Interfacial Coupling Redirects Biaxial Deformation toward Diffusion-Suppressing Phase Reconstruction in Equimass Poly(3-hydroxybutyrate- co -4-hydroxybutyrate)/Poly(lactic acid) Blends

Combining poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB) with poly(lactic acid) (PLA) offers a biodegradable route to balance deformability, stiffness, and optical clarity, but interfacial instability can compromise structural integrity during biaxial deformation. Here, a 50/50 P34HB/PLA blend was reactively compatibilized with the multifunctional bio-based epoxy compound TE3, simultaneously drawn to 3 × 3 at 65 °C, and annealed at 90 °C. The compatibilized blend exhibited refined phase morphology and uniform drawability. Stretching transformed particulate PLA-rich domains into in-plane lamella-like structures, increased the P34HB orientation factor to 0.792, and increased the effective SAXS long period from 5.92 to 7.14 nm. Annealing largely preserved the crystalline orientation, with an orientation factor of 0.789. The final film achieved a tensile strength of 87.3 MPa, an elongation at break of 71.7%, and a transmittance of 81.9% at 650 nm. Oxygen permeability decreased to 4.1 × 10–12 cm3·cm/(cm2·s·cmHg). Within the solution–diffusion framework, this decrease was associated with an 87.7% reduction in the time-lag-derived diffusion coefficient, despite an increase in apparent oxygen solubility. These results support a proposed pathway linking reactive compatibilization, deformation-induced phase reconstruction, and retention of oriented structures to diffusion-suppressed oxygen transport, providing a basis for balancing mechanical and barrier properties in equimass P34HB/PLA films.