Mechanical role of disordered cellulose in macrofibrils of softwood secondary cell walls

Cellulose microfibrils (CMFs) in wood macrofibrils are semi-crystalline including crystalline and disordered regions, yet the mechanical role of disordered cellulose remains elusive. Here, we employed reactive molecular dynamics simulations to systematically investigate the mechanical role of disordered cellulose by developing atomic models of softwood macrofibrils with varying cellulose crystallinity. Simulation results showed that there is a critical balance between the strength and toughness of macrofibrils governed by the cellulose crystallinity. Lowering cellulose crystallinity decreased the longitudinal modulus and tensile strength of macrofibrils but increased the ultimate strain. In contrast, toughness exhibited a non-monotonic dependence on crystallinity and reached a peak at approximately 90%. Hydrogen-bond analysis revealed that disordered cellulose mainly led to fewer hydrogen bonds within CMFs, thereby weakening the strength of macrofibrils. Fracture analysis further revealed that disordered regions acted as crack initiation points and energy dissipation regions, making macrofibrils exhibit more ductile behavior. In addition, a theoretical model based on mixture rule incorporating the effect of interphase was proposed to predict the longitudinal moduli of macrofibrils. These findings provide fundamental molecular insights into the structure-property relationships of macrofibrils in wood secondary cell walls and shed light on the design of wood-based materials with tailored mechanical properties.