Supertough PLA-Silane Nanohybrids by in Situ Condensation and Grafting

Brittleness is a key barrier for poly(lactic acid) (PLA) toward broader applications. Supertough PLA was achieved by simply mixing a low amount (0.5-1 wt %) of organoalkoxysilane with PLA. Three organosilanes, (3-aminopropyl)triethoxysilane (APTES), 3-(triethoxysilyl)propyl isocyanate (ICPTES), and trimethoxymethylsilane (MTMS), were selected for this study to understand how the functional group on a silane affects the behavior of the PLA-silane hybrids. Remarkable improvements in ultimate tensile strain (up to 12 folds) and tensile toughness (up to 10 folds) were observed in APTES- and ICPTES-modified PLA without any loss in tensile strength and modulus. Glass transition temperatures measured by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) did not show any obvious decrease. We propose that in situ condensation of organosilane and grafting of PLA to form a silica-PLA core shell nanocomplex may be the reason for the improved mechanical properties. Scanning electron microscopy (SEM) showed evidence of nanofibrils at fractured surfaces. Dynamic light scattering (DLS) indicated nanoparticle formation (bimodal, 50-200 nm and <10 nm) in dilute solution, while transmission electron microscopy (TEM) provided clearer evidence of the nanosized silica formed in situ. Rheological studies also showed increased chain entanglement in the polymer melts, which contributed to 1 order of magnitude higher complex viscosity and storage modulus. The simple PLA toughening strategy and the new mechanism revealed in this study will open a door to novel performance polymer materials and broader use of PLAs.