Enhancing Tensile Strength and Toughness via Moderate Orientation of Amorphous Molecular Chains in Biobased Biodegradable Poly(lactic acid)

The preparation of high-toughness and high-strength poly(lactic acid) (PLA) is of great significance for promoting its application in packing, construction, and medical fields. Compared to blending with polymeric elastomers, hot stretching is an effective method to improve the toughness of PLA without sacrificing its strength. However, it is still unclear which part of the structural change after hot stretching is responsible for the increased toughness of PLA. In this work, the semicrystalline structure and mechanical properties of the PLA samples stretched at different temperatures were studied. First, the semicrystalline structure of the PLA samples was studied by combining differential scanning calorimetry and wide-angle X-ray diffraction analyses. When the stretching temperature was relatively low (e.g., 65 °C), the mesophase, an intermediate phase between crystal and amorphous, was formed. Furthermore, significant stretch-induced crystallization was observed due to the enhanced chain mobility when the stretching temperature was 65–75 °C. With further increasing the stretching temperature, the crystallinity of the PLA samples decreased sharply to nearly 0 due to the rapid chain relaxation. Intriguingly, the PLA samples stretched at 85 °C showed the highest elongation at break 238.4% and high strength 75.8 MPa, which was attributed to its moderate molecular chain orientation not only hindering physical aging but also retaining the entanglement network of PLA molecular chains. The findings in this work provide an insightful understanding of the improved toughness of PLA, which will facilitate the large-scale practical application of PLA.