Polymer Consistent Force Field Research Articles

Molecular dynamics study on the influence of thermal aging on the mechanical properties of epoxy resins for high voltage bushing.

Thermal aging significantly deteriorates the mechanical properties and service performance of epoxy resins used for the high-voltage bushing. Current studies on the thermal aging behavior of epoxy resins mainly focus on experimental observations. However, an in-depth understanding of the mechanism of thermal aging of epoxy resins requires the monitoring of structural evolution of epoxy resins during thermal aging at the molecular level. To thoroughly analyze the intrinsic factors affecting structural evolution and the effect of thermal aging on the mechanical properties of epoxy resin for high-voltage bushing, epoxy resin models with different crosslinking degrees were established and thermal aging treatments at various temperatures and time periods were carried out by molecular dynamics simulation. It was found that the tensile strength of the epoxy resin was enhanced with the increase of the crosslinking degree, which was related to the elevation of the proportion of C-N and O-H bonds in its structure. With the increase of thermal aging temperature, the tensile strength of the epoxy resin decreased, which was related to the formation of weak bonds. At the early stage of thermal aging and after a period of high-temperature thermal aging, the strength of epoxy resin significantly decreases. The thermal aging of the epoxy resin is accelerated under external loading. In addition, the crosslinking degree and curing agent also affect the thermal aging resistance of epoxy resins. The results of this study can provide guidance for predicting and improving the thermal aging resistance of epoxy resins. Materials Studio was used to construct molecular models and complete crosslinking reactions. DGEBA and 44DDS (or 33DDS) were mixed at a ratio of 2:1, followed by crosslinking reaction. During this process, the Nose method was used to control temperature, the Berendsen method was used to control pressure, and the polymer consistent force field (PCFF) was used to control the motion and force of atoms. Isobaric-isothermal ensemble (NPT ensemble) was used to heat up epoxy resin models to various thermal aging temperatures of 400K, 500K, 600K and 700K. The models were maintained at these temperatures for different thermal aging times of 100ps, 200ps, 300ps, 400ps, 500ps, 600ps, 700ps and 800ps. Afterwards, the models were cooled down to 300K and subjected to uniaxial tensile testing at this temperature with a strain rate of 1 × 109s-1. The structural configurations and stress-strain data during the tensile process were recorded. The flow stress of the material was derived by counting the average stress in the 20-50% strain interval.

Investigation on liquid thermal conductivity of ethylene glycol (EG)/water mixtures: A comparative experimental and molecular dynamics simulation study

Ethylene glycol (EG) and water mixtures can be used as the antifreeze, refrigerant and coolant due to their good environmental, thermodynamic and transport performance. The mixture’s accurate thermal conductivity is indispensable for the optimal design of the relevant heat exchangers. However, thermal conductivity of EG/water mixtures is lacking and is seen to limit the further industrial application. In this paper, the transient hot wire method (THW) was used to measure the thermal conductivities of five EG/water mixtures (with ethylene glycol mass fraction wEG = 0, 0.25, 0.50, 0.75, 1.00) in the temperature range from 253.15 K to 353.15 K and pressures up to 8.0 MPa. A comparison was made between the present experimental data and literature values, yielding a good agreement. The AADs are within 2.65% for EG/water mixture systems. For convenience of engineering calculations, the present experimental data were correlated as a function of temperature and pressure. The average absolute deviations (AADs) between the experimental thermal conductivities and the calculations are 0.50%, 0.10%, 0.60%, 0.40% and 0.32% for mixture systems with ethylene glycol mass fraction wEG = 0.00, 0.25, 0.50, 0.75, 1.00, respectively. In addition, non-equilibrium molecular dynamics was applied to study the thermal conductivities of ethylene glycol/water binary mixtures based on PCFF (polymer consistent force field) and SPC/E (extended simple point charge) model, showing a good agreement with the experimental data. This study can provide fundamental thermal conductivity data for the heat transfer application of ethylene glycol aqueous solution in the industrial field.

