How weak hydration interfaces simultaneously strengthen and toughen nanocellulose materials

Hygroscopicity and the resulting pronounced structural deformation and mechanical attenuation limit the applications of wood and nanocellulose materials. Understanding the micromechanical mechanisms of interfibrillar hydration from the perspective of interfacial hygromechanics is a prerequisite for achieving tailored macroscopic properties. Here, we demonstrated a counterintuitive mechanism that weak hydration interfaces can simultaneously strengthen and toughen nanocellulose materials due to the transition of interfacial deformation mode. The interfibrillar sliding barrier and shear strength are undoubtedly reduced by increasing interfacial hydration and shift from stick–slip motion at low humidity to more continuous slip at high moisture content. These transitions in the shear response suppress local deformation at the interface, facilitate the homogenization of fibril deformation, and ultimately enhance the interfacial load transfer capability of long fibrils through the cooperativity of hydrogen bonds. The redistribution of local stress across the interface also reduces the strain concentration in fibrils, promoting fibril pullout and bridging. This novel mechanism is applicable to other hydrogen bond-dominated building blocks for the bottom-up design of advanced nanocomposites that are both strong and tough.