可同时促进结晶成核和生长的纳米粒子合成及其诱导聚乳酸结晶研究

The intrinsic flaws of poly(L-lactide acid) (PLLA), i.e., slow crystallization rate and low crystallinity result in low heat distortion temperature and poor gas-barrier resistance, severely limiting its practical application. Nanoparticles with high specific surface area have been demonstrated as nucleation agents to increase the nucleation density of PLLA; however, the resultant crystallinity of PLLA was not increased substantially. This is because the nanoparticles could serve as heterogeneous nucleation sites, but have little influence on the crystal growth. In this proposal, we proposed to synthesize highly efficient nano-sized nucleation agents by covalently grafting the chain promoter, poly(ethylene glycol) (PEG), onto graphene oxide and cellulose nanocrystals. It is expected that the prepared PEG-grafted nanoparticles could promote nucleation and accelerate crystal growth of PLLA simultaneously. Conformational ordering at the early stage of crystallization, nucleation, crystallization kinetics, and interfacial crystalline morphology of PLLA induced by PEG-grafted nanoparticles were investigated by utilizing in-situ characterization methods, such as synchrotron radiation infrared microspectroscopic imaging and X-ray microdiffraction. In these efforts, we intended to understand the relationship between the structure and content of PEG-grafted nanoparticles and the preordering and crystallization of PLLA, thus unveiling the physical image of PEG-grafted nanoparticle induced PLLA crystallization. The obtained fundamentals based on the aforementioned scientific issues will offer theoretical guidelines and experiment basis for manufacturing the PLLA products with high crystallinity and superior performance.

聚乳酸结晶速率慢、结晶度低的缺点导致其热变形温度低、阻隔性差，很大程度上限制了其制品的应用。纳米粒子可作为聚乳酸的成核剂，大量增加晶核密度，但仍难显著提高其最终结晶度。这是由于纳米粒子虽能有效诱导成核，但不能促进晶体生长。本项目拟将结晶促进剂聚乙二醇接枝到纳米粒子（氧化石墨烯和纳米纤维素）表面，提高纳米粒子表面附近高分子链的运动能力，获得能同时促进结晶成核和生长的高效纳米成核剂。利用高通量、高时间和空间分辨的同步辐射显微红外和X射线衍射等表征手段原位研究表面修饰纳米粒子诱导聚乳酸结晶初期的分子链构象有序、结晶成核、晶体生长动力学及界面结晶形貌等科学问题。获得表面修饰纳米粒子的结构和含量与聚乳酸预有序和结晶的关系，揭示其诱导聚乳酸结晶的机理，为制备高结晶度、高性能的聚乳酸制品提供理论基础和实验依据。

聚乳酸（PLLA）结晶速率慢、结晶度低的缺点导致其热变形温度低、阻隔性差，很大程度上限制了其制品的应用。为加速促进PLLA结晶，本项目通过外加高表面能的异相成核剂和剪切外场诱导PLLA结晶和调控其晶体形貌，主要取得了如下重要发现：（1）发现高表面能的碳纳米管选择性诱导PLLA/PDLA共混物择优生成立构复合晶（sc晶），其熔点相比PLLA均质晶体高近50 ˚C，从临界成核浓度的角度揭示了sc晶优先生成的动力学模型，为调控聚乳酸共混物sc晶的调控提供了新方法。（2）在线跟踪了界面剪切场诱导PLLA的晶体形貌变化，发现界面剪切场诱导PLLA形成了特殊的柱晶，首次建立了剪切速率和PLLA柱晶特征结构的定量关系，发现了柱晶显著提高了界面粘结强度。（3）在线跟踪了界面剪切场诱导PLLA/PDLA sc晶的晶体形貌变化，发现生成sc柱晶的临界剪切阈值，获得sc柱晶特征结构随界面剪切速率变化的定量关系，发现了所生成的sc横晶显著改善了PLA的耐溶剂性和界面粘结强度，为制备界面增强PLLA材料提供了理论指导。（4）将sc晶均匀包覆于天然苎麻纤维表面，利用其高成核作用，诱导PLLA生成了致密的横晶，为制备纤维增强PLLA复合材料提供了重要参考。在获得上述结晶规律的基础上，利用均匀分散在基体中的氧化石墨烯（GO）诱导PLLA生成高度排列的片晶。并结合非晶区的高浓度GO，二者共同起到纳米阻隔墙的作用，增加了气体分子的扩散渗透路径，有效降低了氧气渗透系数达89.9%，为制备高阻隔PLLA包装薄膜材料提供技术支持。本项目即完成了预计研究内容，有所有扩展。本项目研究成果共发表SCI论文12篇，其中影响因子（IF）大于5.0的论文5篇，发表会议论文7篇。培养硕博士研究生9名（博士6名、硕士3名），其中已毕业6名。