Molecular Dynamics Study on the Effect of Interfacial Cellullose Polymers in Strengthening the Stress Transfer Between Alumina Nanoparticles and Epoxy

AbstractCellulose polymers are widely used to fabricate green composites, implemented as fiber, matrix, and adhesive material between them. In this study, cellulose polymers are used as interphase material between spherical nanoparticles of hydroxylated alumina (Al2O3) and epoxy. Molecular dynamics simulations using the large‐scale atomic/molecular massively parallel simulator (LAMMPS) are utilized to investigate the effect of the cellulose content on the stiffness and tensile strength of epoxy/alumina nanocomposites. Polymer consistent forcefield (PCFF) and its supplement provided by MedeA software (PCFF+) define the interactions between the atoms. The simulation results indicate that incorporating cellulose into epoxy would increase the stiffness and strength properties considering that the cellulose polymers do not experience any agglomeration in the hosting matrix and have good interfacial adhesion with epoxy and alumina. This interfacial adhesion is needed since inserting cellulose polymers between alumina nanoparticles and epoxy would increase the porosity in the interphase region, limiting the stress transfer at the interphase and reducing the tensile strength of epoxy‐alumina nanocomposites.

Mechanical properties and deformation behaviors of the hydroxyl-terminated polybutadiene and ammonium perchlorate interface by molecular dynamics simulation

The interfacial structure and deformation behaviors have considerable influences on the structural reliability and durability of the hydroxyl-terminated polybutadiene (HTPB) solid rocket propellant. In this work, an all-atom molecular dynamics (MD) simulation was adopted to investigate mechanical properties and damage behaviors of the HTPB and ammonium perchlorate (AP) interface. The modified polymer consistent force field was implanted to describe intra- and intermolecular interactions. The effects of loading rate, temperature, and moisture content on the interfacial strength and tensile curves were discussed. The simulation results show that the interfacial strength and modulus exhibit an increasing tendency when the loading rate increases. However, increasing the temperature and moisture content will lead to a decrease in the interfacial strength. Two interfacial tensile modes distinguished by the mass density curve were adopted and compared to reveal the internal deformation process. The AP-HTPB interfacial deformation initially occurs inside the HTPB bulk due to its lower mechanical strength than the interface and then gradually expands to the interface region. Furthermore, a linear extrapolation based on phenomenological results was used to reveal the discrepancy in scale between experiments and the MD simulation.

Effect of vacuum and heat treatment on a single chain of cellulose: Molecular dynamics simulation

Molecular dynamics simulation was used to better understand a single, non-crystalline cellulose molecular chain and its response to high-temperature treatment. The system temperature was varied from 430 K to 510 K, and the temperature interval was 20 K. Under the polymer consistent force field (PCFF), the dynamics simulation of each temperature was completed under the constant pressure/constant temperature dynamics (NPT). The experimental results showed that the mechanical properties of cellulose heat-treated at high temperature in a vacuum environment initially increased and then decreased with the increase of temperature. When the temperature was at 450 K, the mechanical properties reached an optimal state. Moreover, its mechanical properties were noticeably related to the connection of hydrogen bonds in the cellulose molecular chain and the thermal motion of the molecular chain.

Molecular dynamics simulations on photoinduced switchable Tg and self-healing behaviors of azobenzene-containing polymers

Developing polymers with switchable glass transition temperature (Tg) can solve scientific challenges such as fracture healing of polymers with high Tg and processing of hard polymers without plasticizer at room temperature. The azobenzene-containing polymers, or the so-called azopolymers, are smart materials that show photoswitchable Tg and photoinduced motions, which can be used in fabrication of photoactuators. Here, healable and reprocessable models were constructed by using linear azopolymers. Molecular dynamics (MD) simulation was used to study the behaviors of photo-induced bending, self-healing, and directional photofluidization of the azopolymers. A temporary modification of the C–N=N–C dihedral potential was applied to achieve the isomerization of azopolymer by using polymer consistent force-field (PCFF). The simulation results show that isomerization of azobenzene groups can lead to photoswitchable Tg of the azopolymers, which resulting in shape changes and different mechanical responses. Furthermore, the photoswitchable Tg makes the azopolymers much easier to heal under UV irradiation, and restore the mechanical properties under visible light irradiation. Finally, the mechanism of photoliquefaction was investigated, our results imply that the photoinduced trans–cis-trans cycles can reduce the mechanical properties and Tg values of azopolymers.

