Polybutadiene center blocks

Among thermoplastic elastomers, the styrene-based block copolymers constitute the largest quantity of TPEs. Their chemical structure makes those polymers with unsaturated center blocks (polybutadienes (SBS) or polyisoprene (SIS)) much more oxidation prone than those with saturated elastomer components (ethylene/butyl-ene (SEES) or ethylene/propylene (SEPS)) [86]. 

A gel permeation chromatogram obtained by Heller (13) indicated that Kraton 101 contained 1% polystyrene, 22% polystyrene/1,4-poly-butadiene, and 77% polystyrene/1,4-polybutadiene/polystyrene, apparently because the material is manufactured by coupling of the diblock. The diblock would be present if the coupling reaction is not 100% efficient. Kraton 101 would thus be essentially a blend of ABBA and AB block copolymers. Assuming that Kraton 102 is manufactured in the same way, the molecular weight of the ABBA block copolymer results as about 72,000 from the above data, the polystyrene blocks having a molecular weight of about 12,000, and the polybutadiene center block one of about 48,000. 

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 containing 20-40% styrene are also useful as impact modifiers for ignition-resistant HIPS as well as other polystyrene resins. Kraton rubbers are available from Shell Chemical Co. and Vector rubbers are available from Dexco Polymers (a joint venture between The Dow Chemical Co. and The Exxon Chemical Co.). Both grades are used extensively for this purpose as are other similar products from other suppliers. Kraton rubbers are usually triblock copolymers of SBS type. The center block is polybutadiene with terminal blocks of polystyrene (49-51). By far the largest use of block copoljuners is in the form of TIPS resins. There are a number of suppliers, with Phillips Petroleum and BASF... 

Examples of the thermoplastic elastomers include polystyrene-fi/ocA -polybutadiene-h/oc -polystyrene (SBS) or the saturated center block counterpart (SEES). In the latter, the EB stands for ethylene-butylene, where a combination of 1,2 and 1,4 copolymerization of butadiene on hydrogenation presents the appearance of a random copolymer of ethylene and butylene (see Table 9.4). 

Table 13.12 also delineates triblock copolymers based on polystyrene-l)toc/c-polybutadiene-6tocA -polystyrene. Polyisoprene may be substituted for the polybutadiene, and/or the center block may be hydrogenated. The hydro-... 

Two other triblock polymers were prepared in cyclohexane. Run no. 13 used polyisoprene in place of polybutadiene as the center block. Run no. 14 used vinyltoluene in place of styrene in the SAMS end block copolymer. The polymerization conditions and results are shown in Table 5. 

The samples used in this study are listed in Table I with selected characterization data. Also given are our notations for the samples. S and B represent polystyrene and polybutadiene, respectively. The block polymers are denoted S/Bi, S/B2, etc. The letters identify the polymer components with the first letter indicating the center segment. The polymers were isolated from the polymerization solution by precipitation into cold (0°C), well-stirred methanol. After careful drying in a vacuum oven (25°C), film samples were prepared by melt pressing at temperatures ranging from 25 °C for homopolybutadiene to 150 °C for homopolystyrene. 

A typical triblock copolymer may consist of about 150 styrene units at each end of the macromolecule, and some 1000butadiene units in the center. The special physical properties of these block copolymers are due to inherent incompatibility of polystyrene with polybutadiene or polyiso-prene blocks. Within the bulk material there are separations and aggregations of the domains. The polystyrene domains are dispersed in continuous matrixes of the polydienes that are the major components. At ambient temperature, below the Tg of the polystyrene, these domains are rigid and immobilize the ends of the polydiene segments. In effect, they serve both as filler particles or as crosslinks. Above Tg of polystyrene, however, the domains are easily disrupted and the material can be processed as a thermoplastic polymer. The separation into domains is illustrated in Fig. 5.4. 

Styrene Butadiene Thermoplastic n (S/B, SB) A group of thermoplastic elastomers introduced in 1965 (Shell chemical Co, Thermolastic ). They are linear block copolymers of styrene and butadiene, produced by lithium-catalyzed solution polymerization, with a sandwich molecular structure containing a long Polybutadiene center surrounded by shorter polystyrene ends. The materials are available in pellet form for extrusion, injection molding, and blow molding, and S/B sheets are thermoformable. 

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