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... For example, studies on the mechanical behavior of amorphous polymer materials [25,26], the microscopic deformation characteristics of inorganic minerals or metals [27,28], the tensile strength of a single carbon nanotube [29,30] or one layer of graphene [31,32], and the nanoscale shear enhancing mechanism of carbon nanotubes in cement [33], all using the MD simulation method, have been reported. A large amount of significant efforts to develop the united atom [34,35] and coarse-grained [36][37][38] have been carried out. When the united atom or coarse-grained MD simulation methods are used, groups of atoms are lumped into single interaction sites with the simplified interaction potentials, and thus, the total number of degrees of freedom of the simulated models can be reduced. ...
... The united atom and coarse-grained MD methods require less computation, due to which, the time and length scales of the models can be increased to simulate the bulk behavior. With the united atom or coarse-grained MD simulation methods, the nanoscale mechanical properties and deformation mechanism of amorphous polyethylene under the tensilecompressive load [25] or pure tensile load [35] have been understood. In the study of Park et al. [38], the thermomechanical behavior of the shape-memory polyurethane copolymers was also studied using the coarse-grained MD simulation method. ...
... For the configuration containing 100 PU molecule chains, the uniaxial tension simulation at two other strain rates (2 × 10 11 /s and 1 × 10 10 /s) was also conducted to study the influences of strain rate. The tensile rate adopted here is in the range suggested by previous studies [25,35]. ...
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At present, polyurethane (PU) has been extensively used as a grouting material in civil engineering. The mechanical properties of PU are the key to achieving the desirable grouting effect. This study presents the research results of the mechanical behavior of PU matrix under tensile, successive cyclic tensile, and stress relaxation at the nanoscale, using the coarse-grained molecular dynamics simulation method. The influences of the number of molecule chains and strain rate on the tensile mechanical properties are discussed, and the tensile deformation mechanism of PU matrix is revealed. The tensile strength of PU matrix is independent of loading path, and after yielding, the strain of PU matrix contains the elastic strain, plastic strain, and viscous strain. In the stress relaxation process, the evolution of the axial stress is mainly caused by the varied van der Waals interactions. The stress relaxation behavior of PU matrix can be described by the viscoelastic model consisting of one elastic element in parallel with one Maxwell element.
... The stoichiometric mixing ratio of methyl methacrylate and cumene hydroperoxide was 35:1, with a low initial density of 0.5 g/cm 3 . ry 15, 2022 submitted to Polymers 2 of 18 that the tensile strength of concrete/PMMA interfaces is due to van der Waals forces 39 and that silica/PMMA interfaces exhibit a higher tensile strength than calcite/PMMA 40 interfaces in mode I [5]. In this paper, we simulate PMMA injection and PMMA/silica 41 interface shear deformation. ...
... The DREIDING force field was successfully employed for both organic and inorganic materials [21]. The reduced form of the DREIDING potential energy Equation [25,39] can be expressed as follows: ...
... In the third equation above, E nb is the van der Waals energy (Lennard-Jones potential), in which r ij is the distance between the ith and jth atoms with charges q i and q j . r c is the cutoff distance, equal to 12 Å in this study [17,39,40]. ...
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The mechanical properties of cementitious materials injected by epoxy have seldom been modeled quantitatively, and the atomic origin of the shear strength of polymer/concrete interfaces is still unknown. To understand the main parameters that affect crack filling and interface strength in mode II, we simulated polymethylmethacrylate (PMMA) injection and PMMA/silica interface shear deformation with Molecular Dynamics (MD). Injection simulation results indicate that the notch filling ratio increases with injection pressure (100 MPa–500 MPa) and temperature (200 K–400 K) and decreases with the chain length (4–16). Interface shear strength increases with the strain rate (1×108 s−1–1×109 s−1). Smooth interfaces have lower shear strengths than polymer alone, and under similar injection conditions, rough interfaces tend to be stronger than smooth ones. The shear strength of rough interfaces increases with the filling ratio and the length of the polymer chains; it is not significantly affected by temperatures under 400 K, but it drops dramatically when the temperature reaches 400 K, which corresponds to the PMMA melting temperature for the range of pressures tested. For the same injection work input, a higher interface shear strength can be achieved with the entanglement of long molecule chains rather than with asperity filling by short molecule chains. Overall, the mechanical work needed to break silica/PMMA interfaces in mode II is mainly contributed by van der Waals forces, but it is noted that interlocking forces play a critical role in interfaces created with long polymer chains, in which less non-bond energy is required to reach failure in comparison to an interface with the same shear strength created with shorter polymer chains. In general, rough interfaces with low filling ratios and long polymer chains perform better than rough interfaces with high filling ratios and short polymer chains, indicating that for the same injection work input, it is more efficient to use polymers with high polymerization.
... That observation can be partly attributed to the fact that the temperature of the polymers exceeds their glass transition temperatures (T g ) when U (i) P is over 1 km/s. T g of the polymers are typically below 350 K. 22,40,53 However, it should be noted that T g increases with increasing pressure. 54,55 For example, the PE sample experiences a shock pressure of ∼4 GPa when U (i) P is 1 km/s, and therefore, T g can be higher than the corresponding value under the atmospheric pressure. ...
... However, it should be noted that the tensile strength of the three polymers, obtained from uniaxial tensile tests simulated at higher temperatures, is significantly smaller. 22,40,53,60 For example, polyurethane and polyurea lose more than 80% of their tensile strength (under uniaxial loading) when the temperature increases from 300 to 500 K. 22,40 The ideal (or ultimate) tensile strength of a material (σ id ) can be estimated from the P-V Hugoniot, 25,56 which can be expressed as ...
Article
Macroscopic experimental results of the plate impact tests of polymers are generally interpreted using the free surface approximation and the acoustic approximation. However, their validity over a range of shock pressures has not been thoroughly investigated yet. We conducted molecular dynamics simulations of plate impact tests of polyethylene to obtain molecular-level insights on those two common approximations associated with the interpretation of shock pressure and spall strength. Our results revealed that the free surface approximation could slightly underpredict the shock pressure in the polymer. The spall strength computed from the free surface velocity history can be significantly smaller than the actual tensile stress in the region of spallation.
... Nowadays, MD is widely used for simulations of material systems from the atomistic level to the mesoscale. One of its main advantages is that time is an explicit variable, allowing simulations of equilibrium properties and time-dependent ones; this is an essential characteristic for the simulation of viscoelastic materials, as shown in several polymer mechanical studies [30][31][32][33][34][35][36][37][38][39][40]. MD not only predicts the exact configuration of a system at a given time instant, but also allows estimating the system behavior through statistical or averaging methods [41]. ...
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Helices made of polymeric elastic filaments have been the target of considerable and growing interest in past years, given several potential applications. Although the underlying mechanisms responsible for the formation of such helices are still not sufficiently understood, recent results suggest they may result from buckling instabilities emerging from torsion in the extremities or when there is asymmetry across the filament’s cross section. Also, the occurrence of perversions (regions where the helical handedness changes) has attracted considerable interest in a number of theoretical works, but the possibility of creating more than a single perversion, and thus control the geometry of helices and perversions in the resulting filament, has been given much less attention, despite its clear importance. In this paper, we present coarse-grained Molecular dynamics (MD) simulations that show it is possible to replicate the formation of helices and perversions within certain conditions, and which complement information available from experimental approaches. We show how the helical radius can depend on the strength and the asymmetry of the pairwise interactions, the filament’s aspect ratio, and the strain rate of recovery, and we discuss in detail how perversions occur. The bonding potential parameters were found to have a small effect on the number of perversions, while the strain rate exhibited a significant effect, namely, an increase in 200-fold of the strain rate can induce as many as eight times more perversions for an aspect ratio of 200 (and three times more perversions for an aspect ratio of 50). The increase in the pair-wise interaction stiffness leads to lower loop diameters and higher number of loops, while an increase in the pair-wise equilibrium distance leads to larger loop diameters and consequently a lower number of loops; however, both these parameters exhibit a strong dependence on the aspect ratio. It was also found that an increase in the surface modification by 30% leads to an increase in circa 2.3 times the number of formed loops, while the average loop diameter decreases by circa 40%. From these results emerges a better understanding of how to tailor the geometry of the studied polymer elastic filaments, vital information for the design of next-generation nano-mechanical systems, such as those obtained by nano-patterning of soft materials. Graphical abstract Different number of perversions resulting from different strain rates during deformation (strain rate of case a is 200 times that of case b)
... In previous studies, Hao Wang [35] analyzed uniaxial tensile simulations of asphaltaggregate models using molecular dynamics methods and concluded that model size, loading rate and temperature have some influence on tensile strength results, which is similar to the findings of Ye-shou Xu [38], D. Hossain [39] and others for tensile simulations of rubber models and polymer models. However, the effect of different loading conditions on the simulation results during uniaxial tensile simulations regarding asphalt binder models has not been clarified. ...
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In order to study the characteristics and laws of nanocrack generation and self-healing behavior of asphalt materials under tensile action, the molecular dynamics (MD) method was used to simulate the continuous “tensile failure—self-healing” process, and this study remedies the shortcomings of existing experimental and observational methods. It is found that the MD-reproduced formation process of asphalt binder nanocrack contains four stages: “tensile extension”, “nanocrack generation”, “crack adding, expanding and penetrating” and “cracking failure”. The influence of tensile conditions on the tensile cracking simulation of an asphalt binder model was analyzed, and it was found that low temperature and high loading rate would increase the tensile strength of the asphalt binder model. In addition, the MD-reproduced healing process of asphalt binder nanocracks can be divided into four stages: “surface approach”, “surface rearrangement”, “surface wetting” and “diffusion”, which is similar to the healing process of polymers. Finally, from the perspective of energy change, the change rule of dominant van der Waals energy in the self-healing process was studied. Based on the existing research, the influence of damage degree on the healing performance of asphalt binder and its mechanism were further analyzed. The research results further enrich the theoretical research on microlevel cracking and healing of asphalt materials, and have certain theoretical value for the further development of self-healing asphalt materials.
... The methodology of Hossain et al. 37 was selected to quantify the entanglement of the system. According to their definition, the entanglement parameter was defined as the total fraction of flexion nodes. ...
