Strain Engineering of Ion-Coordinated Nanochannels in Nanocellulose.

Expanding the interlayer spacing plays a significant role in improving the conductivity of a cellulose-based conductor. However, it remains a challenge to regulate the cellulose nanochannel expanded by ion coordination. Herein, starting from multiscale mechanics, we proposed a strain engineering method to regulate the interlayer spacing of the cellulose nanochannels. First-principles calculations were conducted to select the most suitable ions for coordination. Large-scale molecular dynamics simulations were performed to reveal the mechanism of interlayer spacing expansion by the ion cross-linking. Combining the shear-lag model, we established the relationship between interfacial cross-link density and interlayer spacing of an ion-coordinated cellulose nanochannel. Consequently, fast ion transport and current regulation were realized via the strain engineering of nanochannels, which provides a promising strategy for the current regulation of a cellulose-based conductor.

Hierarchical and reconfigurable interfibrous interface of bioinspired Bouligand structure enabled by moderate orderliness.

Introducing natural Bouligand structure into synthetics is expected to develop high-performance structural materials. Interfibrous interface is critical to load transfer, and mechanical functionality of bioinspired Bouligand structure yet receives little attention. Here, we propose one kind of hierarchical and reconfigurable interfibrous interface based on moderate orderliness to mechanically reinforce bioinspired Bouligand structure. The interface imparted by moderate alignment of adaptable networked nanofibers hierarchically includes nanofiber interlocking and hydrogen-bonding (HB) network bridging, being expected to facilitate load transfer and structural stability through dynamic adjustment in terms of nanofiber sliding and HB breaking-reforming. As one demonstration, the hierarchical and reconfigurable interfibrous interface is constructed based on moderate alignment of networked bacterial cellulose nanofibers. We show that the resultant bioinspired Bouligand structural material exhibits unusual strengthening and toughening mechanisms dominated by interface-microstructure multiscale coupling. The proposed interfibrous interface enabled by moderate orderliness would provide mechanical insight into the assembly of widely existing networked nanofiber building blocks toward high-performance macroscopic bioinspired structural assemblies.

Near-room-temperature water-mediated densification of bulk van der Waals materials from their nanosheets.

The conventional fabrication of bulk van der Waals (vdW) materials requires a temperature above 1,000 °C to sinter from the corresponding particulates. Here we report the near-room-temperature densification (for example, ∼45 °C for 10 min) of two-dimensional nanosheets to form strong bulk materials with a porosity of <0.1%, which are mechanically stronger than the conventionally made ones. The mechanistic study shows that the water-mediated activation of van der Waals interactions accounts for the strong and dense bulk materials. Initially, water adsorbed on two-dimensional nanosheets lubricates and promotes alignment. The subsequent extrusion closes the gaps between the aligned nanosheets and densifies them into strong bulk materials. Water extrusion also generates stresses that increase with moulding temperature, and too high a temperature causes intersheet misalignment; therefore, a near-room-temperature moulding process is favoured. This technique provides an energy-efficient alternative to design a wide range of dense bulk van der Waals materials with tailored compositions and properties.

Bioinspired polysaccharide-based nanocomposite membranes with robust wet mechanical properties for guided bone regeneration.

Polysaccharide-based membranes with excellent mechanical properties are highly desired. However, severe mechanical deterioration under wet conditions limits their biomedical applications. Here, inspired by the structural heterogeneity of strong yet hydrated biological materials, we propose a strategy based on heterogeneous crosslink-and-hydration (HCH) of a molecule/nano dual-scale network to fabricate polysaccharide-based nanocomposites with robust wet mechanical properties. The heterogeneity lies in that the crosslink-and-hydration occurs in the molecule-network while the stress-bearing nanofiber-network remains unaffected. As one demonstration, a membrane assembled by bacterial cellulose nanofiber-network and Ca2+-crosslinked and hydrated sodium alginate molecule-network is designed. Studies show that the crosslinked-and-hydrated molecule-network restricts water invasion and boosts stress transfer of the nanofiber-network by serving as interfibrous bridge. Overall, the molecule-network makes the membrane hydrated and flexible; the nanofiber-network as stress-bearing component provides strength and toughness. The HCH dual-scale network featuring a cooperative effect stimulates the design of advanced biomaterials applied under wet conditions such as guided bone regeneration membranes.

Biomimetic Ultratough, Strong, and Ductile Artificial Polymer Fiber Based on Immovable and Slidable Cross-links.

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 ß-sheet nanocrystallites and a slidable mechanically interlocked network based on polyrotaxane to imitate the dissipative stick-slip motion of the ß-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.

Intrinsic kink deformation in nanocellulose.

Sharp bends can be widely observed in isolated cellulose nanofibrils (CNFs) after mechanical treatment, referred to as kink dislocations that are previously found in wood cell walls under compression. The non-Gaussian distribution of kink angle implies some inherent deformation behaviors of cellulose nanocrystals (CNCs) hidden in the formation of kink dislocations in CNFs. We herein perform molecular dynamics simulations to investigate the kink deformation of nanocellulose. It is interesting to find an intrinsic deformation mode of Iß CNCs under uniaxial compression, in which the metastable structure of kinked CNCs turns out to be the triclinic Iα phase with twin boundaries originated from interlayer dislocation-induced allomorphic transition. An intrinsic kink angle (~60°) is defined based on geometric traits of stable kinked CNCs. Moreover, the weakened intrachain hydrogen bonds in twin boundaries lead to exposed glycosidic bonds and damaged hydrogen-bonding networks, which would act as the origin of kink defects in nanocellulose.

Strengthening and Toughening Hierarchical Nanocellulose via Humidity-Mediated Interface.

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.