Blocks copolymers - Thermoplastic polymers, elastomers and additives

Many of developments leading to the materialization of thermoplastic elastomers are due to chance or unexplained phenomena. Most thermoplastic elastomers are block copolymers or graft. The terminology used routinely uses to represent a type A units and B and C similarly. Thus, AB represents a diblock copolymer, triblock ABC one in which all segments have been polymerized monomers different and ABA two endblocks A and one midblock B. Also used English initial monomer to represent the copolymer; and S ( styrene) and B ( butadiene ) hence the name SBS.

Are common following representations: where (n) random graft block A in the block B

The medium is typically a polydiene block, either polybutadiene or polyisoprene, resulting in the well known family of styrene-butadiene-styrene (SBS) and styrene-isoprene-styrene (SIS). Other SBC that have been successful commercially include ethylene-butylene (SEBS), ethylene-propylene (SEPS), polyisobutylene (SIBS) and ethylene-ethylene-propylene (SEEPS).It is essential that the hard and soft blocks are immiscible, so that, on a microscopic scale, the polystyrene blocks form separate domains. These domains are attached to the ends of the elastomer chains and form binding points multifunctional, providing physical links rubber. When heated, the polystyrene domains soften and SBC can be processed as thermoplastics. When solidified, the SBC show good elastomeric properties as the polystyrene domains are reformed back resistance.The tensile strength of the SBC is much greater than the extent unreinforced vulcanized rubbers. Most SBC have an elongation at break ranges of more than 800% and is comparable to the resistance of the vulcanized rubbers. SBCs exhibit Newtonian flow behavior not due to their extreme segmental incompatibility. The melt viscosity is much higher than that of polybutadienes, polyisoprene and random copolymers of styrene and butadiene. SBCs are rarely used as pure materials. Most of them can be easily blended with other polymers, oils, fillers, resins, colorants and processing aids to meet the physical and mechanical properties required.

Polymerization of these materials occurs by two pathways: the anionic polymerization and catalysis organo-metallic basically with lithium alkyls. These complex polymers generally have a rare feature: they have two distinct Tg due to its segmented composition of two types of polymer, one of which provides crosslinking points and the other has an elastic linear chain.