Praperation and Properties of Thermoplastic Elastomers with Multi-arm Star Architecture from δ-Caprolactone

The most commonly used thermoplastic elastomers (TPEs) are ABA linear triblock copolymers, which usually suffer from undesirable stress relaxation, poor creep resistance and large residual strain. One of the solutions to address the above problems is to use multi-arm star block copolymers as TPEs. In this contribution, a series of multi-arm star block copolymers (PδCL-b-PLLA)_n was prepared by one-pot sequential ring-opening copolymerization of bio-based δ-caprolactone (δCL) and L-lactide (L-LA) using 1,4-benzenedimethanol, trimethylolpropane or pentaerythritol as the initiators. The well-defined structures of obtained (PδCL-b-PLLA)_n copolymers were carefully characterized by ¹H and ¹³C-NMR spectra. Compared to their linear analogues, the three-arm and four-arm star block copolymers exhibited the improved thermal stabilities. The microphase separation of PδCL soft block and PLLA hard block was supported by the differential scanning calorimetry as well as small-angle X-ray scattering. The effects of molecular weight, fraction of hard block (f_hard) and topology on the mechanical properties of (PδCL-b-PLLA)_n copolymers were investigated using uniaxial and cyclic tensile experiments. The (PδCL-b-PLLA)_n copolymers with f_hard of 0.45 behaved as thermoplastics while those with f_hard of 0.37 behaved as thermoplastic elastomers. The tensile strength of (PδCL-b-PLLA)_n copolymers considerably increased as the increase of their molecular weights. When f_hard of (PδCL-b-PLLA)_n was 0.37, the star block copolymers exhibited higher tensile strength, better elastic recovery as well as lower residual strain than the corresponding ABA linear block copolymers with comparable molecular weights. Remarkably, (PδCL₁₉₆-b-PLLA₁₀₀)₃ exhibited high tensile strength (26.8 ± 0.2） MPa, high elastic recovery (96.7 ± 0.2） %, high resilience (71.0 ± 0.1） % as well as low residual strain (6.4 ± 0.2） %, which were comparable to the commodity styrene based TPEs. Compared to four-arm star block copolymers, three-arm star block copolymers exhibited higher tensile strength and better elastic recovery, which was probably ascribed to the reduced segregation strengths that originated from the lower molar mass of each arm for four-arm star block copolymers.