核-壳型长链超支化生物降解共聚酯增韧剂的分子设计及其改性聚乳酸机理研究

Due to the poor impact toughness of poly(lactic acid) (PLA), its widespread applications of are significantly limited. The currently available impact modifiers capable to make PLA attain super-tougheness, are derived from non-biodegradable petroleum-based polymers with linear molecular structure. Moreover, the melt flowability of toughened PLA materials tends to become poorer with respect to pure PLA. As a class of polymers characteristic of highly branched and spherical-like molecular structure, hyperbranched polymers (HBP) are expected to solve the flow-related problems during the present PLA toughening. However, the reported HBP-type modifiers for toughening PLA in the literature, have either higher Tg values or undesirable compatibility with poly(L-lactide) (PLLA) matrix, thus resulting in unsatisfactory toughening efficiency. In this context, with methyl ricinoleate containing flexible long-chain structure and D-lactide as starting materials, a kind of biodegradable and core-shell toughener with good flowability and superior toughening effect will be synthesized in this project. The long-chain hyperbranched core with low Tg of such novel toughener would impart excellent impact toughness and flowability to PLA. On the other hand, the in-situ formed stereocompex between poly(D-lactide) (PDLA) shell and PLLA matrix is expected to markedly improve interfacial compatibility and the crystallization of the matrix, both of which contribute to the further improvement in impact strength. And by establishing the correlation of molecular characteristics of the toughener and processing conditions with physical properties (such as mechanical, rheological and crystallization properties) and microstructure, its modification mechanisms on PLA will be determined. The findings of this project will offer innovative ideas and fundamental scientific basis to the molecular design and applications of biobased HBP as well as the achievement of high-performance and “greener” PLA-based materials.

聚乳酸（PLA）因冲击韧性差，极大限制了其广泛应用。目前PLA超韧化改性剂源于不可生物降解的线性石油基高分子,且增韧后易导致PLA熔体流动性变差。超支化聚合物（HBP）高度支化的球状分子结构有望解决PLA增韧中流动性问题。但现有HBP增韧剂Tg偏高或与左旋PLA（PLLA）基体相容性欠佳，导致增韧效果差。为此，本项目拟以含柔性长链的蓖麻油酸甲酯和右旋丙交酯作为原料，合成一种可生物降解、流动性好的高效核-壳型增韧剂，其低Tg的长链超支化核能赋予PLLA基体良好的冲击韧性和流动性，而右旋PLA壳与PLLA原位形成的立构复合晶体，能明显改善界面相容性和基体结晶性能，有利于冲击韧性的进一步提高；同时研究增韧剂的分子结构特征和加工条件等与共混物力学、流变、结晶性能及微观结构之间内在关联，探明其增韧改性PLA的机理。该研究将为生物基HBP分子设计、应用及PLA材料绿色高性能化，提供新的思路和理论依据。

本项目针对现有聚乳酸增韧剂多源于不可再生的线性石油基高分子，且共混后易导致PLA熔体粘度增加，而有望改善聚乳酸增韧中流动性的目前超支化聚合物增韧剂普遍存在Tg偏高（即链段柔顺性差）或与聚乳酸基体相容性欠佳，导致增韧效果差，为此我们设计了一种源于蓖麻油酸甲酯的核-壳型全生物基超支化共聚酯增韧剂。立项前，尚未见此类聚乳酸增韧剂的报道。在国家自然科学基金的资助下，项目组按照计划书的研究内容，开展了该新型生物基增韧剂合成、表征及其改性左旋聚乳酸（PLLA）的系统研究与探索，完成了各项研究任务和实现预期目标。主要研究内容与结果为：. （1）以蓖麻油酸甲酯（MR）为原料，分别采用环氧开环和“巯-烯”光点击路线合成了AB2型单体，探究了不同实验条件对两种AB2型单体转化率和产率的影响，然后通过自缩聚反应制备了超支化聚酯（HBPE），探究了酯交换催化剂种类与聚合温度对HBPE分子结构和物理性能的影响。结果表明，“巯-烯”光点击法更加简便高效和环保，所制备HBPE支化度位0.35~0.42，且具有较高分子量（Mn=13,800~16,300 g/mol）、更低Tg（-50.6oC）、更好热稳定性（Tonset=341.2oC），且与线性MR基聚酯相比，HBPE显示更低粘度；（2）选择性能最佳的HBPE为核，以其羟基引发右旋丙交酯开环聚合，在HBPE表面接枝右旋聚乳酸，得到了用于PLA的核-壳型超支化共聚酯增韧剂。随着HBPE核封端羟基数目的增加，所制备的核-壳增韧剂Tg增加，热稳定性降低；（3）与HBPE相比，核-壳型增韧剂与PLLA共混后所得改性PLA样品的界面相容明显改善，且共混物熔体粘度较纯PLLA更低。其中，采用封端为5个羟基HBPE所合成的核-壳增韧剂制备的改性PLLA样品断裂伸长率和缺口冲击强度分别较纯PLLA增加了约22和2倍, 而拉伸强度仍保留了纯PLLA的75%。该成果不仅为新型生物基增韧剂分子设计和聚乳酸“绿色”、高效改性提供新的思路和参考。. 迄今，已在国内外期刊已发表和录用论文13篇，其中SCI论文9篇；申请发明专利2件（其中1件已授权）；培养硕士生3名。