Investigation of Thermal-Induced Crystallization Limit in Melt-Extruded Poly(L-lactic Acid) Monofilaments for...

High-performance biodegradable polymers have garnered significant attention as viable alternatives to traditional metal fibers in implantable medical devices. The orientation formation process (OFP) has been employed to facilitate rapid molecular chain orientation and crystallization above glass transition temperature, enabling the production of exceptionally strong and tough monofilaments with promising applications in biodegradable self-expanding occluders and aortic stents. However, the temperature is a critical factor influencing the final performance of these fibers. This study investigates the effect of annealing temperature on the chain structure of extruded Poly(L-lactic acid) (PLLA) monofilaments. These findings indicate that constrained annealing induces ordered arrangement of molecular chains and generates numerous cavities within the monofilament. Notably, when subjected to constrained annealing for 1 h, the crystallinity of the monofilaments reaches its maximum value of 39.54±1.97%. This suggests that there exists an upper limit for crystallization under isolated thermal influences. This limitation primarily arises from a substantial presence of entangled chains within the extruded monofilaments that cannot effectively crystallize at elevated temperatures. The annealing process promotes alignment of crystal lamellae in a pre-oriented manner, thereby enhancing subsequent directional alignment during OFP. Moreover, the stents and occluders prepared by braided PLLA monofilaments demonstrate outstanding mechanical and recovery performance. These insights provide valuable experimental references for further exploration into OFP mechanisms applicable to high-performance semi-crystalline polymers intended for biodegradable medical devices, meeting the ever increasing material demands of the evolving society.