LAUSR

Polylactide (PLA) becomes brittle shortly after physical aging, posing significant challenges for practical applications. This issue can be effectively overcome through a pre‐melt‐stretching process, known as mechanical rejuvenation. However, the underlying mechanisms remain poorly understood due to the intricate multilevel structures in pre‐stretched PLA and their evolution during physical aging. Herein, PLA containing 12% D‐isomer units is utilized as a model system to eliminate the influence of structures such as mesophase and crystals. The samples remain fully amorphous throughout the pre‐stretching and subsequent aging processes. Notably, during physical aging, the pre‐stretched samples retain their ductility, while the isotropic samples exhibit increased embrittlement. Thermal analysis is employed to elucidate the changes in the amorphous phase during aging. The results reveal the impact of the amorphous segmental mobility on the ductility change during aging, which is primarily governed by the fraction of mobile amorphous phase (XMAF), with a critical threshold determining the ductile‐to‐brittle transition. This work would shed light on the toughening of physically aged glassy polymers.