Alexey Lyulin
Technische universiteit Eindhoven, Applied Physics, Faculty Member
We propose a computational procedure to assess size effects in polymer-carbon nanotube (CNT) composites. The characteristic dimension of CNTs, in the order of nanometers in the radial direction, calls for a detailed analysis of the atomic... more
We propose a computational procedure to assess size effects in polymer-carbon nanotube (CNT) composites. The characteristic dimension of CNTs, in the order of nanometers in the radial direction, calls for a detailed analysis of the atomic structure of the polymer in the region surrounding the CNT. This region, known as the interphase layer, plays a central role in the overall response of polymer-CNT composites [1] and can be related to the occurrence of size effects. In this contribution, the interphase layer and the CNTinduced size effects are characterized by means of Molecular Dynamics (MD) simulations on a polymerCNT representative unit. An optimization procedure is then employed to define a mechanically equivalent continuum model which describes the CNT, the polymer-CNT interface, and the interphase as a threedimensionalequivalentfiber[2]. Havingestablishedthemechanicalequivalencebetweentheatomisticand thecontinuummodel,weinvestigatesizeeffectsinthemechanicalpropertiesofaCNT-polymercomposite with realistic CNT volume fractions. In particular, we use the Generalized Finite Element Method [3] to efficiently handle the inclusion of the equivalent fibers in a polymer matrix
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We propose a computational procedure to assess size effects in polymer-carbon nanotube (CNT) composites. The characteristic dimension of CNTs, in the order of nanometers in the radial direction, calls for a detailed analysis of the atomic... more
We propose a computational procedure to assess size effects in polymer-carbon nanotube (CNT) composites. The characteristic dimension of CNTs, in the order of nanometers in the radial direction, calls for a detailed analysis of the atomic structure of the polymer in the region surrounding the CNT. This region, known as the interphase layer, plays a central role in the overall response of polymer-CNT composites [1] and can be related to the occurrence of size effects. In this contribution, the interphase layer and the CNTinduced size effects are characterized by means of Molecular Dynamics (MD) simulations on a polymerCNT representative unit. An optimization procedure is then employed to define a mechanically equivalent continuum model which describes the CNT, the polymer-CNT interface, and the interphase as a threedimensionalequivalentfiber[2]. Havingestablishedthemechanicalequivalencebetweentheatomisticand thecontinuummodel,weinvestigatesizeeffectsinthemechanicalpropertiesofaCNT-polymercomposite with realistic CNT volume fractions. In particular, we use the Generalized Finite Element Method [3] to efficiently handle the inclusion of the equivalent fibers in a polymer matrix
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We have performed the molecular-dynamics simulations for the atactic polystyrene (aPS) films supported by one substrate (SF, supported film) and films capped by two substrates (CF, capped film). The simulations of supported films have... more
We have performed the molecular-dynamics simulations for the atactic polystyrene (aPS) films supported by one substrate (SF, supported film) and films capped by two substrates (CF, capped film). The simulations of supported films have been carried out with the purpose to study the influence of confinement on the glass-transition temperature (Tg). We define the Tg by measuring the film density and thickness. We show that the Tg value of aPS films weakly depends on the film thickness and remains almost constant for films down to 2 nm, which is in agreement with recent experimental study. The simulations of capped films have been performed to study the statistical and mechanical properties of polymer chains under shear. The capped film has been loaded with different normal pressures (25–170 MPa) and sheared with different shear velocities (5 × 10−4–1 × 10−1 nm ps−1). In the absence of shear the internal structure of the aPS SF and CF films is different. We found that the internal structure, density and order parameter of the aPS CF films do not change with the small shear deformations.
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Abstract NPT molecular-dynamics simulations have been carried out, using the Gromacs package, of the coarse-grained united-atom model of a polymer melt in the vicinity of the glass transition and confined between two crystalline... more
Abstract NPT molecular-dynamics simulations have been carried out, using the Gromacs package, of the coarse-grained united-atom model of a polymer melt in the vicinity of the glass transition and confined between two crystalline substrates. The pair interactions between all united atoms are described with the help of the Lennard-Jones (LJ) potential. For a corresponding bulk polymer at fixed potential cutoff distance the temperature dependence of density is not influenced by different methods of shifting the potential but is affected by the long range energy and pressure correction. At the same time the shift to higher densities and higher glass-transition temperatures is observed upon increasing the LJ cutoff distance from 2.5 σ to 5 σ . Depending on the potential cutoff value the polymer-film density can be both below and above the corresponding bulk density at fixed temperature; these dependences are also very sensitive to modification of the potential at a fixed cutoff distance. The effect increases with decreasing film thickness, thus resulting in a different thickness dependence of the glass-transition temperature at different cutoff values.