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Molecular dynamics simulation was applied to study the irreversible strain through loading and unloading cyclic tests of polyurethane (PU) reinforced with halloysite nanotube (HNT). The influences of the stretching cycle rate, different temperatures, the volume of halloysite nanotube and the density rate of the hard and soft domain of PU were studied on the permanent set. The results illustrate that the residual strain was increasing when the stretching loading is increasing, for example, with increasing strain load to 250% the residual strain increased to 55%. In contrast, the increasing volume fraction of HNT and hard part content of PU lead to lower residual strain. The recovery of the permanent set is achievable by increasing temperature from 1 K to 200 K residual strain is decreased to 52%. An Ogden constitutive and the theory of pseudo-elasticity were adopted to simulate this composite in the ABAQUS software. This model has proposed a reasonable prediction of plastic deformation.
... This defines a range of values, {−0.003, −0.001, 0.001, 0.003}, applied to each strain pattern 35 These are then used to generate the metric tensor: G, GPa. ...
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In CO2-enhanced coalbed methane (CO2-ECBM) engineering, accurate knowledge of the interaction mechanism of CO2 and coal matrix is crucial for improving the recovery of CH4 and contributing to the geological sequestration of CO2. This study is performed to prove the accuracy of molecular simulation and calculate the variation characteristics of pore structure, volumetric strain, mechanical properties, Fourier transform infrared (FT-IR) spectra, and the system free energy by molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) methods. According to the obtained results, a relationship between pore structure, swelling strain, mechanical properties, chemical structure, and surface free energy was established. Then, the correlation of various coal change characteristics was analyzed to elucidate the interaction mechanism between CO2 and coal. The results showed that (1) the molecular simulation method was able to estimate the swelling mechanism of CO2 and coal. However, because the adsorption capacity of the molecular simulate is greater than that of the experiment and the raw coal is softer than the macromolecular structure, the molecular results are slightly better than the experimental results. (2) As pressure increased from 0 to 4 MPa, the intramolecular pores and sorption-induced strain changed significantly, whereas when the pressure increased from 4 to 8 MPa (especially at 6-8 Mpa), there was an increase of the intermolecular pores and mechanical properties and transition from elastic to plastic. In addition, when the pressure was >8 MPa, the coal matrix changed slightly. ScCO2 with a higher adsorption capacity results in greater damage and causes larger alterations of coal mechanical properties. (3) The change of the coal matrix is essentially controlled by the surface free energy of the molecular system. E valence affects the aromatic structure and changes the volume of the intramolecular pores, thus affecting the sorption-induced strain change rate. E non affects the length of side chains and the disorder degree of coal molecules and changes the volume of the intramolecular pores, thus affecting the mechanical property change rate. Our findings shed light on the dynamic process of coal swelling and provide a theoretical basis for CO2 enhancing the recovery of CH4 gas in coal.
... Its accuracy affects the accuracy of the simulation results. For polyethylene, we referred to the research of D. Hossain et al. on the polymer deformation mechanism [47]. The force field parameters were selected, as shown in Table 1. ...
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Understanding the properties of polymer–metal interfacial friction is critical for accurate prototype design and process control in polymer-based advanced manufacturing. The transient polymer–metal interfacial friction characteristics are investigated using united-atom molecular dynamics in this study, which is under the boundary conditions of single sliding friction (SSF) and reciprocating sliding friction (RSF). It reflects the polymer–metal interaction under the conditions of initial compaction and ultrasonic vibration, so that the heat generation mechanism of ultrasonic plasticization microinjection molding (UPMIM) is explored. The contact mechanics, polymer segment rearrangement, and frictional energy transfer features of polymer–metal interface friction are investigated. The results reveal that, in both SSF and RSF modes, the sliding rate has a considerable impact on the dynamic response of the interfacial friction force, where the amplitude has a response time of about 0.6 ns to the friction. The high frequency movement of the polymer segment caused by dynamic interfacial friction may result in the formation of a new coupled interface. Frictional energy transfer is mainly characterized by dihedral and kinetic energy transitions in polymer chains. Our findings also show that the ultrasonic amplitude has a greater impact on polymer–metal interfacial friction heating than the frequency, as much as it does under ultrasonic plasticizing circumstances on the homogeneous polymer–polymer interface. Even if there are differences in thermophysical properties at the heterointerface, transient heating will still cause heat accumulation at the interface with a temperature difference of around 35 K.
... The density of the amorphous PE system depends on the initial number of united atoms present, number and type of chains. After equilibrating, the density of the amorphous PE system was found to be 0.843 g/cm 3 , which is very close to the value reported in the literature [12]. Figure 1a shows the relaxed and equilibrated amorphous PE system, and Figure 1b shows the corresponding density variation with respect to the equilibration time. ...
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Packaging material has a significant role in maintaining or altering the shelf life of different products. Polymer materials are extensively used as packaging materials for different perishable and non-perishable products both during transportation and storage. This article aims at developing a new polymer composite which can be used as packaging material. This new composite addresses the challenge of controlling oxygen diffusion rates during the storage of perishable goods such as vegetables, meat and produce, etc. The proposed new composite primarily consists of nonacosan-10-ol and polyethylene. Molecular dynamics simulations (MDS) are performed by mixing 5.2%, 17.1%, 29.2%, 40.8% and 45.2% (wt/wt) of nonacosan-10-ol to amorphous polyethylene. Mechanical properties such as Young’s modulus/glass transition temperature, and gas transport properties such as diffusion coefficient and diffusion volume are estimated from the MDS and diffusion related simulations consisting of different oxygen concentrations in polyethylene-alone system and polyethylene- nonacosan-10-ol blends. The impact of adding different weight percent of nonacosan-10-ol to polyethylene is quantitatively assessed and optimal composition of the proposed additive is suggested corresponding to minimal oxygen diffusion rate, high elastic modulus and good thermal stability.
... After the yield plateau stage, with the further increase in the strain, the axial stress starts to increase again. This stage is defined as the strain hardening stage (Stage V) [45,46]. Under uniaxial compression, the TFPU grouting materials usually undergo the strain hardening [7,8], and it is thought to be due to the compaction of the samples. ...
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Two-component foaming polymer (TFPU) grouting material is increasingly used in civil engineering. Its compressive strength is key to achieving the desired enhancing effect. The constitutive model of TFPU grouting material is a theoretical basis to evaluate the strength performance, which, however, is not fully understood. Here the uniaxial compression experiment of TFPU samples of different densities (0.11–0.53 g·cm ⁻³ ) was conducted. Based on the stress–strain curves, the damage evolution equation of each sample was obtained by function fitting, followed by the establishment of statistical damage constitutive model. The model was simplified to a universal function with density as the argument. Results show that the stress–strain curves contain the initial compression stage, linear elastic stage, yield stage, yield plateau stage, and strain hardening stage regardless of the varied density. The variation laws of the damage with strain conform to the form of first-order decay exponential function. The theoretical stress–strain curves are in good agreement with the experimental ones, indicating that the statistical damage constitutive model can well reflect the mechanical behavior of TFPU grouting material. With this constitutive model, the mechanical properties of TFPU grouting material can be obtained according to the density alone, which is more convenient for practical engineering applications.
... The polyethylene model with mixed crystalline and amorphous regions was built from the view of microscopic or submicroscopic. Preliminary model was with 4 × 4 × 80 crystalline and 80 irregular molecular chains of 100-monomer, and the initial amorphous model density is set to 0.8 g/cm 3 [27,28]. In the simulation, it was subjected to 0.5 × 10 4 fs of constant pressure relaxation under a regular system synthesis (NVT) with reaction conditions set to 500 K with the time step of 0.5 fs, followed by 0.5 × 10 4 fs of constant pressure relaxation under an isothermal isobaric system synthesis (NPT) with reaction conditions set to 500 K with the time step of 0.5 fs. ...
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Mechanical stresses generated during manufacturing and laying process of high voltage cables can result in degradation of insulation properties, affecting the stable operation of the transmission system. Traditional test methods for testing the effect of mechanical stress on the insulation properties of polyethylene still have some shortcomings to be explored and it is able to explain the changes of the insulation properties of polyethylene under mechanical stress from a microscopic perspective. In order to further study the effect of stress on the insulation properties of polyethylene, microstructural changes, the breakdown field strength, conductivity and charge distribution of polyethylene at different elongation rates are investigated by a combination of experimental and molecular dynamics simulations. The results show that the increase in stress leads to a decrease in crystallinity and microcrystalline size of the material decrease. The untwisting and orientation of the polyethylene molecular chains during the stretching process can create cavities, resulting in an uneven sample distribution and thickness reduction, leading to a reduction in the breakdown field strength. Meanwhile, some crystal regions are transformed into amorphous regions. The loose amorphous regions facilitate the directional migration of carriers, resulting in the increase of conductivity. When the elongation ratio is smaller, the distance between the molecular chains increases and the trap depth of the specimen becomes shallower. This facilitates the migration of ions and electrons and increases the rate of decay of the surface potential. When the stretch is further increased, the trap depth will become larger, decreasing the decay rate of the surface potential and reducing the insulation properties of the polyethylene. Meanwhile, the molecular dynamics model of semi-crystalline polyethylene was developed to observe the microstructure and energy changes during the stretching process. The conclusions in terms of tensile tests were verified from a microscopic perspective.
... The hydrogen atoms of CH 2 (including CH 3 ) in each monomer are united with a carbon atom to form a single atom in the polyethylene molecular chain model, after which the Dreiding potential [21] is employed to describe the interatomic interactions of the polyethylene molecular chain. In prior molecular dynamics simulation literature, the Dreiding potential [22,23] has been extensively employed and proved effective, and it is advantageous for modeling the spatial conformation of polymer molecular chains. At the same time, compared with the all-atom model and FENE model, studies have shown that the atomic structure of polymer melts or biomolecular aggregates can be effectively simulated for long time and large length scales by united-atom modeling with an efficient backmapping methodology [20]. ...
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Wall slip directly affects the molding quality of plastic parts by influencing the stability of the filling flow field during micro injection molding. The accurate modeling of wall slip in nanochannels has been a great challenge for pseudoplastic polymer melts. Here, an effective modeling method for polymer melt flow in nanochannels based on united-atom molecular dynamics simulations is presented. The effects of driving forces and wall–fluid interactions on the behavior of polyethylene melt under Poiseuille flow conditions were investigated by characterizing the slip velocity, dynamics information of the flow process, and spatial configuration parameters of molecular chains. The results indicated that the united-atom molecular dynamics model could better describe the pseudoplastic behavior in nanochannels than the commonly used finitely extensible nonlinear elastic (FENE) model. It was found that the slip velocity could be increased with increasing driving force and show completely opposite variation trends under different orders of magnitude of the wall–fluid interactions. The influence mechanism was interpreted by the density distribution and molecular chain structure parameters, including disentanglement and orientation, which also coincides with the change in the radius of gyration.
