Elongation, butadiene block copolymers

Styrenic TPEs are the most widely used. One such material, commercialized by BASE in 1999, is a styrene-butadiene block copolymer with a styrene content of about 70%, intended for thin film for food packaging. It has high oxygen and water permeability, and excellent toughness and optical properties. Cling films with EVA layers on the outside are also available, which provide complete recovery of deformation at elongations up to 400%, and elongation at break of over 650%. 

The same dependence of tensile elongation characteristics on block copolymer structure that was observed in this work has been reported for styrene/butadiene block copolymers. Table 3. The importance of the hard/soft/hard sequence is evident since the hard/ soft (A/B) sequence has very low elongation. This corresponds in this work to incompletely extended polymer. Table 2. 

The resulting TPE can either be used alone or blended with aliphatic oil and polypropylene. In the former case a higher tensile strength and elongation at break are obtained in comparison with the commercially available styrene-hydrogenated butadiene-styrene block copolymers, especially at high temperatures. 

Orientations in elongated mbbers are sometimes regular to the extent that there is local crystallization of individual chain segments (e.g., in natural rubber). X-ray diffraction patterns of such samples are very similar to those obtained from stretched fibers. The following synthetic polymers are of technical relevance as mbbers poly(acrylic ester)s, polybutadienes, polyisoprenes, polychloroprenes, butadiene/styrene copolymers, styrene/butadiene/styrene tri-block-copolymers (also hydrogenated), butadiene/acrylonitrile copolymers (also hydrogenated), ethylene/propylene co- and terpolymers (with non-conjugated dienes (e.g., ethylidene norbomene)), ethylene/vinyl acetate copolymers, ethyl-ene/methacrylic acid copolymers (ionomers), polyisobutylene (and copolymers with isoprene), chlorinated polyethylenes, chlorosulfonated polyethylenes, polyurethanes, silicones, poly(fluoro alkylene)s, poly(alkylene sulfide)s. 

Although an ethylene vinyl acetate copolymer was immiscible in NR blends, addition of a 6 phr ethylene vinyl acetate block copolymer enabled compatibilization of heterogeneous NR/acrylonitrile butadiene rubber blends. These blends increased the tensile strength, the elongation at break and tear strength due to an increase in the interfacial adhesion between the blended components by increasing the rigidity of the matrix in the presence of the ethylene vinyl acetate copolymers. ... 

When Szwarc et al. discovered [15,16], or rediscovered [17,18], the anionic living polymerization, a completely different preparation of these elastomers was proposed the study of TPEs passed from infancy to maturity. These authors used sodium metal naphthalene diinitiators to prepare poly (styrene-l>-isoprene-6-styrene), which was probably the first TPE with a perfectly defined structure. However, this copolymer could not be commercialized, as most of the poly-isoprene units were -3,4-, with poor elastomeric properties. It is only when the polymerization was initiated by alkyllithium that poly(styrene-l>-isoprene- -styrene) and poly (styrene-butadiene- -styrene) were obtained with the classical TPE properties very high tensile strength and elongation at break, very rapid elastic recovery, and no chemical crosslinking. Bailey et al [19] announced the existence of these materials in 1966 and Holden et al [20] published the corresponding theory in 1967 and extended it to other block copolymers. 

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