Biaxial Stretching‐Induced Structural Regulation Governs Nanoscale Viscoelastic Deformation and Creep Behavior of Poly (Lactic Acid) Films

ABSTRACT Understanding the nanoscale viscoelastic deformation behavior of biaxially oriented poly (lactic acid) (BOPLA) films is crucial for elucidating the relationship between stretching‐induced structural evolution and localized mechanical responses. In this work, BOPLA films with different biaxial stretching ratios were prepared, and the effects of crystalline evolution and amorphous‐chain mobility on localized deformation were investigated by combining structural characterization with instrumented nanoindentation. Increasing the stretching ratio promoted strain‐induced crystallization and reorganized the crystalline orientation, increasing the crystallinity from 31.3% to 40.2% while progressively restricting amorphous‐chain mobility. Correspondingly, the maximum indentation depth, indentation strain rate, and creep displacement systematically decreased, with the creep displacement decreasing from 616.98 to 454.57 nm. Loading‐rate‐dependent analysis further revealed that rapid loading limited molecular relaxation during loading and enhanced delayed deformation during the subsequent holding stage. The apparent creep stress exponent decreased from 23.56 to 17.82 with increasing stretching ratio, indicating reduced apparent stress sensitivity of creep deformation. Overall, the results establish a processing–structure–nanoscale deformation relationship in which stretching‐induced structural constraints suppress molecular relaxation and enhance resistance to localized time‐dependent deformation.