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Pretty much every talk on the mechanics of polymer materials will feature, at one point or another, an image of a plate of spaghetti and/or a snake. Two iconic metaphors, representing the two quintessential determinants of the mechanical... more
Pretty much every talk on the mechanics of polymer materials will feature, at one point or another, an image of a plate of spaghetti and/or a snake. Two iconic metaphors, representing the two quintessential determinants of the mechanical quality of classical polymer materials: structure and dynamics. Classical polymer texts will emphasize the fact that the long polymer strands become entangled, impeding each other’s ability to explore space much more than ordinary non-extended particles do. As a result, polymer solutions and melts – in addition to the viscous characteristics expected for these ultimately liquid systems – exhibit mechanical properties usually found in solids even in the absence of chemical cross-linking and do so over broad ranges of timescales. These liquidlike traits, possibly enhanced by elevated temperatures, facilitate the structural relaxations that provide malleability and ensure the easy processing of entangled polymer materials: desirable properties in their own right but also beneficial to the recyclability of polymeric materials. For these and many other reasons, regimes of deformability and plasticity are highly sought-after in application. Those same applications, however, generally also require that – once formed into a product – the materials are strong, are tough and provide good long-term integrity. To remedy the long-time liquidlike behaviour, and to impart additional resilience, the chains may be permanently linked together with chemical cross-links to form so-called network materials. In the end, the dynamical mechanical response of the fully formed material is determined by a combination of chain length, chain density, cross-linking functionality and density, solvent properties, the mechanical properties and the topology of the individual polymer strands in a complex interplay that continues to surprise and confuse.
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Influence of the deformation on the intrachain mobility is a problem of a fundamental importance. Brownian dynamics simulations of deformed polymer chains were performed for chain models with three types equal rotational potential: (i)... more
Influence of the deformation on the intrachain mobility is a problem of a fundamental importance. Brownian dynamics simulations of deformed polymer chains were performed for chain models with three types equal rotational potential: (i) symmetrical potential with three equal rotational isomers, (ii) Ryckaert-Bellemans potential (PE chain), Adolf-Ediger potential (polyisoprene chain). The dependences of fractions of different conformers on the deformation were obtained. The rates of transitions between rotational isomers were calculated by using the hazard plot technique. Influence of the torsional potential on the mechanism of conformational mobility is discussed.
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ABSTRACT The dependence of the normal modes relaxation time, τ, on the scale of motion, Λ, for a freely jointed polymer chain in the presence of an orienting field of dipole symmetry was obtained both analytically and by the method of... more
ABSTRACT The dependence of the normal modes relaxation time, τ, on the scale of motion, Λ, for a freely jointed polymer chain in the presence of an orienting field of dipole symmetry was obtained both analytically and by the method of Brownian dynamics. Comparison with the results for the case of a quadrapole field obtained earlier shows that the field symmetry influences mainly the prefactor in the dependence τ(Λ) reflecting the mobility of single chain element in the field.
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Keywords: molecular-dynamics simulation; polystyrene film; protein–polymer interactions The atomistic molecular-dynamics simulations of the initial stage of myoglobin adsorption on amorphous polystyrene surfaces with varying... more
Keywords: molecular-dynamics simulation; polystyrene film; protein–polymer interactions The atomistic molecular-dynamics simulations of the initial stage of myoglobin adsorption on amorphous polystyrene surfaces with varying hydrophobicity are presented. The polystyrene surfaces as non-oxidized (hydrophobic) and oxidized (hydrophilic) films, both in united-atoms and dummy-hydrogen atoms representations are modeled. The protein is placed initially at different distances and orientations from the polymer. We monitor the interactions between the protein and the polystyrene surface for the same polystyrene surface in contact with the protein in different initial orientations and for one initial orientation of the protein in contact with different polystyrene surfaces. By comparing the stability and the number of myoglobin-polystyrene atomic contacts and the interaction energies, it is found that the initial contact of the protein with the hydrophobic polystyrene surfaces is stronger than with the hydrophilic ones. The orientations of the myoglobin in which the more rigid protein parts face the polymer exhibit stronger initial contact with the polymeric surface.
