Deformation of a solid usually leads to an increase in the internal potential energy U in of the ... more Deformation of a solid usually leads to an increase in the internal potential energy U in of the solid. In inelastic and plastic deformation, the increase in U in is due to the formation of excited structural defects in the solid under the action of the external force (e.g., in crystals, dislocations form). The storage of U in has been studied experimentally in detail for crystalline metals 2], rubbers , and glassy organic polymers . However, experimental measurements on polymer systems cannot determine the contributions of inter actions of various types to the increase in U in . Today, computer simulation can identify such contributions and can suggest what types of interactions con trol the energy storage and influence the mechanical behavior of material . In this work, we performed molecular dynamic simulation of uniaxial compres sive and tensile deformation of a full atom model of amorphous glassy polymethylene and analyzed the contributions of interactions of various types to the increase in its potential energy.
The structure and mechanical properties of clay nanoparticles is a sub- ject of growing interest ... more The structure and mechanical properties of clay nanoparticles is a sub- ject of growing interest because of their numerous applications in engineering. We present the results of molecular dynamics simulation for a single nanoplate of pyro- phyllite - a 2:1 clay mineral consisting of two tetrahedral sheets SiO4 and intervening octahedral AlO6 sheet. The model under consideration consists of one
Molecular dynamics computer simulations have been carried out of a chemically realistic many-chai... more Molecular dynamics computer simulations have been carried out of a chemically realistic many-chain nonentangled model of glassy atactic polystyrene under the influence of uniaxial mechanical deformation. Both the initial elastic and the postyield (up to 100% of ...
Molecular-dynamics (MD) simulations have been performed for two amorphous polymers with extremely... more Molecular-dynamics (MD) simulations have been performed for two amorphous polymers with extremely different mechanical properties, atactic polystyrene (PS) and bisphenol A polycarbonate (PC), in the isotropic state and under load. The glass transition temperatures, Young moduli, yield stresses and strain-hardening moduli are calculated and compared to the experimental data. Both chemistry-specific and mode-coupling aspects of the segmental mobility in the isotropic case and under the uniaxial deformation have been identified. The mobility of the PS segments in the deformation direction is increased drastically beyond the yield point. A weaker increase is observed for PC.
Two-dimensional structural defects in a polyethylene crystal (twist domain walls) are investigate... more Two-dimensional structural defects in a polyethylene crystal (twist domain walls) are investigated by means of analytic treatment and molecular dynamics simulation. The formation of the domain wall as a result of relaxation of the crystal from different initial states is studied and parameters of this type of defects are calculated.
The possibility of supersonic motion of vacancies in a polyethylene crystal is revealed by means ... more The possibility of supersonic motion of vacancies in a polyethylene crystal is revealed by means of analytical investigation and numerical simulation. It is demonstrated that in the crystalline field of immovable neighbors, a vacancy with a core size of about 70 CH 2 groups and a velocity in the range of 1.02-1.05 sound velocity preserves itself for the time scale of about 1 ns. It is demonstrated that this type of structural defect is similar to coupled supersonic solitons described earlier in the one-dimensional chains with combined gradient and nongradient nonlinearity. An analytic approach is proposed for prediction of their shape and velocity. The simulation of the crystal with all degrees of freedom released demonstrates that the supersonic vacancy is still distinguishable. Its lifetime is less than 5 ps but still may be significant for physical applications.
The structure and mechanical properties of clay nanoparticles is a subject of growing interest be... more The structure and mechanical properties of clay nanoparticles is a subject of growing interest because of their numerous applications in engineering. We present the results of molecular dynamics simulation for a single nanoplate of pyrophyllite - a 2:1 clay mineral consisting of two tetrahedral sheets of SiO4 and an intervening octahedral AlO6 sheet. Simulations were performed in the temperature interval from 5 to 750 K using the ionic-type potentials of Cygan et al. On this basis the temperature dependences of structural parameters, characterizing both tetrahedral and octahedral sheets as well as single lamella, have been studied. Two slightly different structures were observed in this wide temperature interval. The mechanical properties of the nanoplate were calculated from stress-strain diagrams, which have been obtained at relatively slow rates of deformation (for molecular simulations). Using different types of loading, we calculated the full elasticity tensor and estimated the influence of temperature on its components. We estimated also the bending and torsion stiffnesses of the nanoplate as specific characteristics of this type of particle. Because the nanoplate is atomically thin, a reasonable determination of the thickness is a nontrivial problem, both in the modeling of mechanical properties and in physical interpretation of the obtained data. We propose a procedure for its calculation.