Effects of temperature and hard segment content on the interfacial mechanical properties of graphene/thermoplastic polyurethane composites: A molecular dynamics study

The effect of system temperature and hard segment content (HSC) on the interfacial mechanical behavior of graphene/thermoplastic polyurethane (TPU) nanocomposites is investigated by Molecular Dynamics (MD) simulations. The graphene opening and sliding pull-out simulations are carried out. The polymer consistent force field (PCFF) has been applied and validated by calculating the glass transition temperature of the neat TPU system. For better results interpretation, the maximum pull-out force and average pull-out force was used to characterize the interfacial strength of graphene/TPU nanocomposite in the opening and sliding mode, respectively. Besides, the number distribution of the hydrogen bonds (H-bonds) was used to clarify the reasons for the variation of pull-out force with different HSCs, and observe the evolution of the structural morphology. Based on the results obtained, the interfacial strength can be enhanced by decreasing the temperature or increasing the HSC both in the opening and sliding mode. Furthermore, the delay in the failure of interfacial strength due to the enhancement of entanglement strength can be achieved by adjusting the HSC of TPU to improve the uniform distribution of H-bonds in the opening mode. Additionally, we successfully applied the variation of the number of benzene rings with sliding distance to quantitatively describe the effects of temperature and HSC on the potential relationship between the interfacial strength and the mobility of polymer chains within the polymer-graphene interface in the sliding mode.

Full-Atomistic Optimized Potentials for Liquid Simulations and Polymer Consistent Force Field Models for Biocompatible Shape-Memory Poly(ε-caprolactone).

Thermally induced shape memory poly(ε-caprolactone) (PCL)-based polymers are one of the most extensively researched families of biocompatible materials. They are degradable under physiological conditions and have high applicability in general biomedical engineering, with cross-linked PCL networks being particularly useful for tissue engineering. In this study, we used the optimized potentials for liquid simulations (OPLS) force field, which is well suited for describing intermolecular interactions in biomolecules, and the class II polymer consistent force field (PCFF) to investigate the properties of telechelic PCL with diacrylates as reactive functionalities on its end groups. PCFF has been specifically parameterized for simulating synthetic polymeric materials. We compare the findings of all-atom molecular dynamics simulations with known experimental data and theoretical assumptions to verify the applicability of both these force fields. We estimated the melt density, volume, transition temperatures, and mechanical characteristics of two-branched PCL diacrylates with a molecular weight of 2481 Da. Our findings point to the utility of the aforementioned force fields in predicting the properties of PCL-based polymers. It also opens avenues for developing PCL cross-linked polymer models and employing OPLS to investigate the interactions of synthetic polymers with biomolecules.

The study of polydimethylsiloxane nanocone distortion in the demolding process using molecular dynamics method

Experimental results show that after demolding process from the polymethylmethacrylate mold, the poly(dimethlysiloxane) (PDMS) nanocone replica yields 150%–160% larger in height as compared to the mold size dimensions before rupture. The observation from the experiment gives the direction for this study to investigate the distortion of PDMS nanocone structures in the demolding soft lithography process using molecular dynamics simulation. The aim of this study is to employ the molecular dynamics simulations and study the stress–strain curve of the nanocone structures that were subjected to uniaxial stress. Two force fields (polymer consistent forcefield and condensed-phase optimized molecular potentials for atomistic simulation studies) were utilized for the modeling. The results from the molecular dynamics simulation show that when the PDMS nanocone is subjected to tensile stress, it shows characteristics of flexible plastic pattern curve with significant yielding. This study also found that ultimate tensile stress for PDMS nanocones is within the value found in the bulk structure of 4.335–6.478 MPa.

Quantifying oxygen diffusion in bitumen films using molecular dynamics simulations

Bitumen in asphalt pavements reacts slowly with atmospheric oxygen, resulting in oxidative ageing. This oxidative reaction is strongly dependent on the physical diffusion of the oxygen into the bitumen. This study aims to use molecular dynamics (MD) simulation to investigate the oxygen diffusion into the bitumen film and analyse the effects of anti-ageing compounds (AACs) on the oxygen diffusion. The MD diffusion simulations using a Polymer Consistent Force Field (PCFF) were conducted on a bitumen-air bi-layer model at different temperatures. Fick’s second law was used to calculate the diffusion coefficient of the oxygen in the bitumen film. It is found that the oxygen diffusion coefficients ranged from 6.67 × 10−10 to 7.45 × 10−11 m2/s for the unmodified and AAC-modified bitumens at the simulating temperatures of 25, 50 and 100 °C. Irganox acid and DLTDP (Dilauryl thiodipropionate):furfural showed two different anti-aging mechanisms, i.e., reducing the oxygen physical diffusion and controlling the chemical oxidative reaction. Reducing the oxygen diffusivity by constructing a network in the bitumen to retard oxygen diffusion and increase the transport path is an efficient way to slow down the bitumen aging without the antioxidant consumption. This work proposed a MD-based computational approach, contributing to 1) determination of the oxygen diffusion coefficient of the existing bitumen that is extremely challenging for the experimental measurement and 2) instruction of developing new antioxidant.

