Preparation of Super-tough Poly(lactic acid) Using Malic Acid-based Copolyester via Reactive Blending

Reactive compatibilization is an efficient way to enhance the interfacial adhesion and improve the toughness of the blends. In this study, a fully bio-based aliphatic polyester (PBSePM) containing malic acid moiety was synthesized. Then, super-tough poly(lactic acid) (PLA)/malic acid-based aliphatic polyester blends (PLA/PBSePM) were prepared by reactive blending in the presence of hexamethylene diisocyanate (HDI). The reactive blending behavior was investigated by torque, proton nuclear magnetic resonance (H-1-NMR), Fourier transform infrared spectroscopy (FTIR), and wide-angle X-ray diffraction (WAXD). The effects of HDI addition on the tensile properties, rheological behavior, phase morphology, and melting and crystallization behavior of PLA/PBSePM blends were studied by dynamic rheometer, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). The results showed that the gel fraction and complex viscosity of the blends increased with the increase of the amount of HDI. In the reactive blending processing, HDI reacted with PBSePM and PLA. The in situ PLA-PBSePM copolymer was formed, which acted as a compatibilizer to enhance the interfacial adhesion between PLA and PBSePM. With the increase of HDI, the phase interface of the blends became blurred, and the compatibility was significantly improved. The cavities emerged in the tensile fracture surfaces and impact fracture surfaces of the reactive blends, which would dissipate energy during the stretching and impacting evolution. With the increase of HDI content, the interfacial adhesion was too strong, which delays the occurrence of matrix yielding; hence, the toughness decreased. All the blends presented a great tensile toughness with an elongation at break higher than 300%. The impact toughness of the blends significantly increased first and then slightly decreased with the increasing amount of HDI. The notched impact strengths of all the reactive blends are higher than 53 kJ/m(2), indicating a super-tough behavior. The highest notched impact strength of 81.5 kJ/m(2) was achieved while adding 2.9 wt% HDI. With the increase of HDI, the crystallization capacity of PLA decreased, the glass transition temperature and cold crystallization temperature increased first and then almost remained unchanged; meanwhile, the melting enthalpy decreased. This study suggested that incorporated malic acid into aliphatic polyester and then reactive blending would be an efficient way to obtain bio-based super-tough PLA materials.