左旋/右旋聚乳酸共混物中立构复合诱导的分级结晶、复杂多晶态结构形成与演变

Stereocomplex (SC) crystallization has been an effective approach to improve the physical performances of polymeric materials. Clarifying the crystallization kinetics, crystalline structural formation and evolution of stereocomplexable polymer systems is of fundamental importance for the processing and preparation of SC materials with high performances. Formations of stereocomplexes (SCs) and homocrystallites (HCs) are competing in the crystallization of enantiomeric polymer blends. SCs formed in the initial stage of crystallization can have confinement effects on the subsequent crystallization of polymer chains. Under the specific conditions, such confinement effects can cause the fractional crystallization and the formation of unusual polymorphic structure. In this project, we would like to select the poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) blends with various molecular weights and mixing ratios as the target system and investigate the fractional crystallization kinetics, formation and evolution of different polymorphs (especially the unconventional beta, gamma polymorphs of HCs). Polymorphic phase diagram of PLLA/PDLA blend system crystallized under different conditions will be established. The in-situ experimental methods to analyze the complicated polymorphic structures, structural evolution and transition of PLLA/PDLA blend system will be established or perfected. This study will clarify the fractional crystallization, complicated polymorphic behavior and their mechanisms for the PLLA/PDLA blend system, and will establish the preliminary relationships between fractional crystallization, polymorphic crystalline structure and physical properties of PLLA/PDLA blended materials. The results obtained in this study will help to further understand the behavior and processes of confined, fractional crystallization and SC crystallization in the multi-component polymer systems; they will also provide significant guidance for the preparation and processing of SC materials.

立构复合（SC）结晶是提高高分子材料综合性能的有效途径，阐明可立构复合聚合物体系的结晶动力学、结构形成与演变是高性能SC材料制备的基础。在手性异构高分子共混物中，SC与同质结晶（HC）互为竞争，结晶初期形成的SC晶体对链段的后期结晶具有受限效应，在一定条件下，该受限效应可导致体系的分级结晶和不同晶型的形成。本课题以不同分子量和组成的左/右旋聚乳酸（PLLA/PDLA）共混物为研究对象，系统研究共混体系的分级结晶动力学和多晶态（特别是非常规的beta、gamma型HC）结构的形成与演变，建立其复杂多晶态结构的相图；完善原位分析PLLA/PDLA共混物复杂多晶态结构与演变的实验方法。本研究将阐明PLLA/PDLA共混体系分级结晶和复杂多晶态行为及其机理，初步建立分级结晶、多晶态结构与材料性能之间的关系，将有助于进一步理解多组分高分子体系的受限、分级结晶和SC结晶行为，指导SC材料的加工制备。

立构复合（SC）结晶是提高高分子材料综合性能的有效途径，阐明可立构复合聚合物体系的结晶动力学、结构形成与演变是高性能SC材料制备的基础。在手性异构高分子共混物中，SC与同质结晶（HC）互为竞争，结晶初期形成的SC晶体对链段的后期结晶具有受限效应。在特定的条件下，这种受限效应可以诱导聚合物的分期结晶和不同晶型的形成。本项目以不同分子量、共混比例的聚左旋/右旋乳酸（PLLA/PDLA）共混物为模型体系，研究了其分期结晶动力学、复杂多晶态结构（特别是非常规晶型，如β-HC）的形成与演变。发现中分子量PLLA/PDLA等量共混物由于不同晶型的相继结晶可表现出独特的分期结晶现象。在低结晶温度（Tc, 75-100 ºC）下，由于结晶初期先形成的SC的受限效应，中分子量共混物形成SC和β-HC。高Tc（110-160 °C）下，由于链滑移能力增强，形成SC和α/αʹ-HC。低中分子量、中间热处理温度（170–200 °C）和合适的PDLA含量（0.1–0.2）也有利于PLLA/PDLA非等量共混物形成β-HC。β-HC的形成是由于结晶初期形成的SC的受限效应、β-HC和SC之间构象匹配。β-HC是亚稳态晶型，在升温过程中通过熔融-重结晶机理转变为热力学稳定的α-HC。. 此外，本项目选择高分子量PLLA/PDLA等量共混物为模型体系，并研究了链缠结对其结晶动力学和结晶结构的影响。通过与常规缠结样品相比，发现了解缠结提高共混物的结晶速率和SC的结晶度，揭示了链缠结在高分子量共混物的SC结晶中的关键作用。本项目结合同步辐射SAXS/WAXD和原位FTIR等手段，完善和构建了原位、实时分析PLLA/PDLA共混物复杂多晶态结构形成与演变的实验方法，特别是结构相似度高的HC多晶态。本项目有助于深入理解多组分高分子体系的受限、分期结晶和SC结晶行为，指导SC材料的加工制备。