Generation of Tough, Stiff Polylactide Nanocomposites through the In Situ Nanofibrillation of Thermoplastic Elastomer

Polylactide (PLA) resins are among the most desirable biopolymers due to their biobased and compostable nature, excellent stiffness, and tensile strength. However, the widespread application of PLA has long been hindered by its inherent brittleness. While multiple routes have been successfully developed for the toughening of PLA, this toughening has always come at the cost of compromising the stiffness and strength of the matrix. In this work, we report a robust and scalable method for the development of PLA nanocomposites with an unprecedented combination of stiffness and toughness. Using the in situ nanofibrillation technique, we generated PLA composites containing nanofibrils of thermoplastic polyester elastomer (TPEE). Due to the high aspect ratio of these nanofibrils, they form physically percolated networks at low weight fractions (∼2.8 wt %) which dramatically change the mechanical behavior of the material. We found that, upon network formation, the material transitions from brittle to ductile behavior, dramatically increasing its toughness with only a marginal decrease in Young's modulus. We investigate the peculiar rheological behavior and crystallization kinetics of these blends, and propose an extension of the critical ligament thickness mechanism, wherein intrinsic toughening arises at the fiber-matrix interface in the presence of entangled elastomer networks.