聚乳酸立构三嵌段共聚物的结晶和相分离竞争机制研究

Poly(lactide) (PLA) has been widely concernned due to the biocompatibility and biodegradability. There are there enantiomeric poly(lactides): poly(D-lactide) (crystallizable), poly(L-lactide) (slow crystallizable) and poly(D,L-lactide) (amorphous), which have good miscibility due to the same composition but different crystallization behavior. Firstly, the phase separation, crystallization behavior and crystallization dynamics in PLA-PEG-PLA triblock copolymers with PLLA, PDLA and PDLLA as end blocks will be investigated, respectively. Secondly, the triblock copolymers composed of the three enantiomeric poly(lactides), which have a homogeneous chemical composition but different crystallinities for each blocks, will be designed and synthesized in this work. The phase separation, crystallization behavior and crystallization dynamics will be investigated. Thirdly, there is a good miscibility and a weak interaction between blocks in enantiomeric poly(lactides) triblock copolymers. The monte carlo (MC) simulation method will be used to investigate the phase separation behavior and the chain conformation in triblock copolymer with weak interaction between blocks, which will be helpful to the improvement of competition mechanism between phase separation and crystallization. This study is different from the traditional investigation on phase separation and crystallization behavior of the coplymer composed by different chemical components, which will be helpful to the further understand of the copolymerization modification of PLA and the competition mechanism between crystallization and phase separation behavior.

聚乳酸(PLA)以其良好的生物相容性和生物可降解性受到广泛关注。PLA有左旋聚乳酸(PLLA)(结晶性)、右旋聚乳酸(PDLA)(结晶性)和消旋聚乳酸(PDLLA)(非晶)三种立构体。三者因有相同的化学组成而热力学相容性好，却有不同的结晶性。首先，本申请拟定量考察以PLLA、PDLA、PDLLA为端嵌段的PLA-PEG-PLA共聚物中相分离、结晶及结晶动力学。第二，本项目拟将三种立构体设计合成为聚乳酸三嵌段立构共聚物，既整链具有相同的化学组成，但各嵌段具有不同结晶能力，考察立构共聚物的分相、结晶行为及结晶动力学。第三，立构体组成的共聚物各嵌段相容性较好，为弱相互作用系统，本申请拟利用MC方法模拟弱相互作用系统的分相及构象分布情况，对完善微相分离和结晶之间的竞争机制提供帮助。聚乳酸立构共聚物区别于传统上不同化学成分组成的共聚物，将进一步完善聚乳酸共聚改性机理和共聚物相分离及结晶竞争机制。

聚乳酸(PLA)作为一种具有较好生物相容性和生物可降解的生物塑料，广泛用作人体组织工程材料、3D打印材料。提高聚左旋乳酸(PLLA)的结晶度可以使其耐热性和强度性能有所提高。本工作完成了各种均聚物、聚乙二醇(PEG)/PLLA共混物、聚己内酯(PCL)/PLLA共混物和PLLA/聚消旋乳酸(PDLLA)共混物和PLLA/聚右旋乳酸(PDLA)立构三嵌段及PEG/PLLA/PDLA五嵌段共聚物的合成及表征，考察了均聚物共混物及共聚物中的分相及结晶行为。研究结果表明，相容性PEG、不相容PCL作为增塑剂对PLLA结晶速率、结晶度有明显的促进作用，PLLA结晶过程中PEG、PCL与之发生相分离行为；PDLA与PLLA形成立构复合晶体(SC)，结晶度和熔融温度显著提高；在PEG存在下，PLLA/PDLA也形成SC晶体，且结晶度提高，进一步确定了PEG的促进作用，同时PEG易与PLLA/PDLA发生相分离行为，从聚乳酸链中分离出来，并发生结晶；PDLLA的存在对PLLA的结晶有一定的抑制作用。PLLA/PDLA立构三嵌段及PEG/PLLA/PDLA五嵌段共聚物都可形成SC晶体，但是与共混物比较其熔融温度和结晶度降低，结合蒙特卡洛(MC)模拟结果分析其原因在于：三嵌段及多嵌段立构共聚物中为相容性嵌段，易于产生分子链间的缠结，因此形成SC晶体，但是形成多嵌段共聚物后分子链活动能力较差，结晶能力降低。本工作还进行部分拓展工作：利用芯片量热技术(Flash DSC)对PLLA均聚物、PLLA/PEG两嵌段和三嵌段共聚物的结晶动力学进行了比较研究，发现PEG-PLLA两嵌段共聚物中PLLA结晶速率最大，PLLA均聚物结晶速率最低，三嵌段共聚物结晶速率介于之间；还考察了POSS纳米粒子接枝聚合物对PLLA结晶行为的影响，发现POS接枝物的引入可有效提高PLLA的结晶能力；最后对石墨烯复合材料的吸波特性进行了一定的研究，拟进一步开发其与PLLA结合后的性能。本研究结果可为新型生物可降解、生物相容性、结晶性材料的设计提供基础理论依据，也可以为聚乳酸这种工程塑料的性能改善提供一定的理论依据。