Synthesis Of Polylactide-based Biorenewable Thermoplastic Elastomers And Their Structure-Property Study

Biorenewable polymers are widely used due to their reproducibility and environment-friendly.In particular,polylactide(PLA)is a typical renewable and biodegradable polymer with the capacity to substitute limited petroleum-based polymers in the packing and textile applications.In this dissertation,lactide and isoprene were used to prepare a series of thermoplastic semicrystalline elastomers combined ROP and RAFT polymerization.Block copolymers composed of two or more chemically diverse polymer segments which are thermodynamically immiscible can phase-separate into numerous fascinating morphologies with excellent mechanical properties.Therefore,the effects of molecular weight of soft segment and crystallization properties of hard segment on the thermal,mechanical properties and morphologies of PLA-based elastomers,and the structural evolution of microscopic phase structures during tensile deformation were detailedly investigated.The specific content includes the following aspects:1.Triblock copolymers with sustainable monomers from bioresources were synthesized using a convenient and robust strategy.PLA-based macromolecular chain transfer agent(PLA-CTA)was first synthesized by ring-opening polymerization(ROP)using 1,4-phenylenebis(methylene)bis(2-hydroxyethyl)dicarbonotrithioate(PMHD)with two terminal hydroxyl groups as small molecular chain transfer agent.And then reversible addition-fragmentation chain transfer(RAFT)polymerization was applied to synthesize polylactide-block-polyisoprene-block-polylactide(PLA-b-PI-b-PLA)triblock copolymers by using PLA-CTA and isoprene.1H NMR,FT-IR,and DSC results all indicated the successful preparation of PLA-b-PI-b-PLA triblock copolymers.2.PLA-b-PI-b-PLA triblock copolymers synthesized combined ROP and RAFT polymerization could behave from thermoplastic to thermoplastic elastomer tuned by changing PI chain length,with the highest tensile strength of 13 MPa or elongation at break of 1424%,suggesting their broad potential applications.TEM and monotonic tensile test results were carried out to study the structure of microphase separation and mechanical properties for PLA-based triblock elastomers.3.When the semicrystalline PLA-b-PI-b-PLA triblock elastomers were prepared by casting THF solutions into the film samples,microphase separation and crystallization are simultaneously performed during the solvent evaporation,and there was a competition between the microphase separation and crystallization.After eliminate the PLA crystallization by quenching,wide-angle X-ray diffraction(WAXD),small-angle X-ray scattering(SAXS)and ultra-small angle X-ray scattering(USAXS)could be performed to study the microphase structure of the semicrystalline elastomer.4.In order to characterize the elastic properties of the elastomer,the step-cyclic tensile tests were applied to the samples to investigate the elastic recovery properties of PLA-b-PI-b-PLA triblock elastomers.The elastic properties of triblock elastomers could be tuned by the different molecular weight of PI soft segment.The microphase-separated structure were also changed during the stretching processes for triblock elastomers.In-situ small-angle X-ray scattering(in-situ SAXS)was conducted to study the changes of microphase structure for PLA-b-PI-b-PLA triblock elastomers during stretching processes.5.The PI segment could change the crystallization process of PLA segment to some extent,and the crystallization kinetics were different from PLA-CTA homopolymer.The kinetics of isothermal crystallization were characterized by differential scanning calorimetry(DSC),and analyzed using the Avrami equation.Changes of PLA crystal form for triblock copolymer at different crystallization temperatures were investigated using wide-angle X-ray diffraction(WAXD).