低温加工条件下高分子量对映体聚乳酸立构复合晶体的可控形成及其机理

Stereocomplex crystallites produced in the crystallization of enantiomeric poly (L-lactide) (PLLA) and poly (D-lactide) (PDLA), is characterized by its high melting point, 50 C higher than that of PLLA or PDLA homocrystallites. Stereocomplex crystallization has been proven to be one of the most effective methods to enhance the properties of polylactides (PLA). However, stereocomplex formation is limited by the preparation methods and molecular weight of homopolymers. Melt blending requires high temperature processing (220-250 C), which easily induces significant degradation of the homopolymer. A low temperature approach (160-210 C) to prepare stereocomplex of high-molecular-weight PLLA and PDLA at temperatures much lower than those adopted in conventional melt blending, will be detailedly studied in this project. The process takes advantage of the temperature window between the melting temperature of the stereocomplex crystallites and the melting temperature of homocrystallites, which is much wider than any other polymorphic polymers known, to produce the stereocomplex crystallites when the homopolymers can no longer crystallize. In order to clarify the stereocomplexation mechanism, the effect of the interactions between enantiomeric helical chains of PLLA and PDLA on the stereocomplex formation, and the competition between stereocomplex crystallites and homocrystallites during crystallization will be investigated. In addition, to control stereocomplex crystallization, the influence of the molecular structure of PLLA and PDLA, the processing field factors such as shear, and the combined action of processing field factors including temperature and shear on the crystalline structure of PLLA/PDLA blends will be discussed.

聚乳酸(PLA）的对映异构体聚左旋乳酸( PLLA) 和聚右旋乳酸( PDLA)形成的立构复合晶体，熔点较单组分晶体高约50℃。立构复合结晶已被证实是提高聚乳酸性能的有效方法。然而，聚乳酸立构复合晶体的形成受到成型方法（须220-250℃高温熔融加工，易导致PLA的降解）及聚乳酸本身分子量的限制。本项目利用立构复合晶体与单组份晶体熔融温度之间较宽的温度窗口，采用低温方法(160-210℃)在单组份晶体不能结晶而立构复合晶体可发生结晶的条件下形成高分子量对映体聚乳酸的立构复合晶体。拟通过研究PLLA与PDLA分子链间相互作用对立构复合结晶的影响、单组份结晶与立构复合结晶间的竞争机制，以明确该低温方法的立构复合机理；并进一步研究PLLA和PDLA分子结构、剪切等其它外场加工因素以及各种加工外场因素综合作用对高分子量PLLA/PDLA共混物结晶结构的影响，以实现立构复合晶体的可控形成。

聚乳酸(PLA）的对映异构体聚左旋乳酸( PLLA) 和聚右旋乳酸( PDLA)形成的立构复合晶体，熔点较单组分晶体高约50℃。立构复合结晶已被证实是提高聚乳酸性能的有效方法。然而，聚乳酸立构复合晶体的形成受到成型方法（须220-250℃高温熔融加工，易导致PLA的降解）及聚乳酸本身分子量的限制。本项目前期利用立构复合晶体与单组份晶体熔融温度之间较宽的温度窗口，采用低温方法(160-210℃)在单组份晶体不能结晶而立构复合晶体可发生结晶的条件下形成高分子量对映体聚乳酸的立构复合晶体。本项目通过研究结晶方式（熔融结晶和冷结晶）和结晶温度以及分子链结构对立构复合晶体形成的影响，进一步阐明了立构复合结晶机理。基于对结晶过程中同质晶体和立构复合晶体之间的结晶竞争的理解，通过添加PEG增加链段活动能力、添加PMMA降低链段活动能力以及自成核等手段调控同质结晶和立构复合结晶之间的竞争，实现了熔融结晶过程单一立构复合晶体的形成。并通过过冷度调控有机小分子成核剂在PLLA/PDLA熔体中再结晶形成的晶体形态，实现了对立构复合结晶的成核效率和晶体形态调控。此外，通过流变学手段发现了低温方法中形成的立构复合晶体的网络特性，进一步在低温熔融加工过程中通过PLA和PEO之间的宏微观和宏观相分离制备多层次多孔材料。多孔材料的多层次形态包含熔融加工过程中立构复合晶体网络调控的介孔和宏观相分离形成的大孔。基于立构复合晶体网络的多孔PLA材料表现出很好的疏水特性，其较高的耐热性和耐化学性极大地扩展了在某些极端条件和催化体系中的吸附应用。项目支持下，已发表论文22篇（SCI收录21篇），申请中国发明专利5项，其中授权3项。项目为立构复合聚乳酸材料的工业化生产提供了方法借鉴，而且由于立构复合晶体材料所具有的高耐热性，也为聚乳酸的工程化应用奠定了基础。