Molecular Motion Research Articles

Inserting urethane into 1,2,3‐triazole click‐cured solid propellant binder to form microphase separation and enhance mechanical and combustible performances

AbstractHuisgen‐click curing systems are widely used in solid propellant binders due to their low humidity sensitivity and mild reaction conditions. However, the rigid triazole rings from the Huisgen‐click reaction decrease the molecular moving ability and increase decomposition temperature, lowering the mechanical and combustion performance. Thus, we bonded the Huisgen‐click system and polyester binders with urethane, forming microphase separation and improving chemical stability. The results show that the elongation at break, tensile strength, and Young's modulus of the binders increase by 533.3%, 201.2%, and 1263.9%, respectively. Furthermore, the urethane reduces the thermal resistance index of the binders from 191.2 to 179.2°C, which means better combustion performance. Further mechanism study reveals that the urethane leads to microphase separation, which aggregates the hard segments and weakens their restriction effect on the molecular motion of polyether. Such a microphase separation brings comprehensive improvement in the mechanical properties of the solid propellants and probably can be used in other multi‐component systems of energetic materials.

Molecular dynamics simulation of CO2 dissolution-diffusion in multi-component crude oil

In order to study the dissolution-diffusion process and mechanism of CO2 in multi-component crude oil, a model of multi-component crude oil system with octane as the main component and 16 other alkanes as a compound was constructed by using molecular dynamics simulation method. We estimated the CO2 density distribution in crude oil model and the shift in crude oil model volume change. We then investigated the microscopic influence mechanism of CO2 dissolution-diffusion on the volume expansion of crude oil by simulating the action of CO2 dissolution-diffusion in the multi-component crude oil model. Based on the variation law of mean square displacement between crude oil molecules, the dissolution and diffusion coefficients of CO2 were predicted, and the influence of CO2 dissolution-diffusion on crude oil mobility was analyzed. It is found that temperature intensifies the molecular thermal motion and increases the voids between alkane molecules, which promotes the dissolution of CO2 and encourages CO2 molecules to transmit, making the crude oil expand and viscosity decrease, and improving the flow ability of crude oil; with the enhancement of given pressure, the potential energy difference between the inside and outside of the crude oil model becomes larger, and the voids between alkane molecules become larger, which is favorable to the dissolution of CO2. Nevertheless, the action of CO2 molecules’ diffusing in the crude oil sample is significantly limited or even tends to zero, besides, the mobility of crude oil is affected due to the advance of external pressure. The mechanism of CO2 dissolution and diffusion in multi-component crude oil is revealed at the microscopic level, and provides theoretical guidance for the development of CO2 flooding.

Topochemical Syntheses of Polyarylopeptides Involving Large Molecular Motions: Frustrated Monomer Packing Leads to the Formation of Polymer Blends.

We report the topochemical syntheses of three polyarylopeptides, wherein triazolylphenyl group is integrated into the backbone of peptide chains. We synthesized three different monomers having azide and arylacetylene as end-groups from glycine, L-alanine and L-valine. We crystallized these monomers and the crystal structures of two of them were determined by single-crystal X-ray diffractometry. Due to the steric constraints, both of these monomers crystallized with two molecules, viz. conformers A and B, in the asymmetric unit. Consistently, in both cases, the A-conformers are antiparallelly π-stacked and B-conformers are parallelly slip-stacked, exploiting weak interactions. Though the arrangements of molecules in the pristine crystals were unsuitable for topochemical reaction, upon heating, they undergo large motion inside the crystal lattice to reach a transient reactive orientation and thereby the self-sorted conformer stacks react to give a blend of triazole-linked polyarylopeptides having two different linkages. Due to the large molecular motion inside crystals, the product phase loses its crystallinity.

