Question

Explain why triblock copolymer Styrene-Butadiene-Styrene is a thermoplastic elastomer, and why the diblock copolymer Styrene-Butadiene is not.

Answer 1

In thermoplastic block copolymers, like SBS, different polymers are incompatible ie. do not dissolve into one another, blocks of the same type tend to aggregate and separate into small “domains.” Styrene-butadiene-styrene (SBS), a “tri-block” copolymer composed of butadiene repeating units in the center portion of the chain and styrene units at the ends. Polystyrene is a tough hard plastic while Polybutadiene is rubbery, and this gives SBS its rubber-like properties. Polystyrene and polybutadiene are incompatible, so that the polystyrene end-groups associate together to form domains of glassy polystyrene in a sea of elastic polybutadiene. The polybutadiene center portions thus form a connected elastomeric network held together by rigid domains of polystyrene end-blocks, which are relatively stable up to the glass transition temperature of polystyrene (about 100 °C, or 212 °F). Thus, it behaves like elastomeric rubbers at room temperature, but when heated, can be processed like plastics. Most types of rubber are difficult to process because they are crosslinked. But SBS type thermoplastic elastomers manage to be rubbery as they are not crosslinked.

SBS is made with living anionic polymerization.

In the diblock copolymer Styrene-Butadiene, styrene/butadiene ratio influences the properties of the polymer with high styrene content. Therefore, the rubbers are harder and less rubbery.