Moisture-Induced Counterintuitive Effects on the Strength and Toughness of Cellulose Nanopaper

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.