In Situ Formation of Soft–Rigid Hybrid Fibers Decorated by Sparse Lamellae of PLLA: Achieving Ductile and Heat-Resistant Materials with High Strength

Abstract

Constructing an oriented crystalline structure with high crystallinity in poly(l-lactide) (PLLA) is one efficient way to improve its mechanical strength, but flexibility is sacrificed because orderly arrangement restricts molecular mobility seriously. In this work, a unique hierarchical structure of soft–rigid hybrid fibers decorated by sparse lamellae of PLLA is in situ constructed successfully to improve mechanical strength and flexibility simultaneously. Through interfacial stereocomplexation between the PLLA matrix and poly(3-hydroxybutyrate-co-3-hydroxyvalerate)–poly(d-lactide) (PHBV-PDLA) copolymers, followed by a strong flow field during the multistage stretching extrusion (MSE), soft–rigid hybrid fibers of PHBV/interfacial stereocomplex crystallites (i-SCs) are formed to not only act as a reinforcement filler but also favor energy-dissipating deformation mechanisms. Moreover, under the coupling effect of hybrid fibers and intense flow field, the sparse PLLA lamella with a stiff shish is obtained. Consequently, nanofibrillar PLLA/PHBV-PDLA composites with a highly oriented structure exhibit high strength (94.1 MPa), considerable ductility (65.7%), and excellent heat resistance (E′140 °C > 310 MPa), compared to sea–island structured PLLA/PHBV-PDLA composites with an isotropic structure (59.3 MPa, 11.7%, and E′140 °C > 60 MPa). Furthermore, nanofibrillar PLLA/PHBV-PDLA composites can maintain ductility (57.7%) and strength (98.3 MPa) for 886 days at ambient temperature. The strengthening and toughening mechanisms based on the unique structure are proposed by investigating the microstructural changes before and after a tensile test. This work provides significant guidance for strong, ductile, and heat-resistant PLLA-based materials for durable applications.