... This method has been well used in previous studies [33]. The FV calculation under the application of external stress was performed for polymer and amorphous materials [34], which have an FV distribution through the whole system. In this work, this method is also applied firstly for GB investigation since the FV in GB is expected to change in the GB failure process. ...
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In the present work, the evolution of atomic structures and related changes in energy state, atomic displacement and free volume of symmetrical grain boundaries (GB) under the effects of external strain in body-centered cubic (bcc) iron are investigated by the molecular dynamics (MD) method. The results indicate that without external strain, full MD relaxations at high temperatures are necessary to obtain the lower energy states of GBs, especially for GBs that have lost the symmetrical feature near GB planes following MD relaxations. Under external strain, two mechanisms are explored for the failure of these GBs, including slip system activation, dislocation nucleation and dislocation network formation induced directly by either the external strain field or by phase transformation from the initial bcc to fcc structure under the effects of external strain. Detailed analysis shows that the change in free volume is related to local structure changes in these two mechanisms, and can also lead to increases in local stress concentration. These findings provide a new explanation for the failure of GBs in BCC iron systems.
... The stress-strain behavior and Young's modulus results are illustrated in Figure 3a,b. As shown in Figure 3b, taking the P(VDF-HFP)/5 vol%-A@Z as an example, the stressstrain curve has four distinct regimes [25]: elastic regime, yield regime, softening regime and hardening regime. In the elastic regime, the stress increases nearly linearly with increasing applied strain. ...
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Polymer materials with excellent physicochemical and electrical properties are desirable for energy storage applications in advanced electronics and power systems. Here, Al2O3@ZrO2 nanoparticles (A@Z) with a core-shell structure are synthesized and introduced to a P(VDF-HFP) matrix to fabricate P(VDF-HFP)/A@Z nanocomposite films. Experimental and simulation results confirm that A@Z nanoparticles increase the crystallinity and crystallization temperature owing to the effect of the refined crystal size. The incorporation of A@Z nanoparticles leads to conformational changes of molecular chains of P(VDF-HFP), which influences the dielectric relaxation and trap parameters of the nanocomposites. The calculated total trapped charges increase from 13.63 μC of the neat P(VDF-HFP) to 47.55 μC of P(VDF-HFP)/5 vol%-A@Z nanocomposite, indicating a substantial improvement in trap density. The modulated crystalline characteristic and interfaces between nanoparticles and polymer matrix are effective in inhibiting charge motion and impeding the electric conduction channels, which contributes to an improved electrical property and energy density of the nanocomposites. Specifically, a ~200% and ~31% enhancement in discharged energy density and breakdown strength are achieved in the P(VDF-HFP)/5 vol%-A@Z nanocomposite.
... As shown in Fig. 15b, the breakage of covalent bonds left the aromatic layer existing as free radicals and allowed the aromatic layer to rearrange, which could evolve into more parallel stacking structure as a result of the π-π stacking interaction . Moreover, it has long been recognized that strain could impart a preferential orientation to the macromolecule of coals (Blanche, 1995;Bustin et al., 1995) and polymers (Jawhari et al., 1995;Hossain et al., 2010). Though accompanying the generation of structural defects, plastic deformation occurs more easily, the strain magnitude paralleling the bedding plane does not vary significantly among different samples (5.6-8.0%). ...
Preprint
Despite the vast research studying the influence of stress on the physical structure, little is known whether and how stress works on the chemical structure in coals. In the present study, some insights are given by investigating the macromolecular structure evolution characteristics and mechanism of a Chinese anthracite (Ro,max = 3.6%) during deformation experiments at temperatures of 300–400 °C, confining pressure of 150 MPa, and strain rates of 2.5–20*10⁻⁶ s⁻¹. Additional heat treatments (without stress) were also performed at the same temperature range. Revealed by vitrinite reflectance, XRD, and Raman spectroscopy, a smaller and less ordered macromolecular structure was observed within experimental conditions. Upon heating alone, Ro,max and stacking diameter (La) increased together, while stacking height (Lc) and structure ordering (ID/IG and FWHM of D and G peaks) decreased with increasing temperature, indicating structure rearrangement and relaxation. In contrast, the macromolecular structure in deformation experiments showed a two-stage evolution with decreasing strain rate. Stage 1 was characterized by the obvious decreases in Ro,max, La, and Lc, but minor changes in Raman spectra, implying stress might break in-plane and inter-layer chemical bonds at high strain rates. Stage 2 was identified by the relative increases in Ro,max, La, and FWHM of D peak, in combination with Lc, mainly indicating the generation of structural defects at low strain rates allows the aromatic layer to accommodate the stress without breakage. Our results suggest stress can act on the macromolecules directly by breaking or distorting the chemical bonds depending on the strain rate.
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Polymer nanocomposites with excellent mechanical performances have been increasingly sought after in engineering applications such as biotechnology, aerospace, and automotive areas. Through molecular dynamics (MD) simulation, this work systematically assessed the tensile performance of poly(methyl methacrylate) (PMMA) nanocomposite reinforced by randomly dispersed two-dimensional diamond - diamane. It is found that randomly dispersed diamane effectively enhances the tensile properties of PMMA with surface functionalization, and the enhancement effect can be remarkably augmented by cross-linking. Simulations reveal that the enhancement effect can be effectively tailored by the alignment of the diamane fillers. The PMMA nanocomposites exhibit much better tensile performance when the diamane fillers are uniformly aligned along the in-plane direction of the filler. Additional investigations show that larger diamane filler is preferred without cross-linking, while smaller diamane filler should be considered when cross-linking is present. Overall, the impacts of different factors on the tensile properties of PMMA nanocomposites are analysed in-depth in this work, which provides atomistic insights for the preparation of polymer nanocomposites with desired mechanical properties.
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We study and compare four coarse-grained models of cis-1,4-polyisoprene distinguished by mapping schemes for locating superatoms. First, coarse-grained potentials are obtained by iterative Boltzmann inversion method for the polymer melt. For all the coarse-grained models considered, time-scale factors based on translational and rotational motion were found to be different. However, coarse-grained potentials were unable to reproduce stress–strain behaviour of the underlying detailed model due to weak attractive nature of the nonbonded part of the coarse-grained potential. Consequently, the nonbonded potentials were optimized using particle swarm optimization to match the tensile behaviour of a detailed model of the polymer below glass transition temperature. While the modified potentials seemed to better predict the mechanical behaviour, the ability to accurately predict simultaneously the structural distributions also, depends on the mapping scheme.
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In this work, the mechanism and optimal ratio of dendritic amino‐terminated aromatic polyamide G2 as damping agent to improve the damping properties of chlorinated butyl rubber (CIIR) were studied. G2 can effectively improve the damping properties of CIIR composite by hydrogen‐bonding interactions. The types and quantities of hydrogen bonds in the gradient proportional G2/CIIR_Cell systems were counted, while the contribution of single factor hydrogen bond to the damping characteristics of G2/CIIR composite was explored. Finally, the proportion of G2/CIIR composite with optimal damping properties was selected to be 22/100, which contained 6 α‐type hydrogen bonds and 12 β‐type hydrogen bonds. Meanwhile, this CIIR composite showed excellent mechanical properties, as well as its glass transition temperature (Tg) was 223.7845 K. Thus, it was a kind of CIIR composite material with high damping properties and wide application prospects. Dendrimer G2 was used as polymer additive to improve the damping properties of CIIR by hydrogen bonding mechanism. The contribution of different types and quantities of hydrogen bonds to the damping properties of CIIR was studied. The damping properties of G2/CIIR(22/100) composite was improved while its mechanical properties was excellent. Dendrimer G2 was used as polymer additive to improve the damping properties of CIIR by hydrogen bonding mechanism. The contribution of different types and quantities of hydrogen bonds to the damping properties of CIIR was studied. The damping properties of G2/CIIR(22/100) composite was improved while its mechanical properties was excellent. This article is protected by copyright. All rights reserved
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Electroless plating of membranes offers a viable pathway to create flexible electrodes for soft sensors and actuators, as well as flexible electronics and batteries. Ionic polymer metal composites are a promising class of active materials, realized through electroless plating of ion‐exchange membranes. The plating and electrode‐membrane interface play a key role on their performance, but computational tools to inform the selection of the plating material and optimize the plating process are currently lacking. Here, this gap is filled through the study of the electrode‐membrane interface in different types of ion‐exchange membranes via molecular dynamics simulations. Both commercially available cation‐ and research‐grade anion‐exchange membranes are studied here. For platinum coating, it is predicted that cation‐exchange membranes will have a superior interface than anion‐exchange membranes, in terms of metal penetration into the membrane, reliability of actuation performance, and interface stability. The results are in line with previous endeavors documenting the higher stability of the interface for cation‐ than for anion‐exchange membranes, easier plating processes, and better electrochemical performance when working with cation‐exchange membranes. The proposed computational framework offers a versatile environment for testing different types of coatings for specific membranes, toward optimizing the performance of electrochemical devices with plated flexible electrodes. Plating of ion‐exchange membranes is a critical step to create flexible electrodes for soft sensors and actuators. Yet, no computational tool to optimize the plating process is currently available. Molecular dynamics simulations could serve as a benchmark to inform the selection of plating materials, as demonstrated in this comparative study that explores the stability of different composites of ion‐exchange membranes.
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The removal of organic pollutants is a major challenge in wastewater treatment technologies. Coagulation by plant proteins is a promising technique for this purpose. The use of these proteins has been experimentally investigated and reported in the literature. However, the determination of the molecular interactions of these species is experimentally challenging and the computational approach offers a suitable alternative in gathering useful information for this system. The present study used a molecular dynamic simulation approach to predict the potentials of using Moringa oleifera (MO), Arachis hypogaea, Bertholletia excelsa, Brassica napus, and Helianthus annuus plant proteins for the coagulation of organic pollutants and the possible mechanisms of coagulation of these proteins. The results showed that the physicochemical and structural properties of the proteins are linked to their performance. Maximum coagulation of organic molecules to the proteins is between 50–100%. Among five proteins studied for coagulation, Brassica napus and Helianthus annuus performed better than the well-known MO protein. The amino acid residues interacting with the organic molecules play a significant role in the coagulation and this is peculiar with each plant protein. Hydrogen bond and π—interactions dominate throughout the protein–pollutants molecular interactions. The reusability of the proteins after coagulation derived from their structural quality analysis along with the complexes looks promising and most of them are better than that of the MO. The results showed that the seed proteins studied have good prediction potentials to be used for the coagulation of organic pollutants from the environment, as well as the insights into their molecular activities for bioremediation.
