Molecular dynamics simulation insight into the temperature dependence and healing mechanism of an intrinsic self-healing polyurethane elastomer.

An intrinsic self-healing polyurethane (PU) elastomer was synthesized in our previous work. In this work, three-dimensional (3D) micro-crack models based on experimental samples were further introduced to investigate their self-healing behavior, mechanism, and temperature dependence by molecular dynamics (MD) simulations. In particular, the number, type, strength, and lifetime of hydrogen bonds as well as the microscopic behavior of molecular diffusion in the self-healing process were investigated. It was found that the self-healing capacity of PU mainly results from intermolecular electrostatic interactions, and the hydrogen bond plays a key role in electrostatic interactions. There is an optimum ratio of soft and hard segments at which the number of hydrogen bonds is appropriate and the self-healing capacity is optimum. Besides, the temperature has an optimal value at which the self-healing rate of PU is the fastest. The exchanges of hydrogen bonds, which endowed PU with self-healing capacity, were further revealed intuitively. We found that the exchanges of hydrogen bonds are reversible and more likely to occur on the urethane groups. This study deepened the understanding of the self-healing character of PU at the molecular level.

Understanding Mechanism of Adsorption in the Decolorization of Aqueous Methyl Violet (6B) Solution by Okra Polysaccharides: Experiment and Theory.

Optimal conditions for ultrasonic-assisted extraction of polysaccharide from Chinese okra were found using response surface methodology. The okra polysaccharide (OPS) was used for the adsorption of methyl violet 6B (MV). Conditions for maximal adsorption efficiency of MV were established. The mechanism of MV adsorption was investigated by the characterization and physicochemical analysis of OPS before and after the adsorption of MV. Both infrared (IR) analysis and molecular dynamics (MD) simulation suggest that MV adsorption by OPS was an electrostatic interaction between MV and oxygen-containing groups of OPS. Further, the results of first-principles calculation were in agreement with IR spectroscopy measurements and MD simulation, which were all consistent with the suggested adsorption mechanism. Optimization of okra extraction conditions, maximized efficiency of MV adsorption by OPS, and the understanding of the adsorption mechanism are the highlights of this work, providing a reference for promising applications of OPS in the treatment of wastewater in textile, paper, and other industries.

Unveiling the Origin of the High Catalytic Activity of Ultrathin 1T/2H MoSe2 Nanosheets for the Hydrogen Evolution Reaction: A Combined Experimental and Theoretical Study.

2 D transition metal dichalcogenide materials with layered nanostructures and specific phases usually exhibit excellent catalytic activities for the hydrogen evolution reaction (HER). A facile solvothermal process was used to prepare ultrathin noble-metal-free 2 D biphasic MoSe2 nanosheets composed of a metastable metallic 1T phase and a semiconducting 2H phase. High metallic 1T phase content and few-layer-thick MoSe2 nanosheets were obtained by tuning the amount of NaBH4 used in the reaction. The optimal integration of a metallic 1T phase and an environmentally stable 2H phase in MoSe2 electrocatalysts provides abundant active sites and good conductivity beneficial for the HER. The combination of experimental results and DFT calculations implies that the electrocatalytic activity for the HER on the MoSe2 surface goes through a collaborative Heyrovsky and Volmer reaction process. The theoretical studies suggest that the presence of 1T-MoSe2 could reduce the band energy relative to 2H-MoSe2 and, consequently, accelerate the sluggish HER kinetics of 2H-MoSe2 . This work provides valuable and novel insights into the understanding of the structure-activity relationships in 2 D transition metal dichalcogenide electrocatalysts.

Evidence of Spin Frustration in a Vanadium Diselenide Monolayer Magnet.

Monolayer VSe2 , featuring both charge density wave and magnetism phenomena, represents a unique van der Waals magnet in the family of metallic 2D transition-metal dichalcogenides (2D-TMDs). Herein, by means of in situ microscopy and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X-ray and angle-resolved photoemission, and X-ray absorption, direct spectroscopic signatures are established, that identify the metallic 1T-phase and vanadium 3d1 electronic configuration in monolayer VSe2 grown on graphite by molecular-beam epitaxy. Element-specific X-ray magnetic circular dichroism, complemented with magnetic susceptibility measurements, further reveals monolayer VSe2 as a frustrated magnet, with its spins exhibiting subtle correlations, albeit in the absence of a long-range magnetic order down to 2 K and up to a 7 T magnetic field. This observation is attributed to the relative stability of the ferromagnetic and antiferromagnetic ground states, arising from its atomic-scale structural features, such as rotational disorders and edges. The results of this study extend the current understanding of metallic 2D-TMDs in the search for exotic low-dimensional quantum phenomena, and stimulate further theoretical and experimental studies on van der Waals monolayer magnets.

Between scylla and charybdis: hydrophobic graphene-guided water diffusion on hydrophilic substrates.

The structure of water confined in nanometer-sized cavities is important because, at this scale, a large fraction of hydrogen bonds can be perturbed by interaction with the confining walls. Unusual fluidity properties can thus be expected in the narrow pores, leading to new phenomena like the enhanced fluidity reported in carbon nanotubes. Crystalline mica and amorphous silicon dioxide are hydrophilic substrates that strongly adsorb water. Graphene, on the other hand, interacts weakly with water. This presents the question as to what determines the structure and diffusivity of water when intercalated between hydrophilic substrates and hydrophobic graphene. Using atomic force microscopy, we have found that while the hydrophilic substrates determine the structure of water near its surface, graphene guides its diffusion, favouring growth of intercalated water domains along the C-C bond zigzag direction. Molecular dynamics and density functional calculations are provided to help understand the highly anisotropic water stripe patterns observed.