Xiangrui Zheng

Mechanically interlocked networks (MINs) built from [2]rotaxane cross-linkers combine high extensibility with enhanced toughness, yet how the interplay between ring mobility and bond scission influences their mechanical responses remains unclear. Here, we employ coarse-grained molecular dynamics simulations to investigate the deformation mechanisms of MINs with varying bond strength, strain rate, and graft density, where a crossover is observed in the deformation mechanism as bond strength increases. Specifically, the weak cross-links lead to scission-dominated deformation with negligible ring sliding, whereas MINs with strong cross-links exhibit profound ring sliding and inhibited bond scission, contributing to the enhanced toughness. Moreover, bond scission and ring sliding exhibit a strain-rate response with the lowest rate favoring scission over sliding, while reducing graft density delays the onset of both processes. These findings provide molecular insights for optimizing MIN mechanics by balancing movable-junction dynamics and cross-link stability.

Understanding the dependence of the properties of nanocellulose structural materials on moisture content is very important. Here, we demonstrate the effects of moisture on cellulose nanopaper strength and toughness. Interestingly, we observe initial increases followed by subsequent decreases in the tensile strength, toughness, and fracture toughness of cellulose nanopaper as the moisture content increases from 0% to 35%, thus revealing the counterintuitive yet nonmonotonic effects of moisture on the mechanical properties of cellulose nanopaper. Further experimental characterization and coarse-grained molecular dynamics simulations suggest that this counterintuitive behavior stems from the competition between the intrinsic strength of each cellulose nanofiber and the interfacial strength between fibers at the nanofiber level. Optimal mechanical properties are observed when the interfacial strength is balanced at moderate moisture levels, allowing efficient slippage between cellulose nanofibers. These findings provide fundamental insights into the moisture-dependent properties of structural materials with biopolymeric building blocks.

Cellulose-based materials have been widely studied due to their biodegradability and excellent mechanical strength. However, the intrinsic brittleness of cellulose limits its applications. In this study, the bacterial cellulose-chitosan films were prepared by a casting method. The cellulose films with ≤ 30% chitosan exhibited a simultaneous increase in strength and toughness. The slippage of cellulose chains and dense network structure contribute to the improved strength and toughness of the cellulose-chitosan films. Besides, coarse-grained molecular dynamics simulations were performed to investigate the effects of different molecular factors (i.e., chain length, molecular interaction strength, and density) on the mechanical properties of cellulose-chitosan films. These molecular factors exhibited opposite effects on the tensile strength and strain at break. In particular, modifying the density of cellulose-chitosan films could simultaneously improve the strength and toughness significantly without sacrificing the elastic modulus. The findings provide physical insights into the strengthening and toughening effects of the cellulose-chitosan films.