Force Field Molecular Dynamics Research Articles

Reactive force-field MD simulation on the pyrolysis process of phenolic with various cross-linked and branched structures

Based on the reactive force-field (ReaxFF) molecular dynamics, the thermostability of cross-linked and non-cross-linked phenolic was compared to study the effect of cross-linked and branched structures on the pyrolysis mechanism. The results show that during the pyrolysis process, phenolic tends to break into low-boiling molecules with less than 4 carbon atoms and polymerizes into macromolecules with more than 1500 carbon atoms. The results of residual carbon rate and products distribution confirm that carbon skeleton of phenolic resin with high degree of crosslinking and branching is better preserved and has better thermostability. The reaction kinetics path of phenolic pyrolysis was reasonably summarized, revealing the generation mechanism of low-boiling molecules with less than 4 carbon atoms and large carbon clusters, which is in good agreement with the experimental results of activation energy and chemical bond energy. The reaction kinetics data obtained in this work provide a reliable basis for multi-scale reaction simulation.

Comparing oxidation of aluminum by oxygen and ozone using reactive force field molecular dynamics simulations

Ozone has attracted more scholars’ attention due to its less environmental injury during oxidation. Oxidation with ozone has been more regarded because ozone evaluates oxygen and can form a terrific oxide layer even at lower temperatures compared to oxygen. This work investigated the Al(100) surface oxidation simulations by O2 and O3 molecules at 400, 600, and 800 K temperatures with the reactive force field (ReaxFF) method. In separate simulation boxes, one hundred ozone molecules and 150 oxygen molecules have been placed in a space above the Al(100) surfaces to examine this metal’s behavior with them (gases incipient density of 0.47 g/cm3). We found further growth of the oxide layer in the case of ozone-aluminum. Subsequently, we surveyed the correlation between temperatures and the development of alumina layers in the case of oxygen and ozone in separate sets through ReaxFF simulation to untangle the mechanism of oxidation kinetics of the Al(100) surface by ozone. Even though there are demonstrated constraints on growing an oxide layer with ozone and oxygen, it is possible to produce a thicker oxide layer at lower temperatures using ozone. Here, we used LAMMPS as our program package for all simulations and used reactive molecular dynamics simulation, which is a relatively inexpensive tool for studying complex conditions that are impossible to reproduce with other techniques.

Reactive behaviors and mechanisms of cellulose in chemical looping combustions with iron-based oxygen carriers: An experimental combined with ReaxFF MD study

Biomass energy is one of the important and feasible renewable energy resources. Chemical looping combustion (CLC) can meet the requirements of efficient CO2 capture with low energy consumption. The combination of the two factors is intuitive and important for realizing carbon neutral energy processing and conversion. Cellulose is the component with the highest content in biomass. However, there is still a lack of clear understanding of the thermal conversion behavior and microscopic reaction mechanism of cellulose in CLC. In this work, the behaviors and mechanisms of cellulose in CLC with iron-based OCs were studied systematically based on the comprehensive technics of thermo-gravimetric analysis (TGA) experiments, kinetic models and molecular dynamic simulations. The reaction behaviors, kinetic mechanism, and complex reactive networks of cellulose under complex environmental interface conditions of CLC were revealed. Three different stages were revealed for thermal weight loss and reaction behavior of cellulose in CLC via TGA. The effects of different conversion rates and reaction stages on activation energy during CLC were described based on two model-free integration methods. The dynamic evolution of the CLC process was discussed by ReaxFF MD (reactive force field molecular dynamics) simulations. A complex reaction network of cellulose depolymerization and CO2 release in CLC was obtained.

