Bond Orientation Research Articles

Understanding creep in vitrimers: Insights from molecular dynamics simulations

Vitrimers offer malleability and recyclability due to covalent adaptable networks, however, they are prone to low-temperature creep. In this work, we investigate the molecular mechanisms of vitrimer creep using molecular dynamics simulations. We model the interplay between dynamic bonding with mechanical loading using a topology-based reaction scheme. The creep behavior is compared against cross-linked epoxies with dynamic reactions to understand the unique aspects related to dynamic bonding. It is found that the free volume that arises from tensile loads is reduced in vitrimers through dynamic bond rearrangement. An important feature that distinguishes the secondary creep behavior between epoxies and vitrimers is the orientation of the dynamic bonds during loading. In vitrimers, the dynamic bonds preferentially align orthogonal to the loading axis, decreasing the axial stiffness during secondary creep, resulting in larger creep strain compared to epoxies. Over longer timescales, such increased strain leads to void growth, resulting in tertiary creep.

Impact of Imine Bond Orientations and Acceptor Groups on Photocatalytic Hydrogen Generation of Donor-Acceptor Covalent Organic Frameworks.

Covalent organic frameworks (COFs) have emerged as one of the most studied photocatalysts owing to their adjustable structure and bandgaps. However, there is limited research on regulating the light-harvesting capabilities of acceptor building blocks in donor-acceptor (D-A) isomer COFs with different bond orientations. This investigation is crucial for elucidating the structure-property-performance relationship of COF photocatalysts. Herein, a series of D-A isostructural COFs are synthesized with different imine bond orientations using benzothiadiazole and its derivatives-based organic building units. Extended light absorption is achieved in COFs with acceptor groups that have strong electron-withdrawing capacities, although this resulted a decreased hydrogen generation efficiency. Photocatalytic experiments indicated that dialdehyde benzothiadiazole-based COFs, HIAM-0015, exhibit the highest hydrogen generation rate (17.99mmol g-1 h-1), which is 15 times higher than its isomer. The excellent photocatalytic performance of HIAM-0015 can be attributed to its fast charge separation and migration. This work provides insights into the rational design and synthesis of D-A COFs to achieve efficient photocatalytic activity.

Electric field modulated configuration and orientation of aqueous molecule chains.

Understanding how external electric fields (EFs) impact the properties of aqueous molecules is crucial for various applications in chemistry, biology, and engineering. In this paper, we present a study utilizing molecular dynamics simulation to explore how direct-current (DC) and alternative-current (AC) EFs affect hydrophobic (n-triacontane) and hydrophilic (PEG-10) oligomer chains. Through a machine learning approach, we extract a 2-dimensional free energy (FE) landscape of these molecules, revealing that electric fields modulate the FE landscape to favor stretched configurations and enhance the alignment of the chain with the electric field. Our observations indicate that DC EFs have a more prominent impact on modulation compared to AC EFs and that EFs have a stronger effect on hydrophobic chains than on hydrophilic oligomers. We analyze the orientation of water dipole moments and hydrogen bonds, finding that EFs align water molecules and induce more directional hydrogen bond networks, forming 1D water structures. This favors the stretched configuration and alignment of the studied oligomers simultaneously, as it minimizes the disruption of 1D structures. This research deepens our understanding of the mechanisms by which electric fields modulate molecular properties and could guide the broader application of EFs to control other aqueous molecules, such as proteins or biomolecules.

The Mechanism of Heterogeneous Ice Nucleation by Fatty Alcohol Monolayers.