Molecular dynamics simulations of a semi-flexible main chain LCP (liquid crystalline polymer) hav... more Molecular dynamics simulations of a semi-flexible main chain LCP (liquid crystalline polymer) have been carried out using a newly developed model named solo-LJ-spring-GB model. The new model represents the molecular chain in the form of GB-spring-LJ-spring-...
We revisit the problem of the molecular mechanism of the chain diffusion between crystalline and ... more We revisit the problem of the molecular mechanism of the chain diffusion between crystalline and amorphous fractions in semicrystalline polyethylene (PE). There exists a long-standing controversy on the nature of the topological point defects which diffuse along the chain stems in crystallites and shift the stems. Namely, the conformational (including gauche conformations) twistecompression (interstitial-like) and the smooth (soliton-like) twistetension (vacancy-like) localized defects were offered for this role. However, none of the proposed models for the process could explain all the experimental facts which seemed unclear and contradictory. Moreover, it was discovered recently that in PE samples of uncommon morphology (electron beam irradiated samples, fibers and single crystals) the diffusion process has the activation energy about 3 times less than that in common melt-crystallized samples. No explanation ever followed. We have carried out molecular dynamics (MD) simulation of both the defects in a realistic model of PE crystal and obtained estimates for their formation energies and diffusion coefficients. These estimates together with analysis of available experimental data allow to solve both the problems and to propose models for molecular mechanisms of the observed diffusion processes. The agents of the 'old' diffusion process are the smooth twistetension defects. Shifts in a chain stem of a crystallite in a common sample are initiated at the interface to an amorphous region through extended thermal motion of the chain stem in the amorphous region. If the motion causes a strong pull (with a twist) at the chain stem in the crystallite, such motion produces a smooth defect of twistetension on this stem. The proposed molecular model conforms with available mechanical experiments if one accepts that the process corresponds to the most low temperature (a 1 ) from the a-peaks observed. The 'new' diffusion process results from diffusion of the conformational twistecompression defects in crystallites. The needed sequence of conformations appears near a crystallite as a result of a quick gamma process. Because the state of the semicrystalline polymer is unstable, the position of the boundary between the crystalline and disordered regions fluctuates so that segments of chains pass from disordered to crystalline state (and vice versa). The conformational defects in disordered region are captured through expansion of the crystalline region where they become stable and diffuse along the chains. Our MD estimate for the activation energy of the process E act 8.65 kcal/mol is in a good agreement with the experimental value 7 kcal/mol. The diffusion coefficients of both the defects are too high to have effect on the statistics of both of these very slow processes. Therefore the statistics of the 'old' process is the statistics of strong thermal pulls at chain stems in crystallites, and the statistics of the 'new' process is related to the statistics of fluctuations of the position of the boundaries between crystalline and disordered fractions.
Journal of Experimental and Theoretical Physics, 2002
A molecular dynamics simulation was performed to estimate the effective mass and the diffusion an... more A molecular dynamics simulation was performed to estimate the effective mass and the diffusion and friction coefficients of point defects in macromolecular chains of crystalline polyethylene. The results were compared with theoretical mass and kinetic coefficient predictions for topological solitons, with which these defects were identified. The results are used to discuss the soliton model of dielectric α c relaxation in weakly oxidized polyethylene.
We carry out a molecular dynamics simulation of the first stages of constrained melting in crysta... more We carry out a molecular dynamics simulation of the first stages of constrained melting in crystalline polyethylene (PE). When heated, the crystal undergoes two structural phase transitions: from the orthorhombic (O) phase to the monoclinic (M) phase, and then to the columnar (C), quasi-hexagonal, phase. The M phase represents the tendency to the parallel packing of planes of PE zigzags, and the C phase proves to be some kind of oriented melt. We follow both the transitions O→M and M→C in real time and establish that, at their beginning, the crystal tries (and fails) to pass into the partially ordered phases similar to the RI and RII phases of linear alkanes, correspondingly. We discuss the molecular mechanisms and driving forces of the observed transitions, as well as the reasons why the M and C phases in PE crystals substitute for the rotator phases in linear alkanes.
Recently [1], we studied the plastic deformation of crystalline polyethylene (PE) by molecular-dy... more Recently [1], we studied the plastic deformation of crystalline polyethylene (PE) by molecular-dynamic modeling and showed that the major mechanism of initiation of this process in the presence of only edge dislocations is dissociation of dislocation dipoles formed by dislocations whose Burgers vectors are parallel to the axes of polymer chains. The contribution of screw dislocations to plastic deformation of PE has not been studied. As follows from experimental data [2], this contribution is significant. Indeed, the screw dislocations parallel to polymer chains in crystalline PE have a minimum line energy density; thus, they should dominate plastic deformation.
