Triblock styrene-butadiene

Butadiene copolymers are mainly prepared to yield mbbers (see Styrene-butadiene rubber). Many commercially significant latex paints are based on styrene—butadiene copolymers (see Coatings Paint). In latex paint the weight ratio S B is usually 60 40 with high conversion. Most of the block copolymers prepared by anionic catalysts, eg, butyUithium, are also elastomers. However, some of these block copolymers are thermoplastic mbbers, which behave like cross-linked mbbers at room temperature but show regular thermoplastic flow at elevated temperatures (45,46). Diblock (styrene—butadiene (SB)) and triblock (styrene—butadiene—styrene (SBS)) copolymers are commercially available. Typically, they are blended with PS to achieve a desirable property, eg, improved clarity/flexibiHty (see Polymerblends) (46). These block copolymers represent a class of new and interesting polymeric materials (47,48). Of particular interest are their morphologies (49—52), solution properties (53,54), and mechanical behavior (55,56). 

FIG. 12-17. Logarithmic plots of the storage (f>0 and loss D") tensile compliance against frequency for a triblock styrene-butadiene copolymer reduced to Tq = 8S C. The block lengths S/B/S have molecular weights 16,000/78,000/16,000 and the morphology corresponds to spherical polystyrene donuiins in a polybutadiene matrix. Temperatures of measurement from —82.89 to 86.44°C are identified. (Pesko and Tschoegl." ) Reproduced, by permission, from the International Journal of Polymeric Materials. 

With this in mind, Tanaka and co-workers [43] proposed a new method for the characterisation of the sequence distribution of styrene units in styrene-butadiene copolymers by a combination of selective ozonolysis of the double bonds in butadiene units and GPC measurements of the resulting products. His method is based upon high resolution GPC analysis of the alcohols corresponding to styrene sequences obtained by scission of all the carbon-carbon double bonds of butadiene units. The ozonolysis-GPC method has already been proven to be a very powerful tool to characterise the sequence distribution of styrene units and the tacticity in random, partially blocked, and triblock styrene-butadiene copolymers [39, 44-48] in this study a new analytical method of the sequence distribution of 1,2 units in polybutadiene was investigated on the basis of the ozonolysis-GPC method. 

Figure 3.8. Schematic representation of the polystyrene domain structure in styrene-butadiene-styrene triblock copolymers. (After Holden, Bishop and Legge )...

Styrene-butadiene-styrene triblocks and the related S-I-S and SEBS materials (Section 11.8). 

Block copolymer chemistry and architecture is well described in polymer textbooks and monographs [40]. The block copolymers of PSA interest consist of anionically polymerized styrene-isoprene or styrene-butadiene diblocks usually terminating with a second styrene block to form an SIS or SBS triblock, or terminating at a central nucleus to form a radial or star polymer (SI) . Representative structures are shown in Fig. 5. For most PSA formulations the softer SIS is preferred over SBS. In many respects, SIS may be treated as a thermoplastic, thermoprocessible natural rubber with a somewhat higher modulus due to filler effect of the polystyrene fraction. Two longer reviews [41,42] of styrenic block copolymer PSAs have been published. 

Chu et al. [24] correlated viscosity-morphology and compatibility of PS-PB blends. The effect of styrene-butadiene triblock copolymer in PS-PB was studied, and it was found that the domain size decreases with an increase of compatibilizer loading. The blending methods influenced the morphology due to the difference in the extent of mixing. 

Linear triblock copolymers of the type styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene (SIS) are produced commercially by anionic polymerization through sequential addition of monomers in the reaction chamber [10] as shown below ... 

FIGURE 5.7 Phase separation in styrene-butadiene-styrene (SBS) triblock copolymer. The isolated spherical styrene domains form the hard phase, which act both as intermolecular tie points and filler. The continuous butadiene imparts the elastomeric characteristics to this polymer. MW = molecular weight. (From Grady, B.P. and Cooper, S.L., Science and Technology of Rubber, Mark, J.E., Erman, B., and Eirich, F.R. (eds.). Academic Press, San Diego, CA, 1994. With permission.)... 

