Structure–Processing–Property Relationships in PVOH/PBAT Blends Plasticized with Dipentaerythritol, ε-Caprolactone, and Water

Water-soluble poly(vinyl alcohol) (PVOH) offers high tensile strength and an excellent oxygen barrier, but its dense hydrogen-bond network places its melting point close to its decomposition temperature, making it difficult to melt-process. Poly(butylene adipate-co-terephthalate) (PBAT) is melt-processable and ductile but has low strength and dimensional stability that restrict its use. To combine their strengths, PBAT/PVOH blends (10–40 wt % PBAT) were melt-compounded with a 20 wt % ternary plasticizer of dipentaerythritol, ε-caprolactone, and water in equal parts, benchmarked against a water-only control at 40 wt % PBAT. Across FTIR, XRD, DSC, steady-shear and oscillatory rheology, DMA, tensile, SEM, AFM, TGA, and gas-barrier measurements, the ternary plasticizer governs the structure–property relationship by redistributing the PVOH hydrogen-bond network rather than breaking it; it broadens and shifts the O–H stretching envelope, suppresses PVOH crystallinity and recrystallization, depresses the glass transition by 17–27 °C, and broadens the relaxation spectrum, with no cross-linking or covalent grafting. Composition sets the phase topology─dispersed PBAT domains at 20–30 wt % and a cocontinuous structure at 40 wt %─while the plasticizer controls interfacial adhesion, converting the weakly bonded, void-laden interfaces of the water-only control into well-adhered ones. Among the series, the 20 wt % blend (PBAT-20) gives the best stiffness–toughness balance (modulus ≈ 0.23 GPa, ultimate tensile strength ≈ 20 MPa) together with an oxygen transmission rate of ≈6.4 cm3 m–2 day–1 for a 400 μm film, equivalent to ≈102 cm3 m–2 day–1 when normalized to a 25 μm thickness. The water-vapor transmission rate of the same film remains high (6.06 g m–2 day–1), so the films reported here are suited to oxygen-sensitive but moisture-tolerant applications rather than to moisture-critical ones. These findings position ternary hydrogen-bond engineering as a reaction-free, melt-processing route to property-tunable PBAT/PVOH films for biodegradable flexible packaging.