Optimizing the mechanical properties of cellulose nanopaper through surface energy and critical length scale considerations

Cellulose nanopaper exhibits outstanding stiffness, strength, and toughness that originate from the exceptional properties of constituent cellulose nanocrystals (CNCs). However, it remains challenging to link the nanoscale properties of rod-like CNCs and their structural arrangements to the macroscale performance of nanopaper in a predictive manner. Here we address this need by establishing an atomistically informed coarse-grained model for CNCs via a strain energy conservation paradigm and simulating CNC nanopaper properties mesoscopically. We predict how the mechanical properties of CNC nanopaper with nacre-inspired brick-and-mortar structure depend on CNC overlap length and interfacial energy. We show that the modulus and strength both increase with increasing overlap length, but saturate at different critical length scales where a transition from non-covalent interfacial sliding to CNCs fracture is the key influencing mechanism. Maximum toughness is achieved when the interface and CNC failure are tuned to occur at the same time through balanced failure. We propose strategies for maximizing nanopaper mechanical performance by tuning interfacial interactions of constitutive CNCs through surface modifications that improve shear transfer capability. Our model generates broadly applicable insights into factors governing the performance of self-assembling paper materials made from 1D nanostructures.