High Strain Rate Biaxial Stretching of Poly(L‐lactide) Induces Spherical Mesophase Formation

ABSTRACT The formation of spherical mesophase in amorphous rubbery‐state poly(L‐lactide) (PLLA) under high strain rate biaxial stretching was explored employing infrared spectroscopy, 2D wide‐angle x‐ray scattering (2D‐WAXD), polarized optical microscopy (POM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and thorough rheological analysis. Results indicate that spherical mesophase formation was markedly enhanced at extensional strain rates of 0.1–2 s −1 , leading to a substantial increase in nucleation density. POM and SEM observations demonstrated that biaxial stretching induced spherical mesophase domains, with the average diameter increasing as the extensional strain rate rose from 0.01 to 2 s −1 . However, mesophase content declined when the extensional strain rate exceeded 2 s −1 . A multistage model was developed, wherein conformational transitions from tt/tg to the lower‐energy gt/gg states were kinetically preferred. Strain rates below 2 s − 1 promoted chain disentanglement and significantly reduced melt viscosity, thereby lowering energy barriers for conformational rearrangement. Conversely, strain rates exceeding 2 s − 1 provided insufficient relaxation time, impeding conformational transitions and structural reorganization. These findings yield vital insights for regulating PLLA film microstructure, essential for optimizing its performance characteristics.