ABA-Type Thermoplastic Elastomers Composed of Poly(ε-caprolactone-co-δ-valerolactone) Soft Midblock and Polymorphic Poly(lactic acid) Hard End blocks

A series of triblock copolymers consisting of the poly(ε-caprolactone-co-δ-valerolactone) (PCVL) soft midblock and poly(lactic acid) (PLA) hard end blocks were synthesized via the sequential ring-opening polymerization. PCVL random copolyester was prepared by the copolymerization of ε-caprolactone (εCL) and δ-valerolactone (δVL). PCVL has the lower crystallization temperature but comparable crystallizability with the poly(ε-caprolactone) homopolymer, indicating the isomorphic crystallization of εCL and δVL. Molecular weight and chemical composition of PLA–PCVL–PLAs were controlled by the feed ratio of lactide monomer to PCVL macroinitiator. PLA–PCVL–PLAs exhibit the typical characteristics of thermoplastic elastomer (TPE). Crystallization behavior, thermal, and mechanical properties of PLA–PCVL–PLAs are readily modulated by changing the length, tacticity, and stereostructure of PLA end blocks. PLA end blocks retard the crystallization of PCVL midblock in PLA–PCVL–PLAs. Stereocomplex crystallites of PLA are conclusively formed in the racemic blends of PLA–PCVL–PLAs, leading to the increased melting temperature and thermal stability. SAXS data indicates that the microphase-separated domain size of PLA–PCVL–PLAs enhances with increasing the PLA block length and the stereocomplexation impedes forming the ordered microphase-separated structure in the racemic blends. Tensile strength, Young’s modulus, and heat-resistant temperature of PLA–PCVL–PLA TPEs increase, whereas their strains at break and transparency decrease with the crystallization or stereocomplexation of PLA end blocks.