31 research outputs found
Multiscale equilibration of highly entangled isotropic model polymer melts
We present a computationally efficient multiscale method for preparing
equilibrated, isotropic long chain model polymer melts. As an application we
generate Kremer-Grest melts of chains with entanglements and
- beads per chain, which cover the experimentally relevant bending
rigidities up to and beyond the limit of the isotropic-nematic transition. In
the first step, we employ Monte Carlo simulations of a lattice model to
equilibrate the large-scale chain structure above the tube scale while ensuring
a spatially homogeneous density distribution. We then use theoretical insight
from a constrained mode tube model to introduce the bead degrees of freedom
together with random walk conformational statistics all the way down to the
Kuhn scale of the chains. This is followed by a sequence of simulations with
carefully parameterized force-capped bead-spring models, which slowly introduce
the local bead packing while reproducing the larger scale chain statistics of
the target Kremer-Grest system at all levels of force-capping. Finally we can
switch to the full Kremer-Grest model without perturbing the structure. The
resulting chain statistics is in excellent agreement with literature results on
all length scales accessible in brute-force simulations of shorter chains.Comment: Revised manuscript. Submitted to Journal of Chemical Physic
A Monte Carlo Study on the Effects of Excluded Volume Interactions on the Scattering from Block Copolymer Micelles and Branched Polymers
A computational tool for symbolic derivation of the small angle scattering from complex composite structures
Analysis of small angle scattering (SAS) data requires intensive modelling to
infer and characterize the structures present in a sample. This iterative
improvement of models is a time consuming process. Here we present the
Scattering Equation Builder (SEB), a C++ library that derives exact analytic
expressions for the form factor of complex composite structures. The user
writes a small program that specifies how sub-units should be linked to form a
composite structure and calls SEB to obtain an expression for the form factor.
SEB supports e.g. Gaussian polymer chains and loops, thin rods and circles,
solid spheres, spherical shells and cylinders, and many different options for
how these can be linked together. In the present paper, we present the
formalism behind SEB, and give simple case studies such as block-copolymers
with different types of linkage and more complex examples such as a random walk
model of linked sub-units, dendrimers, polymers and rods attached to
surfaces of geometric objects, and finally the scattering from a linear chain
of 5 stars, where each star is build by four diblock copolymers. These examples
illustrate how SEB can be used to develop complex models and hence reduce the
cost of analyzing SAS data.Comment: 18 pages, 13 figure
Stress Relaxation in Entangled Polymer Melts
We present an extensive set of simulation results for the stress relaxation
in equilibrium and step-strained bead-spring polymer melts. The data allow us
to explore the chain dynamics and the shear relaxation modulus, , into
the plateau regime for chains with entanglements and into the terminal
relaxation regime for . Using the known (Rouse) mobility of unentangled
chains and the melt entanglement length determined via the primitive path
analysis of the microscopic topological state of our systems, we have performed
parameter -free tests of several different tube models. We find excellent
agreement for the Likhtman-McLeish theory using the double reptation
approximation for constraint release, if we remove the contribution of
high-frequency modes to contour length fluctuations of the primitive chain.Comment: 5 pages, 3 figure
A Formalism for Scattering of Complex Composite Structures. 2 Distributed Reference Points
Recently we developed a formalism for the scattering from linear and acyclic
branched structures build of mutually non-interacting sub-units.{[}C. Svaneborg
and J. S. Pedersen, J. Chem. Phys. 136, 104105 (2012){]} We assumed each
sub-unit has reference points associated with it. These are well defined
positions where sub-units can be linked together. In the present paper, we
generalize the formalism to the case where each reference point can represent a
distribution of potential link positions. We also present a generalized
diagrammatic representation of the formalism. Scattering expressions required
to model rods, polymers, loops, flat circular disks, rigid spheres and
cylinders are derived. and we use them to illustrate the formalism by deriving
the generic scattering expression for micelles and bottle brush structures and
show how the scattering is affected by different choices of potential link
positions.Comment: Paper no. 2 of a serie
Strain-dependent localization, microscopic deformations, and macroscopic normal tensions in model polymer networks
We use molecular dynamics simulations to investigate the microscopic and
macroscopic response of model polymer networks to uniaxial elongations. By
studying networks with strands lengths ranging from to 200 we cover
the full crossover from cross-link to entanglement dominated behavior. Our
results support a recent version of the tube model which accounts for the
different strain dependence of chain localization due to chemical cross-links
and entanglements