Thermoplastic Elastomers with Composite Crystalline-Glassy Hard Domains and Single-Phase Melts

We report the synthesis and characterization of thermoplastic elastomers (TPEs) containing both crystalline and glassy hard segments, with the aim of capturing the mechanical properties of conventional all-amorphous triblock TPEs, while forming the solid-state structure by crystallization from a single-phase melt. To accomplish this, we used living ring-opening metathesis polymerization (ROMP) and subsequent hydrogenation to synthesize symmetric pentablock copolymers with the architecture crystalline-glassy-rubbery-glassy-crystalline. Analogous crystalline-rubbery-crystalline triblocks show a high initial modulus, yielding, and poor recovery, resulting from platelike crystalline hard blocks. By contrast: with the pentablock architecture and appropriate selection of block lengths, crystallization from a single-phase melt causes a layer rich in the glassy block to form around the crystallites, limiting their lateral growth and generating composite hard domains with both crystalline and glassy components. The pentablocks show the low initial modulus, strain-hardening behavior, and small permanent set desired for TPEs, while retaining an easily processed single-phase melt.