| Thermoplastic elastomer (TPE) is a functional polymer material with good elasticity. The block copolymer-based TPE is consisted of the hard and soft blocks with different chemical structures. The hard block of TPE is usually the semicrystalline polymer or amorphous polymer with high glass transition temperature (Tg). The soft block of TPE is generally the amorphous polymer with low Tg. Poly(lactic acid) (PLA) has three enantiomers, including the semicrystalline poly(L-lactic acid) (PLLA), poly(D-lactic acid) (PDLA), and amorphous poly(D,L-lactide) (PDLLA). Meanwhile, the equimass blend of PLLA and PDLA can form stereocomplex crystallites with a high melting point (Tm). Therefore, PLA can be used as the ideal hard block of TPE. The microphase-separated structure and property of PLA-based TPE can be adjusted by changing the stereochemistry and crystallizability of PLA blocks.The α,ω-dihydroxy poly(ethylene-co-butylene) (PEB) with a high molecular weight was first prepared. A series of poly(lactic acid)-poly(ethylene-co-butylene)-poly(lactic acid) (PLA-PEB-PLA) TPEs with different copolymer composition and stereo structure of PLA segments, including the PLLA-PEB-PLLA and PDLA-PEB-PDLA with semicrystalline PLA block and the PDLLA-PEB-PDLLA with amorphous PLA block, were synthesized by the ring-opening polymerization of lactide using PEB as the macroinitiator. As indicated by the differential scanning calorimeter (DSC) results, the triblock copolymers display two TgS, corresponding to the PLA and PEB blocks. Wide-angle X-ray diffraction (WAXD) results demonstrate that PLLA or PDLA forms a-form homocrystallites and PDLLA, PEB are amorphous in the prepared TPEs. The microphase-separated morphology of PLA-PEB-PLA TPEs is studied by the small angle X-ray scattering (SAXS) and transmission electrical microscopy (TEM). which strongly depends on the copolymer composition and stereo-regularities of PLA segments. As the volume fraction of PLA (fpla) is increased, the microphase-separated morphology of PLA-PEB-PLA TPEs changes from spherical, hexagonally packed cylindrical, to lamellar structures. PLA-PEB-PLA triblock copolymers show the properties of TPEs. Their Young"s moduli and tensile strengths increase while the strain at break (εb) decreases with increasing fpla.PDLLA-PEB-PDLLA shows the good flexibility with a strain at break of 394%. Viscoelastic properties of PLA-PEB-PLA copolymers are characterized by dynamic mechanical thermal analysis (DMTA). Storage modulus of PLA-PEB-PLA copolymers increases with increasing fpla.Storage modulus of PLLA-PEB-PLLA copolymer is apparently higher than that of PDLLA-PEB-PDLLA copolymer. Interestingly, PLA-PEB-PLA TPEs show the shape memory behavior. As compared to the PLLA-PEB-PLLA TPE with semicrystalline PLLA block, the PDLLA-PEB-PDLLA TPE with amorphous PDLLA block has higher shape recovery ratio. In the PLLA-PEB-PLLA/PDLA-PEB-PDLA equimass blends, PLLA and PDLA only form the stereocomplex crystallites and Tm of PLA block is increased to 220℃. Due to the preferential stereocomplexation of PLLA and PDLA segments before microphase separation, the morphological orderness was diminished in the PLLA-PEB-PLLA/PDLA-PEB-PDLA blends. Compared with the PLLA-PEB-PLLA copolymers, the PLLA-PEB-PLLA/PDLA-PEB-PDLA blend has a significantly increased tensile strength, Young’s modulus, and storage modulus.Furthermore, the α,ω-dihydroxy poly(caprolactone-co-valerolactone) (PCVL) was prepared by the ring opening copolymerization of ε-caprolactone and δ-valerolactone using 1,6-hexanediol as the initiator. A series of poly(lactic acid)-poly(caprolactone-co-valerolactone)-poly(lactic acid) (PLA-PCVL-PLA) TPEs containing different copolymer compositions and PLA stereostructures, including the PLLA-PCVL-PLLA and PDLA-PCVL-PDLA with semicrystalline PLA block and PDLLA-PCVL-PDLLA with amorphous PLA block, were prepared. DSC results indicate that PCVL is semicrystalline but has a low degree of crystallinity. Tg and Tm of PCVL are about-60.0 and 23.4℃, respectively. As indicated by the WAXD results, PLLA or PDLA block forms the α-form homocrystallites in PLLA-PCVL-PLLA and PDLA-PCVL-PDLA copolymers. SAXS results demonstrate that the microphase-separated structure is less ordered in PLA-PCVL-PLA copolymer. PLLA-PCVL-PLLA copolymer shows the properties of TPEs, with a tensile strength of 4.5～7.6 MPa and a strain at break larger than 700%. The storage modulus of PLA-PCVL-PLA copolymers increases with increasing the mass fraction of PLA. With the crystallization of PLA block and the increase of PLA mass fraction, the transparency of PLA-PCVL-PLA copolymer decreases. In the equimass blends of PLLA-PCVL-PLLA and PDLA-PCVL-PDLA, the stereocomplex crystallites are formed between the PLLA and PDLA segments, leading to the high Tm of PLA block. As compared to the PLLA-PCVL-PLLA copolymers, the PLLA-PCVL-PLLA/PDLA-PCVL-PDLA blend also has a good flexibility and its εb is about 700%. |
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