Toughness Amplification via Controlled Nanostructure in Lightweight Nano-Bouligand Materials.

The enhanced properties of nanomaterials make them attractive for advanced high-performance materials, but their role in promoting toughness has been unclear. Fabrication challenges often prevent the proper organization of nanomaterial constituents, and inadequate testing methods have led to a poor knowledge of toughness at small scales. In this work, the individual roles of nanomaterials and nanoarchitecture on toughness are quantified by creating lightweight materials made from helicoidal polymeric nanofibers (nano-Bouligand). Unidirectional (  = 0°) and nano-Bouligand beams (  = 2°-90°) are fabricated using two-photon lithography and are designed in a micro-single edge notch bend (µ-SENB) configuration with relative densities between 48% and 81%. Experiments demonstrate two unique toughening mechanisms. First, size-enhanced ductility of nanoconfined polymer fibers increases specific fracture energy by 70% in the 0° unidirectional beams. Second, nanoscale stiffness heterogeneity created via inter-layer fiber twisting impedes crack growth and improves absolute fracture energy dissipation by 48% in high-density nano-Bouligand materials. This demonstration of size-enhanced ductility and nanoscale heterogeneity as coexisting toughening mechanisms reveals the capacity for nanoengineered materials to greatly improve mechanical resilience in a new generation of advanced materials.