Organic ice nucleating substances (INSs) are thought to play an essential role in cloud formation and, hence, precipitation and climate. Organic INSs are an important but poorly understood class of INSs in the atmosphere. To study organic INSs with exposed hydroxylated surfaces, researchers have previously used fatty alcohol monolayers as model systems. For alcohol monolayers, ice nucleation temperatures increase with increasing alkyl chain length and show a high-low oscillation following the number (odd-even) of carbon atoms in the alkyl chains. We employ atomistic models, together with molecular dynamics simulations, to investigate ice nucleation by C20H41OH, C30H61OH, and C31H63OH monolayers. As expected, we find that ice nucleation by alcohol monolayers depends on the lattice match to ice, and a poorer lattice match can at least partially account for the reduced ice nucleation ability of C20H41OH monolayers compared to monolayers of the longer chain alcohols. More interestingly, our simulations identify a limited range of alcohol configurations that readily nucleate ice via the basal plane. For configurations outside this range, ice nucleation did not occur on the time scale of our simulations (i.e., 5000 ns). The configurational feature that crucially influences ice nucleation is the angle between the alcohol C-O bond and the interfacial plane. C-O bonds directed sharply toward or away from the water phase strongly inhibit ice nucleation. In contrast, ice nucleation is easily observed for a relatively narrow band of C-O bond orientations centered about the surface plane. For comparable surface configurations, the ice nucleating abilities of C30H61OH and C31H63OH monolayers are practically identical, but the existence of a narrow band of ice-compatible surface configurations can perhaps explain why odd-chain alcohol monolayers are better INSs than even-chain alcohol monolayers. Earlier simulations have shown that for alcohols differing by a single carbon atom, the odd-chain monolayer is less rigid than the even-chain monolayer. This suggests the possibility that for odd-chain alcohol monolayers, the orientation of the C-O bonds can more easily adjust into the ice-compatible range than their even-chain counterparts, accounting for their enhanced ice nucleating ability.

Orientation Sensitive SEIRA Sensors Based on Single-Walled Carbon Nanotube Near Fields.

Molecular vibrations that bear information about intrinsic properties of chemical compounds are challenging to detect at submonolayer densities. Surface-enhanced infrared absorption (SEIRA) spectroscopy has been proven to be a viable approach to enhance and detect weak vibration signals. Here, we report a SEIRA sensor based on mid-infrared surface plasmon resonances supported by single-walled carbon nanotubes (SWCNTs). Due to the 1D nature of SWCNTs, their plasmon modes are highly polarized with the electromagnetic fields spatially confined to nanometer scales. Leveraging these characteristics of SWCNTs, we observe a polarization selective coupling between their surface plasmons and vibrational modes of chemical bonds introduced onto their surfaces. A maximum modulation of ∼15% to the plasmon resonance peak is obtained for a submonolayer chemical group coverage. These findings suggest that SWCNTs may potentially serve as a highly sensitive SEIRA platform for revealing intricate information about molecular compositions and bond orientations.

Dispersion, Dynamics, and Mechanics of Polymer Nanocomposites Filled with Rigid-Soft Janus Nanoparticles via Molecular Dynamics Simulation.

The introduction of nanoparticles (NPs) presents boundless possibilities for enhancing the performance of polymer nanocomposites (PNCs). Consequently, the design of novel NPs becomes of paramount significance for PNCs. In our study, we employ the dumbbell two-component model of Janus nanoparticles (JNPs) and design rigid-soft JNPs as fillers. Using coarse-grained molecular dynamics simulations, we systematically investigate the dispersion, dynamics, and mechanical properties of these novel PNCs. First, we determine the optimal dispersion conditions by studying rcutoff and εnp. The simulation indicates that when the interaction between polymer chains and JNPs is a repulsive potential, the JNPs tend to aggregate together, forming a cluster with soft NPs inside and rigid NPs outside. Conversely, under attractive interactions, JNPs show superior dispersion uniformity compared to the repulsive system, and as εnp increases, the dispersion improves. Then, the mean square displacement (MSD) indicates that JNPs effectively impede the mobility of polymer chains, with the degree of hindrance increasing as εnp grows; this effect is more pronounced in attractive systems. Comparing JNPs of different particle sizes, we find that smaller JNP systems exhibit higher temperature sensitivity. Furthermore, there exists a critical particle size (Dnp ≈ 5σ) under a constant filling fraction at which the NPs exert the most pronounced restriction effect on the polymer. Next, upon examining the mechanical behavior, we find that the rigid-soft JNPs demonstrate notable elasticity and variability compared to traditional NPs. This observation is confirmed through measurements of the bond orientation and mean square radius of gyration of the soft segments of JNPs. In summary, this research provides a comprehensive understanding of the intricate interplay among various factors, offering valuable insights for optimizing JNP dispersion and enhancing the mechanical properties of PNCs.

