Microstructure transformation in biaxially oriented poly(lactic acid) films governed by pre-crystallization

Precise microstructure control in bio-based and biodegradable poly(lactic acid) (PLA) offers a viable pathway to replace non-degradable conventional plastics. The pre-crystalline structure of the precursor film plays a decisive role in the biaxial orientation (BO) process of polymers, yet its regulatory mechanism via melt draw ratio (MDR) during casting, a core but under-explored parameter in biaxially oriented poly(lactic acid) (BOPLA) films fabrication, remains unclear. This study reveals how MDR during casting governs the microstructure and properties of BOPLA films by tailoring the pre-crystallization. We demonstrate that the chain orientation induced by MDR is substantially relaxed during the preheating stage of BO process, while the regulated initial nucleation density, grain growth and physical crosslinking network density lead to differences in the biaxial orientation behavior and performance of PLA. A low MDR (≤4) produces a sparse physical crosslinking network, which delays strain hardening and enables uniform deformation at a stretching ratio of up to 4 × 4 during the BO process. In contrast, a high MDR (≥6) generates a dense network that triggers premature strain hardening and fracture at lower biaxial stretch ratios. Consequently, BOPLA films from low-MDR precursors exhibit in-plane isotropic orientation, uniform formation, and an optimal property balance, whereas those from high-MDR precursors exhibit compromised structural integrity and deteriorated performance despite more pronounced grain refinement. Distinct from previous studies that mainly focused on component modification or post-processing optimization, this work clarifies the precursor structure-performance relationship via pre-crystallization regulation, providing a practical principle for high-performance biaxially oriented polymers through precise MDR control.