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This paper discusses some of the fundamental mechanisms of atomic-scale deformation associated with inter and intramolecular bond forces under applied loads in polyurea, through simulations using a detailed all-atom molecular dynamics model. The study demonstrates the evolution of conformational changes in the polymer molecular structure, as well as how internal variables associated with bond length, bond angle, bond dihedral, chain entanglements of the soft regime of the molecule, and free volume evolve with tensile loading at high strain-rates. Furthermore, it explores the effect of strain rates, simulation constraints, and nanofillers (buckyballs) on the overall polyurea response and properties. The observations made in this study can be used in the design of materials that can exhibit superior performance in extreme environments.
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Polyethylene plays important roles in human lives all over the world due to its good properties and wide applications. In this study, polyethylene chains under different temperatures (400, 450, 500, 550 and 600 K) and pressures (0, 1, 10, 50 and 100 atm) were simulated by using molecular dynamic simulation method, and the properties such as micro-structure, coordination number, stable conformation, density, and self-diffusion coefficient were detailed. The results show that when the temperature increases from 400 K to 600 K at 1 atm, the average bond distance increases from 1.5299 Å to1.5313 Å, and the coordination number declines from 17.4 to 13.9, making the density decrease from 0.82 g/cm³ to 0.72 g/cm³. The self-diffusion coefficient increases from 0.13 nm²/ns to 0.34 nm²/ns. When the pressure increases from 0 atm to 100 atm at the temperature of 500 K, the non-bonded interaction energy decreases from −1.171 kcal/mol to −1.186 kcal/mol, and the coordination number rises from 15.5 to 15.8, making the density increase from 0.772 g/cm³ to 0.782 g/cm³. The self-diffusion coefficient decreases from 0.39 nm²/ns to 0.36 nm²/ns. The knowledge of polyethylene chain at different temperatures and pressures will boost better understand the industrial solution polymerization process and solid waste pyrolysis engineering of plastics.
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The poor toughness of polylactic acid (PLA) has led to imposing certain restrictions on its use in various applications. In similar cases, introducing nanostructures is known to be a practical strategy for tailoring the mechanical features of polymeric materials. Herein, the contributory impacts of the presence of pristine and surface-treated boron nitride nanosheets on the mechanical behavior of PLA was explored through molecular dynamics simulations. Comparatively, it was observed that embedding 3 wt.% untreated boron nitride nanosheet (BNNS) improved Young’s modulus and toughness of the polymer by 24.5% and 42%, respectively. Both chemical and physical surface treatments resulted in significantly lower polymer chains mobility during the deformation by intensifying interfacial interactions. Moreover, the obtained findings illustrated that applying either of chemical hydroxyl functional groups or PLA chains onto BNNS would contribute to the enhancement of PLA toughness. Among all the analyzed systems, approximately 5% functionalized nanosheets indicated the best toughening role by increasing PLA absorbed energy per unit volume by 131% on average although the enhancing role of chemically functionalized nanofiller was slightly better. The present findings could give a deeper insight into obtaining more efficacious designs for tailoring mechanical properties of polymer-based nanocomposites.
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Polymeric nanofibers demonstrate core-shell structure when their diameter drops below the nanoscale. Understanding the corresponding mechanism at the atomistic scale paves the way for achieving the long-term goal of adjusting their microstructure and controlling their properties purposefully. To explain the current contradictory views on the origins of the nanofiber core-shell structure, a molecular dynamics (MD) study is performed. In the MD simulation, cold-drawing and hot-drawing methods are implemented to investigate the role of evaporation rate, stretching force and external temperature on the formation of polymeric nanofiber’s core-shell structure. The results show that the distribution of the solvent atoms inside the as-formed nanofiber is strongly associated with the evaporation rate. The increase of the stretching force during the hot-drawing or cold-drawing process has a significant influence on the polymeric chain orientation inside the nanofiber. The external temperature has little influence on nanofiber formation. The final microstructure of nanofibers relies on the interplay between the stretching force and the evaporation rate. When the evaporation is dominant, fiber tends to form a tabulated structure with a less dense core embedded with a denser shell layer. However, when the stretching force is dominant, the fiber may form a denser structure with chains more aligned.
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The increasing request for dielectric polymers is drawing research efforts on designing new copolymers with high energy density and long-term cyclic stability. Herein, the all-atom molecular dynamics simulation coupled with density functional theory is applied to investigate how the chain sequence structure affects the dielectric, viscoelastic, and energy-saving properties of poly(ethylene-co-methyl acrylate). For various chain sequences, their dielectric ratios, actuation sensitivities, and hysteresis loss have been quantitatively evaluated to analyze the corresponding dielectric efficiencies, mechanical flexibilities, and cyclic stabilities, respectively. It is demonstrated that dielectric efficiency is mainly determined by dipolar polarization depending on charge distribution and surface electrostatic potential, while mechanical flexibility is associated with the coupling effect of dielectric strength and Young's modulus. Accordingly, a few of chain sequences with optimized performance have been picked out, providing the guidance for actual syntheses of promising dielectric copolymers.
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Full-atomic molecular dynamics simulations were conducted to investigate the time evolution of microscopic damage in polyetheretherketone (PEEK) polymers under cyclic loading conditions. Three characteristics were used to quantify microscopic damage: entropy, distribution of the end-to-end distance of polymers, and the volume fraction of voids. Our results show that the degree of disentanglement of polymers and the volume fraction of voids increase with cyclic loading, which may lead to entropy generation. Uniaxial tensile strength simulations of the polymer system before and after cyclic loading were performed. The tensile strength after cyclic loading was lower than that before loading. Furthermore, two systems with the same entropy and different loading histories showed almost the same strength. These results imply that entropy generation is expressed as the total microscopic damage and can potentially be employed for effective evaluation of the degradation of material characteristics.
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The impact of nanoparticles (NPs) composed of atoms with covalent bonding is investigated numerically and theoretically. We use recent models of covalent bonding of carbon atoms and elaborate a numerical model of amorphous carbon (a-C) NPs, which may be applied for modeling soot particles. We compute the elastic moduli of the a-C material which agree well with the available data. We reveal an interesting phenomenon-stress-dependent adhesion, which refers to stress-enhanced formation of covalent bonds between contacting surfaces. We observe that the effective adhesion coefficient linearly depends on the maximal stress between the surfaces and explain this dependence. We compute the normal restitution coefficient for colliding NPs and explore the dependence of the critical velocity, demarcating bouncing and aggregative collisions, on the NP radius. Using the obtained elastic and stress-dependent adhesive coefficients we develop a theory for the critical velocity. The predictions of the theory agree very well with the simulation results.
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The multiaxial deformation behaviors of amorphous epoxy polymers were characterized using planar biaxial compression tests and finite element (FE) analyses of cruciform specimens. The multiaxial limit of elastic deformation was investigated considering buckling instability of the planar biaxial compression tests. An offset-based yield criterion was proposed to determine the initial yield surface considering the nonlinear elastic deformation characteristics observed in the deformation tests. An FE analysis confirmed using the results of the tests revealed that the proposed method captured the initial multiaxial yielding in the central gauge of the specimen, demonstrating that existing pressure-dependent classical yield functions, such as the paraboloidal and conical yield functions, cannot appropriately describe the initial yield surface of the epoxy polymer. The yield function previously customized from molecular dynamics (MD) data exhibited a relatively good agreement with the initial yield surface. The determined shape of the initial yield surface and its agreement with the MD data–driven yield function were not consistently maintained during subsequent yielding. The results indicate that a new customized yield function properly describing the broader initial yield surface than those of the classical yield functions and its shape transition to plastic behavior is necessary to accurately reflect the entire multiaxial deformation behavior.
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The aim of this article was to reveal the dynamic and structural response of hexagonal boron nitride (h-BN) reinforced polyethylene (PE) nanocomposites under shock/impact loading. Micro- and nano-scale deformation dynamics of h-BN reinforced PE nanocomposites were captured using experimental and atomistic-based combined approaches. The experimental approach was adopted to investigate the effect of fabrication techniques and the concentration of boron nitride nanoplatelets (BNNP) on the impact behavior of PE-based nanocomposites. The impact strength of nanocomposites was studied with the help of differential scanning calorimetry in conjunction with the plastic deformation governing mechanism captured using the micrographs of scanning electron microscopy (SEM). It was reported from the experiments that the crystallinity of BNNP/PE nanocomposites is significantly affected by the fabrication technique. The BNNP/PE nanocomposites fabricated with the solvent blending method show superior impact strength as compared to samples fabricated using the colloidal solution method at the same BNNP weight concentration of 5%. It was also revealed from the SEM observations that the addition of BNNP to the PE matrix shifts the plastic deformation mechanism from a combination of craze and drawing of fibrils in pure PE to brittle failure in BNNP/PE nanocomposites. To complement the experimental observations, molecular dynamics-based simulations were performed to study them effect of orientation and distribution of boron nitride nanosheets (BNNS) on shock mitigation capabilities of BNNS/PE nanocomposite. It was predicted from MD simulations that a parallel oriented nanosheet remains longer in contact with the shock, which helps in dissipating energy from the shock wave. As compared to stacked, dispersed nanosheets have superior shock attenuation capabilities. These findings could be helpful to design a roadmap for shock wave mitigation through impedance mismatch and energy disruption across the multiple interfaces of BNNS and polymer matrix.
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In this paper, the hybrid atomistic-continuum (HAC) method is used to investigate the properties of fluid flow inside a cavity on different surfaces. To extract the microscopic quantities of the flow on surfaces with different conditions (rough or smooth), a combination of Materials Studio 2017 and LAMMPS software and to extract the macroscopic results of the flow, OpenFOAM software has been used. In this paper, a strategy is used to reduce the thickness of atomic domain, which can reduce the time consuming associated with HAC solver by up to 17%. The interaction of argon gas with different surfaces of copper, iron, nickel, platinum, silicon, aluminum and amorphous polyethylene was also examined using the two mixing laws of Lorentz–Berthelot (LB) and Fender–Halsey (FH), and the effects of surface material and the type of mixing law on the formation of flow inside the cavity were investigated. Based on different simulations, it was shown that the FH mixing law can predict the potential energy more accurately than the LB mixing law. It was also shown that the TMAC increases by 4 and 3%, respectively, by creating the protrusions on the iron surface and creating the xenon coating on it.