Decoupling effects of C3H3/C4H5/i-C4H5/CN radicals on the formation and growth of aromatics: A ReaxFF molecular dynamics study

This paper demonstrates the effects of C3H3/C4H5/i-C4H5/CN radicals on the formation and growth of aromatic hydrocarbons, particular types of polycyclic aromatic hydrocarbons (PAHs) such as heavier-PAHs, cyclopentafused PAHs (CP-PAHs), through reactive force field molecular dynamics simulations under 2250K and 2500K. Adding non-acetylene radicals enhanced the early-stage mass growth of aromatics to varying degrees. However, the later-stage mass growth and aromaticity degree were weakened by adding radicals except for the C3H3 radical. The reactivity of acetylene and additional radicals showed that the additional C3H3 radicals accelerated the consumption of radical pools, and the interaction reaction between C3H3 was responsible for the consumption of radical pools for the C3H3 additive case. The consumption of i-C4H5 was significantly faster than that of C4H5, whereas the consumption of acetylene in the two simulation cases was essentially the same. Thus, the i-C4H5 simulation case formed more intermediate species C6H7 than C4H5 radical. As for the formation of less-studied PAHs, i-C4H5 radicals generated more heavier PAHs and CP-PAHs than straight butyl C4H5 isomers. Finally, the effect of nitrogen-containing radical CN, which normally resides in ammonia fuel, was analyzed. Several factors explained the stronger enhancement in aromatic mass growth, but the relatively lowest aromatization degree resulted from the additional CN radicals. The reactions between CN and acetylene formed carbon–nitrogen species that participated less in the formation and growth of aromatics. The potential reactions between CN radical and aromatics led to ring-opening reactions and the formation of nitrogen-membered rings, which accelerated the mass growth of carbon clusters but inhibited the development of aromaticity degree. The least amount of CP-PAHs was found in CN cases compared with hydrocarbon radical cases.

Oxidative aging mechanism of asphalt binder using experiment-derived average molecular model and ReaxFF molecular dynamics simulation

Oxidative aging in asphalt binders has significant negative impacts on asphalt pavement performance. This study investigated oxidative aging of virgin asphalt binder based on average molecular model through Reactive Force Field (ReaxFF) molecular dynamics simulation, which allows for continuous bond breakage and formation. First, experimental characterizations were used to identify the average molecular model of virgin asphalt. Subsequently, using this model, individual and bulk asphalt molecules were subjected to oxidative aging simulations through ReaxFF molecular dynamics. Moreover, the simulated results were validated by structural parameters and physical properties, including carbonyl index, sulfoxide index, density, glass transition temperature and shear viscosity. Results showed that the proposed average molecular model could characterize the structural changes and physical properties of asphalt binders well. The number and specific locations of sulfoxide groups and ketone groups in each SARA fraction were identified. Sulfoxide groups formed earlier than ketone groups. Oxidative aging reaction can be accelerated by increasing the temperature. Temperature had greater influence on the generation of ketone groups than sulfoxide groups. Initial hydroxyl groups on aromatic rings would be oxidized to ketone groups first, followed by hydroxyl groups on benzyl carbons. Not all benzyl carbons were susceptible to oxidation, and some may not be oxidized to ketone groups. Hydroxyl group was found to be the intermediate oxidation product before the formation of ketone group, and it can be used to establish the short-term aged asphalt model.

Scalable Bayesian Uncertainty Quantification for Neural Network Potentials: Promise and Pitfalls.

Neural network (NN) potentials promise highly accurate molecular dynamics (MD) simulations within the computational complexity of classical MD force fields. However, when applied outside their training domain, NN potential predictions can be inaccurate, increasing the need for Uncertainty Quantification (UQ). Bayesian modeling provides the mathematical framework for UQ, but classical Bayesian methods based on Markov chain Monte Carlo (MCMC) are computationally intractable for NN potentials. By training graph NN potentials for coarse-grained systems of liquid water and alanine dipeptide, we demonstrate here that scalable Bayesian UQ via stochastic gradient MCMC (SG-MCMC) yields reliable uncertainty estimates for MD observables. We show that cold posteriors can reduce the required training data size and that for reliable UQ, multiple Markov chains are needed. Additionally, we find that SG-MCMC and the Deep Ensemble method achieve comparable results, despite shorter training and less hyperparameter tuning of the latter. We show that both methods can capture aleatoric and epistemic uncertainty reliably, but not systematic uncertainty, which needs to be minimized by adequate modeling to obtain accurate credible intervals for MD observables. Our results represent a step toward accurate UQ that is of vital importance for trustworthy NN potential-based MD simulations required for decision-making in practice.

