaviour is dealt... 

The result of this approach for the three-block copolymer SBS (styrene-butadiene-... 

Fluids with a small yield stress may, however, be stable, namely if this yield stress exceeds the pressure differences. An example the three-block copolymer SBS exhibits a minor yield stress, which is hardly detectable, but sufficient to prevent the growth of distortions. As discussed in 9.1.4, it is caused by the polystyrene domains. It has also been observed that, when blending SBS with another polymer, it shows, even at very low concentrations, a pronounced tendency to form a continuous phase, in other words it does not break up into dispersed particles. 

Poly (iso butylene) glycol is a suitable rubbery segment in thermoplastic elastomers. An oligomer such as 56 with a tertiary chloro group at both ends could be employed also as the initiator of cationic polymerization of a-methylstyrene (a-MeSt) in the synthesis of a three-block copolymer of poly(a MeSt)-polyisobutylene-poly(a-MeSt)52). 

Monofunctional 1-hydroxycellulose triesters, such as tributyrate and propionate acetate derivatives, have been coupled to bis(4-isocyanotophenyl)disul-flde to obtain macroinitiators for the radical syntheses of three block copolymers of the type cellulose-initiator-initiator-cellulose [140]. 

Films of the three block copolymers were cast from chloroform, a mutual solvent for PS and PEO,( ) and the measured and 0. core level spectra are shown in Figure 2. The spectra show the characteristic peak of PEO, the shake-up satellite of PS, and an easily deconvoluted doublet for the core levels in PS and PEO. It is apparent from the spectra that he PS concentration at the copolymer surface increases as the PS in the copolymer increases. More importantly, however, an analysis of the spectral data clearly shows that the surface compositions are significantly richer in PS than would be predicted based on a knowledge of the bulk compositions of the block copolymers. In Figure 3 is shown a plot of the surface-vs-bulk compositions for the diblock copolymers. ... 

Figure 21.4 Corrected melt viscosity as a function of shear stress and temperature for the three block copolymers studied. Reproduced with permission from Legge, Holden and Schroeder, Thermoplastic Elastomers A Comprehensive Review, Hanser Verlag, Munich, 1987...

An inverted sequence of the same procedure has also been used [139] to prepare the same three-block copolymers. Indeed, thermal polymerization of MMA by ABME gives rise to a polymer mainly containing only one benzoin methyl ether moiety per macromolecule, since growing MMA radicals terminate mostly by disproportionation. Thus, terminally photoactive poly(MMA) is used to obtain the photoinitiated block copolymerization of styrene. In this case, a 90% yield of block copolymers is obtained, appreciably higher than in the preceding method, fully consistent with the usual assumption that the termination in styrene polymers occurs by combination. In fact, coupling of the growing styryl radicals with the less reactive poly(MMA)-bound methoxy benzyl radicals also contributes to the formation of block copolymers. 

The diiunctional photoiniferter such as //-xylylene bis(A,A -diethyldithio-caibamate) (XDC) has led to several ABA-type three-block copolymers by successive photoinitiated polymerizations [147]. 

A reversed sequence of the same procedure has also been followed [151] by using ABME in conjunction with EMP+PF and CHO in order to obtain, under UV irradiation, poly(CHO) containing an azo linkage in the main chain. As the photolysis of the az group at 350 nm irradiation is not achievable due to the higher absorption coefficient of the residual benzoin methyl ether end groups, poly(CHO) is thermally decomposed in the presence of styrene, to give mainly three-block copolymers (Scheme 46). 

Similarly, the incorporation of diaryldisulfide moieties on a cellulosic prepolymer and its irradiation in the presence of styrene or chloroprene is reported [153] to give multiphase three-block copolymers (Scheme 49). 

Block copolymers are an example of multicomponent polymer systems for which the problem of separate investigation of each component (block) of the copolymer is of particular interest. The PL method permits a successive separation of each block of the copolymer for the investigation of the IMM of the latter using luminescent markers . The PL method was applied to the study of the three-block copolymers of the ABA and BAB types where A and B are PMMA and polystyrene(PS)blocks. 

Table 13. Relaxation times characterizing the intramolecular mobility of the polymer chain, for the homopolymer and the block of the corresponding chemical structure and molecular weight M in three-block copolymers (PMMA-PS-PMMA) and (PS-PMMA-PS) at 25 °C ij,ed 0.38 cP. a) ratMA, b) PS. The polymer concentration in solution is 0.005% the component under investigation with a luminescent marker is denoted by ( )...

It can be concluded that the best emulsifying properties are obtained with two-block copolymers, and that these are superior to three-block copolymers and to graft copolymers. Furthermore, a two-block copolymer is most efficient if its composition is about 50 50 and if its molecular weight is higher than those of the corresponding homopolymers. These results agree with those obtained recently by Skoulios and co-workers (II). An explanation of these phenomena by Meier is based on the different precipitation rates of the various polymers during solvent evaporation (12). 

In 1961, using lithium catalyst, a series of sty-rene-isoprene (SI) and styrene-butadiene (SB) block copolymers were synthesized [Bull and Holden, 1977]. The resins had T = -90 to -i-90°C. Full-scale production started in 1965. Since then, numerous two- and three- block copolymers have been developed. More recently, hydrogenated and during the last few years maleated block copolymers are being offered. With the world consumption of 330 kton/y, the block copolymers constitute the largest part of the commercial TPE market. Large quantity of SBS resin is used... 

SBS styrene-butadiene- styrene three block copolymer... 

Hadziioannou G, Skoulios A (1982) Molecular weight dependence of lamellar stmcture in styrene/isoprene two- and three-block copolymers. Macromolecules 15 258-262... 

Thus, there is a rich variety of very interesting structures even for the simple diblock-copolymer. And we have a simple tool for controlling the microstructure of a block-copolymer melt. All you need to do is to vary the length ratio between A and B blocks. Even richer variety of structures exist in three-block copolymers — it is actually too rich to discuss in this book. 

Figure 5-33. Electron micrographs of films of two-block and three-block copolymers of styrene and butadiene cut vertically (left) or parallel (right) to the film surface (M. Matsuo). Top row SBS polymer with S/B= 80/20 mol/mol. Spherical domains of poly(butadiene) segments embedded in a matrix of poly(styrene) segments center row SB polymer with S/B = 60/40 mol/mol. Rods (cylinders) of poly (butadiene) segments in a matrix of poly (styrene) segments bottom row SBS polymer with S/B = 40/60 mol/mol. Lamellae of poly(butadiene) segments alternate with lamellae of poly(styrene) segments.

Until now, the morphological studies have been performed mainly on block-copolymers contairting two chentically different blocks A and B only. One can expect the revelation of quite novel morphological structures for three-block copolymers, including three mutually incompatible units (ABC). And there are few references on such polymers. 

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