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Cycloolefin copolymer (COC) could be a best promising commercial polymer dielectric for metallized film capacitors at elevated temperature according to the molecular structure, but the dielectric energy storage about COC remains a huge challenge due to the lack of processing strategies of its ultrathin films. Herein, we demonstrate that COC dielectric film of around 10 μm can be fabricated by solution casting followed by uniaxial stretching and steadily service at 140 °C. Excitedly, the charge-discharge efficiency (η) of COC film is up to 97.8 % at 600 MV/m at 25 °C, which is superior to the commercially available biaxially oriented polypropylene (BOPP). Remarkably, at 140 °C, COC film exhibits a maximum discharge energy density (Ue) of 2.32 J/cm³ and a η of 85.7 %, far outperforming the heat-resistant polyimide (PI) with a maximum Ue of 1.00 J/cm³ and a η of 24.1 %. Our work provides a novel strategy for the manufacture of ultrathin COC film and could promote the commercialization of high-temperature polymer capacitor films for the energy storage.
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Plastic deformation of polyethylene in uniaxial and biaxial loading conditions is studied using molecular dynamics simulation. Effects of tensile strain rates from 1 × 10 ⁵ to 1 × 10 ⁹ s ⁻¹ , and mass density in the range of 0.923–0.926 g/cm ³ on mechanical behaviour and microstructure evolution are examined. Two biaxial tensile deformation modes are considered. One is through simultaneous stretching in both the x and y directions and the other sequential stretching, firstly in the x-direction and then in the y-direction while strain in the x-direction remains constant. Tangent modulus and yield stress that are determined using the stress–strain curves from the molecular dynamics simulation show a strong dependence on the deformation mode, strain rate and mass density, and all are in good agreement with results from the experimental testing, including fracture behaviour which is ductile at a low strain rate but brittle at a high strain rate. Furthermore, the study suggests that the stress–strain curves under uniaxial tension and simultaneous biaxial tension at a relatively low strain rate contain four distinguishable regions, for elastic, yield, strain softening and strain hardening, respectively, whereas under sequential biaxial tension, stress increases monotonically with the increase of strain, without noticeable yielding, strain softening or strain hardening behaviour. The molecular dynamics simulation also suggests that an increase in the strain rate enhances the possibility of cavitation. Under simultaneous biaxial tension, with the strain rate increasing from 1 × 10 ⁶ to 1 × 10 ⁹ s ⁻¹ , the molecular dynamics simulation shows that polyethylene failure changes from a local to a global phenomenon, accompanied by a decrease of the void size and increase of uniformity in the void distribution. Under sequential biaxial tension, on the other hand, the density of the cavities is clearly reduced.
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The nanoscale morphologies of block copolymer (BCP) thin films are determined by chain architecture. Experimental studies of thin film blends of different BCP chain types have demonstrated that blending can stabilize new motifs, such as coexistence phases. Here, we deploy coarse-grained molecular dynamics (MD) simulations in order to better understand the self-assembly behavior of BCP blend thin films. We consider blends of lamella- and cylinder-forming BCP chains, studying their morphological makeup, the chain distribution within the morphology, and the underlying polymer chain conformations. Our simulations show that there are local concentration deviations at the scale of the morphological objects that dictate the local structure, and that BCP chains redistribute within the morphology so as to stabilize the structure. Underlying these effects are measurable distortions in the BCP chain conformations. The conformational freedom afforded by BCP blending stabilizes defects and allows coexistence phases to appear, while also leading to kinetic trapping effects. These results highlight the power of blending in designing the morphology that forms.
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The mechanical response of polyethylene nano-fibers with the same chain length but different chain numbers are studied by using steered molecular dynamics simulations. The shrinking or stretching forces acted on the chain ends are investigated according to the chain-end distance and temperature under isothermal or continuous warming-cooling conditions, respectively. An inflection point is found in the Force-Distance response when temperature is below 500 K. This inflection point is related to the balance between entropy force and inter-monomer interaction and it reflects the strong effect of crystallization on the mechanical response of the nano-fibers. The force at inflection point is also affected by the buckling effect due to increased stiffness when crystallization occurs. The two stages found in the Force-Temperature response and the difference between the shrinking and stretching forces indicate the hysteresis of crystallization and melting. The forces at different shrinking and stretching rates reveal the entropy contribution upon the mechanical response, indicated by the Ramachandran plot of dihedrals. The chain-conformation entropy is majorly contributed by dihedrals and is quantified by the information entropy of dihedrals, which has a highly similarity to the mechanical Force-Temperature response. The enlarged forces in multiple chains over a single chain are attributed to the enhanced dihedral-conformation entropy. Our study provides a new insight to the dynamically mechanical response of polymer nano-fibers according to the effect of crystallization and entropy contribution.
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Nanoparticles are prone to aggregation in the matrix under the action of surface energy, chemical bonding, electrostatic attraction, and van der Waals forces. Based on the molecular dynamics method, the tensile properties of graphene oxide (GO)‐reinforced nylon 66 composites were simulated and calculated, and the stress–strain curves, atomic stress clouds, and changes in the number of hydrogen bonds were obtained for the composites under tensile loading, and the effects of GO agglomeration on the mechanical properties of GO/PA66 composites were analyzed. When the GO mass fraction accounted for 90.3%, 80.5%, and 70.6% of the model, respectively, the mechanical properties of the composites decreased by 15.4%, 4.5%, and 4.2%, respectively. Because GO agglomeration in the composites made it difficult to bond effectively between GO and PA66, the number of hydrogen bonds and the type of hydrogen bonds within the model, varied with the degree of aggregation. The changes in the number of hydrogen bonds and the type of hydrogen bonds during uniaxial stretching were simulated by molecular dynamics. The number of hydrogen bonds and the type of hydrogen bonds directly affect the mechanical properties of the composites. It can be seen from the atomic stress cloud diagram that the aggregation of GO makes the mechanical properties of GO fail to exert effectively. The main stress is on the C atoms in the outermost layer of GO, while the stress on the inner layer C atoms is relatively small. As the degree of GO agglomeration decreases, the stress on each C atom of GO lamellae tends to be uniform, which effectively brings into play the excellent mechanical properties of GO and improves the mechanical properties of the composite. GO/PA66 model.
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Pre-deformed glassy polymers exhibit distinct stress responses with opposite loading directions, referred to as the Bauschinger effect. Although this phenomenon has been known for decades, the underlying microscopic origin remains largely elusive. In this work, we perform coarse-grained molecular dynamics (CGMD) tension and compression simulations on a typical glassy polymer polycarbonate. The intermedia variables of self-entanglement and network orientation are extracted to describe the internal microstructure change during deformation. The results show that the competition between intra-chain deformation and inter-chain friction leads to the occurrence of yielding, while strain hardening is governed by the increase of inter-chain friction. Motived by the physical mechanisms revealed by the CGMD simulations, we further develop a mean-field shear transformation zone (STZ) model which contains the crucial internal variable of self-entanglement. The theoretical model well captures the yielding, strain hardening and the Bauschinger effect observed in MD simulations. By comparing the mechanical responses of the polycarbonates under tension and compression, we contribute the substantial Bauschinger effect to the distinct deformation mechanisms in these loading processes. The increase in yield strength during tensile-reloading is governed by the decrease of self-entanglement, which leads to enhanced inter-chain friction, while the decreased yield strength during compressive-reloading is associated with the increase of self-entanglement, causing reduced inter-chain friction. Overall, this work promotes the fundamental understanding of the complex mechanical responses of glassy polymers and also provides a new continuum-level theoretical framework for amorphous solids.
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Viscoelasticity-induced structural change in Zr55Cu30Ni5Al10 metallic glass (Zr-MG) and amorphous selenium (a-Se) is investigated using synchrotron X-ray diffraction. By analyzing the two-dimensional diffraction pattern, two types of structural anisotropy with the feature of the residual elastic strain or heterogeneous intensity of diffraction rings are revealed. The origin of the structural anisotropy is attributed to the topological rearrangement in the Zr-MG and conformation rearrangement in the a-Se. Our findings bring a structural identity to the phenomenological structureless deformation defect widely used in different amorphous materials.
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A larger number of studies successfully prepared various polymer materials with excellent self-healing properties, but the study on the underlying self-healing mechanism remains comparably backward and still unclear. In this study, we prepared a self-healing polyurethane-urea (PUU) elastomer based on noncovalent bonds. Then, a coarse-grained model of PUU was successfully constructed using the iteration Boltzmann inversion (IBI) method. Microphase separation and mechanical properties of PUU were reproduced using this model by coarse-grained molecular dynamics (MD) simulation. The three-stage healing mechanism comprised the following: (1) movement of the material to close the gap, (2) interdiffusion of the polymer, and (3) bond exchange. The mechanism was revealed by determining the effects of hard segment content on the microstructure (chain entanglement, interactions of soft and hard segments, chain motility) and healing capacity over healing time. In the initial stage of healing, the polymer chains were disentangled, and the degree of entanglement of the healed samples decreased. A novel experimental strategy confirmed the transition of hydrogen bonds from disorder to order during the healing process. The motility of the cut polymer chains (low molecular weight), especially the cut soft segment, and the disordered hydrogen bonds played a key role in the healing capacity. The increased content of the ordered hydrogen bonds led to the formation of a hard segment network, which was not conducive to healing. Finally, the promoting mechanism of external factors, such as heating and trace amount of solvent, on the healing of PUU was explained. Our work systematically and profoundly reveals the self-healing behavior and mechanism of microphase-separated PUU at the molecular level.
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The fast-growing construction industry has a vast potential to rise to the plastics challenge by using them in both primary and recycled forms as a sustainable solution to some challenges in the built environment. Improving existing plastics and developing innovative polymers and polymer nanocomposites requires knowledge of interatomic interactions and their influence on macroscopic properties. Coarse-grained (CG) models offer a more computationally efficient alternative to their all-atom counterparts for simulating larger, more representative models of these materials. However, the parameterization and calibration process of CG force fields (CG-FFs) commonly entails solving a nonconvex optimization problem involving numerous local minima, rendering traditional optimization techniques impractical and iterations based on educated guesses inefficient. Here, we develop an approach to efficiently parameterize a CG-FF by coupling a metaheuristic algorithm as the calibrator (optimizer) with support vector regression-based surrogate models trained using molecular dynamics data. The merit of the approach is demonstrated by parameterizing a CG-FF potential for polyvinyl chloride (PVC) as a representative general-purpose plastic with many applications in the construction industry. The generalizability of the CG-FF to large PVC models in both pristine and carbon nanotube-filled composite forms is demonstrated. The CG-FF also accurately reproduces glass transition temperature and thermal conductivity as unseen properties of PVC.