Molecular Dynamics Simulation of the Nanoindentation of Coal Vitrinite

Coal deformation is closely correlated with the distribution of organic maceral groups, however, molecular dynamics (MD) simulations of vitrinite nanoindentation have rarely been conducted. In this study, the vitrinite substrate for indentation was constructed utilizing polymer consistent force field (PCFF), and a spherical ghost indenter was used for loading. The results showed that: 1) In the indentation process, some of the vitrinite atoms overcame the energy barrier to move, with the most important deformation mechanism including the sliding, bending, and reorientation of vitrinite molecular chains, leading to the formation of a shearing transformation zone (STZ), which was also found to contain structural defects and stacking of aromatic structures. 2) The distribution of atomic displacements in the vitrinite substrate could be subdivided into distinct regions, with slippage at the region boundaries producing shear bands. 3) The surface morphology and mechanical properties obtained from the nanoindentation simulation were similar to experimental results from the literature, indicating that MD simulations are a powerful tool for studying coal nanoindentation. The results from this study increase the current scientific understanding of the mechanical properties of vitrinite by providing a new perspective that elucidates the nanoscale structural evolution occurring during the indentation process.

Molecular dynamics simulations of mechanical properties of epoxy-amine: Cross-linker type and degree of conversion effects

Molecular dynamics (MD) simulations are conducted to study the thermo-mechanical properties of a family of thermosetting epoxy-amines. The crosslinked epoxy resin EPON862 with a series of cross-linkers is built and simulated under the polymer consistent force field (PCFF). Three types of curing agents (rigidity1,3-phenylenediamine (1,3-P), 4,4-diaminodiphenylmethane (DDM), and phenol-formaldehyde-ethylenediamine (PFE)) with different numbers of active sites are selected in the simulations. We focus on the effects of the cross-linkers on thermo-mechanical properties such as density, glass transition temperature (T g), elastic constants, and strength. Our simulations show a significant increase in the T g, Young’s modulus and yield stress with the increase in the degree of conversion. The simulation results reveal that the mechanical properties of thermosetting polymers are strongly dependent on the molecular structures of the cross-linker and network topological properties, such as end-to-end distance, crosslinking density and degree of conversion.

Molecular dynamics simulation of the interface properties of continuous carbon fiber/ polyimide composites

The resin-fiber interface is pivotal to the performance of composites. Molecular dynamics simulations were used to study the relationship between interface structure and interface mechanical properties of carbon fiber/polyimide composites. The rationality of the models and the polymer consistent force field were verified by the glass transition temperature, the density of polyimide and the pull-out test. It was found that the interfacial shear strength (IFSS) with grafted functional groups was greatly intensified with the order of -C6H5 > -COOH > -CH3 > -OH > -NH2. The IFSS increased linearly with the increasing number of grafted groups. Grafted polyimide monomers can intensify interfacial performance more effectively. We also calculated the interaction energy by density functional theory method and the surface roughness to reveal the enhancement mechanism of the interface, which is the synergistic effect of the non-bond interaction and surface roughness.

