Synergistic Toughening of Poly(lactic acid)-Cellulose Nanocrystal Composites through Cooperative Effect of Cavitation and Crazing Deformation Mechanisms

In this work, we present poly(lactic acid) (PLLA)-cellulose nanocrystal (CNC) composites with simultaneous improve-ments in tensile toughness and impact strength using dual CNC-based fillers. The dual CNC-based fillers were designed to toughen PLLA by control of (a) the morphology and (b) the interfacial interactions in the PLLA composites. The first filler, denoted as CNCaq-rD, was comprised of rigid CNC and a rubber layer (rD), which exhibited immiscibility with the PLLA matrix, albeit with weak interactions. The second filler, denoted as CNCaq-rD-PDLA, was comprised of rigid CNC, a middle rubber layer (rD), and an outer layer of PDLA blocks that could form strong interactions with PLLA matrix via stereocomplexation. The dual fillers superbly improved both tensile toughness and impact strength of PLLA through synergistic effects. Incorporating 10% CNCaq-rD and 10% CNCaq- rD-PDLA into the PLLA matrix, the strain at break (a measure of tensile toughness) and impact strength increased by 100-fold and 3-fold, respectively. Scanning electron microscopy (SEM) and small-angle X-ray scattering (SAXS) analysis revealed dual deformation mechanisms, (i) cavitation between CNCaq-rD and the PLLA matrix and (ii) matrix crazing induced by CNCaq- rD-PDLA. Our study suggests that these dual mechanisms result in dramatic property enhancements of the PLLA under tensile and impact conditions. The resultant PLLA-based composites with improved thermomechanical properties could have potential applications in packaging and in the medical fields.