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Based on the demand for materials in the aerospace field, we are researching the brittleness problem of phthalonitrile resin (PN75), a high-temperature resistant resin material. Combining molecular dynamics simulations with experiments in time-consuming and costly studies has become a meaningful way to reduce research and development cycle time and cost. In a world where high-performance computer simulations are capable of enormous computational scales with guaranteed reliability, experiments and characterization at the atomic-molecular scale are still challenging tasks. Revealing microscale mechanisms through atomic simulations and providing guidance for laborious, time-consuming, and expensive experiments is one of the current scientific hotspots. The free volume pore distribution of PN75 was analyzed using molecular dynamics (MD) simulations to reveal the primary mechanism of PN75 during toughening. Improving the scalability of the free volume cavity of PN75 creates an appreciable energy dissipation mechanism. The combination of experimental and MD simulation results verifies the usefulness of simulation as a guide for experiments. MD simulations can reduce complex experimental and characterization efforts, reveal experimental mechanisms at the nanoscale, and guide the design of polymers.
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The origin of the mechanical properties of highly cross-linked epoxy networks was theoretically investigated from a subcontinuum perspective. By use of all-atom molecular dynamics (MD) simulations, the macromolecular network of epoxy formed during the cross-linking reactions was classified into subgroups according to their bonding relationship. The deformation energy density applied to the entire system under mechanical loading is expressed by the contribution of each subgroup. The load transfer capabilities according to the chemical bonding state between resin and hardener were then quantified at the atomic level. On the basis of the results, an analytic blending model was established that can predict mechanical properties of the cross-linked epoxy according to its chemical composition and associated network topology. It was confirmed that the proposed model successfully predicts the mechanical properties of materials for the range of composition ratios that can be considered in actual synthesis as well as an in-depth analysis of the individual molecular components.
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To investigate the potential mechanism affecting the mechanical properties of polyethylene (PE) are beneficial to its practical application. In this paper, the atomic motion is decomposed to reveal the underlying mechanisms of how PE mechanical properties are affected by temperature, strain rate, and inter-chain interaction, using molecular dynamics (MD) simulations. It is found that the underlying mechanism of temperature affecting mechanical properties of PE chains is further demonstrated to closely relate with the radial motion of PE atoms. The increasing motion range of PE atoms in the radial direction would result in a decrease of breaking stress and breaking strain of PE chains in the axial direction. In addition, the strain rates would suppress the radial movement range of the PE atoms, which strengthens the mechanical properties of the PE chain. In contrast, the introduction of interaction between PE chains by increasing the chain number causes the radial motion of atoms to be more intense, which reduces the strength of the polyethylene chains. These simulation results are highly expected to provide novel insight to understand the mechanism of materials' mechanical properties.
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Polyethylene (PE) is a candidate liner material for Type IV storage devices. In this case, all-atom molecular dynamics simulations are employed to study the properties of Polyethylene with the presence of H2, including tensile properties, glass transition, diffusion in different PE and bubbling during rapid depressurization. The presence of H2 deteriorates the polyethylene matrix's tensile performance and decreases the glass transition temperature. The branch, side chain and small molecules promote the diffusion of H2 in the amorphous region by introducing more free volume below Tg. With a sufficient length, the length of polymer chain has minor effect on the diffusion of H2. Graphene, as a 2D reinforcement, could decrease the diffusion of H2 but suffers from poor interfacial bonding. Finally, H2 bubble(s) formed from the over-saturated H2 molecule and were observed in both the exclusive and free volume and stabilised at low pressure during rapid depressurization. According to the result obtained in this work, branchless HDPE is expected to give superior performance while the viscosity, which is important during processing, could be tailored by molecular weight. Processing technique leads to orientation is preferred, such as injection moulding.
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Among various graphene-based flexible electronic devices, the graphene/polymer substrate is one of the most common microstructures. Its interfacial mechanical properties directly determine the performance and reliability of these devices. Accordingly, the interaction between single-layer two-dimensional material such as graphene and polymer substrate has become an urgent research field in the manufacture and application of flexible electronic devices. In this study, traction-separation (T–S) models are established to study the interfacial mechanical behaviors of graphene/polyethylene terephthalate (PET) substrate structures using molecular dynamics (MD) simulations. The calculated interface parameters are verified by simulating the blister test with MD simulation and finite element analysis (FEA). Two common types of defects (Stone-Wales (S–W) and single vacancy (S–V)) are considered. By monitoring the aromatic ring distribution (order parameter and concentration) in PET substrate, the results reveal that under the normal loading, the S-W defect in graphene enhances the interfacial strength, while the separation energy is not sensitive to the existence of the S–W defect. The S–V defect in graphene degrades both the normal interfacial strength and separation energy owing to the loss of carbon atoms. Under the shear loading, it is found that the surface roughness of graphene caused by defects is an essential factor affecting the interfacial shear properties at the nanoscale. In addition, the results indicate that a low-concentration S–V defect can increase the surface roughness of graphene to obtain stronger mechanical interlocking, thereby enhancing the interfacial shear properties.
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Stretching is the most important way of forming polymer films, and the mechanism of stretch-induced structural evolution is a long-standing fundamental issue in the stretching process of polymer films. Current methods are limited in sampling frequency or transparent materials and difficult to in situ measure the molecular chain structure evolution. In this study, we proposed a new principle for measuring the stretching process of polymer materials based on dielectric anisotropy. Firstly, the strain-dielectric anisotropy linear relation during small elastic deformation was found, and an extremely small strain (∼0.1%) can be measured with a high sampling frequency (∼8.3 Hz). Then, the molecular chain orientation-induced dielectric anisotropy for the underlying mechanism was confirmed by molecular dynamics simulations and in situ polarized Raman spectroscopy experiments. The proposed method was successfully applied in measuring tube compliance and dynamic health monitoring by measuring small elastic strain accurately, exhibiting the potential for wide applications.
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Nose has modified Newtonian dynamics so as to reproduce both the canonical and the isothermal-isobaric probability densities in the phase space of an N-body system. He did this by scaling time (with s) and distance (with V¹D/ in D dimensions) through Lagrangian equations of motion. The dynamical equations describe the evolution of these two scaling variables and their two conjugate momenta p/sub s/ and p/sub v/. Here we develop a slightly different set of equations, free of time scaling. We find the dynamical steady-state probability density in an extended phase space with variables x, p/sub x/, V, epsilon-dot, and zeta, where the x are reduced distances and the two variables epsilon-dot and zeta act as thermodynamic friction coefficients. We find that these friction coefficients have Gaussian distributions. From the distributions the extent of small-system non-Newtonian behavior can be estimated. We illustrate the dynamical equations by considering their application to the simplest possible case, a one-dimensional classical harmonic oscillator.
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A theory of yielding of glassy polymers by thermally-activated production of local molecular kinks is described. The theory predicts the yield stress at absolute zero to be dependent only on the shear modulus and the Poisson's ratio, and is capable of describing the temperature, pressure, and strain rate dependences of the flow stress from absolute zero to near the glass transition temperature.
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Plastic deformation in a structurally well-relaxed two-dimensional atomic glass was simulated by a computer molecular dynamics approach. The simulation, which was carried through yielding and to substantial plastic strains, demonstrated that the principal mechanism of plastic strain production is by local partly dilatant shear transformations nucleated preferentially in the boundaries of liquid-like material separating the small quasi-ordered domains that form when the glass is well relaxed. Under imposed forward shear-strain increments, local shear transformations in atomic clusters were found to be mostly in the same direction as the applied stress. There were, however, substantial levels of shear transformations in other random directions, including many opposed to the applied stress. In all instances, however, nucleaton of shear transformations reduced the Gibbs free energy monotonically, which is governed largely by the locked-in excess enthalpies of the glassy state. At shear strains above 15%, localization of shear into bands was observed to begin. This steadily intensified and formed well-defined sharp shear bands into which all the shear strain became concentrated by the end of the simulation at a strain of 27%. A strong correlation was found between the tendency for shear localization and retained shear-induced dilatation.
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We have simulated semi crystalline polyethylene (PE) using Monte Carlo method with the Metropolis dynamics. The mechanical response is evaluated in the first steps of deformation. The simulation results are compared with the experimental curves at different conditions concerning temperature and deformation rate. The obtained results allow us to suggest that the method of MC together with the model and the dynamics employed give a satisfactory description of the mechanical properties of lineal polymers as the polyethylene. The simulations MC allows to differentiate the effect of the diverse parameters as the temperature, the test time, chain length and density which affect the mechanical behavior. The study of the non-reversible strain up to reading the material fracture is projected for future work.
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This paper presents the formulation of a constitutive model for amorphous thermoplastics using a thermodynamic approach with physically motivated internal state variables. The formulation follows current internal state variable methodologies used for metals and departs from the spring-dashpot representation generally used to characterize the mechanical behavior of polymers like those used by Ames etal. in Int J Plast, 25, 1495–1539 (2009) and Anand and Gurtin in Int J Solids Struct, 40, 1465–1487 (2003), Anand and Ames in Int J Plast, 22, 1123–1170 (2006), Anand etal. in Int J Plast, 25, 1474–1494 (2009). The selection of internal state variables was guided by a hierarchical multiscale modeling approach that bridged deformation mechanisms from the molecular dynamics scale (coarse grain model) to the continuum level. The model equations were developed within a large deformation kinematics and thermodynamics framework where the hardening behavior at large strains was captured using a kinematic-type hardening variable with two possible evolution laws: a current method based on hyperelasticity theory and an alternate method whereby kinematic hardening depends on chain stretching and material plastic flow. The three-dimensional equations were then reduced to the one-dimensional case to quantify the material parameters from monotonic compression test data at different applied strain rates. To illustrate the generalized nature of the constitutive model, material parameters were determined for four different amorphous polymers: polycarbonate, poly(methylmethacrylate), polystyrene, and poly(2,6-dimethyl-1,4-phenylene oxide). This model captures the complex character of the stress–strain behavior of these amorphous polymers for a range of strain rates.
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Recent developments that increase the time and distance scales accessible in the simulations of specific polymers are reviewed. Several different techniques are similar in that they replace a model expressed in fully atomistic detail with a coarse-grained model of the same polymer, atomistic → coarse-grained (and beyond!), thereby increasing the time and distance scales accessible within the expenditure of reasonable computational resources. The bridge represented by the right-pointing arrow can be constructed via different procedures, which are reviewed here. The review also considers the status of methods which reverse this arrow, atomistic ← coarse-grained. This “reverse-mapping” recovers a model expressed in fully atomistic detail from an arbitrarily chosen replica generated during the simulation of the coarse-grained system. Taken in conjunction with the efficiency of the simulation when the system is in its coarse-grained representation, the overall process permits a much more complete equilibration of the system (larger effective size of Δt) when that equilibration is performed with the coarse-grained replicas (II → III) than if it were attempted with the fully atomistic replicas (I → IV).