Comparison of coarse-grained models for cis-1,4-polyisoprene-graphite system

ABSTRACT Structural and dynamical properties of cis-1,4-polyisoprene-graphite system were compared by four coarse-grained models. Coarse-graining of graphene layers replaced four carbon atoms by a single bead while four mapping schemes were used for the polymer. Pressure-correction was applied to the nonbonded part of the coarse-grained potential among coarse-grained polyisoprene beads. Among structural properties density profiles, bond orientation and conformation tensor as function of distance from the graphite surface were studied. Bulk values were achieved at around 2 nm from the graphite surface. Higher degree of coarse-graining showed comparatively larger fluctuations in data. Among dynamical properties mean square displacement and autocorrelation of end-to-end unit vectors for varied distances from the graphite surface were studied. Dynamics study indicated that pressure-correction method used to optimise pressure of the system resulted in slower dynamics for the higher degree of coarse-graining.

Finite Temperature String with Order Parameter as Collective Variables for Molecular Crystal: A Case of Polymorphic Transformation of TNT under External Electric Field.

An external electric field is an effective tool to induce the polymorphic transformation of molecular crystals, which is important practically in the chemical, material, and energy storage industries. However, the understanding of this mechanism is poor at the molecular level. In this work, two types of order parameters (OPs) were constructed for the molecular crystal based on the intermolecular distance, bond orientation, and molecular orientation. Using the K-means clustering algorithm for the sampling of OPs based on the Euclidean distance and density weight, the polymorphic transformation of TNT was investigated using a finite temperature string (FTS) under external electric fields. The potential of mean force (PMF) was obtained, and the essence of the polymorphic transformation between o-TNT and m-TNT was revealed, which verified the effectiveness of the FTS method based on K-means clustering to OPs. The differences in PMFs between the o-TNT and transition state were decreased under external electric fields in comparison with those in no field. The fields parallel to the c-axis obviously affected the difference in PMF, and the relationship between the changes in PMFs and field strengths was found. Although the external electric field did not promote the convergence, the time of the polymorphic transformation was reduced under the external electric field in comparison to its absence. Moreover, under the external electric field, the polymorphic transformation from o-TNT to m-TNT occurred while that from m-TNT to o-TNT was prevented, which was explained by the dipole moment of molecule, relative permittivity, chemical potential difference, nucleation work and nucleation rate. This confirmed that the polymorphic transformation orientation of the molecular crystal could be controlled by the external electric field. This work provides an effective way to explore the polymorphic transformation of the molecular crystals at a molecular level, and it is useful to control the production process and improve the performance of energetic materials by using the external electric fields.

Bevel-edge epitaxy of ferroelectric rhombohedral boron nitride single crystal.

Within the family of two-dimensional dielectrics, rhombohedral boron nitride (rBN) is considerably promising owing to having not only the superior properties of hexagonal boron nitride1-4-including low permittivity and dissipation, strong electrical insulation, good chemical stability, high thermal conductivity and atomic flatness without dangling bonds-but also useful optical nonlinearity and interfacial ferroelectricity originating from the broken in-plane and out-of-plane centrosymmetry5-23. However, the preparation of large-sized single-crystal rBN layers remains a challenge24-26, owing to the requisite unprecedented growth controls to coordinate the lattice orientation of each layer and the sliding vector of every interface. Here we report a facile methodology using bevel-edge epitaxy to prepare centimetre-sized single-crystal rBN layers with exact interlayer ABC stacking on a vicinal nickel surface. We realized successful accurate fabrication over a single-crystal nickel substrate with bunched step edges of the terrace facet (100) at the bevel facet (110), which simultaneously guided the consistent boron-nitrogen bond orientation in each BN layer and the rhombohedral stacking of BN layers via nucleation near each bevel facet. The pure rhombohedral phase of the as-grown BN layers was verified, and consequently showed robust, homogeneous and switchable ferroelectricity with a high Curie temperature. Our work provides an effective route for accurate stacking-controlled growth of single-crystal two-dimensional layers and presents a foundation for applicable multifunctional devices based on stacked two-dimensional materials.

Manipulating the Properties of Polymer Vitrimer Nanocomposites by Designing Dual Dynamic Covalent Bonds.

