Optimal loading of PET for enhanced barrier and mechanical properties of MXD6 films

• Controlled addition of PET allows an unavoidable processing residue to be used beneficially. • A specific PET content plays an important role in determining film structure and related properties. Property improvements can be achieved while the optical clarity of the films is largely retained. The industrial production of high-barrier poly(m-xylylene adipamide) (MXD6) films is challenged by the difficulty in maintaining material purity, as trace polyethylene terephthalate (PET) impurities are often inadvertently introduced during processing, which can significantly alter the film's performance. This study shows that controlled PET addition (0–10 wt%) can turn this limitation into an advantage. We identify a critical PET concentration of 6 wt%, below which finely dispersed PET particles act as effective heterogeneous nucleating agents, accelerating MXD6 crystallization and improving interfacial adhesion. This structural optimization synergistically enhances film properties: the oxygen and water vapor transmission rates decrease by 31.2% and 57%, respectively, while tensile strength increases by 29% to 105.4 MPa, relative to pure MXD6. Beyond 6 wt%, PET agglomeration reduces nucleation efficiency and restricts MXD6 chain mobility, leading to performance deterioration. This non-monotonic behavior, governed by the competition between nucleation promotion and mobility restriction, establishes 6 wt% PET as the optimal loading. Our work provides a practical strategy for designing high-performance MXD6 films by utilizing a common impurity as a targeted modifier, without sacrificing optical clarity.