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We perform molecular dynamics simulations of the glass transition through isobaric and isochoric cooling of a model polymeric material. In general, excellent agreement between the simulation results and the existing experimental trends is observed. The glass transition temperature (Tg)(Tg) is found to be a function of pressure under isobaric conditions and specific volume under isochoric conditions. Under both isobaric and isochoric conditions, the trans-state fraction and the torsional contributions to the energy undergo abrupt changes at the glass transition temperature. We analyze these data to show that the glass transition is primarily associated with the freezing of the torsional degrees of the polymer chains which is strongly coupled to the degree of freedom associated with the nonbonded Lennard-Jones potential. We attribute the greater strength of the glass transition under constant pressure conditions to the fact that the nonbonded Lennard-Jones potential is sensitive to the specific volume, which does not change during cooling under isochoric conditions. Comparison of the isochoric and isobaric data demonstrate that the thermodynamic state is independent of cooling path above Tg,Tg, while path-dependent below Tg.Tg. The simulation data show that the free volume at the isobaric glass transition temperature is pressure dependent. We also find that a glass transition occurs under isochoric conditions, even though the free volume actually increases with decreasing temperature. © 1999 American Institute of Physics. Peer Reviewed http://deepblue.lib.umich.edu/bitstream/2027.42/70861/2/JCPSA6-110-14-7058-1.pdf
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This authoritative, widely cited book has been used all over the world. The fourth edition incorporates the latest developments in the field while maintaining the core objectives of previous editions: to correlate properties with chemical structure and to describe methods that permit the estimation and prediction of numerical properties from chemical structure, i.e. nearly all properties of the solid, liquid, and dissolved states of polymers. * extends coverage of critical topics such as electrical and magnetic properties, rheological properties of polymer melts, and environmental behavior and failure * discusses liquid crystalline polymers across chapters 6, 15, and 16 for greater breadth and depth of coverage * increases the number of supporting illustrations from approximately 250 (in the previous edition) to more than 400 to further aid in visual understanding.
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The continuous development of the modern process industries has made it increasingly important to have information about the properties of materials, including many new chemical substances whose physical properties have never been measured experimentally. This is especially true of polymeric substances. This chapter discusses the properties of polymers and correlates the properties of known polymers with their chemical structure, to establish structure–property relationships. These correlations may be purely empirical, purely theoretical, or partly empirical and partly theoretical. This chapter also describes the methods for the estimation and prediction of the more important properties of polymers in the solid, liquid, and dissolved states, in cases where experimental values are not to be found. This description of correlations and methods for estimation and prediction are based on the rationale that the design of manufacturing and processing equipment requires considerable knowledge of the processed materials and related compounds. Also, this knowledge is essential for the application and final use of these materials.
The role of the torsional potential in bulk polymer chain dynamics is investigated via molecular dynamics simulation using polyethylene as a model system. A number of three-fold barrier values, both greater and less than the standard one, were invoked. The one-fold potential that determines the gauche vs trans energy difference was also varied. For each of the selected torsional potentials, the MD volumetric glass transition temperature, Tg, was located. It was found that Tg is quite sensitive to the three-fold barrier magnitude, moving from below 100 K to nearly 400 K as the barrier goes from zero to twice the standard value. However Tg was found to be quite insensitive to the gauche trans energy difference. Details of the conformational dynamics were studied for the case of a zero torsional potential. This included the rate and location of conformational transitions, the decay of the torsional angle autocorrelation function (ACF) and the cooperativity of conformational transitions, all as a function of temperature. The temperature dependence of the conformational transition rate remains Arrhenius at all temperatures. The relaxation time characterizing the torsional angle ACF decay exhibits WLF temperature behavior. The conformational transitions are randomly distributed over the bonds at high temperature, but near Tg they become spatially heterogeneous and localized. The transitions show next-neighbor correlation as well as self-correlated forward-backward transitions. All of these features are similar to those found in previous simulations under the standard torsional potential.
Article
The authors report the parameters for a new generic force field, DREIDING, that they find useful for predicting structures and dynamics of organic, biological, and main-group inorganic molecules. The philosophy in DREIDING is to use general force constants and geometry parameters based on simple hybridization considerations rather than individual force constants and geometric parameters that depend on the particular combination of atoms involved in the bond, angle, or torsion terms. Thus all bond distances are derived from atomic radii, and there is only one force constant each for bonds, angles, and inversions and only six different values for torsional barriers. Parameters are defined for all possible combinations of atoms and new atoms can be added to the force field rather simply. This paper reports the parameters for the nonmetallic main-group elements (B, C, N, O, F columns for the C, Si, Ge, and Sn rows) plus H and a few metals (Na, Ca, Zn, Fe). The accuracy of the DREIDING force field is tested by comparing with (i) 76 accurately determined crystal structures of organic compounds involving H, C, N, O, F, P, S, Cl, and Br, (ii) rotational barriers of a number of molecules, and (iii) relative conformational energies and barriers of a number of molecules. The authors find excellent results for these systems.
Article
Starting from the united atom model, we construct a coarse-grained model for a flexible polymer chain, in which some successive CH2 atoms are combined into an effective segment. To connect the coarse-grained model with the atomistic model, we propose a scheme to obtain the effective potentials acting between bonded and nonbonded segments from atomistic molecular dynamics simulation for a single isolated chain. We assume that the total effective potential is a sum of potential components for independent coarse-grained variables. The effective bond potentials are determined by simply taking the logarithm of the corresponding distribution functions calculated from the atomistic simulations. On the other hand, to consider the characteristic entropy effects of the polymer chain system, the effective nonbonded potentials are evaluated using the canonical ensemble average for fixed distance between the segments. We confirmed that the coarse-grained model using these effective potentials can reproduce the radii of gyration and various distribution functions of the coarse-grained variables over a wide temperature range. We also confirmed that the effective potentials obtained for the isolated chain system are applicable to the melt system. A detailed analysis of the distribution functions showed that the effective bond length and the effective torsion angle correlate strongly with the effective bond angle. In order to improve the quality of our coarse-grained potentials, these correlations should be taken into account.
Article
The behavior of a glassy polyethylene-like polymer undergoing active compressive deformation was investigated via molecular dynamics simulation. Several important features can be identified within the stress–strain response of the system. Namely, the system deforms elastically, yields, softens, and then at large strains exhibits strain hardening. Simulations reveal that the actively deforming polymer exhibits several distinct characteristics at the molecular scale. Active deformation is found to significantly increase the transition rate between different dihedral angle states as well as promote the propagation of dihedral angle flips along the chain. When deformation is stopped, the transition rates decrease and propagation of these transitions along the chain is once again hindered. Below the glass transition temperature, transitions are heterogeneously distributed within the system. However, a local density-transition rate correlation study shows that this transitional heterogeneity is not attributable to heterogeneity in the local density. Instead, the high local transition rates must be caused by stresses propagated along the chain backbone as indicated by changes in neighbor correlations with stress. The yield stress is determined as a function of strain rate between strain rates of 108s−1 and 5×1010s−1. The activation volume within the context of the Eyring model is calculated to be 0.21 nm3 for this system.
Article
A detailed atomistic approach has been used to investigate the kinematics of plastic deformation in glassy atactic polypropylene to 20% strain. The microstructural stress-strain behaviour was found to consist of smooth reversible portions bounded by irreversible sharp stress drops indicating plastic rearrangement of the structure. Averaging the stress-strain behaviour over an ensemble of 1-815 nm microstructures showed a yield point in the neighbourhood of 5-7% strain. The transformation shear strain for plasticistructural rearrangements was found to be broadly distributed, averaging 1-5% shear strain with a standard deviation of 2-6% shear strain. Combining this result with the activation volume measurements of common glassy polymers showed the size of the plastically transforming region to have a diameter of about 10 nm, thus involving several thousand segments. The transformation shear strain was independent of the system size. Scrutiny of the molecular segment motions associated with plastic rearrangements showed no recurring simple kinematical configurations and no correlation of the local atomic strain to topological features of the chain.
Article
Elementary process of plastic deformation in a computer simulated metallic amorphous model which was subjected to shear deformation has been investigated by analysing spatial distributions of atomic displacement, atomic strain, atomic stresses, atomic shear modulus and atomic coordination expressed by Voronoi polyhedra. Plastic shear deformation took place at localized sites in a spherical shape with a diameter of 3–4 atomic distances. The deformation sites are characterized by an atomic pressure distribution around thc site which is arranged so as to assist the local deformation, and at the same time by a local mechanical instability expressed by a low atomic shear modulus which in turn is related to the Voronoi polyhedra type. It was also observed that adjacent deformation sites were transformed sequentially in the manner of a chain reaction, resulting in macroscopic slip.
Article
We have carried out stochastic dynamics and molecular dynamics simulations of n‐tridecane (C13H28) as isolated chains, in bulk melts, and in confined melts between solid surfaces, employing both a united atom (UA) model and an explicit atom (EA) model, in order to compare chain conformations, packing, orientational correlations, and self‐diffusion predicted by the UA and EA models. The EA model, which explicitly takes into account all hydrogens, exhibits nearly identical results for chain conformations to those from the UA model. However, only the EA model, which shows considerably enhanced interchain packing and orientational correlations in the melts over those for the UA model, reproduces very closely the height and the width of the interchain peak in the experimental x‐ray scattering profile. Dynamically, inclusion of explicit hydrogens decreases the self‐diffusion constants in the melts by a factor of 6–8, resulting in reasonably good agreement with the experimental value. Moreover, in the melts confined between solid surfaces, the presence of explicit hydrogens leads to much more pronounced layering of both the monomer segments and the entire molecules, which are strongly oriented along the solid surfaces.
Article
We present a theory for how a shear stress alone can induce structural changes in a glassy polymer to break up the rigidity of the glass and allow flow. We consider a molecular model in which the shear‐stress field is introduced as a bias on the rotational conformation of backbone bonds. It is argued that the fraction of flexed bonds is transiently increased from that in the glass and that this, additionally causing a volume increase, produces a polymer structure resembling the liquid at some temperature above the glass transition. Using these considerations along with several well‐known empirical relations, we have calculated the plastic properties of polystyrene and polymethyl methacrylate. These calculated properties correspond reasonable well with the cold‐drawing data available.
