Pressure Annealing Enables Synergistic Regulation of Stereocomplex Crystallization and Orientation in Polylactide

Abstract Polylactide (PLA), a promising biobased polymer, is limited by the difficulty in simultaneously achieving a high content of stereocomplex crystals (SCs) and high molecular chain orientation. Herein, solid phase extrusion (SPE) is employed to create highly oriented poly(l-lactide)/poly(d-lactide) (PLLA/PDLA) precursor films, while subsequent pressure annealing promotes stereocomplexation and suppresses orientation relaxation, enabling the fabrication of PLA films enriched with highly oriented SCs. During pressure annealing, moderate temperatures (120–140 °C) enhance chain mobility and promote molecular rearrangement. Meanwhile, pressure suppresses orientation relaxation, enabling the co-growth of homocrystals (HCs) and SCs and resulting in a highly oriented structure with enhanced SC crystallinity and orientation (fH-SC = 0.94). At higher temperatures (150–160 °C), pressure annealing induces a melt–recrystallization process in which oriented HCs selectively transform into SCs. Although SC formation competes with orientation retention, pressure promotes SC growth under a pre-oriented and confined environment, thereby mitigating orientation loss. An optimal pressure of approximately 100 MPa achieves a balance between SC perfection and orientation preservation. The resultant films deliver favorable piezoelectric performance and outstanding solvent resistance. This work lays a theoretical foundation for fabricating PLA films abundant in oriented SCs.