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.

Tuning instability in suspended monolayer 2D materials.

Monolayer two-dimensional (2D) materials possess excellent in-plane mechanical strength yet extremely low bending stiffness, making them particularly susceptible to instability, which is anticipated to have a substantial impact on their physical functionalities such as 2D-based Micro/Nanoelectromechanical systems (M/NEMS), nanochannels, and proton transport membrane. In this work, we achieve quantitatively tuning instability in suspended 2D materials including monolayer graphene and MoS2 by employing a push-to-shear strategy. We comprehensively examine the dynamic wrinkling-splitting-smoothing process and find that monolayer 2D materials experience stepwise instabilities along with different recovery processes. These stepwise instabilities are governed by the materials' geometry, pretension, and the elastic nonlinearity. We attribute the different instability and recovery paths to the local stress redistribution in monolayer 2D materials. The tunable instability behavior of suspended monolayer 2D materials not only allows measuring their bending stiffness but also opens up new opportunities for programming the nanoscale instability pattern and even physical properties of atomically thin films.

Ductile amorphous boron nitride microribbons.

The broad applications of ceramic materials in functional devices are often limited by their intrinsic brittleness. Amorphous boron nitride (a-BN), as a promising ceramic has shown high thermal stability and excellent dielectric properties that can be applied to microfabricated aerogel and nano dielectric layers, while its mechanical properties at small scales are yet to be studied. Here we report synthesized a-BN microribbons can have a uniform elongation at a breaking strain of more than 50% upon tension, exhibiting outstanding ductility. Such a-BN microribbons with lengths ranging from tens to hundreds of micro-meters were prepared via the small molecule precursors sol-gel method. Through in situ uniaxial tensile measurements, we demonstrated that a-BN microribbons also display a surprising flaw-tolerance behaviour. Combining high-resolution atomic characterization with molecular dynamics simulations, we reveal that the large tensile plasticity of a-BN originates from the topological deformation induced multiple energy-dissipation mechanisms including unfolding and reorientation of local curly h-BN layers and their interlayer debonding, slippage as well as the intralayer tearing. Our findings provide new insights to develop ductile amorphous covalent-bonded materials for emerging applications.

Theoretical study of hydrogen abstraction by HO2 radicals from primary straight chain amines CnH2n+1-NH2 (n = 1-4).

Hydrogen abstraction reactions by HO2 radicals from four primary amines including methylamine (MA), ethylamine (EA), n-propylamine (PA), and n-butylamine (BA), are investigated and the effect of the functional group on rate constants at different reaction sites is examined. A hybrid functional BH&HLYP coupled with cc-pVTZ as the basis set is utilized to determine geometry optimizations, frequencies, and connections between transition states and corresponding local minima. By comparing the reaction energies obtained by several density functional theory methods to those obtained using the gold-standard CCSD(T)/CBS(T-Q) method, the M08-HX/maug-cc-pVTZ combination is identified as the best suitable method with a mean unsigned deviation of 0.81 kcal mol-1. This method is then applied to construct the potential energy surface for all the reaction systems. High-pressure limit rate constants at 500-2500 K are calculated through variation transition state theory and conventional transition state theory, including a one-dimensional hindered rotor treatment and asymmetrical Eckart tunneling correction. The branching ratio analysis suggests that the hydrogen abstraction at the C site adjacent to the NH2 functional group (α reaction site) dominates the reactions. Linear Bell-Evans-Polanyi and Bell-Evans correlations are observed for the hydrogen abstractions at the C reaction sites. Furthermore, a scheme to estimate the rate constants for the CnH2n+1-NH2 + HO2 reaction system is presented.

Bioinspired Impact-Resistant and Self-Monitoring Nanofibrous Composites.

Lightweight and impact-resistant materials with self-monitoring capability are highly desired for protective applications, but are challenging to be artificially fabricated. Herein, a scalable-manufactured aramid nanofiber (ANF)-based composite combining these key properties is presented. Inspired by the strengthening and toughening mechanisms relying on recoverable interfaces commonly existing in biological composites, mechanically weak but dense hydrogen bonds are introduced into the ANF interfaces to achieve simultaneously enhanced tensile strength (300 MPa), toughness (55 MJ m-3 ), and impact resistance of the nanofibrous composite. The achieved mechanical property combination displays attractive advantages compared with that of most of previously reported nanocomposites. Additionally, the nanofibrous composite is designed with a capability for real-time self-monitoring of its structural safety during both quasi-static tensile and dynamic impact processes, based on the strain/damage-induced resistance variations of a conductive nanowire network inside it. These comprehensive properties enable the present nanofibrous composite with promising potential for protective applications.

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.

Micromechanical Landscape of Three-Dimensional Disordered Graphene Networks.

Disordered carbons can be considered under the modeling framework of disordered graphene networks (DGNs) due to the continuous three-dimensional connectivity and high graphitization. Correlating microstructures and mechanical behaviors of DGNs to their topology is pivotal to revealing more intrinsic features hidden by disorder. Herein, starting from basic deformations and topology, we investigate DGNs with various densities to explore their micromechanical landscape. Both the tension and shear of DGNs exhibit prolonged plastic platforms through local tearing of microstructures. However, compression displays special plastic damages of forming kinklike puckers and sp3-bonded carbon, resulting in a tension-compression asymmetry of DGNs. Out-of-plane topological defects contribute to the main negative-curvature topology in deformed DGNs. Moreover, there are novel scaling laws where both the Young's modulus and strength (logarithms) follow an inversely proportional scaling with respect to average angular defects. Ashby charts demonstrate that the mechanical properties of DGNs can reach the theoretical limit region, surpassing those of most conventional materials.

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.

Superstrong Noncovalent Interface between Melamine and Graphene Oxide.

There have been growing academic interests in the study of strong organic molecule-graphene [or graphene oxide (GO)] systems, owing to their essential noncovalent nature and the consequent chemomechanical behavior within the interface. A more recent experimental measurement [ Chem 2018, 4, 896-910] reported that the melamine-GO interface exhibits a remarkable noncovalent binding strength up to ∼1 nN, even comparable with typical covalent bonds. But the poor understanding on the complex noncovalent nature in particular makes it challenging to unveil the mystery of this high-performance interface. Herein, we carry out first-principles calculations to investigate the atomistic origin of ultrastrong noncovalent interaction between the melamine molecule and the GO sheet, as well as the chemomechanical synergy in interfacial behavior. The anomalous O-H···N hydrogen bonding, formed between the triazine moiety of melamine and the -OH in GO, is found cooperatively enhanced by the pin-like NH2-π interaction, which is responsible for the strong interface. Following static pulling simulations validates the 1 nN level rupture strength and the contribution of each noncovalent interaction within the interface. Moreover, our results show that the -OH hydrogen bonding will mainly augment the interfacial adhesion strength, whereas the -NH2 group cooperating with the -OH hydrogen bonding and conjugating with the GO surface will greatly improve the interfacial shear performance. Our work deepens the understanding on the chemomechanical behaviors within the noncovalent interface, which is expected to provide new potential strategies in designing high-performance graphene-based artificial nacreous materials.