Article
We present here an optimized united atom model that is able to reproduce properties of melts of n‐alkane chains of varying molecular weights. This model differs from previous models in that the Lennard‐Jones well depth for the terminal methyl group (0.2264 kcal/mol) differs from that of the methylene units (0.093 kcal/mol). The position of the minimum is at 4.5 Å for both units. Properties of n‐C44H90 melts from this model are compared with experiments and those from an explicit atom model. Good agreement with experiment is obtained for static properties of the melt, specifically P–V–T behavior, chain conformations, and x‐ray scattering profiles. The large‐scale dynamics, as measured by self‐diffusion, are found to agree reasonably well with experimental results, being about 30% faster with our best united atom force field. Analysis of the end‐to‐end vector orientation autocorrelation function in terms of the Rouse model yields a monomer friction coefficient somewhat greater than that determined from the rate of self‐diffusion, reflecting the fact that the n‐C44H90 chains are not sufficiently long to behave as Gaussian coils. Detailed local chain dynamics for n‐C44H90 melts, as measured by the P1(t) and P2(t) orientation autocorrelation functions for C–H vectors, are found to agree reasonably well with results from simulations using an explicit atom model, and yield spin‐lattice relaxation times T1 and nuclear Overhauser enhancement values in reasonable agreement with experimental 13C NMR measurements. As with large scale dynamics, local dynamics are faster in general (about 20%) than experimental results. © 1995 American Institute of Physics.
Article
This work presents an application of recently developed ideas about how to map real polymer systems onto abstract models. In our case the abstract model is the bond fluctuation model with a Monte Carlo dynamics. We study the temperature dependence of chain dimensions and of the self-diffusion behavior in the melt from high temperatures down to 200 K. The chain conformations are equilibrated over the whole temperature range, which is possible for the abstract type of model we use. The size of the chains as measured by the characteristic ratio is within 25% of experimental data. The simulated values of the chain self-diffusion coefficient have to be matched to experimental information at one temperature to obtain a scaling for the Monte Carlo time step. The melt viscosity from the simulations as determined by applying the Rouse model is then in good qualitative agreement with experimental data over the experimentally available temperature range. The activation energy as extracted from an Arrhenius fit is different because the simulations are done at constant volume. Both experimental data and the simulation, which covers a far greater temperature range, show Arrhenius behavior for the viscosity and no indication of a finite nonzero Vogel–Fulcher temperature. For one temperature (T=509 K) various time-dependent mean-square displacements are available from atomistic molecular-dynamics simulations, and are shown to be in excellent agreement with the results from the coarse-grained model. © 1997 American Institute of Physics.
Article
Since to form a hole the size of a molecule in a liquid requires almost the same increase in free energy as to vaporize a molecule, the concentration of vapor above the liquid is a measure of such ``molecular'' holes in the liquid. This provides an explanation of the law of rectilinear diameters of Cailletet and Mathias. The theory of reaction rates yields an equation for absolute viscosity applicable to cases involving activation energies where the usual theory of energy transfer does not apply. This equation reduces to a number of the successful empirical equations under the appropriate limiting conditions. The increase of viscosity with shearing stress is explained. The same theory yields an equation for the diffusion coefficient which when combined with the viscosity and applied to the results of Orr and Butler for the diffusion of heavy into light water gives a satisfactory and suggestive interpretation. The usual theories for diffusion coefficients and absolute electrical conductance should be replaced by those developed here when ion and solvent molecule are of about the same size.
Article
Molecular dynamics simulation of bulk liquid and glass of long‐chain molecules has been performed. The system consists of linear chains of up to 50 spherical segments, each subject to forces due to bond stretching, bending, and torsion, and to nonbonded interaction, according to a truncated Lennard‐Jones potential, between segments in neighboring chains and between segments separated by more than three bonds along the chain. The parameters are chosen to mimic polymethylene, the segment representing a CH2 unit. Behaviors suggestive of liquid‐to‐glass transition were exhibited by (i) cessation of trans–gauche conformational transitions, (ii) changes in the temperature coefficients of the density and internal energy, and (iii) effective vanishing of the segmental self‐diffusion coefficient. The ‘‘freezing in’’ of these properties occurs at decreasing temperatures in the order given above, indicating the decreasing size of domains of cooperative motion required. The dependence of the transition temperature on the chain length and on the flexibility of the chain (effected by switching of the torsional potential off) obtained is in accord with experimental observations. Below the transition temperature the system behavior depends on the path through which the state was reached, suggesting that simulation of relaxation effects could be achieved in longer runs.
Article
The local chain dynamics of bulk amorphous polymers has been studied by means of molecular dynamics simulation in the vicinity of the glass transition temperature Tg. Two models of polymers are used, one mimicking a polyethylene chain and the other a hypothetical freely‐rotating chain, both of infinite chain length. The structural relaxations are examined by means of the time‐correlation function of vectors embedded in the polymer backbone and of the distribution of the angles by which these vectors reorient after a time interval t. Some degree of chain mobility is seen to persist even as the temperature is lowered to Tg and below. In addition to the rotational diffusion process that takes place as a result of a series of small step motions, some large‐angle jump motions are also seen to occur in both models. The jump motions, which are obscured by prevalent faster modes of motions at high temperatures, become clearly revealed in the reorientation angle distributions as the temperature is lowered. In the polyethylene model, the large‐angle jump is directly associated with conformational transitions, while in the freely‐rotating chain model, in which no torsional barrier exists, the jump probably arises because of the local potential minima created by the surrounding chains. The conformational transitions in the polyethylene model are highly cooperative among the segments neighboring along the chain, especially so as the temperature is lowered through Tg.
Article
Polymer materials range from industrial commodities, such as plastic bags, to high-tech polymers used for optical applications, and all the way to biological systems, where the most prominent example is DNA. They can be crystalline, amorphous (glasses, melts, gels, rubber), or in solution. Polymers in the glassy state are standard materials for many applications (yogurt cups, compact discs, housings for technical equipment, etc.).
Article
The global orientational order that develops in polycarbonate under plastic deformation has been measured quantitatively by CSA and dipolar DECODER experiments. The results are in substantial agreement with the predictions of an affine entanglement network model. Athermal atomistic simulations, on the other hand, tend to overestimate the orientational order. The orientation behavior in glassy polycarbonate seems to be essentially the same as that in the melt.
Article
A three-dimensional polybead model of the structure and large strain deformation of amorphous polymeric networks is constructed and evolved via Monte Carlo techniques. The model has successfully simulated various deformation conditions and been found to qualitatively capture the proper state of deformation, rate of deformation, and temperature dependence of real amorphous polymeric materials. Partitioning of the stress calculations indicates that strain softening following yield is a result of the evolution of intermolecular contributions to the response whereas the strain hardening phenomenon is a result of evolution in the intramolecular contributions. These calculations provide a fundamental basis for development of continuum-level plasticity models and, indeed, support assumptions currently used in successful constitutive models of the elastic−viscoplastic behavior of polymers.
Article
Atomistic Monte Carlo (MC) simulations of uniaxial tension of an amorphous linear polyethylene (PE)-like polymer glass have been carried out. A united-atom model has been used where PE chains are represented by beads connected by flexible springs. Highly efficient end-bridging MC moves have been used to first equilibrate the polymer in the melt and then cool to a temperature below its glass transition temperature. A mix of efficient MC moves has also been used to simulate the deformation dynamics. Upon uniaxial deformation the stress response to the strain is initially linear elastic, subsequently as the strain increases further yielding is observed, and finally strain hardening is developed. The simulated Young modulus and Poisson ratio take realistic values. Furthermore, the temperature and strain rate dependencies of stress−strain curves have been investigated, and the results are in qualitative agreement with the experimental observations. Chain conformation and energy and stress partitioning with increasing strain are followed in detail. During the deformation the chains adopt more extended conformations, and the fraction of dyads in the trans state increases. In the elastic region mechanical work done on the sample is primarily stored as nonbonded internal energy, whereas from the yield region onward the intrachain contributions start to play a role.
Article
The viscosity and self-diffusion coefficient were measured for polyethylene samples in the molecular weight range 200 < M < 120 000. The viscosity η was determined by capillary flow or in a parallel-plate rheometer; the diffusion coefficient was obtained from the attenuation of NMR spin echoes in a pulsed-field gradient. The molecular weight dependence of the viscosity at 175°C can be described by two power-law regions, η ∝ M1.8 below M = 5000 and η ∝ M3.6 above 5000. The diffusion coefficient on the other hand can be characterized by a single power law D ∝ M-2 over the entire range studied. Comparisons with molecular theory were made by examining the product ηD. At low molecular weights ηD agrees well with predictions of the Rouse model. At high molecular weights ηD increases with molecular weight, approaching the prediction of the reptation model from below. The temperature dependence of the viscosity follows the Vogel-Fulcher equation, η ∼ eB′/(T-T0), with parameters that are consistent with the free volume theory of the liquid state. When the data at low molecular weights are compared at a constant free volume state, the viscosity and diffusion coefficient are proportional to M and M-1 as required by the Rouse model.
Article
Methods are developed for the prediction of the elastic constants of an amorphous glassy polymer by small-strain deformation of microscopically detailed model structures. A thermodynamic analysis shows that entropic contributions to the elastic response to deformation can be neglected in polymeric glasses. A statistical mechanical analysis further indicates that vibrational contributions of the hard degrees of freedom are not significant, so that estimates of the elastic constants can be obtained from changes in the total potential energy of static microscopic structures subjected to simple deformations. Mathematical procedures are developed for the atomistic modeling of deformation and applied to glassy atactic polypropylene. Predicted elastic constants are always within 15% of the experimental values, without the use of adjustable parameters. An estimate of the thermal expansion coefficient is also obtained. Inter- and intramolecular contributions to the mechanical properties are examined, and it is found that coexistence in the bulk reduces the effects of individual chain idiosyncrasy.
Article
A method is developed for the detailed atomistic modeling of well-relaxed amorphous glassy polymers. Atactic polypropylene at -40°C is used as an example. The model system is a cube with periodic boundaries, filled with segments from a single "parent" chain. An initial structure is generated by using a modified Markov process, based on rotational isomeric state theory and incorporating long-range interactions. This structure is then "relaxed" by potential energy minimization, using analytical derivatives. Computing time is kept relatively small by stagewise minimization, employing a technique of "blowing up" the atomic radii. Model estimates of the cohesive energy density and the Hildebrand solubility parameter agree very well with experiment. The conformation of the single chains in the relaxed model system closely resembles that of unperturbed chains. Pair distribution functions and bond direction correlation functions show that the predominant structural features are intramolecular and that long-range orientational order is completely absent.