聚右旋乳酸嵌段及多臂共聚物对聚左旋乳酸的共混改性研究

Poly(lactic acid) (PLA) has got more and more attention due to its good biocompatibility, biodegradability, mechanical and processing properties. However, because of its inherent brittle nature and low thermal stability PLA needs to be modified to be suitable for use in various applications. Ikada et al. found that a stereocomplex is formed from enantiomeric PLAs, due to the peculiar strong interaction between PLLA and PDLA chains. The stereocomplexed PLLA/PDLA blend has melting temperature (Tm)(220-230oC) by ca. 50 oC higher than those of pure PLLA and PDLA (170-180oC).In this project,we suggest a new concept for toughening semi-crystalline PLLA.A PDLA-PEG-PDLA triblock copolymer capable of forming a stereocomplex with the PLLA matrix was used as the sole toughening agent to improve the physical toughness of PLLA. The stereocomplex forming triblock copolymer could perform multiple functions.It can act as a nucleating agent,the compatibilizer, as well as a toughening agent.The stereocomplex involving PDLA and PLLA have specifc affinity for each other and with significantly higher melting temperature as compared to either PLLA or PDLA. By employing PDLA-PEG-PDLA triblocks, the end blocks act as compatibilizers, forming the favorable stereocomplex,while the soft midblock may serve as the toughening agent..Star-shaped(multi-arm) polymers, bearing strictly defined numbers of linear arms, provide useful model structures for studies of the branched macromolecular structures. In a dynamically growing area of biocompatible and biodegradable polymers, synthesis and characterization of star-shaped(multi-arm) aliphatic polyesters are particularly advanced. However, all of the stereocomplex investigations were limited till now to the linear polylactides. For stereocomplexes based on star-shaped polymers an enhancement of some physical properties could be expected because of multiarm topology of the components.In this project,multi-arm poly(D-lactide)(MA-PDLA) with different PDLA arms are blended with linear poly(L-lactide)s.Crystallization behaviors,melt rheology and thermal stability are examined. The incorporation of MA-PDLA into linear PLLA may form stereocomplex as the nucleation agent and result in the improvement of the crystallization of the linear PLLA.Influence of multi-arm PDLA addition in the rheological behavior and processing behavior were studied.The melt complex viscosity of linear PLLA was decreased or increased PLLA through controlling the relative viscosity of the MA-PDLA.Above all,the aim of this project is to improve the thermal stability,processability and mechanical property of PLLA materials synthetically through blending of PDLA-PEG-PDLA and MA-PDLA.

聚乳酸由于其良好的生物相容性、生物降解性以及优异的机械性能和加工性能，受到越来越多的关注。但是PLA 均聚物存在不少缺陷，如热稳定性差，结晶速度慢，加工性能差等大大阻碍了它的实际应用。文献报道聚左旋乳酸（PLLA）与聚右旋乳酸(PDLA)的共混物形成立体复合物后可以明显提高PLA的结晶性能和熔点，因此本项目拟在PLLA中引入三嵌段共聚物PDLA-PEG-PDLA以及多臂PDLA形成立体复合物对PLLA进行改性，研究PDLA-PEG-PDLA和多臂PDLA在PLLA熔融与冷却过程中对PLLA结晶行为的影响与控制，以提高加工过程的结晶速度和产物的耐热使用温度。通过调控共混比例，分子量及其分布，三嵌段共聚物的嵌段长度和比例，多臂PDLA的臂长和臂数以及成型加工温度等条件，设计出具有不同结晶结构和性能的新型聚乳酸共混体系。最终实现对PLLA 加工性能、耐热性能和力学性能的综合改善。

聚乳酸由于其良好的生物相容性、生物降解性以及优异的机械性能和加工性能，受到越来越多的关注。但是 PLA 均聚物存在不少缺陷，如热稳定性差，结晶速度慢，加工性能差等大大阻碍了它的实际应用。文献报道聚左旋乳酸（PLLA）与聚右旋乳酸(PDLA)的共混物形成立体复合物后可以明显提高PLA 的结晶性能和熔点，因此本项目 通过在 PLLA 中引入三嵌段共聚物 PDLA-PEG-PDLA 以及多臂 PDLA 形成立体复合物对 PLLA 进行改性，系统研究了 PDLA-PEG-PDLA 和多臂 PDLA 在 PLLA 熔融与冷却过程中对 PLLA结晶行为的影响与控制，大幅提高了加工过程的结晶速度和产物的耐热使用温度。通过调控共混比例，分子量及其分布，三嵌段共聚物的嵌段长度和比例，多臂 PDLA 的臂长和臂数以及成型加工温度等条件，设计出了具有不同结晶结构和性能的新型聚乳酸共混体系。最终实现了对 PLLA 加工性能、耐热性能和力学性能的综合改善。另外，在PDLA/PLLA共混物中，随着三臂聚乳酸含量的逐渐增加，共混物中总体的相分离分子量逐渐升高，但共混物平均每个臂中发生相分离的临界分子量逐渐减小，即Mlinear/linear blends > Mlinear/3-armed blends > M3-armed/3-armed blends，这可能是由于多臂聚乳酸的空间位阻较大，链缠结严重，导致立体复合进行的不充分，而线性聚乳酸位阻较小，可充分复合。

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