The interfacial load-transfer enhancement mechanism of amino-functionalised carbon nanotube reinforced epoxy matrix composites: A molecular dynamics study

This work provides a comprehensive and comparative study on the interfacial load transfer properties of amino-functionalised carbon nanotube (CNT)/epoxy composites via molecular dynamics simulations. Three models were constructed: (i) a pristine CNT embedded in epoxy resin, (ii) an amino-functionalised CNT embedded in epoxy resin without crosslinking interactions between amino groups and the epoxy resin matrix, and (iii) an amino-functionalised CNT embedded in epoxy resin with crosslinking interactions. The pull-out process was simulated using a steering molecular dynamics simulation to examine the sidewall load transfer effect of the CNTs. The travelling of amino groups along the CNTs was simulated by changing the topology of the functionalised CNT using a polymer consistent force field during the pull-out process. The mean interfacial shear stress, pull-out energy, stress, and atomic displacement were analysed. The results show that the interfacial shear strength as well as the compatibility and cooperativity between the CNTs and the matrix can be efficiently enhanced by introducing amino functional groups. In particular, a dramatic enhancement can be obtained by considering the crosslinking interactions. In addition, the interfacial load transfer between the end of the CNTs and the matrix was studied. The axial normal stress at the end of the CNTs was calculated by placing one diglycidyl ether bisphenol A molecule near the end of the CNT. The results show that there was a large interaction between the end of the CNTs and matrix, which cannot be neglected. Thus, the assumption of zero traction between the fibre end and matrix in the classical shear lag model should be modified at the nanoscale.

Failure of the Asphalt–Aggregate Interface under Tensile Stress: Insight from Molecular Dynamics

Interfacial strength between asphalt binder and aggregate plays a vital role in maintaining the mechanical integrity of asphalt mixture. Given the lack of accurate testing instruments for the interaction of asphalt–aggregate interface, the adhesive interaction and failure evolution occurring at this interface has not been fully understood. In this study, molecular dynamics (MD) simulation was utilized to elucidate the mechanical and deformation behavior of the asphalt–aggregate interface under tensile stress from the atomic perspective. The interface system was constructed with a 12-component asphalt molecular model bonding on a silica substrate. This asphalt molecular model, combining the polymer consistent force field (PCFF) adopted to describe the inter-/intra-action of the system, was first validated. A stress-separation law of this interface can be obtained by tracing the atomic force during the tensile process. From this stress-separation law, the interfacial strength and work of adhesion can be derived. The influences of model size, loading rate, asphalt film thickness, and moisture were investigated. It was found that the interfacial failure type transfers from adhesive failure to cohesive failure as the loading rate decreases to a certain level. Moreover, the interfacial strength is highly associated with the failure type. The interfacial strength of the adhesive failure is about five times that of the cohesive failure, which demonstrates the traditional method of improving the adhesion performance of asphalt on aggregate through increasing its viscosity from the aspect of atomic modeling. Furthermore, the water molecules absorbed at the interface are crucial to the durability of the asphalt–aggregate system. This study provides deep insight into the interfacial failure of the asphalt–aggregate system and could serve as an initial step in multiscale modeling using bottom-up approaches for asphalt mixture.

Effects of interlayer density and surfactant on coupled thermal stress and moisture absorption in modified montmorillonite/polypropylene nanocomposite

AbstractModified montmorillonite/polypropylene nanocomposites (NCs) are increasingly used in industrial applications such as subsea pipelines because hexadecyltrimethyl ammonium montmorillonite (HDTMA+‐Mt) enhances thermomechanical and barrier properties of the amorphous polymer. Two coupled physics of moisture adsorption and thermal loading are investigated. Molecular dynamics simulates HDTMA+‐Mt polymer NC using three force fields including polymer consistent force field and condensed‐phase optimized molecular potentials, and embedded‐atom method. Mechanical properties and self‐diffusion coefficient are investigated at temperature levels of 100 and 298 K, and water content of 0.021 and 0.133 g/g. These properties are evaluated at 1.0 atm pressure for four different volume fractions (vol%) of the HDTMA+‐Mt. The modeling procedure is verified by obtaining the glass transition temperature (Tg) of the NC by scanning the temperature from 200 K (glassy state) up to 325 K (rubbery state). It is observed that the Tg is very close to the experimental value available in the literature. The result of the modeling shows that the increase of clay content of the NC decreases the self‐diffusion coefficient of the material. It is seen that the clay nanoparticle can significantly hinder the degradation of mechanical properties of the NC even when both temperature and water content increase.

