Leveraging intrinsic lignin component for toughening and hydro-stabilizing cellulose nanopaper

Nature has engineered hierarchical and heterogeneous lignocellulose structure, among which lignin significantly contributes to mechanical behavior due to its unique molecular structure and micro-distribution pattern. Inspired by this fact, we unveil the potential of intrinsic lignin to toughen and hydro-stabilize cellulose nanopaper (CNP)-a crucial advancement for its deployment in demanding structural applications. By examining CNPs assembled from nanofibrils with controlled variations in size distribution and lignin content, we show that an optimal level of intrinsic lignin simultaneously enhances both the elastic stiffness and inelastic deformability of CNP. This strengthening and toughening effect is attributed primarily to the significant influence of lignin on the topological structure and interfacial behavior of the constituent nanofibrils. In particular, we clarify the lignin-mediated toughening mechanism in CNP through a combined experimental-simulation approach: amorphous lignin undergoes conformational evolution during interfibrillar sliding, which effectively absorbs and dissipates energy, thereby significantly enhancing the toughness of the hierarchical cellulose matrix. Additionally, the hydro-stability of CNP is enhanced by lignin with intrinsic amphiphilicity, which plays a critical role in moderating interfacial wettability and thereby delaying the hygroscopic kinetics of cellulose assemblies. Therefore, this work presents a dual-purpose strategy that harnesses the inherent functionality of lignocellulosic components to simultaneously toughen and hydro-stabilize the resulting nanomaterials.