Photo-excited charge carrier lifetime enhanced by slow cation molecular dynamics in lead iodide perovskite FAPbI3

Using muon spin relaxation measurements on formamidinium lead iodide [FAPbI3, where FA denotes HC(NH2)2], we show that, among the five structurally distinct phases of FAPbI3 exhibited through two different temperature hysteresis, the reorientation motion of FA molecules is quasi-static below ≈50 K over the time scale of 10−6 s in the low-temperature (LT) hexagonal (Hex-LT, <160 K) phase, which has a relatively longer photo-excited charge carrier lifetime (τc∼10−6 s). In contrast, a sharp increase in the FA molecular motion was found above ≈50 K in the Hex-LT phase, LT-tetragonal phase (Tet-LT, <140 K), the high-temperature (HT) hexagonal phase (Hex-HT, 160–380 K), and the HT-tetragonal phase (Tet-HT, 140–280 K), where τc decreases with increasing temperature. More interestingly, the reorientation motion is further promoted in the cubic phase at higher temperatures (>380/280 K), while τc is recovered to comparable or larger than that of the LT phases. These results indicate that there are two factors that determine τc, one related to the local reorientation of cationic molecules that is not unencumbered by phonons and the other to the high symmetry of the bulk crystal structure.

Topochemical Syntheses of Polyarylopeptides Involving Large Molecular Motions: Frustrated Monomer Packing Leads to the Formation of Polymer Blends

AbstractWe report the topochemical syntheses of three polyarylopeptides, wherein triazolylphenyl group is integrated into the backbone of peptide chains. We synthesized three different monomers having azide and arylacetylene as end‐groups from glycine, L‐alanine and L‐valine. We crystallized these monomers and the crystal structures of two of them were determined by single‐crystal X‐ray diffractometry. Due to the steric constraints, both of these monomers crystallized with two molecules, viz. conformers A and B, in the asymmetric unit. Consistently, in both cases, the A‐conformers are antiparallelly π‐stacked and B‐conformers are parallelly slip‐stacked, exploiting weak interactions. Though the arrangements of molecules in the pristine crystals were unsuitable for topochemical reaction, upon heating, they undergo large motion inside the crystal lattice to reach a transient reactive orientation and thereby the self‐sorted conformer stacks react to give a blend of triazole‐linked polyarylopeptides having two different linkages. Due to the large molecular motion inside crystals, the product phase loses its crystallinity.

Photochemistry sample sticks for inelastic neutron scattering.

Every material experiences atomic and molecular motions that are generally termed vibrations in gases and liquids or phonons in solid state materials. Optical spectroscopy techniques, such as Raman, infrared absorption spectroscopy, or inelastic neutron scattering (INS), can be used to measure the vibrational/phonon spectrum of ground state materials properties. A variety of optical pump probe spectroscopies enable the measurement of excited states or elucidate photochemical reaction pathways and kinetics. So far, it has not been possible to study photoactive materials or processes in situ using INS due to the mismatch between neutron and photon penetration depths, differences between the flux density of photons and neutrons, cryogenic temperatures for INS measurements, vacuum conditions, and a lack of optical access to the sample space. These experimental hurdles have resulted in very limited photochemistry studies using INS. Here we report on the design of two different photochemistry sample sticks that overcome these experimental hurdles to enable in situ photochemical studies using INS, specifically at the VISION instrument at Oak Ridge National Laboratory. We demonstrate the use of these new measurement capabilities through (1) the in situ photodimerization of anthracene and (2) the in situ photopolymerization of a 405nm photoresin using 405nm excitation as simple test cases. These new measurement apparatus broaden the science enabled by INS to include photoactive materials, optically excited states, and photoinitiated reactions.

Laboratory Study and Molecular Dynamics Simulation of High- and Low-Temperature Properties of Polyurethane-Modified Asphalt

This study investigated the modification mechanism of polyurethane (PU) on the high- and low-temperature properties of asphalt based on experimental and molecular dynamics (MD) simulations. PU-modified asphalt was prepared using an in situ polymerization method. The high- and low-temperature properties of asphalt containing various PU contents were investigated via a series of laboratory tests. The asphalt molecular model, the PU molecular model, and the PU–asphalt mixes system were modeled using software. Based on reasonable models, the influence of PU content on the physical modulus and molecular motion of asphalt at high temperatures was researched using MD simulations. The glass transition temperature of PU-modified asphalt with different contents was obtained using software, and the free volume theory and molecular diffusion movement were used to interpret the improvement mechanism of PU content on the low-temperature performance of asphalt. The results indicate that PU effectively can improve the high- and low-temperature properties of asphalt. PU has a significant positive effect on the physical modulus of asphalt, and the effect becomes more significant with the increase of PU content. In addition, PU can form a tight structure with asphalt and increase the stability of the asphalt system. The free volume ensures the free movement of PU molecules in the asphalt, allowing PU to be dispersed homogeneously in the asphalt. However, the free volume decreases when the PU content exceeds 15% by weight, which limits the improvement of the low-temperature performance of asphalt. The results of molecular simulation can explain the enhancement mechanism of PU, providing a reference for the performance enhancement of PU-modified asphalt.

