160 research outputs found
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
Mapping atomistic to coarse-grained polymer models using automatic simplex optimization to fit structural properties
We develop coarse-grained force fields for poly (vinyl alcohol) and poly
(acrylic acid) oligomers. In both cases, one monomer is mapped onto a
coarse-grained bead. The new force fields are designed to match structural
properties such as radial distribution functions of various kinds derived by
atomistic simulations of these polymers. The mapping is therefore constructed
in a way to take into account as much atomistic information as possible. On the
technical side, our approach consists of a simplex algorithm which is used to
optimize automatically non-bonded parameters as well as bonded parameters.
Besides their similar conformation (only the functional side group differs),
poly (acrylic acid) was chosen to be in aqueous solution in contrast to a poly
(vinyl alcohol) melt. For poly (vinyl alcohol) a non-optimized bond angle
potential turns out to be sufficient in connection with a special, optimized
non-bonded potential. No torsional potential has to be applied here. For poly
(acrylic acid), we show that each peak of the radial distribution function is
usually dominated by some specific model parameter(s). Optimization of the bond
angle parameters is essential. The coarse-grained forcefield reproduces the
radius of gyration of the atomistic model. As a first application, we use the
force field to simulate longer chains and compare the hydrodynamic radius with
experimental data.Comment: 34 pages, 3 tables, 16 figure
Mapping atomistic to coarse-grained polymer models using automatic simplex optimization to fit structural properties
We develop coarse-grained force fields for poly (vinyl alcohol) and poly
(acrylic acid) oligomers. In both cases, one monomer is mapped onto a
coarse-grained bead. The new force fields are designed to match structural
properties such as radial distribution functions of various kinds derived by
atomistic simulations of these polymers. The mapping is therefore constructed
in a way to take into account as much atomistic information as possible. On the
technical side, our approach consists of a simplex algorithm which is used to
optimize automatically non-bonded parameters as well as bonded parameters.
Besides their similar conformation (only the functional side group differs),
poly (acrylic acid) was chosen to be in aqueous solution in contrast to a poly
(vinyl alcohol) melt. For poly (vinyl alcohol) a non-optimized bond angle
potential turns out to be sufficient in connection with a special, optimized
non-bonded potential. No torsional potential has to be applied here. For poly
(acrylic acid), we show that each peak of the radial distribution function is
usually dominated by some specific model parameter(s). Optimization of the bond
angle parameters is essential. The coarse-grained forcefield reproduces the
radius of gyration of the atomistic model. As a first application, we use the
force field to simulate longer chains and compare the hydrodynamic radius with
experimental data.Comment: 34 pages, 3 tables, 16 figure
Corrections to Scaling in the Hydrodynamic Properties of Dilute Polymer Solutions
We discuss the hydrodynamic radius of polymer chains in good solvent,
and show that the leading order correction to the asymptotic law ( degree of polymerization, ) is an ``analytic''
term of order , which is directly related to the discretization
of the chain into a finite number of beads. This result is further corroborated
by exact calculations for Gaussian chains, and extensive numerical simulations
of different models of good--solvent chains, where we find a value of for the asymptotic universal ratio , being the
chain's gyration radius. For chains the data apparently extrapolate to
, which is different from the Gaussian value 1.5045,
but in accordance with previous simulations. We also show that the
experimentally observed deviations of the initial decay rate in dynamic light
scattering from the asymptotic Benmouna--Akcasu value can partly be understood
by similar arguments.Comment: 13 pages, 10 figures. submitted to J. Chem. Phy
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
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
Deriving effective mesoscale potentials from atomistic simulations
We demonstrate how an iterative method for potential inversion from
distribution functions developed for simple liquid systems can be generalized
to polymer systems. It uses the differences in the potentials of mean force
between the distribution functions generated from a guessed potential and the
true (simulated) distribution functions to improve the effective potential
successively. The optimization algorithm is very powerful: convergence is
reached for every trial function in few iterations. As an extensive test case
we coarse-grained an atomistic all-atom model of poly (isoprene) (PI) using a
13:1 reduction of the degrees of freedom. This procedure was performed for PI
solutions as well as for a PI melt. Comparisons of the obtained force fields
are drawn. They prove that it is not possible to use a single force field for
different concentration regimes.Comment: 32 pages including 12 figure
Reaction behavior modeling of metal hydride based on FeTiMn using numerical simulations
In this contribution, we perform computer simulations to expedite the development of hydrogen storages based on metal hydride. These simulations enable in-depth analysis of the processes within the systems which otherwise could not be achieved. That is, because the determination of crucial process properties require measurement instruments in the setup which are currently not available. Therefore, we investigate the reliability of reaction values that are determined by a design of experiments.
Specifically, we first explain our model setup in detail. We define the mathematical terms to obtain insights into the thermal processes and reaction kinetics. We then compare the simulated results to measurements of a 5-gram sample consisting of iron-titanium-manganese (FeTiMn) to obtain the values with the highest agreement with the experimental data. In addition, we improve the model by replacing the commonly used Van’t-Hoff equation by a mathematical expression of the pressure-composition-isotherms (PCI) to calculate the equilibrium pressure.
Finally, the parameters’ accuracy is checked in yet another with an existing metal hydride system. The simulated results demonstrate high concordance with experimental data, which advocate the usage of approximated kinetic reaction properties by a design of experiments for further design studies. Furthermore, we are able to determine process parameters like the entropy and enthalpy
Monocular visual acuity of persons 4-74 years, United States, 1971-1972
Visual acuity levels with usual correction, if any, as determined in the opthalmology examination before dilation by race, geographic region, family income, and other selected demographic variables.[Jean Roberts and Jacqueline Ludford].Includes bibliographical references.197730168
Coarse Graining of Nonbonded Inter-particle Potentials Using Automatic Simplex Optimization to Fit Structural Properties
We implemented a coarse-graining procedure to construct mesoscopic models of
complex molecules. The final aim is to obtain better results on properties
depending on slow modes of the molecules. Therefore the number of particles
considered in molecular dynamics simulations is reduced while conserving as
many properties of the original substance as possible. We address the problem
of finding nonbonded interaction parameters which reproduce structural
properties from experiment or atomistic simulations. The approach consists of
optimizing automatically nonbonded parameters using the simplex algorithm to
fit structural properties like the radial distribution function as target
functions. Moreover, any mix of structural and thermodynamic properties can be
included in the target function. Different spherically symmetric inter-particle
potentials are discussed. Besides demonstrating the method for Lennard--Jones
liquids, it is applied to several more complex molecular liquids such as
diphenyl carbonate, tetrahydrofurane, and monomers of poly(isoprene).Comment: 24 pages, 3 tables, 14 figures submitted to the Journal of Chemical
Physics (JCP
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