174 research outputs found
Structure and mobility of cyclohexane as a solvent for Trans-Polyisoprene
Solutions of {\it trans}polyisoprene in cyclohexane are investigated in
atomistic detail at different compositions at two different temperatures. We
investigate the influence of polymer concentration on the dynamics of the
solvent molecules and the structure of the solvation shell. The double bonds
along the polymer backbone are preferentially approached by the solvent
molecules. The mobility of cyclohexane molecules decreases with increasing
polymer concentration at ambient conditions. The reorientation of molecules
becomes more anisotropic with concentration as the polymer hinders the
reorientation of the molecular plane. At elevated temperatures the influence of
the polymer is weaker and the reorientation of the solvent is more isotropic.
Additionally, a fast and efficient way to set up atomistic simulations is shown
in detail in which the initial simulations increase in length and in the
simulation time-step. The excess energy from initial overlaps is removed by
resetting the velocities at regular intervals.Comment: 6 pages, 3 figure
Properties of Poly (isoprene) - Model Building in the Melt and in Solution
Properties of 1,4-\textit{trans} poly (isoprene) at ambient conditions are
determined by simulations on two length scales based on two different models: a
full-atomistic and a mesoscopic one. The models are linked via a mapping scheme
such that one mesoscopic bead represents one chemical repeat unit. Melts as
well as solutions of several chain lengths were investigated and mapped
individually to the meso-scale. The resulting models are compared to each
other. The meso-scale models could be simulated over a large variety of chain
lengths and time-scales relevant for experimental comparison. Concerning static
properties, we determined the persistence length of our systems and the scaling
behavior of the radius of gyration. The latter was compared to experiments and
the agreement is satisfactory. Furthermore, we find deviations from Rouse
dynamics at all chain lengths at ambient conditions.Comment: 11 pictures 7 figure
Density of states of a binary Lennard-Jones Glass
We calculate the density of states of a binary Lennard-Jones glass using a
recently proposed Monte Carlo algorithm. Unlike traditional molecular
simulation approaches, the algorithm samples distinct configurations according
to self-consistent estimates of the density of states, thereby giving rise to
uniform internal-energy histograms. The method is applied to simulate the
equilibrium, low-temperature thermodynamic properties of a widely studied glass
former consisting of a binary mixture of Lennard-Jones particles. We show how a
density-of-states algorithm can be combined with particle identity swaps and
configurational bias techniques to study that system. Results are presented for
the energy and entropy below the mode coupling temperature.Comment: 6 pages, 3 figures, accepted by J Chem Phy
Constant Pressure Hybrid Molecular Dynamics-Monte Carlo Simulations
New hybrid Molecular Dynamics-Monte Carlo methods are proposed to increase
the efficiency of constant-pressure simulations. Two variations of the isobaric
Molecular Dynamics component of the algorithms are considered. In the first, we
use the extended-ensemble method of Andersen [H. C. Andersen J. Chem. Phys.
{\bf 72},2384 (1980)]. In the second, we arrive at a new constant-pressure
Monte Carlo technique based on the reversible generalization of the
weak-coupling barostat [H. J. C. Berendsen et. al J. Chem. Phys. {\bf 81},
3684(1984)]. This latter technique turns out to be highly effective in
equilibrating and maintaining a target pressure. It is superior to the
extended-ensemble method, which in turn is superior to simple volume-rescaling
algorithms. The efficiency of the proposed methods is demonstrated by studying
two systems. The first is a simple Lennard-Jones fluid. The second is a mixture
of polyethylene chains of 200 monomers.Comment: 10 pages, 4 figure
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
Orientation Correlation in Simplified Models of Polymer Melts
We investigate mutual local chain order in systems of fully flexible polymer
melts in a simple generic bead-spring model. The excluded-volume interaction
together with the connectivity leads to local ordering effects which are
independent of chain length between 25 and 700 monomers, i.e. in the Rouse as
well as in the reptation regime. These ordering phenomena extend to a distance
of about 3 to 4 monomer sizes and decay to zero afterwards.Comment: 5 pages, 3 figure
Density of States Monte Carlo Method for Simulation of Fluids
A Monte Carlo method based on a density-of-states sampling is proposed for
study of arbitrary statistical mechanical ensembles in a continuum. A random
walk in the two-dimensional space of particle number and energy is used to
estimate the density of states of the system; this density of states is
continuously updated as the random walk visits individual states. The validity
and usefulness of the method are demonstrated by applying it to the simulation
of a Lennard-Jones fluid. Results for its thermodynamic properties, including
the vapor-liquid phase coexistence curve, are shown to be in good agreement
with high-accuracy literature data.Comment: 5 pages, 3 figures, accepted by J Chem Phy
Local Structure and Dynamics of Trans-polyisoprene oligomers
Mono- and poly-disperse melts of oligomers (average length 10 monomers) of
trans-1,4-polyisoprene are simulated in full atomistic detail. The force-field
is developed by means of a mixture of ab initio quantum-chemistry and an
automatic generation of empirical parameters. Comparisons to NMR and scattering
experiments validate the model. The local reorientation dynamics shows that for
CH vectors there is a two-stage process consisting of an initial decay and a
late-stage decorrelation originating from overall reorientation. The atomistic
model can be successfully mapped onto a simple model including only beads for
the monomers with bond springs and bond angle potentials. End-bridging Monte
Carlo as an equilibration stage and molecular dynamics as the subsequent
simulation method together prove to be a useful method for polymer simulations.Comment: 25 pages, 15 figures, accepted by Macromolecule
Modeling of polystyrene under confinement: Exploring the limits of iterative boltzmann inversion
We explore the limits of a purely structure based coarse-graining technique, the iterative Boltzmann inversion (IBI), in the coarse-graining of a confined concentrated polystyrene solution. In the first place, some technical considerations and challenges encountered in the course of the optimization process are represented. The concepts of the choice of the initial potentials and the cross-dependency of the interactions as well as the order of optimization are discussed in detail. Furthermore, the transferability of a previously developed CG confined polystyrene solution model, the "parent CG confined model", to different degrees of confinement at constant concentration and temperature is examined. We investigate if a CG force field developed for a confined polymer solution by IBI is sensitive to changes in the degree of localization or arrangement of polymers near the surfaces although the concentration is kept constant. For this purpose, reference atomistic simulations on systems of different confinement levels have been performed. The differences in the structure and dynamics of the chains are addressed. Results are compared with those of an unconfined (bulk) system at the same concentration. The chain dimensions and orientations as a function of the distance from the surfaces are also reported. To the best of our knowledge, this is the first computational study that investigates the structural behavior of polymers in close proximity of the surfaces in a concentrated polymer solution rather than in a melt. Transferability of the parent CG confined model is tested by employing the parent force field in CG simulations of the reference systems. Results indicate that the degree of arrangement of monomers and solvent molecules near the surfaces is an important factor that needs to be paid attention to when considering the application of a CG force field developed by IBI to different degrees of confinement.U.S. Department of Energy, Office of Science, Basic Energy Sciences (DE-FG02-06ER46340); Scientific and Technological Research Council of Turke
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