Polymer vitrimer is a novel material that contains dynamic covalent bonds (DCBs) allowing it to combine the desirable characteristics of both thermoplastics and thermosets. Similar to the traditional polymer nanocomposites, introducing nanoparticles into polymer vitrimer is also an effective strategy to further enhance its properties. However, a comprehensive understanding of matrix and interfacial bond exchange reactions (BERs) to tailor the properties of polymer vitrimer nanocomposites (PVNs) is still lacking. Herein, we utilized coarse-grained molecular dynamics simulations to investigate model PVNs in which there are two different kinds of DCBs in the vitrimer matrix and at the interface. Our results show that the normalized bond autocorrelation function (Csw) confirms the independence of BERs in the vitrimer matrix and in the interface. By varying the bond swap energy barrier () in the matrix or in the interface , or in both , a maximum mechanical property is observed at the moderate value of , , or. Meanwhile, the effect of on the stress relaxation and the bond orientation as a function of the time under a fixed strain is well probed, which both decay more slowly at greater . We simulated the tension-recovery curve to examine the effect of on the hysteresis loss and permanent deformation of PVNs, finding an optimal value to achieve its minimum energy dissipation and maximum recovery ratio. Lastly, we investigated the efficiency of self-healing by building and removing walls from the system. Interestingly, a maximum self-healing efficiency of the stress-strain behavior is observed at moderate . Overall, this study provides valuable insights into the relationship between the structure and properties of PVNs, offering implications for the manipulation of their mechanical properties and enhancement of their self-healing capabilities.

Study on the Selectivity of Molecular Imprinting Materials Determined through Hydrogen Bonding on Template Molecular Structures of Flavonoids.

Molecular imprinting technology is widely used for the specific identification of compounds, but the selective recognition mechanisms of the same compounds still need to be further studied. Based on differences in hydrogen bond size and orientation, molecularly imprinted polymers (MIPs) were designed to adsorb flavonols with the same parent core and different hydroxyl groups. A surface-imprinted material was designed with silicon dioxide as the carrier, myricetin as the template molecule, and methacrylic acid (MAA) as the functional monomer. Scanning electron microscopy (SEM), Brunauer-Emmett-Teller surface area (BET) analyses, Fourier-transform infrared spectroscopy (FT-IR), and other characterization experiments were carried out. The intrinsic mechanism of the MIPs was also explored. The MIPs showed good adsorption of myricetin and other flavonoids through hydrogen bonding and steric hindrance. The adsorption capacity was 3.12-9.04 mg/g, and the imprinting factor was 1.78-3.37. Flavonoids with different hydroxyl groups in different numbers and directions had different hydrogen bond strengths with functional monomers. R2, R4, and R1 on 2-phenylchromogenone had stronger electronegativity, and the hydroxyl group was also more likely to form and have stronger hydrogen bonds. The hydroxyl negativity and the degree of steric hindrance of flavonoids played a major role in the recognition of molecularly imprinted materials. This study is of great significance for the synthesis of and selection of templates for analogous molecular imprinting materials.

Exploration of Choline Chloride Based Type II and Type III Deep Eutectic Solvents: A Computational and Experimental Approach

AbstractDeep eutectic solvents (DESs) of choline chloride with citric acid (CACC), Maleic acid (MACC) and Magnesium chloride hexahydrate (MAGCC) were synthesized in 1 : 1 molar ratio and investigated experimentally and computationally. For the computational study, density functional theory (DFT) calculation with B3LYP/6‐31 (+, +) G (d,p) level of theory for MACC and CACC and at B3LYP/SDD basis set for MAGCC was employed and the FT‐IR spectroscopy was used for experimental evaluation. For understanding the orientation of bonds and hydrogen bonding present in DESs geometrically optimized structures were used. Through frontier molecular orbital (FMO) theory and HOMO−LUMO values and various quantum chemical parameters, viz., ionization potential (IP), electron affinities (EA), electro negativity (χ), chemical potential (μ), global softness (σ) and hardness (η), electrophilicity index (ω) were calculated for all DESs. These computational values made it clear that MAGCC is the most stable among the three DESs due to the high value of global hardness (η) (3.291 eV) and HOMO−LUMO energy gap (Egap) (6.582 eV) and lowest value of softness (σ) (0.152 eV), CACC shows the moderate values (η=2.452, Egap=4.905 and σ=0.204 eV). While MACC is found to be the least stable and most reactive among three DESs with values of η, Egap and σ, 1.970, 3.940 and 0.254 eV, respectively. Furthermore, MESP analysis gives an idea about the electrophilic and nucleophilic surface region of DESs. This study will give valuable information for a better understanding of DESs and the applicability of computational calculations in the future.