Study on surface thermal oxidation of silicon carbide irradiated by pulsed laser using reactive molecular dynamics.

Pulsed lasers are a powerful tool for fabricating silicon carbide (SiC) that has a hard and brittle nature, but oxidation is usually unavoidable. This study presents an exploration of the oxidation mechanism of 4H-SiC in oxygen and water under different temperatures via reactive force field molecular dynamics. Single pulse irradiation experiments were conducted to study the oxygen content of the laser-affected zone through energy dispersive x-ray spectrometry. The results show that laser-induced thermal oxidation is a complex dynamic process with the interactions among H, C, O, and Si atoms. The oxidation zone includes an oxide layer, a graphite layer, and a C-rich layer. With an increase in oxygen concentration, the amorphous oxide layer changes from silicon oxide to silicon dioxide. In addition, the formation of carbon clusters at the interface between SiOx and C-rich layers promotes the desorption of the oxide layer. The mechanism revealed in this study provides theoretical guidance for high-quality processing of 4H-SiC at atomic and close-to-atomic scales.

Impact of oxygen and nitrogen-containing species on performance of NO removal by coal pyrolysis gas

Coal pyrolysis gas is considered a promising reburn fuel with excellent NO reduction performance because of the present of nitrogen-containing species (HCN and NH3) in the pyrolysis gas. In this study, we explored the effects of oxygen and nitrogen-containing species on NO removal performance with HCN and NH3 by reactive force field (ReaxFF) molecular dynamics (MD) simulations. Results indicate that appropriately reducing O2 concentrations and increasing the amount of nitrogen-containing species can benefit the NO reduction performance by coal pyrolysis gas. In addition, the effects of oxygen and nitrogen-containing species content on the NO removal and mechanisms of NO consumption and N2 formation are illustrated during NO reduction with HCN and NH3, respectively. Finally, based on the simulations results, practical operating strategies are proposed to optimize the NO reduction efficiency. In summary, this study provides new insights into NO reduction performance, which may contribute to optimizing the operating parameters to decrease NOx emissions during coal combustion.

Effect of oxidation on mechanical properties of copper nanowire: A ReaxFF (reactive force field) molecular dynamics study

Nanostructures with high surface area to volume ratio, such as oxidized and coated Cu nanowires (NWs), exhibit unique mechanical properties due to their size and surface effects. Understanding the complex oxidation process of Cu NWs at nanoscale and quantifying its resulting effects on mechanical behavior and properties are significantly essential for effective usage of Cu NW devices in a wide range of applications in nanoelectronics. Here, we perform molecular dynamics simulations using ReaxFF (reactive force field) to investigate the oxidation process and mechanisms of [001]-oriented cylindrical Cu NWs and its contribution on the mechanical deformation behavior and material properties as a function of NW sizes. The relatively thin oxide CuxOy layer is formed on the surface of Cu NWs in an O2 environment, creating a core/shell (Cu/CuxOy) NW structure that played a key role in governing the overall tensile mechanical deformation behavior and properties of Cu NW. The formation of oxide layer effects, including the resulting interface and defects, leads to a reduction in the initial dislocation nucleation barrier, which facilitates the onset of plasticity and stress relaxation, ultimately resulting in a negative impact on the tensile strength, Young's modulus, yield stress and strain, and flow stress when compared to pristine counterparts. It is worth noting that the tensile mechanical response and properties of the Cu NWs are highly dependent on the pre-existing oxide shell layer associated with the size of NW, determining the overall mechanical performance and properties of Cu NWs.