Magnetic stimulation of chemical reactions in coal

Purpose. To identify a mechanism in terms of which a signal of weak magnetic field is transformed into response of some components of nanostructure of a carbonaceous matter with further formation of chemical bonds. Methodology. Physical and mathematical modelling procedures of elementary chemical acts have been applied. Regularities of quantum mechanics and dynamics of chemical bonds were also used in addition to a magnetic isotope theory, a diffusion theory of recombination of radical pairs taking into consideration triplet-singlet transit, and nuclear-spin selectivity of chemical reactions. Findings. The physical mechanism of the magnetic scenario of interradial reactions is considered from the viewpoint of their energy stimulation through magnetic fields, i.e. the idea has been implemented according to which the number of radical pairs, able to be recombined into stable molecules, increases significantly if the weak magnetic field exercises certain influence. In addition to stimulation of interradial reactions, the magnetic field impact on organic coal mass-radicals results in stabilization of carbonaceous structures with regular atomic arrangement (being two-dimensional matrices, chains etc.) and their increase. Originality. A physical model of structural and phase changes in coal, depending upon the effect by external weak magnetic field, has been developed. It has been shown that the weak magnetic field signal, being incomparably smaller energetically to compare with the energy of thermal molecular motion, can initiate triplet-singlet transitions, i. e. transform radicals into reactive state. A mechanism has been proposed to form chemical bonds between movable radicals and non-complete bonds of atoms at the surfaces of solid phases of carbonaceous matter. Practical value. Implementation of the obtained results, concerning magnetic coal processing, may be connected with the development of a new procedure to avoid explosive conditions in coal seams. Progress in the field of magnetic processing should involve studies concerning calculation and selection of coal processing parameters (i.e. magnetic field density, frequency, energy, and a processing period) to control efficiently the chemical reactions in the carbonaceous matter. A substantiation degree of coal processing will influence greatly both efficiency of chemical processes and expediency of practical use of the results.

Ways to Assess and Regulate the Performance of a Bi-Mechanism-Induced Borneol-Based In Situ Forming Matrix.

As an alternative to the traditional polymeric-based system, it is now possible to use an in situ forming system that is based on small molecules. Borneol was used as matrix formation in this study. While triacetin was incorporated into the formulation for prolonging the drug release. The objective of this study is to understand the initial period of the solvent exchange mechanism at the molecular level, which would provide a basis for explaining the matrix formation and drug release phenomena. The evaluation of basic physical properties, matrix formation, in vitro drug release, and molecular dynamics (MD) simulation of borneol-based in situ forming matrixes (ISM) was conducted in this study. The proportion of triacetin was found to determine the increase in density and viscosity. The density value was found to be related to viscosity which could be used for the purpose of prediction. Slow self-assembly of ISM upon the addition of triacetin was associated with higher viscosity and lower surface tension. This phenomenon enabled the regulation of solvent exchange and led to sustaining the drug release. In MD simulation using AMBER Tools, the free movement of the drug and the rapid approach to equilibrium of both solvent and water molecule in a solvent exchange mechanism in borneol-free ISM was observed, supporting that sustained release would not occur. Water infiltration was slowed down and NMP movement was restricted by the addition of borneol and triacetin. In addition, the increased proportion of triacetin promoted the diminished down of all substances' movement because of the viscosity. The diffusion constant of relevant molecules decreased with the addition of borneol and/or triacetin. Although the addition of triacetin tended to slow down the solvent exchange and molecular movement from computation modelling results, it may not guarantee to imply the best drug release control. The Low triacetin-incorporated (5%) borneol-based ISM showed the highest ability to sustain the drug release due to its self-assembly and has proper solvent exchange. MD simulation addressed the details of the mechanism at the beginning of the process. Therefore, both MD and classical methods contribute to a clearer understanding of solvent exchange from the molecular to macroscopic level and from the first nanosecond of the formulation contact with water to the 10-day of drug release. These would be beneficial for subsequent research and development efforts in small molecule-based in situ forming systems.

