105 research outputs found
Equation of State Based Slip Spring Model for Entangled Polymer Dynamics
A mesoscopic, mixed particle- and field-based Brownian dynamics methodology
for the simulation of entangled polymer melts has been developed. Polymeric
beads consist of several Kuhn segments, and their motion is dictated by the
Helmholtz energy of the sample, which is a sum of the entropic elasticity of
chain strands between beads, slip springs, and nonbonded interactions. The
entanglement effect is introduced by the slip springs, which are springs
connecting either nonsuccessive beads on the same chain or beads on different
polymer chains. The terminal positions of slip springs are altered during the
simulation through a kinetic Monte Carlo hopping scheme, with rate-controlled
creation/destruction processes for the slip springs at chain ends. The rate
constants are consistent with the free energy function employed and satisfy
microscopic reversibility at equilibrium. The free energy of nonbonded
interactions is derived from an appropriate equation of state, and it is
computed as a functional of the local density by passing an orthogonal grid
through the simulation box; accounting for it is necessary for reproducing the
correct compressibility of the polymeric material. Parameters invoked by the
mesoscopic model are derived from experimental volumetric and viscosity data or
from atomistic molecular dynamics simulations, establishing a "bottom-up"
predictive framework for conducting slip spring simulations of polymeric
systems of specific chemistry. The mesoscopic simulation methodology is
implemented for the case of cis-1,4-polyisoprene, whose structure, dynamics,
thermodynamics, and linear rheology in the melt state are quantitatively
predicted and validated without a posteriori fitting the results to
experimental measurements.Comment: 80 pages, 17 figure
Self-Consistent-Field Study of Adsorption and Desorption Kinetics of Polyethylene Melts on Graphite and Comparison with Atomistic Simulations
A method is formulated, based on combining self-consistent field theory with
dynamically corrected transition state theory, for estimating the rates of
adsorption and desorption of end-constrained chains (e.g. by crosslinks or
entanglements) from a polymer melt onto a solid substrate. This approach is
tested on a polyethylene/graphite system, where the whole methodology is
parametrized by atomistically detailed molecular simulations. For short-chain
melts, which can still be addressed by molecular dynamics simulations with
reasonable computational resources, the self-consistent field approach gives
predictions of the adsorption and desorption rate constants which are
gratifyingly close to molecular dynamics estimates.Comment: 18 pages, 10 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
Diffusion via the space discretization (DSD) method to study the concentration dependence of self-diffusion under confinement
A combined atomistic simulation and quasielastic neutron scattering study of the low-temperature dynamics of hydrogen and deuterium confined in NaX zeolite
- …