Vapor sensing and interface properties of reduced graphene oxide–poly(methyl methacrylate) nanocomposite

Synthesized reduced graphene oxide–poly(methyl methacrylate) (RGO–PMMA) nanocomposites were characterized by differential scanning calorimetry, thermogravimetric analysis, and probed for volatile organic compounds (VOC) sensing. A molecular dynamics simulation is performed to investigate the interaction between PMMA and a graphene surface. The condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS), polymer consistent force-field (PCFF) and consistent valence force-field (CVFF) are used to describe the interaction of the graphene–PMMA. None of the three simulated force fields COMPASS, PCFF, and CVFF exhibits a distinctive behaviour of interaction between graphene and PMMA, but CVFF predicts a higher interaction energy in comparison with the simulated force fields COMPASS and PCFF. Experimentally, the selective response for different VOC has been analysed and the highest response together with the fastest recovery is obtained for tetrahydrofuran. A model is introduced explaining observed features.

Temperature effects on the interfacial behavior of functionalized carbon nanotube–polyethylene nanocomposite using molecular dynamics simulation

The present study investigates the interfacial behavior of functionalized carbon nanotube–polyethylene nanocomposite at different temperatures using molecular dynamics simulations, utilizing the second-generation polymer consistent force field. The carboxylic acid group is used to functionalize the carbon nanotube. In order to calculate interfacial interaction energy and interfacial shear strength of the nanocomposite, various pull-out tests are performed at different temperatures in the range of 1–400 K. The effect of functionalization on the interfacial interaction energy, interfacial shear strength, and glass transition temperature of the nanocomposite are studied in comparison to pristine carbon nanotube–reinforced nanocomposite. Results reveal that for all temperatures and degrees of functionalization, the chirality (i.e. armchair and zigzag) of carbon nanotube has a significant effect on interfacial interaction energy and interfacial shear strength of the nanocomposite. It is also found that functionalizing the carbon nanotube in carbon nanotube–polyethylene nanocomposite enhances its interfacial shear strength at different temperatures. Furthermore, a sudden drop in the value of interfacial interaction energy and interfacial shear strength of the pristine as well as functionalized carbon nanotube–reinforced nanocomposite is observed at a temperature near to its glass transition temperature.

Gas diffusion behavior in green camellia insulating oils

As a new environmentally friendly liquid dielectric material, vegetable insulating oil has been widely used in oil-filled power equipment. In oil-filled power equipment, ageing, faults of overheating and discharge cause the decomposition of insulating oil and insulating paper, resulting in dissolved gases in oils. The diffusion behavior of dissolved gases in oils is helpful for evaluation of health state of oil-filled power equipment. In this study, the molecular dynamics simulation based on polymer consistent force field (PCFF) is adopted to analyze diffusion processes of dissolved gases in camellia insulating oils. The diffusion coefficients and free volume of dissolved gases including hydrocarbons, carbon oxides and hydrogen are calculated. The diffusion trajectory of dissolved gases in oils are also given. In addition, impacts of gas species and temperature on molecular diffusion coefficients of oils were also studied. Results quantitatively describe the diffusion behavior of gases with different molecular weight in the oils under various temperatures. The research provides theoretic support for further application of vegetable insulating oils in power equipment.

Uncertainties in the predictions of thermo-physical properties of thermoplastic polymers via molecular dynamics

We quantify the effect of various sources of uncertainties in the prediction of thermo-physical properties of polymers using molecular dynamics simulations. We quantify how the choice of polymer builder, force field, molecular weight and data analysis affect predicted values of the glass transition temperature (Tg), room temperature density and coefficient of thermal expansion of poly(methyl-methacrylate) (PMMA) and polystyrene (PS). Interestingly, we find that the data analysis introduces significant uncertainties in Tg (approximately 5%) while the other properties are insensitive to it. The force field is the only variable that significantly affects the predictions of density. Polymer-consistent force field (PCFF) resulted in a higher density for PMMA than Dreiding and the opposite trend was observed in PS; in all cases the difference in density was less than 2%. Strongly correlated with density, we find that PCFF leads to a higher Tg than Dreiding for PMMA and both force fields predict similar Tg values for PS. The trends in Tg can be explained by differences in segmental mobility of the melts predicted by the two force fields. We find that the choice of amorphous polymer builder results in uncertainties in predictions comparable to those associated with the force field due to subtle, but persistent, differences in molecular structure. The results presented here provide insight into the physics behind molecular simulations of polymers and quantitative levels of uncertainties associated with individual sources that can help practitioners of molecular simulations interested in using their results in engineering applications.