Quantum dipole interactions and transient hydrogen bond orientation order in cells, cellular membranes and myelin sheath: Implications for MRI signal relaxation, anisotropy, and T1 magnetic field dependence.

Despite significant impact on the study of human brain, MRI lacks a theory of signal formation that integrates quantum interactions involving proton dipoles (a primary MRI signal source) with brain intricate cellular environment. The purpose of the present study is developing such a theory. We introduce the Transient Hydrogen Bond (THB) model, where THB-mediated quantum dipole interactions between water and protons of hydrophilic heads of amphipathic biomolecules forming cells, cellular membranes and myelin sheath serve as a major source of MR signal relaxation. The THB theory predicts the existence of a hydrogen-bond-driven structural order of dipole-dipole connections within THBs as a primary factor for the anisotropy observed in MRI signal relaxation. We have also demonstrated that the conventional Lorentzian spectral density function decreases too fast at high frequencies to adequately capture the field dependence of brain MRI signal relaxation. To bridge this gap, we introduced a stretched spectral density function that surpasses the limitations of Lorentzian dispersion. In human brain, our findings reveal that at any time point only about 4% to 7% of water protons are engaged in quantum encounters within THBs. These ultra-short (2 to 3 ns), but frequent quantum spin exchanges lead to gradual recovery of magnetization toward thermodynamic equilibrium, that is, relaxation of MRI signal. By incorporating quantum proton interactions involved in brain imaging, the THB approach introduces new insights on the complex relationship between brain tissue cellular structure and MRI measurements, thus offering a promising new tool for better understanding of brain microstructure in health and disease.

Local Structure of Nonuniform Fluid Mixtures Containing Associating and Chainlike Molecules from a Multilayer Quasichemical Model.

In this work, we develop a theory for predicting details of the local structure in nonuniform multicomponent fluids that may contain chainlike and associating components. This theory is an extension─to the fluid interfaces and mesoscopic structures of different geometry─of the multilayer quasichemical model originally proposed by Smirnova to describe liquid solution in the vicinity of a planar solid wall. The basis of the theory is the "cut-and-bond" approach, much in spirit of SAFT, where an infinite attraction between the separated monomeric units of a chainlike molecule mimics the chemical bonds of the chain. We describe the equilibrium structure of the mixture, including the spatial distribution of the monomeric units and the local orientation of the chemical bonds in chainlike molecules, and discuss the contribution of chemical bonds to the local chemical potential in a nonuniform fluid. To test the new theory, we apply it to mixtures containing combinations of model components: a strongly associating solvent, an inert substance of varying chain length, and a chainlike amphiphile. To compare predictions from the multilayer model with the results of continuous description of nonuniform fluids, we also address the square-gradient theory and derive an analytical expression for the influence parameter that takes into account pair correlations in the quasichemical approximation. The multilayer quasichemical model developed in this work predicts formation of aggregates in liquid solution and describes the local structure of the interfaces between the coexisting liquid phases in the mixture. Our theoretical predictions agree on a qualitative level with the accumulated knowledge about the structure of different types of systems studied in this work.

Inducing local charge polarization by constructing isomeric covalent organic frameworks with different orientations of imine bonds for enhancing photocatalytic hydrogen evolution

The N-atom orientation of imine bonds towards the acceptor in COFs may induce local charge polarization and delocalization, which could fundamentally improve the exciton dissociation and charge separation, thus achieving the promoted photocatalytic H2 evolution.

Impact of imine bond orientations on photocatalytic hydrogen generation of benzothiadiazole-based covalent organic frameworks constructed using “two-in-one” monomers

Two benzothiadiazole-based COFs, constructed using “two-in-one” monomers, exhibit significantly different photocatalytic hydrogen generation performances due to their distinct imine bond orientations.

