134 research outputs found
Local Reorientation Dynamics of Semiflexible Polymers in the Melt
The reorientation dynamics of local tangent vectors of chains in isotropic
amorphous melts containing semiflexible model polymers was studied by molecular
dynamics simulations. The reorientation is strongly influenced both by the
local chain stiffness and by the overall chain length. It takes place by two
different subsequent processes: A short-time non-exponential decay and a
long-time exponential reorientation arising from the relaxation of medium-size
chain segments. Both processes depend on stiffness and chain length. The strong
influence of the chain length on the chain dynamics is in marked contrast to
its negligible effect on the static structure of the melt. The local structure
shows only a small dependence on the stiffness, and is independent of chain
length. Calculated correlation functions related to double-quantum NMR
experiments are in qualitative agreement with experiments on entangled melts. A
plateau is observed in the dependence of segment reorientation on the
mean-squared displacement of the corresponding chain segments. This plateau
confirms, on one hand, the existence of reptation dynamics. On the other hand,
it shows how the reptation picture has to be adapted if, instead of fully
flexible chains, semirigid chains are considered.Comment: 29 pages, several figures, accepted by Macromolecule
Local chain ordering in amorphous polymer melts: Influence of chain stiffness
Molecular dynamics simulation of a generic polymer model is applied to study
melts of polymers with different types of intrinsic stiffness. Important static
observables of the single chain such as gyration radius or persistence length
are determined. Additionally we investigate the overall static melt structure
including pair correlation function, structure function and orientational
correlation function.Comment: 13 pages, 15 figures, PCCP accepte
How does the chain extension of poly (acrylic acid) scale in aqueous solution? A combined study with light scattering and computer simulation
This work adresses the question of the scaling behaviour of polyelectrolytes
in solution for a realistic prototype: We show results of a combined
experimental (light scattering) and theoretical (computer simulations)
investigation of structural properties of poly (acrylic acid) (PAA).
Experimentally, we determined the molecular weight (M_W) and the hydrodynamic
radius (R_H) by static light scattering for six different PAA samples in
aqueous NaCl-containing solution (0.1-1 mol/L) of polydispersity D_P between
1.5 and 1.8. On the computational side, three different variants of a newly
developed mesoscopic force field for PAA were employed to determine R_H for
monodisperse systems of the same M_W as in the experiments. The force field
effectively incorporates atomistic information and one coarse-grained bead
corresponds to one PAA monomer. We find that R_H matches with the experimental
data for all investigated samples. The effective scaling exponent for R_H is
found to be around 0.55, which is well below its asymptotic value for good
solvents. Additionally, data for the radius of gyration (R_G) are presented.Comment: 17 pages, 3 figures, submitted to Macromolecule
On the nature of Thermal Diffusion in binary Lennard-Jones liquids
The aim of this study is to understand deeper the thermal diffusion transport
process (Ludwig-Soret effect) at the microscopic level. For that purpose, the
recently developed reverse nonequilibrium molecular dynamics method was used to
calculate Soret coefficients of various systems in a systematic fashion. We
studied binary Lennard-Jones (LJ) fluids near the triple point (of one of the
components) in which we separately changed the ratio of one of the LJ
parameters mass, atomic diameter and interaction strength while keeping all
other parameters fixed and identical. We observed that the magnitude of the
Soret coefficient depends on all three ratios. Concerning its sign we found
that heavier species, smaller species and species with higher interaction
strengths tend to accumulate in the cold region whereas the other ones
(lighter, bigger or weaker bound) migrate to the hot region of our simulation
cell. Additionally, the superposition of the influence of the various
parameters was investigated as well as more realistic mixtures. We found that
in the experimentally relevant parameter range the contributions are nearly
additive and that the mass ratio often is the dominating factor.Comment: 27 pages, 9 figures, submitted to J. Chem. Phy
Formation of Chain-Folded Structures from Supercooled Polymer Melts
The formation of chain-folded structures from the melt is observed in
molecular dynamics simulations resembling the lamellae of polymer crystals.
Crystallization and subsequent melting temperatures are related linearly to the
inverse lamellar thickness. Analysis of the single chain conformations in the
crystal shows that most chains reenter the same lamella by tight backfolds.
Simulations are performed with a mesoscopic bead-spring model including a
specific angle bending potential. They demonstrate that chain stiffness alone,
without an attractive inter-particle potential, is a sufficient driving force
for the formation of chain-folded lamellae.Comment: 4 pages, 5 figure
Molecular dynamics analysis of the influence of Coulomb and van der Waals interactions on the work of adhesion at the solid-liquid interface
We investigated the solid-liquid work of adhesion of water on a model silica surface by molecular dynamics simulations, where a methodology previously developed to determine the work of adhesion through thermodynamic integration was extended to a system with long-range electrostatic interactions between solid and liquid. In agreement with previous studies, the work of adhesion increased when the magnitude of the surface polarity was increased. On the other hand, we found that when comparing two systems with and without solid-liquid electrostatic interactions, which were set to have approximately the same total solid-liquid interfacial energy, former had a significantly smaller work of adhesion and a broader distribution in the interfacial energies, which has not been previously reported in detail. This was explained by the entropy contribution to the adhesion free energy; i.e., the former with a broader energy distribution had a larger interfacial entropy than the latter. While the entropy contribution to the work of adhesion has already been known, as a work of adhesion itself is free energy, these results indicate that, contrary to common belief, wetting behavior such as the contact angle is not only governed by the interfacial energy but also significantly affected by the interfacial entropy. Finally, a new interpretation of interfacial entropy in the context of solid-liquid energy variance was offered, from which a fast way to qualitatively estimate the work of adhesion was also presented.Donatas Surblys, Frédéric Leroy, Yasutaka Yamaguchi, and Florian Müller-Plathe, "Molecular dynamics analysis of the influence of Coulomb and van der Waals interactions on the work of adhesion at the solid-liquid interface", The Journal of Chemical Physics 148, 134707 (2018) https://doi.org/10.1063/1.5019185
Thermal energy transport across the interface between phase change material n-heneicosane in solid and liquid phases and few-layer graphene
Molecular dynamics simulations have been performed to investigate the mechanism of thermal energy transport at the interface between n-heneicosane in solid and liquid phases and few-layer graphene at different temperatures under two heating modes (in the “heat-matrix” mode, heat is flowing from the heated heneicosane molecules to the cooled ones through the graphene layers and in the “heat-graphene” mode, the energy is flowing from the heated graphene to the cooled heneicosane). The effect of orientation of the perfect crystal structure (heneicosane molecules are positioned perpendicular and parallel to the graphene basal plane) on the interfacial thermal conductance has been examined. It is observed that the interfacial thermal conductance is 2 orders of magnitude higher under the heat-matrix mode than under the heat-graphene mode, for liquid or solid heneicosane and monolayer graphene. With an increase in the number of graphene layers, the interfacial thermal conductance under the heat-matrix mode decreases and reaches a plateau when the number of the graphene layer is more than eight. This is caused by the decreasing contribution of direct heat transfer from the matrix to matrix across the graphene layers via nonbonded intermolecular interactions. The interfacial thermal conductance becomes similar for both heating modes, once the number of graphene layers in the system is over 15. The influence of temperature on the interfacial thermal conductance is found to be insignificant in the range (175–250 K; 350–400 K). Both the phase and structure of heneicosane significantly influence the interfacial conductance. Spectral analysis suggests that graphene vibrational modes of all frequencies contribute to the interfacial heat transfer
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