Ban H.T., Kase T., Kawabe M., Miyazawa A., Ishihara T., Hagihara H., Tsunogae Y., Murata M. and Shiono T.A. New approach to styrenic thermoplastic elastomers S3Tithesis and characterization of crystalline styrene-butadiene-styrene triblock copolymers. Macromolecules, 39, 171, 2006. 

Pakula T., Saijo K., Kawai H., and Hashimoto T. Deformation behaviour of styrene butadiene styrene triblock copolymer with cyhndrical morphology, Macromo/ecu/er, 18, 1294, 1985. 

Magonov, S.N., Elings, V., Cleveland, J., Denley, D., and Whangbo, M.-H., Tapping-mode atomic force microscopy study of the near-surface composition of a styrene-butadiene-styrene triblock copolymer film, Surf. Sci., 389, 201, 1997. 

The use of lightly crosslinked polymers did result in hydrophilic surfaces (contact angle 50°, c-PI, 0.2 M PhTD). However, the surfaces displayed severe cracking after 5 days. Although qualitatively they appeared to remain hydrophilic, reliable contact angle measurements on these surfaces were impossible. Also, the use of a styrene-butadiene-styrene triblock copolymer thermoplastic elastomer did not show improved permanence of the hydrophilicity over other polydienes treated with PhTD. The block copolymer film was cast from toluene, and transmission electron microscopy showed that the continuous phase was the polybutadiene portion of the copolymer. Both polystyrene and polybutadiene domains are present at the surface. This would probably limit the maximum hydrophilicity obtainable since the RTD reagents are not expected to modify the polystyrene domains. 

Synthesis of vinyl block copolymers is accomplished by living polymerisation, mostly by anionic polymerisation. Several strategies can be used, illustrated here by the example of the Styrene-Butadiene-Styrene (or SBS) triblock copolymer. 

Table 19.3 Typical reaction conditions for the hydrogenation of polybutadiene (PB), styrene-butadiene diblock copolymer (SB), styrene-butadiene-styrene triblock copolymer (SBS) and nitrile butadiene rubber (NBR).

Each domain in a block copolymer exhibits its characteristic Ts and Tm. Thus the triblock of styrene-butadiene-styrene (Kraton) has a Tg of 373 K for the styrene block and a Tg of 210 K for the butadiene block. In the temperature range of 210 to 373 K, the block copolymer has both high-resilience and low-creep characteristics. The copolymer is rubbery and flows at temperatures above 373 K. 

A rubber-like copolymer/carbon fibre composite material has also been prepared [170]. Carbon fibres were added directly to o/w highly concentrated emulsions of block copolymers, such as styrene/butadiene triblocks (SBS), in toluene, followed by precipitation in methanol, drying and hot-pressing. The surfactant was found to aid adhesion between the polymer and carbon fibres. The materials obtained had fairly even distributions of carbon fibres, good mechanical properties and conductivities which increased with increasing carbon fibre length. 

Ostler, M. I., Rogers, C. E. Effects of Interphase Structure on Gas Sorption and Diffusion in Styrene-Butadiene Triblock Polymers, to be published... 

Styrene-butadiene or styrene-(ethylene-co-propylene) block copolymers are common compatibilisers for the commingled recyclate from PCW [27]. Improvement of the mechanical properties of heterogeneous PCR (33% PE+39% PVC+28% PET) or (44% PE+1% PP+28% PET+9% PS+2% PVC+16% other plastics) was performed with EPDM or hydrogenated SBS triblocks [52]. 

It is important to appreciate that polymer produced by an anionic chain-growth mechanism can have drastically different properties from one made by a normal free radical reaction. Block copolymers can be synthesized in which each block has different properties. We mentioned in Chapter 4 that Michael Szwdrc of Syracuse University developed this chemistry in the 1950s. Since that time, block copolymers produced by anionic polymerization have been commercialized, such as styrene-isoprene-styrene and styrene-butadiene-styrene triblock copolymers (e.g., Kraton from Shell Chemical Company). They find use as thermoplastic elastomers (TPE), polymers that act as elastomers at normal temperatures but which can be molded like thermoplastics when heated. We will discuss TPEs further in Chapter 7. 

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