Understanding SiOC atomic structures via synchrotron X-ray and reactive force field potential studies

Silicon oxycarbide (SiOC) is a unique system that can generate various compositions and microstructures via different polymeric precursors and pyrolysis conditions. However, understanding of the atomic structure evolution, such as cluster formation, phase evolution, and atomic coordination, is lacking. In this study, cluster evolution and atomic coordination for different SiOC ceramics pyrolyzed at 1200 °C and 1500 °C were investigated by high energy X-ray diffraction (HE-XRD) and radial distribution function (RDF) analysis. A new Reactive Force Field (ReaxFF) molecular dynamics modeling approach was developed to understand nanocluster separation and early-stage atomic radial coordination. For the first time, we demonstrate that orthorhombic SiO2 forms and converts to growth resistant amorphous SiO2. Cubic β-SiC also forms at lower temperatures than reported, along with a minor amount of hexagonal SiC that has never been reported. The RDF data support such phase evolution understanding. ReaxFF simulation provides direct data on atomic mixing, elemental separation, and effects of precursor molecular structures and compositions, especially in the early stage of the pyrolysis. The simulated RDF data complement the experimental data, revealing the significant presence of C–H bonds along with Si–O and C–C bonds.

Extension of the TraPPE Force Field for Battery Electrolyte Solvents.

Optimizing electrolyte formulations is key to improving performance of Li-/Na-ion batteries, where transport properties (diffusion coefficient, viscosity) and permittivity need to be predicted as functions of temperature, salt concentration and solvent composition. More efficient and reliable simulation models are urgently needed, owing to the high cost of experimental methods and the lack of united-atom molecular dynamics force fields validated for electrolyte solvents. Here the computationally efficient TraPPE united-atom force field is extended to be compatible with carbonate solvents, optimizing the charges and dihedral potential. Computing the properties of electrolyte solvents, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and dimethoxyethane (DME), we observe that the average absolute errors in the density, self-diffusion coefficient, permittivity, viscosity, and surface tension are approximately 15% of the corresponding experimental values. Results compare favorably to all-atom CHARMM and OPLS-AA force fields, offering computational performance improvement of at least 80%. We further use TraPPE to predict the structure and properties of LiPF6 salt in these solvents and their mixtures. EC and PC form complete solvation shells around Li+ ions, while the salt in DMC forms chain-like structures. In the poorest solvent, DME, LiPF6 forms globular clusters despite DME's higher permittivity than DMC.

Revealing reactive mechanism and nitrogen transformation of HSW coal combustions at molecule and particle scales

Understanding reactive mechanism of coal thermochemical conversion was the scientific fundamental for coal high-efficient utilization. However, the evolution of coal macromolecular structure, reactants and products remained unclear, in particularly at particle and molecular scales. The complex reaction networks for transformation of pollution elements were also not established. This work uncovered reactive mechanism and nitrogen transformation of HSW coal combustion at microscopic scales via reactive force field molecular dynamics method. The effects of chemical equivalent and combustion temperature were investigated to explore coal structural evolution, combustion reactants and products during reactive process. It was found that the fracture change of coal structure was observable with the continuous reaction. Based on the analysis of nitrogen-containing molecules in coal combustion, it was confirmed that HCN was an important nitrogen-containing intermediate. Further, the transformation networks of organic nitrogen in combustions were established, and the pathway for formation of HCN, NO and NO2 was obtained.