Deformation of a solid usually leads to an increase in the internal potential energy U in of the ... more Deformation of a solid usually leads to an increase in the internal potential energy U in of the solid. In inelastic and plastic deformation, the increase in U in is due to the formation of excited structural defects in the solid under the action of the external force (e.g., in crystals, dislocations form). The storage of U in has been studied experimentally in detail for crystalline metals 2], rubbers , and glassy organic polymers . However, experimental measurements on polymer systems cannot determine the contributions of inter actions of various types to the increase in U in . Today, computer simulation can identify such contributions and can suggest what types of interactions con trol the energy storage and influence the mechanical behavior of material . In this work, we performed molecular dynamic simulation of uniaxial compres sive and tensile deformation of a full atom model of amorphous glassy polymethylene and analyzed the contributions of interactions of various types to the increase in its potential energy.
The structure and mechanical properties of clay nanoparticles is a sub- ject of growing interest ... more The structure and mechanical properties of clay nanoparticles is a sub- ject of growing interest because of their numerous applications in engineering. We present the results of molecular dynamics simulation for a single nanoplate of pyro- phyllite - a 2:1 clay mineral consisting of two tetrahedral sheets SiO4 and intervening octahedral AlO6 sheet. The model under consideration consists of one
Molecular dynamics computer simulations have been carried out of a chemically realistic many-chai... more Molecular dynamics computer simulations have been carried out of a chemically realistic many-chain nonentangled model of glassy atactic polystyrene under the influence of uniaxial mechanical deformation. Both the initial elastic and the postyield (up to 100% of ...
Molecular-dynamics (MD) simulations have been performed for two amorphous polymers with extremely... more Molecular-dynamics (MD) simulations have been performed for two amorphous polymers with extremely different mechanical properties, atactic polystyrene (PS) and bisphenol A polycarbonate (PC), in the isotropic state and under load. The glass transition temperatures, Young moduli, yield stresses and strain-hardening moduli are calculated and compared to the experimental data. Both chemistry-specific and mode-coupling aspects of the segmental mobility in the isotropic case and under the uniaxial deformation have been identified. The mobility of the PS segments in the deformation direction is increased drastically beyond the yield point. A weaker increase is observed for PC.
Two-dimensional structural defects in a polyethylene crystal (twist domain walls) are investigate... more Two-dimensional structural defects in a polyethylene crystal (twist domain walls) are investigated by means of analytic treatment and molecular dynamics simulation. The formation of the domain wall as a result of relaxation of the crystal from different initial states is studied and parameters of this type of defects are calculated.
The possibility of supersonic motion of vacancies in a polyethylene crystal is revealed by means ... more The possibility of supersonic motion of vacancies in a polyethylene crystal is revealed by means of analytical investigation and numerical simulation. It is demonstrated that in the crystalline field of immovable neighbors, a vacancy with a core size of about 70 CH 2 groups and a velocity in the range of 1.02-1.05 sound velocity preserves itself for the time scale of about 1 ns. It is demonstrated that this type of structural defect is similar to coupled supersonic solitons described earlier in the one-dimensional chains with combined gradient and nongradient nonlinearity. An analytic approach is proposed for prediction of their shape and velocity. The simulation of the crystal with all degrees of freedom released demonstrates that the supersonic vacancy is still distinguishable. Its lifetime is less than 5 ps but still may be significant for physical applications.
The structure and mechanical properties of clay nanoparticles is a subject of growing interest be... more The structure and mechanical properties of clay nanoparticles is a subject of growing interest because of their numerous applications in engineering. We present the results of molecular dynamics simulation for a single nanoplate of pyrophyllite - a 2:1 clay mineral consisting of two tetrahedral sheets of SiO4 and an intervening octahedral AlO6 sheet. Simulations were performed in the temperature interval from 5 to 750 K using the ionic-type potentials of Cygan et al. On this basis the temperature dependences of structural parameters, characterizing both tetrahedral and octahedral sheets as well as single lamella, have been studied. Two slightly different structures were observed in this wide temperature interval. The mechanical properties of the nanoplate were calculated from stress-strain diagrams, which have been obtained at relatively slow rates of deformation (for molecular simulations). Using different types of loading, we calculated the full elasticity tensor and estimated the influence of temperature on its components. We estimated also the bending and torsion stiffnesses of the nanoplate as specific characteristics of this type of particle. Because the nanoplate is atomically thin, a reasonable determination of the thickness is a nontrivial problem, both in the modeling of mechanical properties and in physical interpretation of the obtained data. We propose a procedure for its calculation.
