Highly Oriented Polymorphism in Polyglycolic Acid via Coupling of Chain Entanglement and Extensional Strain

Constructing highly oriented β form represents a promising strategy to address the inherent high density and brittleness of biodegradable poly(glycolic acid) (PGA). In this work, we fabricated PGA with different entanglement densities via melt blending with epoxy-functional copolymer, thereby proposing a novel strategy for achieving high-content oriented β form through the synergistic regulation of chain entanglement and an extensional strain. The high chain mobility characteristic of low entanglement density PGA (<218 mol/m3) enables rapid relaxation, limiting strain-induced crystallization and orientation, with the α form as the dominant crystalline structure. As the entanglement density increases to 218–378 mol/m3, the robust chain entanglement network suppresses the relaxation of oriented chains during stretching. The orientation degree increases from 7.8% to 37.6%. The abundance of oriented chains reduced the nucleation energy barrier for β form formation, leading to a rapid increase in the β form crystallinity from 15.7% to 49.5%, while the α form crystallinity remained unchanged. Although further increasing entanglement densities (>378 mol/m3) enhances chain orientation, it causes a significant reduction in chain mobility, thereby restricting crystal growth and decreasing overall crystallinity. Notably, the excessive chain orientation induces a form transition from the helical conformation of β form to the more extended conformations characteristic of α form. This work provides valuable insights into the relationships between entanglement density, orientation, and polymorphic crystallization of PGA materials, with potential applicability to other semicrystalline polymer systems.