1,038 research outputs found
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
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
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
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
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
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
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