Interfacial enhancement and stiffness-toughness balance in MDI-compatibilized PBAT/sc-PLA blends

Biodegradable poly(butylene adipate-co-terephthalate) (PBAT)/stereo-complex poly(lactic acid) (sc-PLA) blends exhibit poor interfacial stability, and achieving a balance between stiffness and toughness is difficult. To address these challenges, this study proposes a synergistic modification strategy combining 4,4'-methylene diphenyl diisocyanate (MDI) reactive compatibilization with a two-step melt-blending process. Fourier transform infrared (FTIR) spectroscopy and gel permeation chromatography analyses confirmed that MDI reacted with PBAT and PLA during the first step of the melt-blending process. Differential scanning calorimetry, wide-angle X-ray diffraction, and FTIR investigations verified the in-situ formation of sc-PLA within the PBAT matrix during the second melt-blending step. Scanning electron microscopy revealed that MDI significantly improved the interfacial compatibility between PBAT and sc-PLA. Moreover, the PBAT/msc-PLA blend exhibited well-balanced stiffness and toughness: its Young's modulus was enhanced by more than 150% (from 78.5 MPa to 197 MPa) compared with the PBAT/sc-PLA blend, while the breaking strain remained above 500% (approximately 590%). Notably, the excellent low-temperature impact strength of the PBAT/msc-PLA blend outclassed that of the unmodified sample, reaching 28.7 kJ/m² at -40 °C, a sixfold improvement. In addition, incorporating sc-PLA increased the heat resistance (Vicat softening temperature increased from 86.2 °C to 96.3 °C) and hydrolytic degradation behavior (weight loss of 33.3% after 15 days) of the PBAT/msc-PLA blend. This work offers valuable guidance for using sc-PLA to modify other biodegradable polymers and synergistically enhance their mechanical properties, heat resistance, and biodegradability.