W2AlC: A new layered MAX phase to adjust the balance between strength and ductility

To adjust the balance between the strength and ductility of high-temperature material, we apply the first-principles calculations to explore the structural feature, elastic modulus and brittle or ductile behavior of M2AlC (M = Mo, Cr and W) layered structure MAX phase. In addition, the thermodynamic properties of M2AlC carbides are also discussed. The calculated results show that two novel M2AlC carbides: Mo2AlC and W2AlC are predicted. For M2AlC carbide, the M − C bond in layered structure plays an important role in strength and ductility. In particular, the W2AlC exhibits better ductility in while has high elastic modulus. Naturally, the strength and ductility of W2AlC are related to the bond strength and bond orientation of W–C bond in (W–C)–Al-(W–C) layered structure. The weak bond strength of W–C bond in shear direction improves the slip and then improves the ductility of W2AlC carbide with high strength. In addition, the calculated Debye temperature follows the order of Cr2AlC > Mo2AlC ≈ W2AlC. Therefore, we believe that W2AlC carbide with (W–C)–Al-(W–C) layered structure can optimize the balance between the strength and ductility of this M2AlC MAX phase.

CO2 Stress-Driven Room Temperature Ferromagnetism of Ultrathin 2D Gallium Oxide.

Spintronic devices work by manipulating the spin of electrons other than charge transfer, which is of revolutionary significance and can largely reduce energy consumption in the future. Herein, ultrathin two-dimensional (2D) non-van der Waals (non-vdW) γ-Ga2O3 with room temperature ferromagnetism is successfully obtained by using supercritical CO2 (SC CO2). The stress effect of SC CO2 under different pressures selectively modulates the orientation and strength of covalent bonds, leading to the change of atomic structure including lattice expansion, introduction of O vacancy, and transition of Ga-O coordination (GaO4 and GaO6). Magnetic measurements show that pristine γ-Ga2O3 is nonferromagnetic, whereas the SC CO2 treated γ-Ga2O3 exhibits obvious ferromagnetic behavior with an optimal magnetization of 0.025emug-1 and a Curie temperature of 300 K.

Understanding silica aerogel-carbon nanotube composite structures by molecular simulation

Silica aerogel-carbon nanotube composites are remarkable materials but with engineering principles poorly understood, particularly with regards to the structure of the silica aerogel at the interface with the carbon nanotubes (CNTs), and the physical phenomena behind it. This work provides valuable insights on this topic through classical molecular dynamics simulations. The effects on aggregation and adsorption on nanotubes caused by different types of silica primary particles, oxidation degree of the CNTs and solvent composition were explored. Silica monomers interact appreciably only with oxidized nanotubes via H-bonds with O-containing groups, whereas condensed primary particles adsorb almost completely to the surface due to siloxane-aromatic C van der Waals interactions. The adsorbed layer is highly structured owing to the preferential orientation of siloxane bonds tangentially to the CNT surface and formation of intralayer H-bonds.

Layering of hydroxyl groups causes anisotropic molecular clustering in liquid ethanol on hydrophobic surfaces: A molecular dynamics study

The clustering of hydrogen-bonding molecules in liquid is ubiquitous and closely related to the various properties of the liquid. At the solid–liquid interface, the influence of solid surfaces on the interfacial structure of hydrogen-bonding clusters, which has remained underexplored, is key to controlling the interfacial properties of the liquids. In this study, we performed MD simulations to reveal the interfacial structures of ethanol clusters present on hydrophobic solid surfaces. The number density profile of the hydroxyl groups indicated the presence of a layered structure, and the orientation of the hydrogen bonds (HBs) differed between the layers. The ethanol molecules preferred the lateral HBs in the layers enriched with hydroxyl groups, whereas the perpendicular HBs were dominant in the interlayer regions with fewer hydroxyl groups. Analysis of the aspect ratio of the clusters revealed that the hydrophobic surface induced the formation of longer interfacial ethanol clusters in the lateral direction, even when the hydroxyl groups of the ethanol molecules were oriented perpendicular to the surface. The lateral HBs in the enriched regions connected with each other, whereas the perpendicular ones in the interlayer regions did not, resulting in the lateral clustering of the ethanol molecules.