A photochemical method to evidence directional molecular motions

Light driven synthetic molecular motors represent crucial building blocks for advanced molecular machines and their applications. A standing challenge is the development of very fast molecular motors able to perform rotations with kHz, MHz or even faster frequencies. Central to this challenge is the direct experimental evidence of directionality because analytical methods able to follow very fast motions rarely deliver precise geometrical insights. Here, a general photochemical method for elucidation of directional motions is presented. In a macrocyclization approach the molecular motor rotations are restricted and forced to proceed in two separate ~180° rotation-photoequilibria. Therefore, all four possible photoinduced rotation steps (clockwise and counterclockwise directions) can be quantified. Comparison of the corresponding quantum yields to the unrestricted motor delivers direct evidence for unidirectionality. This method can be used for any ultrafast molecular motor even in cases where no high energy intermediates are present during the rotation cycle.

Surface jump mechanism of gas molecules in strong adsorption field of coalbed methane reservoirs

The gas in the coalbed methane (CBM) reservoirs is mainly in the adsorbed state, and the influence of surface jump of adsorbed gas on CBM migration and gas well productivity cannot be ignored. In this paper, firstly, the concepts of strong adsorption field and gas molecular equivalent motion volume are proposed, and the analytical relationship between gas filling degree and equivalent molecular free path is established. Secondly, an analytical model of adsorbed gas concentration and equivalent molecular free path is established, and the linkage mechanism of “ gas filling degree - equivalent molecular free path” of gas occurrence in strong adsorption field is revealed. Thirdly, the molecular dynamics mechanism of gas occurrence and surface jump in the strong adsorption field is revealed, and the coupling effect mechanism of “coal rank-pressure” on microscale surface jump is clarified. It is found that (a) the equivalent motion volume of gas molecules can effectively characterize the gas filling degree in the strong adsorption field; (b) when the activation energy is higher than the isosteric heat of the strong adsorption field, gas molecules enter the adsorption/desorption balance layer and transfer to different restricted areas; (c) when the temperature and adsorbate are determined, the coal properties and pressure are the keys to control the surface jump.

High-Quality Reconstruction for Laplace NMR Based on Deep Learning.

Laplace nuclear magnetic resonance (NMR) exploits relaxation and diffusion phenomena to reveal information regarding molecular motions and dynamic interactions, offering chemical resolution not accessible by conventional Fourier NMR. Generally, the applicability of Laplace NMR is subject to the performance of signal processing and reconstruction algorithms involving an ill-posed inverse problem. Here, we propose a proof-of-concept of a deep-learning-based method for rapid and high-quality spectra reconstruction from Laplace NMR experimental data. This reconstruction method is performed based on training on synthetic exponentially decaying data, which avoids a vast amount of practically acquired data and makes it readily suitable for one-dimensional relaxation and diffusion measurements by commercial NMR instruments.

First-Principles Simulations of Tip Enhanced Raman Scattering Reveal Active Role of Substrate on High-Resolution Images.

Tip-enhanced Raman scattering (TERS) has emerged as a powerful tool to obtain subnanometer spatial resolution fingerprints of atomic motion. Theoretical calculations that can simulate the Raman scattering process and provide an unambiguous interpretation of TERS images often rely on crude approximations of the local electric field. In this work, we present a novel and first-principles-based method to compute TERS images by combining Time Dependent Density Functional Theory (TD-DFT) and Density Functional Perturbation Theory (DFPT) to calculate Raman cross sections with realistic local fields. We present TERS results on free-standing benzene and C60 molecules, and on the TCNE molecule adsorbed on Ag(100). We demonstrate that chemical effects on chemisorbed molecules, often ignored in TERS simulations of larger systems, dramatically change the TERS images. This observation calls for the inclusion of chemical effects for predictive theory-experiment comparisons and an understanding of molecular motion at the nanoscale.

