含大量取向晶体和空间约束韧性分散相的聚乳酸阻隔薄膜研究

Manipulation of microstructure during processing is efficient and versatile way to overcome the well-known weaknesses of biodegradable polylactide (PLA) such as low barrier property, low heat deflection temperature and high brittleness, leading to high performance and new functions of PLA materials. In this project, a protocol of melt extrusion followed by multistage stretching is proposed to generate intensively extensional flow aiming to tailor the microstructure of PLA multiphase system for super-robust barrier film, on one hand, the extensional flow is used to induce lots of oriented lamellae acted as excellent “barrier wall” and reinforcement; on the other hand, in order to achieve desirable ductility without sacrifice of strength, a biodegradable polymer with excellent ductility (e.g. poly(butylene adipate-co-terephthalate)) is chosen, meanwhile, a unique phase morphology with spatial confinement is conceived through intensively extensional flow-induced deformation of dispersion phase, which is expected to absorb much energy in a manner of constrained plastic deformation. Finally, the PLA barrier film with promising balance of ductility and strength can be achieved by the tailored microstructure proposed in this project. Based on the novel strategy to construct the specific microstructure proposed, relative scientific understandings should be carried out to fulfill the proposal, including morphology evolution (hierarchical structure of oriented crystalline structure, dimension and its distribution of dispersed phase) of two-phase polymeric system under extensional flow, the mechanism for enhanced barrier properties in terms of the conceived microstructure and its key factors, deformation behavior of the conceived microstructure under stress and thus the mechanism for simultaneous toughening and reinforcement. The results and new understandings based on this project will provide guidance for the fabrication of PLA barrier film with desirably mechanical properties in industry.

在加工过程中构建特定微观结构克服生物可降解高分子聚乳酸性能不足（如低阻隔性、低热变形温度及高脆性等），是实现聚乳酸高性能化及功能化的高效途径。本项目拟通过形成大量取向晶体和空间约束韧性分散相结构，制备增强增韧聚乳酸阻隔薄膜。即，采用“熔融挤出-多级拉伸”施加强拉伸流动场，一方面促进聚乳酸形成大量取向晶体，提高阻隔性能和强度；另一方面，添加韧性良好的生物可降解高分子（如聚己二酸丁二酯/对苯二甲酸丁二酯）作为分散相，通过强拉伸流动场诱导分散相形变形成空间约束结构，在应力下发生有限塑性形变而高效吸收能量，提高延展性且不显著影响强度。基于该思路，进一步探讨一些基本科学问题，包括拉伸流动场下该两相体系形态结构（取向晶体多层次结构以及分散相尺寸及分布）生成规律，弄清该薄膜阻隔机理和关键结构因素，揭示应力下取向晶体和分散相的协同变形行为和增强增韧机理。研究结果有望为聚乳酸阻隔薄膜生产提供理论支持。

项目立足于在加工过程中在可降解高分子聚乳酸中构建特定的微观结构，达到克服其性能缺陷（如低阻隔性、低热变形温度及高脆性等），实现其高性能化及功能化的目的。项目取得了良好的成果，首先发展了“熔融挤出-多级拉伸”加工新方法，施加强拉伸流动场，使聚乳酸形成大量取向非晶相和取向晶体结构，在韧性提高的同时，保持甚至提高强度和模量，揭示了取向非晶结构在聚乳酸增韧方面起到关键作用。然后，我们采用韧性良好的生物可降解高分子（聚己二酸丁二酯/对苯二甲酸丁二酯）作为分散相，成功制备了聚乳酸复合薄膜，通过强拉伸流动场诱导分散相形变，形成空间约束结构，揭示了取向结构（取向晶体多层次结构以及分散相尺寸及分布）的形成规律，发现了分散相与拉伸流动场对取向晶体形成具有协同作用，该取向晶体结构是阻隔性能提高的关键结构因素。此外，项目构筑了隔离网络结构的聚乳酸复合薄膜，通过疏水隔离网络结构的构建，聚乳酸复合薄膜的气体阻隔性能显著提高，项目并讨论了该结构对阻隔性能的影响机制。最后，我们在注塑加工中引入剪切流动场，探讨了增强增韧取向结构在注塑加工中的工程实现，通过强的剪切力以及分散相协同作用，成功获得大量取向结构，实现了增强增韧。上述研究结果有望为聚乳酸阻隔薄膜的生产提供理论支持。