Effect of Diffusion Constraints and ZnOx Speciation on Nonoxidative Dehydrogenation of Propane and Isobutane over ZnO-Containing Catalysts

Heterogeneously catalyzed gas–solid-phase reactions generally suffered from diffusion limitations in large-scale processes or in academic studies when zeolites were used as catalysts or supports. Here, we elucidated the effects of diffusion of reactants/products in nonoxidative propane (PDH) and isobutane dehydrogenation (iBDH) reactions on the performance of catalysts possessing differently structured ZnOx species on (S-1), dealuminated beta (deAl beta), and ZrO2. The catalysts were prepared through physically mixing ZnO and the support. Force-field molecular dynamics simulations revealed that the effectiveness factor η is larger than 0.99 in the PDH reaction over all catalysts and in the iBDH reaction over the ZnO-deAl beta catalyst, thus suggesting that mass transport limitations do not play any significant role. However, the iBDH reaction over S-1-based catalysts suffers from some diffusion limitations (0.35 < η < 0.9). Such conditions are favorable for cracking reactions responsible for isobutene selectivity loss. To compare intrinsic catalyst activity in the PDH and iBDH reactions over the ZnOx/S-1 catalyst, molecular-level insights into individual reaction pathways were derived from density functional theory calculations. The nature of active ZnOx sites was investigated by X-ray absorption spectroscopy and was established to depend on the kind of support material. Binuclear ZnOx species are formed inside small S-1 pores or on the surface of ZrO2, while three-dimensional multinuclear ZnOx clusters are generated in the β zeolite with larger pores. The latter show higher Zn-related activity in the PDH reaction under conditions free of any diffusion constraints. The developed ZnO–deAl beta showed the space–time yield of propene or isobutene formation of 2 kgC3H6 kgcat–1 h–1 or 6.3 kgi-C4H8 kgcat–1 h–1 at 550 °C and about 70–80% equilibrium alkane conversion with an olefin selectivity of about 90%. The activity values are higher than those reported for the state-of-the-art non-noble metal oxide catalysts tested at the same or even higher temperatures.

Thermal decomposition, flame propagation, and combustion reactions behaviours of stearic acid by experiments and molecular dynamic simulation

To better look into the thermal decomposition and combustion progress of stearic acid (C18H36O2), the experimental tests and Reaction Force Field (ReaxFF) molecular dynamics simulation were conducted. Also, an investigation into the flame propagation characteristics and the reaction mechanisms of stearic acid was explored under different conditions. The thermal decomposition activation energy of stearic acid obtained by the TG tests was 105.01 kJ/mol. This is consistent with the initial decomposition stage reaction activation energy calculated to be 94.273 kJ/mol in the simulation. The flame propagation process of stearic acid combustion was explored and involved three stages: the flame growth stage, the stable flame stage, and the flame decay stage. The flame growth stage was subdivided into the flame growth inside the tube, upper the tube, and mushroom-shaped cloud. Besides, the flame growth stage could correspond to three flame propagation velocity peaks. Furthermore, the simplified reaction mechanism scheme for stearic acid radicals could explain the macro flame propagation behaviours. The main reaction pathway through which the C18H36O2 molecules were decomposed into C2H4 and C2H3, C2H5, and other free radicals was found to correspond to the initial flame growth stage, and the flame decay stage was dominated by the generation of final reaction products (H2O, CO2, and CO2, etc.). Concerning the overall reaction mechanism, the unburned stearic acid was continuously ignited, and the combustion products were continuously generated at the same time, thereby corresponding to the stable flame propagation stage. According to the flame propagation process and the simulation results, combustion reaction mechanisms of stearic acid were constructed.

Production mechanism of high-quality carbon black from high-temperature pyrolysis of waste tire.

High-temperature pyrolysis of waste tires is a promising method to produce high-quality carbon black. In this study, carbon black formation characteristics were investigated during tire pyrolysis at 1000-1300°C with residence times of <1s, 1-2s, and 2-4s. It is shown that with temperature increasing from 1000°C to 1300°C carbon black yield was increased from 10% to 27% with residence times of 2-4s. Carbon black exhibited a core-shell nanostructure over 1100°C and the graphitization degree was promoted with the temperature and residence time. While the mean particle diameter decreased with the temperature to 69nm at 1300°C and further increased by residence time. The molecular-level evolution from tire to initial carbon black was further revealed by reactive force field molecular dynamics simulations. Light oil, gas, and radicals were transformed to initial cyclic molecules and long carbon chains via carbon-addition-hydrogen-migration, H-abstraction-C2H2-addition, and radical-chain reactions, subsequently forming PAHs. The coupling of PAHs aliphatic side chains formed large graphene layers that gradually bent to fullerene-like cores and generated incipient carbon black. The process mechanism from volatiles evolution to carbon black was proposed, which may be helpful for obtaining high-quality carbon black from high-temperature pyrolysis of waste tires.

