Strengthening and Toughening Hierarchical Nanocellulose via Humidity-Mediated Interface

Abstract

Undoubtedly humidity is a non-negligible and sensitive problem for cellulose, which is usually regarded as one disadvantage to cellulose-based materials because of the uncontrolled deformation and mechanical decline. But the lack of an in-depth understanding of the interfacial behavior of nanocellulose in particular makes it challenging to maintain anticipated performance for cellulose-based materials under varied relative humidity (RH). Starting from multiscale mechanics, we herein carry out first-principles calculations and large-scale molecular dynamics simulations to demonstrate the humidity-mediated interface in hierarchical cellulose nanocrystals (CNCs) and associated deformation modes. More intriguingly, the simulations and subsequent experiments reveal that water molecules (moisture) as the interfacial media can strengthen and toughen nanocellulose simultaneously within a suitable range of RH. From the perspective of interfacial design in materials, the anomalous mechanical behavior of nanocellulose with humidity-mediated interfaces indicates that flexible hydrogen bonds (HBs) play a pivotal role in the interfacial sliding. The difference between CNC−CNC HBs and CNC−water−CNC HBs triggers the humidity-mediated interfacial slipping in nanocellulose, resulting in the arising of a pronounced strain hardening stage and the suppression of strain localization during uniaxial tension. This inelastic deformation of nanocellulose with humidity-mediated interfaces is similar to the Velcro-like behavior of a wet wood cell wall. Our investigations give evidence that the humidity-mediated interface can promote the mechanical enhancement of nanocellulose, which would provide a promising strategy for the bottom-up design of cellulose-based materials with tailored mechanical properties.

Crystallite fusion in nanocellulose aggregates

“…In the field of nanotechnology, it is currently a challenge to exploit the potential of such nanoscale crystallites, including biopolymer fibrils and clay platelets, in bulk aggregates or composites by tailoring the interactions between crystallites or with other components. [4][5][6] Ideal energy transfer in crystal aggregates must be realized by crystallization of the grain boundary. If multiple crystallites can be coupled into single fusion crystals by forming a bulk aggregate from their dispersions, scalable polycrystalline materials with more efficient mechanical and thermal energy transfers will be produced.…”

Recovery of the Irreversible Crystallinity of Nanocellulose by Crystallite Fusion: A Strategy for Achieving Efficient Energy Transfers in Sustainable Biopolymer Skeletons**

“…In the field of nanotechnology, it is currently a challenge to exploit the potential of such nanoscale crystallites, including biopolymer fibrils and clay platelets, in bulk aggregates or composites by tailoring the interactions between crystallites or with other components. [ 4 , 5 , 6 ]…”

Recovery of the Irreversible Crystallinity of Nanocellulose by Crystallite Fusion: A Strategy for Achieving Efficient Energy Transfers in Sustainable Biopolymer Skeletons**

“…This feature becomes pronounced when the crystallites have nanoscale dimensions and form a large area of the grain boundary. In the field of nanotechnology, it is currently a challenge to exploit the potential of such nanoscale crystallites, including biopolymer fibrils and clay platelets, in bulk aggregates or composites by tailoring the interactions between crystallites or with other components [4–6] …”