Reactive Compatibilization Enabled Structure and Property Optimization of Poly(lactic acid)/Poly(3-hydroxybutyrate- <i>co</i> -4-hydroxybutyrate) Blend Films via Biaxial Stretching and Annealing

Poly(lactic acid) (PLA), a representative biodegradable polymer, is limited in packaging applications by its brittleness and inadequate gas barrier performance. Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB), a fully biodegradable and biobased polyester with a favorable balance of flexibility and strength, serves as an effective toughening component for PLA. Here, we fabricate high-strength and transparent PLA/P34HB blend films via a synergistic strategy combining reactive compatibilization, simultaneous biaxial stretching, and thermal annealing. During melt blending, epoxidized cardanol sorbate (ECD-SA) undergoes in situ epoxide ring-opening reactions with polyester end groups to generate interfacial covalent coupling structures, which suppress phase separation and improve interfacial integrity. Biaxial stretching (2.5 × 2.5 at 70 °C) induces in-plane chain orientation and stretching-induced crystallization/ordering, while subsequent annealing at 90 °C refines crystallographic order and stabilizes the poststretch noncrystalline structure. The optimized film delivers a balanced property profile: tensile strength of 55.6 MPa, elongation at break of 115.8%, water vapor permeability of 3.2 × 10–14 g·cm/cm2·s·Pa, oxygen permeability of 7.7 × 10–12 cm3·cm/cm2·s·cmHg, and visible-light transmittance of ∼84% at 650 nm. Mechanistically, the barrier enhancement is interpreted through the permeability decomposition P = D × S: despite a moderate increase in S, the net permeability decreases substantially because the reduction is diffusion-governed (strongly suppressed D). This work provides a scalable route to achieving a competitive synergy of strength, ductility, barrier performance, and transparency in biodegradable polyester films for sustainable packaging.