Overcoming the transverse strength loss of Uniaxially stretched PLA films: The role of rigid amorphous fraction in achieving high performance

Abstract

Conventional uniaxial hot stretching of polylactic acid (PLA) films usually leads to the loss of transverse direction (TD) strength, due to the orientation of molecular chains along the machine direction (MD). To address this issue, this study proposed a novel and industrially feasible strategy, i.e., combining melt casting and uniaxial hot stretching, and utilizing the critical role of the rigid amorphous fraction (RAF) in governing the crystal evolution and macroscopic properties during hot stretching, to produce high-performance PLA films without sacrificing the TD strength. Compared with hot-pressed precursors, the melt casting precursors possessed higher RAF, which could promote strain-induced crystallization more quickly, forming a dense PLA crystal network at low stretching ratios (SR ≤ 3). This physical cross-linked network effectively restricted the movement of amorphous chains, particularly along TD, and induced higher stress during the strain hardening stage, thereby facilitating the formation of highly oriented crystals and smaller-sized nanocrystals (denser crystal network structure). Consequently, even at SR of 5, the TD strength of PLA films remained undiminished (55.9 MPa), and the tensile and tear strength in MD reached 144 MPa, 225.6 N/mm, respectively. More importantly, these films exhibited excellent puncture strength (275 N/mm), high transparency (90% at 600 nm), heat resistance (1980 MPa at 80 °C), and superior oxygen barrier properties (1.9 × 10⁻¹⁵ cm³ cm/(cm²·s·Pa)). This work would provide a theoretical and practical pathway for advanced PLA film production.