高剪切场诱导弹性体增韧聚乳酸薄膜层状微结构的形成及对薄膜拉伸断裂机理的影响

Poly(lactic acid)(PLA) is an aliphatic biodegradable polymer which is a promising materials to fabricate environmental packaging films. However, its drawback of brittleness is limiting the application of PLA. Blending with some elastomers or flexible resins is an effective method to improve the ductility of PLA. And the obtained modified PLA materials can be used to produce films with high ductility. In our preliminary study, it was found that the PLA blending melts subjected to high shear stress when them were extruded through a die or narrow slit for film forming, that would lead to deformation and orientation of micro-structure of these melts, and formation of micro-layered structure in the obtained films. There seems to be a direct relation between the layered structure and high ductility of films. In this project, we will research formation mechanism of micro-layered structures of these modified PLA films at the presence of high shear stress. The influences of micro-layered on ductility, as well as cold crystallization behaviors of the films will be confirmed. And changes on performances of the films will be researched when these PLA films are transformed from amorphous materials to crystalline materials. Especially, the toughening effect and influence of micro-layered structures on tensile fracture mechanism of these PLA films with high crystallinity will be researched and discussed in detail. The project’s achievements can provide some reliable theoretical directions and experimental data for production of modified PLA films with high ductility and high heat deformation temperature.

聚乳酸是一种环境友好材料，在薄膜包装领域具有广阔的应用前景。但是其脆性大的缺点限制了PLA的应用。利用弹性体与PLA共混是增韧PLA的有效方法。增韧PLA材料可应用于制备高延展性PLA薄膜制品。我们前期研究发现，在PLA薄膜的加工成型过程中，PLA共混物熔体经过口模窄缝成型时，由于受到较强剪切场作用，熔体微观结构发生取向和变形，导致薄膜内部形成大量的层状结构，而这些层状结构的存在可能与薄膜拉伸韧性的进一步提高有直接关系。本课题拟研究高剪切场作用下，PLA共混物薄膜中层状结构的形成机理及影响因素；研究层状结构与PLA薄膜韧性间的关系；研究层状结构对PLA基体冷结晶行为的影响，探讨PLA薄膜材料由非晶态向高结晶状态演变时，薄膜性能的变化。特别研究高结晶度下，层状结构对薄膜材料拉伸断裂行为的抑制机理和增韧效应。为制备具有高韧性、高耐热形变温度的PLA薄膜提供可靠的理论依据和实验数据。

本项目通过弹性体或者高延展性聚合物对聚乳酸（PLA）进行增韧改性研究。并利用挤出压延或吹膜成型来制备PLA共混改性膜材。研究PLA共混改性材料的加工流变学行为、拉伸性能、微观形貌、结晶性能、耐热形变性能等。实验结果发现在适当的剪切和拉伸作用下，可以促使颗粒状分散相转变为层状、纤维状等取向结构。作为对比，通过模压成型制备含有颗粒状分散相的PLA共混物试样。实验结果发现，当分散相的颗粒尺寸和界面相容性得到优化时，含有颗粒状分散相的共混物模压试样表现出良好的拉伸韧性。这是由于在拉伸应力作用下，低结晶度PLA基体容易发生屈服而形成剪切带，同时，具有良好相界面的细化分散相颗粒可以高效地终止银纹，使得试样的拉伸韧性大幅提高。而具有层状微结构的PLA共混物试样的拉伸韧性要更加优异。在对上述PLA共混物试样热处理后，DSC曲线显示PLA基体的结晶度明显提升。热处理所得高结晶度PLA共混物模压试样的韧性剧烈下降，而热处理后所得高结晶度压延试样的拉伸韧性依旧能够保持相对较高的水平。这是由于在拉伸应力作用下，高结晶度PLA基体的屈服能力大大下降，分散相界面容易脱粘形成大量缺陷点，颗粒分散相不能有效抑制银纹向微裂纹发展，导致含有颗粒状分散相的模压样条的拉伸韧性大幅降低。相较球形/椭球形分散相颗粒，具有更高比表面积的层状等取向微结构更能有效地阻止银纹沿横向扩展为裂纹，使得试样能够在拉伸过程中保持稳定的拉伸延展。此外，在取向结构和结晶结构的共同作用下，PLA共混物试样的强度、模量和耐热性能获得较大提升。本研究为制备具有高韧性、高耐热形变温度的PLA改性薄膜、以及片材制品提供了一定的理论依据和实验数据。