Against the Segmental Relaxation of the Chain Network: Activating the Ductile Mechanism of Both Glassy and Semicrystalline PLLA

The mechanical behavior of semicrystalline poly(l-lactide) (PLLA) is heavily influenced by its complex chain structure, which continues to be a key focus in ongoing research regarding its ability to exhibit ductile behavior. In this work, the brittle-ductile transition (BDT) of the amorphous structure of PLLA is studied in relation to its physical aging, and following the yielding of amorphous structure, the deformation of crystalline PLLA spherulites is closely examined. The significant impact of physical aging on amorphous PLLA’s mechanical properties is evident as the least aged samples show a BDT at –30 °C, aligning more closely with Vincent’s plot expectations, whereas more aged samples only exhibit ductility above 40 °C. Additionally, brittle specimen rupture is observed when the chain pullout force is reached, which increases with decreasing temperature and governs the brittle failure mechanism. The wide range of aging times and testing temperatures enables an in-depth analysis of the crazing behavior, highlighting its temperature and time dependence as an activated process. Upon yielding of the amorphous phase, various deformation modes are observed in the crystalline spherulite studies, confirming the chain network's ability to disintegrate crystalline structures and transition into a less ordered configuration as indicated by the nanoindentation and Raman spectroscopy measurements. This study conceives PLLA, not as an inherently brittle polymer, but as one whose structure needs to restore its constrained activation capacity.