Molecular dynamics simulations of a semi-flexible main chain LCP (liquid crystalline polymer) hav... more Molecular dynamics simulations of a semi-flexible main chain LCP (liquid crystalline polymer) have been carried out using a newly developed model named solo-LJ-spring-GB model. The new model represents the molecular chain in the form of GB-spring-LJ-spring-...
We revisit the problem of the molecular mechanism of the chain diffusion between crystalline and ... more We revisit the problem of the molecular mechanism of the chain diffusion between crystalline and amorphous fractions in semicrystalline polyethylene (PE). There exists a long-standing controversy on the nature of the topological point defects which diffuse along the chain stems in crystallites and shift the stems. Namely, the conformational (including gauche conformations) twistecompression (interstitial-like) and the smooth (soliton-like) twistetension (vacancy-like) localized defects were offered for this role. However, none of the proposed models for the process could explain all the experimental facts which seemed unclear and contradictory. Moreover, it was discovered recently that in PE samples of uncommon morphology (electron beam irradiated samples, fibers and single crystals) the diffusion process has the activation energy about 3 times less than that in common melt-crystallized samples. No explanation ever followed. We have carried out molecular dynamics (MD) simulation of both the defects in a realistic model of PE crystal and obtained estimates for their formation energies and diffusion coefficients. These estimates together with analysis of available experimental data allow to solve both the problems and to propose models for molecular mechanisms of the observed diffusion processes. The agents of the 'old' diffusion process are the smooth twistetension defects. Shifts in a chain stem of a crystallite in a common sample are initiated at the interface to an amorphous region through extended thermal motion of the chain stem in the amorphous region. If the motion causes a strong pull (with a twist) at the chain stem in the crystallite, such motion produces a smooth defect of twistetension on this stem. The proposed molecular model conforms with available mechanical experiments if one accepts that the process corresponds to the most low temperature (a 1 ) from the a-peaks observed. The 'new' diffusion process results from diffusion of the conformational twistecompression defects in crystallites. The needed sequence of conformations appears near a crystallite as a result of a quick gamma process. Because the state of the semicrystalline polymer is unstable, the position of the boundary between the crystalline and disordered regions fluctuates so that segments of chains pass from disordered to crystalline state (and vice versa). The conformational defects in disordered region are captured through expansion of the crystalline region where they become stable and diffuse along the chains. Our MD estimate for the activation energy of the process E act 8.65 kcal/mol is in a good agreement with the experimental value 7 kcal/mol. The diffusion coefficients of both the defects are too high to have effect on the statistics of both of these very slow processes. Therefore the statistics of the 'old' process is the statistics of strong thermal pulls at chain stems in crystallites, and the statistics of the 'new' process is related to the statistics of fluctuations of the position of the boundaries between crystalline and disordered fractions.
Journal of Experimental and Theoretical Physics, 2002
A molecular dynamics simulation was performed to estimate the effective mass and the diffusion an... more A molecular dynamics simulation was performed to estimate the effective mass and the diffusion and friction coefficients of point defects in macromolecular chains of crystalline polyethylene. The results were compared with theoretical mass and kinetic coefficient predictions for topological solitons, with which these defects were identified. The results are used to discuss the soliton model of dielectric α c relaxation in weakly oxidized polyethylene.
We carry out a molecular dynamics simulation of the first stages of constrained melting in crysta... more We carry out a molecular dynamics simulation of the first stages of constrained melting in crystalline polyethylene (PE). When heated, the crystal undergoes two structural phase transitions: from the orthorhombic (O) phase to the monoclinic (M) phase, and then to the columnar (C), quasi-hexagonal, phase. The M phase represents the tendency to the parallel packing of planes of PE zigzags, and the C phase proves to be some kind of oriented melt. We follow both the transitions O→M and M→C in real time and establish that, at their beginning, the crystal tries (and fails) to pass into the partially ordered phases similar to the RI and RII phases of linear alkanes, correspondingly. We discuss the molecular mechanisms and driving forces of the observed transitions, as well as the reasons why the M and C phases in PE crystals substitute for the rotator phases in linear alkanes.
Recently [1], we studied the plastic deformation of crystalline polyethylene (PE) by molecular-dy... more Recently [1], we studied the plastic deformation of crystalline polyethylene (PE) by molecular-dynamic modeling and showed that the major mechanism of initiation of this process in the presence of only edge dislocations is dissociation of dislocation dipoles formed by dislocations whose Burgers vectors are parallel to the axes of polymer chains. The contribution of screw dislocations to plastic deformation of PE has not been studied. As follows from experimental data [2], this contribution is significant. Indeed, the screw dislocations parallel to polymer chains in crystalline PE have a minimum line energy density; thus, they should dominate plastic deformation.
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Papers by N. Balabaev