Aggregation states, thermal molecular motion and carrier properties in functional polymer thin films

Aggregation states, thermal molecular motion and carrier properties in functional polymer thin films

Improved brightness of multiarylpyrroles in the near-infrared region via coordination-induced supramolecular system

Organic fluorescent dyes that emit in the near-infrared region offer many benefits in biological imaging; nevertheless, their low fluorescence quantum yield remains problematic. In this study, we synthesized an aggregation-induced emission dye from multiarylpyrroles (MAPs) and continuously increased the fluorescence intensity by coordinating the MAPs with tris(pentafluorophenyl)borane (BCF). The coordination between MAPs and BCF improved the fluorescence quantum yields without changing the emission wavelength by increasing the intramolecular interactions and restricting the molecular motion. Moreover, experimental and theoretical calculations verified that the ability to produce reactive oxygen species decreased after coordination, which was beneficial for long-term fluorescence imaging tracking. The coordination between near-infrared dyes and BCF not only provides general guidance for improving the quantum yield but also regulates the charge transfer between the singlet and triplet states, which influences the production of reactive oxygen species.

Highly Efficient C/N‐Fused Architecture for Narrowband Deep‐Blue Thermally Activated Delayed Fluorescence

AbstractEmitters with high color purity and small full width at half maximum (FWHM) have attracted considerable attention in recent years. In this work, a novel narrow‐emitting thermally activated delayed fluorescent (TADF) emitter, p‐FLDID, is developed through C/N fusion. The p‐FLDID is developed through core structure modification instead of peripheral substituent engineering. The design approach activates the TADF pathway and regulates emission in the deep‐blue region. The multiresonance (MR) type p‐FLDID emitter exhibits deep‐blue emission with a peak wavelength of 449 nm, FWHM of 26 nm, Stokes shift of 13 nm, and a photoluminescence quantum yield of ≈97%. The rigid structure of p‐FLDID not only restricts the molecular motion of the conjugated backbone structure but also results in a high horizontal emitting dipole orientation of 91%. The fabricated p‐FLDID‐based organic light‐emitting diode exhibits deep blue emission with an external quantum efficiency of 22.0%, a FWHM of 29 nm, and a CIEy of 0.073. The present work demonstrates that high‐efficiency TADF emission with a high‐purity blue color can be realized via C/N fusion.

Hydraulic properties of bentonite buffer material beyond 100 °C

Bentonite buffer materials are important components of engineered barrier systems for the disposal of high-level radioactive waste produced during nuclear power generation. The design temperature of the buffer material is < 100 °C, and increasing the design temperature can reduce the required disposal area. This characteristic necessitates the evaluation of the thermal–hydraulic–mechanical properties of the buffer at temperatures above 100 °C to increase its target temperature. Therefore, the hydraulic properties of Gyeongju (KJ) bentonite buffer material were evaluated in this study, including the soil–water characteristic curve (SWCC) and hydraulic conductivity. An experimental system was manufactured to measure the suction and saturated hydraulic conductivity of KJ bentonite buffer material above 100 °C; the relative humidity of KJ bentonite buffer material was measured at 25–149 °C with an initial water content of 0, 0.06, and 0.12 under constant saturation conditions. The suction decreased as the temperature increased (10%–25% reduction at 99 °C-149 °C). The Van-Genuchten SWCC fitting parameters were also derived at 25 °C-149 °C using previously reported and newly generated experimental results, and the applicability of the modified Van-Genuchten SWCC model in this temperature range was verified. The hydraulic conductivity was proportional to temperature up to 100 °C, in agreement with the theoretical model results. Between 100 °C and 150 °C, the hydraulic conductivity increased nonlinearly because of molecular motion and structural changes inside the sample.