For every retinylidene a force field: Using machine learning to improve molecular simulations of rhodopsins.

G-protein-coupled receptors (GPCRs) are a superfamily of transmembrane proteins that perform diverse signaling roles in organisms, including humans. This makes them an attractive target for pharmaceuticals, with 134 different GPCRs being currently targeted by roughly 700 drugs. Here we study the retinylidene cofactor of GPCR rhodopsin that is responsible for initiating phototransduction. There are many distinct retinal forms—isomers, rotamers, and protonation states—that all have unique electronic landscapes. However, current molecular dynamics force field libraries are incomplete in that they ignore these differences and treat each retinylidene form identically. It is unknown what effect this has on current molecular dynamics simulations of retinal proteins. Our work addresses this problem by using machine learning methods to generate unique force fields for each of forty retinylidene forms. This approach improved on traditional parameterization schemes by using Born-Oppenheimer molecular dynamics trajectories to sample the retinylidene energetic landscapes. To validate our methods, the new force fields were then used in all-atom molecular dynamics simulations of the entire rhodopsin-membrane-water system. From these simulations we were able to recover hydrogen relaxation rates that closely matched experimental solid-state deuterium (2H) NMR measurements. Comparison of force field parameters showed a small but—given the highly allosteric nature of rhodopsin—potentially significant difference between retinylidene forms. We expect this work will improve simulations of many different types of rhodopsins, and that it will lead to an increased appreciation of the complexity of the retinylidene energetic landscape. Work supported by US NIH (R01EY026041) and US NSF (CHE 1904125, MCB 1817862) and Ministry of Education and Science of the Russian Federation (Project FSRM-2020-0006).

Insight into the Competitive and Synergistic Effects during Coal/NH3 Cofiring via Reactive Molecular Dynamics Simulations

In this work, reactive force field molecular dynamics (ReaxFF MD) simulations are performed to study competitive and synergistic effects and their impact on CO, CO2, unburned carbon (UC), and nitrogen oxide (NO) emissions in coal/NH3 cofiring at different conditions. Simulation results show that cofiring results in higher emissions of UC and NO. Increasing the temperature is helpful in reducing UC emissions but has little effect on NO emissions. Although decreasing the O2 equivalent ratio can help in reducing NO emissions, UC emissions increase significantly. The conclusion is that the best O2 equivalent ratio in cofiring is 0.6. Increasing the NH3 cofiring ratio will reduce NO emissions but seriously increase UC emissions. The competition between coal and NH3 for O2 will decrease the coal combustion rate and increase the NH3 combustion rate. The competition effect is further proved by tracing the OH distribution. The synergistic mechanism is revealed. The free radicals and intermediates generated from coal combustion, such as HO2, H2O2, H, and OH, will promote NH3 decomposition and oxidation. The kinetics analysis shows that the competition between NH3 combustion and coal combustion only influences the combustion rate of coal and NH3, while the synergistic effect of coal decreases the activation energy of NH3 combustion. The research results are helpful to further study the mechanism of coal/NH3 cofiring and guide the technology of NH3 blending in coal-fired power plants.

