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.
It remains a challenge to artificially fabricate fibers
with the
macroscopic mechanical properties and characteristics of spider silk.
Herein, a covalently cross-linked double-network strategy was proposed
to disrupt the inverse relation of strength and toughness in the fabrication
of ultratough and superstrong artificial polymer fibers. Our design
utilized a strong fishnet-like structure based on immovable cellulose
nanocrystal cross-links to mimic the function of the β-sheet
nanocrystallites and a slidable mechanically interlocked network based
on polyrotaxane to imitate the dissipative stick–slip motion
of the β-strands in spider silk. The resultant fiber exhibited
superior mechanical properties, including gigapascal tensile strength,
a ductility of over 60%, and a toughness exceeding 420 MJ/m3. The fibers also showed robust biological functions similar to those
of spider silks, demonstrating mechanical enhancement, energy absorption
ability, and shape memory. A composite with our artificial fibers
as reinforcing fibers exhibited remarkable tear and fatigue resistance.
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