A highly sensitive fluorescence “on–off–on” sensing platform for captopril detection based on AuNCs@ZIF-8 nanocomposite

Here, a novel fluorescent sensing strategy is established for the detection of captopril (CP) sensitively on the basis of a nanocomposite of gold nanoclusters (AuNCs) and metal−organic framework (AuNCs@ZIF-8). The aggregation-induced emission (AIE) effect will be triggered when AuNCs is encapsulated by metal−organic framework (MOF) which served as a carrier since it limits the molecular motion of AuNCs, and the fluorescence of AuNCs greatly enhanced about 5-time after forming the nanocomposites of AuNCs@ZIF-8. The strong orange-emission at 562 nm was quenched in the presence of mercury ions through dynamic quenching. After adding captopril, the quenched fluorescence of AuNCs@ZIF-8/Hg2+ system would be restored due to the specific interaction among captopril with mercury ions. Simultaneously, the restored degree of AuNCs@ZIF-8/Hg2+ fluorescence depended on the concentration of captopril. Hence, with AuNCs@ZIF-8 serving as reporter signal, the captopril content can be monitored by an “on-off-on” fluorescence sensing mode with a linear relationship of 1–100 μM, and the limit of detection for captopril was 0.134 μM.

Constraining effects on polymer chain relaxation in crosslinked supramolecular dual networks

Polymer networks containing transient physical and permanent chemical cross-links exhibit unique mechanical properties due to the intrinsic reassociating ability of supramolecular functional groups. Similar to supramolecular gels, these networks allow the controlled release of stored energy and can extend the life of polymer networks in practical applications. In this study, we investigated the rheology, dielectric spectroscopy, stress–strain behavior, and dynamic mechanical analysis of networks based on long polybutylene oxide (PBO) chains functionalized with randomly placed thymine (Thy) side groups. A transient network was formed by proportionally mixing this matrix with short non-entangled linear 1,3,5-diaminotriazine (DAT) head–tail modified PBO chains, exploiting the hetero-complementarity of the DAT–Thy triple hydrogen bond. This transient polymer network was further cross-linked to a dual network via a thiol-ene click reaction to form static covalent bonds. In PBO, the similar polarity of the PBO matrix and the DAT–Thy functional groups ensures that the molecular chain motion is not affected by segregation, resulting in a homogeneous polymer phase without microphase-separated functional group domains. Dielectric relaxation spectroscopy was combined with rheology to quantify the relaxation processes of the interconnected polymers and the strength of the DAT–Thy bonding interactions in the melt. The results showed two distinct plateaux in the relaxation modulus due to contributions from hydrogen and permanent bonds. In the case of the dual network, the lifetime of the hydrogen bond was prolonged and higher activation energy was observed due to the physical cross-link preventing the movement of the long chain.

One master and two servants: One Zr(Ⅳ) with two ligands of TCPP and NH2-BDC form the MOF as the electrochemiluminescence emitter for the biosensing application

Here we put forward an innovative “one master and two servants” strategy for enhancing the ECL performance. A novel ECL luminophore named Zr-TCPP/NH2-BDC (TCPP@UiO-66-NH2) was synthesized by self-assembly of meso-tetra(4-carboxyphenyl)porphine (TCPP) and 4-aminobenzoic acid (NH2-BDC) with Zr clusters. TCPP@UiO-66-NH2 has a porous structure and a highly ordered structure, which allows the molecular motion of TCPP to be effectively confined, thereby inhibiting nonradiative energy transfer. Importantly, TCPP@UiO-66-NH2 has a higher and more stable ECL signal. To further improve the sensitivity of the sensor, we use polydopamine-coated manganese dioxide (PDA@MnO2), which has a double quenching effect, as the quencher. The nucleocapsid (N) protein of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2-N) is one of the ideal markers for the early diagnosis of COVID-19, and its sensitivity detection is of great significance for the prevention and treatment of COVID-19. Thus, we constructed a quenching-type ECL sensor for the ultrasensitive detection of the SARS-CoV-2-N. Its linear range is 10 fg/mL∼1 μg/mL and the calculated detection limit is 1.4 fg/mL (S/N = 3). The spiked recoveries are 97.40–103.8%, with the relative standard deviations (RSD) under 3.0%. More importantly, the technique offers a viable way to identify and diagnose viral infections early.