Polarization Controlled Assembly of Ultrathin Thiorphan Nanostructures on ZnO Surface Facets

Despite the importance of thiorphan as a small molecule with vital biological roles, its interactions with zinc oxide (ZnO) nanomaterials that are prospective in drug delivery and theranostic applications have not yet been sufficiently explored. Here the impact of surface polarity of different ZnO facets on thiorphan adsorption is studied both experimentally by atomic force microscopy (AFM) and angle resolved X-ray photoelectron spectroscopy (XPS) and theoretically by force field molecular dynamics (FFMD) and density functional tight binding simulations (DFTB). Polar ZnO surfaces cause the formation of thiorphan nanodots, where the size of the nanodots depends on the direction of dipoles: small (4 nm) nanodots are formed on Zn-face ZnO, while large (25 nm) nanodots are formed on O-face ZnO. Nonpolar ZnO surfaces cause self-assembly into layered nanoislands with characteristic 4 nm layer thickness, which subsequently merge into rigid nanolayers. The self-assembly is shown to be controlled solely by the effect of surface dipole electric field orientation and magnitude, whereas effects of surface chemistry or solution are negligible. The results thus also show a way for controlling the assembly of thiorphan and other molecular nanomaterials for diverse applications.

Experimental and simulation study on hydroxyl group promoting low-temperature oxidation of active groups in coal

The influence of hydroxyl groups in coal on low-temperature oxidation of other active groups was studied by analyzing the reactivity of six small molecule models under various conditions by using the reactive force field molecular dynamics method. Fourier-transform infrared spectroscopy was performed to investigate the influence of hydroxyl on the coal self-reaction process at low temperatures. Finally, the electrostatic potential, bond length, and frontier orbital energy level of each reaction model were calculated using density functional theory. The calculation results revealed that the reactivity of the alcohol hydroxyl structure was the strongest without and with oxygen, and numerous hydroxyl radicals were generated during the reaction process. Hydroxyl radicals accelerated the reaction rate of the active group to be faster than O2. Hydroxyl radicals can affect the reaction of the active group through two routes, one route is the hydrogen abstraction reaction, and the other route is electrophilic substitution. The amount of the OH–OH structure in coal decreased by 3 % under the oxygen-free condition at 343 K compared with that under the normal temperature (298 K). This phenomenon played a crucial role in the self-reaction process of coal and promoted –COOH and –CH2– reactions. Quantum chemical calculations revealed that the energy gap value of each reaction structure decreased, and the reaction activity increased. Hydroxyl radicals affect the electron cloud density of the active group. The nucleophilic reaction sites of ethyl and methyl structures increased, the CC bond between carboxyl and aldehyde structures and benzene ring was long, and the ability of decarboxylation and dealdehyde was enhanced. Furthermore, the benzene ring of the phenolic hydroxyl group was activated, and the nucleophilic reaction ability of the alcohol hydroxyl group was enhanced.

Insight into pyrolysis behavior of silicone-phenolic hybrid aerogel through thermal kinetic analysis and ReaxFF MD simulations

Phenolic aerogel matrix nanocomposites are modified to improve their thermal stability and oxidation resistance, while the pyrolysis behavior will become more complex. Herein, the pyrolysis behavior of silicone-phenolic hybrid aerogel, as the pyrolyzable component of the nanocomposites, was investigated by thermal analysis experiments (TG-MS, TG-FTIR, and Py-GC/MS), kinetic studies (isoconversional methods and distributed activation energy model (DAEM)), and reactive force field molecular dynamics (ReaxFF MD) simulations. Experimentally, it was found that the main product of silicone-phenolic hybrid aerogel during the first decomposition stage is CH4, which was also validated by ReaxFF MD simulation. The main product CH4 was predominantly formed by hydrogen extraction reaction from silicone aerogel. The main decomposition products and pyrolysis kinetics of silicone-phenolic hybrid aerogel were analyzed by a combination of the multi-DAEM and ReaxFF MD simulations. The five-pseudo-components DAEM was utilized to reveal the decomposition of silicone-phenolic hybrid aerogel in five stages, which were in good agreement with the experimental conversion rates. The rearrangement of the Si-O-Si backbone of silicone aerogel at elevated temperatures forms silica-like species with antioxidant performance. This study can provide reliable guidance for the characterization and design of phenolic aerogel matrix nanocomposites and the selection of antioxidant modifiers.