聚乳酸/硅藻壳复合材料的结晶行为及降解性能研究

  塑料自发明以来，迅速发展成为世界上使用最广泛的材料之一，但大部分塑料废弃后无法自然降解，给生态环境带来了巨大考验。可降解高分子材料是一类有望替代传统化石基塑料的绿色环保材料，其中聚乳酸具备良好的加工性能和生物相容性，成为最具代表和最有发展前景的可降解材料之一。但是，聚乳酸结晶性能较差，导致其脆性大，耐热性差。另外，聚乳酸吸水性差，自然环境下降速率较慢，限制了其大规模应用。为此，本文通过无机粒子改性的方法，将多孔硅藻壳与聚乳酸复合，改善了聚乳酸结晶和降解性能，系统揭示了高比表面积、高表面活性硅藻壳调控聚乳酸结晶和降解的机理。

  首先研究了硅藻壳对聚乳酸结晶行为的影响。硅藻壳具有高比表面积和生物相容性，可均匀分散在聚乳酸中，并与聚乳酸形成了氢键，加强了界面相互作用力。硅藻壳具有多级孔结构，熔融共混后与聚乳酸形成了互穿网络结构，提高了两相结合力。良好的分散性和相容性使得硅藻壳起到异质成核效应，降低成核势垒，提高聚乳酸结晶速率，促进亚稳态a¢晶型向稳态a晶型转变，结晶度可由~14.5%提升至~43.5%，强度和韧性同时得到提高。

  其次探讨了硅藻壳对聚乳酸降解性能的影响。在硅藻壳影响下聚乳酸的降解周期由2年缩短至3个月。经过12周的降解，数均分子量从~147000 g/mol迅速降至~9700 g/mol，低于聚乳酸链缠结的临界分子量，可进一步被自然同化吸收。结合降解过程中形貌、分子结构、分子量、结晶度以及力学性能的变化，系统分析了硅藻壳对聚乳酸降解行为的影响，发现硅藻壳可促进聚乳酸发生非均质体侵蚀降解，并调控聚乳酸晶区和非晶区的降解次序，加快晶区降解。裂纹扩展模拟分析表明，硅藻壳诱导形成的微纤晶尖端应力强度因子KI远超聚乳酸断裂韧性KIC（~100 MPa×mm1/2），导致裂纹失稳扩展，进一步加速吸水降解。

  紧接着分析了天然硅藻壳和化学合成二氧化硅对聚乳酸性能的影响。系统比较了聚乳酸/硅藻壳、聚乳酸/硅藻土和聚乳酸/二氧化硅的结晶性能、力学性能和降解性能等，揭示了天然硅藻壳改性聚乳酸性能的优势。结晶动力学分析表明，硅藻壳可以起到异质成核效应，促进聚乳酸更快形成多而稳定的小尺寸球晶。与纯聚乳酸相比，聚乳酸/硅藻壳的结晶度提高了~188%，抗拉强度和断裂伸长率分别提高了~71%和~9%；而聚乳酸/二氧化硅的结晶度仅提高~34%，相应的抗拉强度和断裂伸长率分别下降了~35%和~43%，变得更脆。降解12周后，聚乳酸/硅藻壳复合材料质量损失高达93%，分子量由~146000 g/mol下降到~7800 g/mol，降解速率快；而聚乳酸/二氧化硅复合材料质量损失为30%，分子量仅降至~91300 g/mol，降解速率慢。

  最后揭示了聚乳酸/硅藻壳复合材料的降解机理。采用力学衰减模型建立了断裂强度和分子量之间的联系，得出了聚乳酸降解过程中力学性能失效的临界分子量为~26000 g/mol。降解机理分析表明，硅藻壳通过诱导聚乳酸结晶改变了晶区和非晶区的分布，提高结晶度后增加了晶区之间“纽带”区的缺陷；另外，硅藻壳诱导的微纤晶促进了裂纹失稳扩展产生缺陷，有助于聚乳酸快速吸水，加速降解。总之，聚乳酸/硅藻壳复合体系具有较好的结晶性能、力学性能以及可调控的降解速率，在绿色可降解高分子材料领域具有广阔的应用前景。

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