How Biaxial Flow Modulates Crystal Nucleation Behavior in Poly(lactic acid)
Yunqing Ma, Jianhe Zhu, Wenyi Li, Menglan Qiao, Chaofan Wang, Liangbin Li, Hang Guo, Kunpeng Cui
Abstract
Flow-induced crystallization (FIC) in polymers is predominantly studied under idealized uniaxial flows. However, the existing theories poorly account for the complex mechanisms associated with multiaxial deformation, such as biaxial film stretching. Through synchrotron radiation in situ wide-angle X-ray scattering (WAXS) analysis of poly(lactic acid) (PLA) films subjected to uniaxial, uniaxial constant width (UCW), simultaneous biaxial (SIM), and sequential biaxial (SEQ) stretching processes, we present three fundamental advances in understanding the FIC behavior: (I) The lateral stress generated during UCW/SIM stretching suppresses crystallization by disrupting molecular orientation, which is more dominant for the FIC behavior compared to conformational ordering. (II) The orthogonal two-step SEQ stretching enables precise control over both the crystallization kinetics and crystal orientation. When first-step stretching avoids nucleation, the second-step stretching generates crystals either parallel to or perpendicular to the initial flow direction, challenging classical FIC paradigms. This directional selectivity depends on the stability of flow-induced precursors (FIPs): subcritical FIPs are disrupted by orthogonal deformation, while stabilized FIPs crystallize preferentially along the first-step stretching axis via enhanced longitudinal conformational ordering. (III) We establish a phase diagram that correlates SEQ stretching parameters with crystallization kinetics and film drawability, providing a predictive framework for optimizing industrial-scale production of high-performance PLA films.