Mechanical Properties of Star Block Polymer Thermoplastic Elastomers with Glassy and Crystalline End Blocks

The mechanical properties of thermoplastic elastomers (TPEs) consisting of 6-arm star block polymers with glassy, crystalline, or composite crystalline glassy physical cross linking (hard) domains were investigated and compared to the analogous linear triblock or pentablock polymers. The 6-arm stars exhibited qualitatively similar solid-state morphologies and phase behavior to their linear counterparts, as demonstrated by small angle X-ray scattering and differential scanning calorimetry. Consequently, the architecture had minimal impact on the small-strain behavior in uniaxial extension at room temperature. As the applied strain increased, the star polymers exhibited more pronounced strain hardening than the corresponding linear TPEs, resulting in an increase in the ultimate strength of 20% for the polymers with crystalline end blocks and 30% when the end blocks were glassy. Each of the three star polymers exhibited superior recovery (i.e., lower residual strain) and lower hysteresis than the corresponding linear TPEs when subjected to repeated strain cycles. The enhancement in the recovery was most significant for the polymers with glassy hard domains. The TPEs with crystalline or crystalline glassy domains recovered more rapidly than the corresponding linear block polymers but showed only modest improvements in the recovery measured after the specimens were allowed to rest for 5 min. These results indicate that the covalent junction at the core of the star strengthens and accelerates the recovery of the network but does not greatly suppress plastic deformation of the crystallites. Overall, this work demonstrates that the mechanical performance of block polymer TPEs can be improved by using a star macromolecular architecture.