Toward Strong Thermoplastic Elastomers with Asymmetric Miktoarm Block Copolymer Architectures

Thermoplastic elastomers (TPEs) are designed by embedding discrete glassy or semicrystalline domains in an elastomeric matrix. Typical styrenic-based amorphous TPEs are made of linear ABA-type triblock copolymers, where the volume fraction <i>f</i> of the glassy domains A is typically less than 0.3. This limitation ultimately restricts the range of mechanical strength attainable with these materials. We had previously predicted using self-consistent field theory (SCFT) that A­(BA′)_<i>n</i> miktoarm block copolymers with an approximately 8:1 ratio of the A to A′ block molecular weights and <i>n</i> ≥ 3 should exhibit discrete A domains at considerably larger <i>f</i> and offer potential for the combination of high modulus, high recoverable elasticity, and high strength and toughness. Using transmission electron microscopy and small-angle X-ray scattering on model polystyrene-<i>b</i>-polyisoprene (PS–PI) miktoarm copolymers, we show that such polymers indeed possess discrete PS domains for <i>f</i> values considerably higher than 0.3. The hexagonal morphology with PS cylinders was achieved for <i>f</i> = 0.5 and <i>n</i> = 3. Mechanical testing indicates that these miktoarm materials are strong, tough, and elastic and thus may be potential candidates for a new generation of thermoplastic elastomers