Synthesis And Characterization Of A New Fully Biodegradable Block Copolymer PLLA-PBA-PLLA

Poly(1,4-butylene adipate)is a kind of aliphatic polyester with temperature-dependent polymorphism.Due to its biodegadatin and biocompatibility,the PBA can be potentially applied in biomedical and eco-friendly materials.Depending on melt-crystallization temperature,PBA exhibits polymorphric crystallization structures:?,?,?+?.Studies have shown that the polymorphic morphology of PBA is closely related to its crystal morphology and properties.At present,most studies regulate the polymorphism of PBA by physical methods such as blending and adding nucleating agents or chemical methods like synthesizing random copolymers.The crystallinity and polymorphic crystalline of PBA in regular block copolymers are rarely studied.In this study,a new triblock copolymer PLLA-PBA-PLLA was synthesized for the first time.On the basis of this,we can investigate the polymorphic morphology and phase transition behavior of the PBA middle segment in different molecular weight ABA type triblock copolymers.The relationship between restricted crystallization,polycrystalline morphology,phase transition and properties in PBA segment under the restriction of PLLA chains at both ends was further analyzedFirst,we synthesized the hydroxyl-terminated PBA by polycondensation and esterification reaction.And then,with the addition of PBA as a macromolecular initiator,the PLLA block is attached to both ends of the hydroxyl-terminated PBA to synthesize the triblock copolymers with different molecular weights.The molecular weight and chemical structure of the synthesized copolymer was respectively determined by gel permeation chromatography(GPC)and H nuclear magnetic resonance spectroscopy(~1H-NMR).Using the differential scanning calorimetry(DSC)to measure the thermal properties(including melting point,melting enthalpy,crystallization temperature,glass transition temperature,etc.)of each block of the copolymer-mainly analyze and investigate the crystallization behavior and crystallization kinetics of the PLLA-limited PBA in block copolymers.The polymorphic crystallization structure and crystal phase transformation of the copolymer were studied by Wide-angle diffraction(WAXD)and Polarized light microscopy(POM).The crystal morphology of copolymer was observed by Fourier transform infrared spectroscopy(FTIR).The thermal gravimetric analyzer(TGA)investigated the thermal stability of the triblock copolymer.Experiments found that the PBA segment in the triblock copolymer with different molecular weights was restricted by PLLA segments at both ends.There still exits polycrystalline morphology of?,?,?+?and phase transition behavior from?to?in the PBA segment of the triblock copolymer.The thermal stability of the synthesized copolymer has not improved.