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Abstract Molecular dynamics computer simulations of amorphous atactic polystyrene have been performed for chains of up to 320 monomer units in a temperature range from 100 to 600 K and in a pressure range from 0.1 to 1000 MPa. The... more
Abstract Molecular dynamics computer simulations of amorphous atactic polystyrene have been performed for chains of up to 320 monomer units in a temperature range from 100 to 600 K and in a pressure range from 0.1 to 1000 MPa. The MD-determined values of the glass transition temperature T g are in a good agreement with experimental PVT data at different values of applied pressure. Local translational mobility has been investigated by measuring the mean square translational displacements of monomers as a function of time. The long-time asymptotic slope of these dependencies is close to 0.6 at T > T g , showing diffusive behaviour. The cage effect when local translational motions are essentially frozen in the glassy state has been studied. Local orientational mobility is studied with the help of Legendre polynomials of the first, P 1 ( t ), and second, P 2 ( t ), order for the bonds in the main chain and along the phenyl side groups. Temperature dependencies of the orientational relaxation times at different values of pressure are well described by the Vogel–Fulcher law.
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A new coarse-grained force field for isotactic polypropylene (iPP) has been developed based on a single unperturbed chain Monte Carlo algorithm. The model reproduces the helical behavior of iPP and predicts structural properties... more
A new coarse-grained force field for isotactic polypropylene (iPP) has been developed based on a single unperturbed chain Monte Carlo algorithm. The model reproduces the helical behavior of iPP and predicts structural properties sufficiently well.
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Recent experiments on polyelectrolyte membranes have clearly shown that at operating temperatures perfluoroimide acid (PFIA) has a higher electrical conductivity than widely used Nafion. In the present paper classical molecular-dynamics... more
Recent experiments on polyelectrolyte membranes have clearly shown that at operating temperatures perfluoroimide acid (PFIA) has a higher electrical conductivity than widely used Nafion. In the present paper classical molecular-dynamics simulations were carried out to study the structural properties of both materials, and the proton and water transport in the corresponding membranes at T=300 K and T=353 K. In this temperature range, the temperature effects on the hydrated internal polyelectrolyte structure were found to be negligible. The PFIA has longer side chains across a wide range of hydration levels which would have promoted more trapping of water and hydronium ions in PFIA. Indeed, the average number of water molecules in the first hydration shell around the side-chain protogenic groups was found to be higher in PFIA than in Nafion. Our simulations showed the formation of large continuous water clusters and connected pore volumes in PFIA at high hydration levels which promote...
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ABSTRACT The orientational mobility of segments in dendrimers are studied by the method of Brownian dynamics, and the results are analyzed in terms of an earlier proposed analytical theory. The orientational autocorrelation function for... more
ABSTRACT The orientational mobility of segments in dendrimers are studied by the method of Brownian dynamics, and the results are analyzed in terms of an earlier proposed analytical theory. The orientational autocorrelation function for the cosine of an angle of segmental rotation in dendrimers of a given generation P 1(t) is controlled by three relaxation processes with the corresponding relaxation times. Characteristic times and the contribution from the above processes to P 1(t) are calculated. The first process refers to the local mobility of a selected segment; the second process, to the rotations of a dendrimer branch, which originates from the selected segment of a given generation; and the third process, to the rotation of a dendrimer macro-molecule as a whole. The proposed approach makes it possible to estimate the relaxation spectrum of a dendrimer by studying the orientational mobility of segments in different generations. The relaxation times can be used to describe various relaxation processes observed by different experimental methods, such as dielectric relaxation, NMR, dynamic birefringence, and polarized luminescence.
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We have performed molecular-dynamics simulations of atactic polystyrene thin films to study the effect of shear rate, pressure, and temperature on the stress-strain behaviour, the relevant energetic contributions and non-affine... more
We have performed molecular-dynamics simulations of atactic polystyrene thin films to study the effect of shear rate, pressure, and temperature on the stress-strain behaviour, the relevant energetic contributions and non-affine displacements of polymer chains during constant-shear deformation. Under this deformation sliding motion is observed at high shear rates between the top substrate and top polymer layer, which disappears when the shear rate decreases. At low shear rates stick-slip motion of the whole film with respect to the bottom substrate takes place. We found that at low shear rates the yield stress logarithmically depends on the shear rate; this behaviour can be explained in terms of the Eyring model. It was also observed that an increase in the normal pressure leads to an increase in the yield stress in agreement with experiments. The contributions to the total shear stress and energy are mainly given by the excluded-volume interactions. It corresponds to a local transla...