Role of Stress Relaxation in Biaxially Stretching-Induced Crystallization of Hard Segments in Thermoplastic Elastomers

Thermoplastic elastomers of multiblock copolymers comprising alternatingly semicrystalline (hard) and molten (soft) blocks, such as olefin block copolymers, polyurethanes, polyester–polyethers, and polyamide–polyethers, are often processed via biaxial stretching for the products of thin films and foams. In the biaxially stretching-induced crystallization of hard segments, stress relaxation commonly plays an important role, but its mechanism has not yet been clearly studied. By comparing the parallel crystallization cases with and without stress relaxation, we performed dynamic Monte Carlo simulations of the biaxially stretching-induced crystallization of hard segments in the concentrate and dilute cases of bulk diblock copolymers to investigate the role of stress relaxation. The results demonstrated that stress relaxation accelerates intramolecular crystal nucleation but significantly suppresses crystallinity, resulting in more and smaller crystallites in both concentrate and dilute hard-segment microdomains. The dilute hard-segment microdomains with suppressed crystallinity will contribute less to the physical cross-links of thermoplastic elastomers and consume less impact energy via melting–recrystallization, thus lower their Young’s modulus and toughness. Furthermore, their smaller crystallites will weaken the yielding strength of physical cross-links. In conclusion, stress relaxation plays a negative role in the production of strong yet tough films and foams of thermoplastic elastomers. The high-speed biaxial stretching or foaming could reduce the extent of stress relaxation and thus guarantee the industrial solidification processes for the highly efficient and high-quality production.