27 research outputs found
Equilibration of High Molecular-Weight Polymer Melts: A Hierarchical Strategy
A strategy is developed for generating equilibrated high molecular-weight
polymer melts described with microscopic detail by sequentially backmapping
coarse-grained (CG) configurations. The microscopic test model is generic but
retains features like hard excluded volume interactions and realistic melt
densities. The microscopic representation is mapped onto a model of soft
spheres with fluctuating size, where each sphere represents a microscopic
subchain with monomers. By varying a hierarchy of CG
representations at different resolutions is obtained. Within this hierarchy, CG
configurations equilibrated with Monte Carlo at low resolution are sequentially
fine-grained into CG melts described with higher resolution. A Molecular
Dynamics scheme is employed to slowly introduce the microscopic details into
the latter. All backmapping steps involve only local polymer relaxation thus
the computational efficiency of the scheme is independent of molecular weight,
being just proportional to system size. To demonstrate the robustness of the
approach, microscopic configurations containing up to chains with
polymerization degrees are generated and equilibration is confirmed by
monitoring key structural and conformational properties. The extension to much
longer chains or branched polymers is straightforward
The Cassie-Wenzel transition of fluids on nanostructured substrates: Macroscopic force balance versus microscopic density-functional theory
Classical density functional theory is applied to investigate the validity of
a phenomenological force-balance description of the stability of the Cassie
state of liquids on substrates with nanoscale corrugation. A bulk free-energy
functional of third order in local density is combined with a square-gradient
term, describing the liquid-vapor interface. The bulk free energy is
parameterized to reproduce the liquid density and the compressibility of water.
The square-gradient term is adjusted to model the width of the water-vapor
interface. The substrate is modeled by an external potential, based upon
Lennard-Jones interactions. The three-dimensional calculation focuses on
substrates patterned with nanostripes and square-shaped nanopillars. Using both
the force-balance relation and density-functional theory, we locate the
Cassie-to-Wenzel transition as a function of the corrugation parameters. We
demonstrate that the force-balance relation gives a qualitatively reasonable
description of the transition even on the nanoscale. The force balance utilizes
an effective contact angle between the fluid and the vertical wall of the
corrugation to parameterize the impalement pressure. This effective angle is
found to have values smaller than the Young contact angle. This observation
corresponds to an impalement pressure that is smaller than the value predicted
by macroscopic theory. Therefore, this effective angle embodies effects
specific to nanoscopically corrugated surfaces, including the finite range of
the liquid-solid potential (which has both repulsive and attractive parts),
line tension, and the finite interface thickness. Consistently with this
picture, both patterns (stripes and pillars) yield the same effective contact
angles for large periods of corrugation.Comment: 13 pages 9 figure
One size fits all: equilibrating chemically different polymer liquids through universal long-wavelength description
Mesoscale behavior of polymers is frequently described by universal laws.
This physical property motivates us to propose a new modeling concept, grouping
polymers into classes with a common long-wavelength representation. In the same
class samples of different materials can be generated from this representation,
encoded in a single library system. We focus on homopolymer melts, grouped
according to the invariant degree of polymerization. They are described with a
bead-spring model, varying chain stiffness and density to mimic chemical
diversity. In a renormalization group-like fashion library samples provide a
universal blob-based description, hierarchically backmapped to create
configurations of other class-members. Thus large systems with
experimentally-relevant invariant degree of polymerizations (so far accessible
only on very coarse-grained level) can be microscopically described.
Equilibration is verified comparing conformations and melt structure with
smaller scale conventional simulations
Hierarchical modeling of polystyrene melts: From soft blobs to atomistic resolution
We demonstrate that hierarchical backmapping strategies incorporating generic
blob-based models can equilibrate melts of high-molecular-weight polymers,
described with chemically specific, atomistic, models. The central idea behind
these strategies, is first to represent polymers by chains of large soft blobs
(spheres) and efficiently equilibrate the melt on mesoscopic scale. Then, the
degrees of freedom of more detailed models are reinserted step by step. The
procedure terminates when the atomistic description is reached. Reinsertions
are feasible computationally because the fine-grained melt must be
re-equilibrated only locally. To develop the method, we choose a polymer with
sufficient complexity. We consider polystyrene (PS), characterized by
stereochemistry and bulky side groups. Our backmapping strategy bridges
mesoscopic and atomistic scales by incorporating a blob-based, a moderately CG,
and a united-atom model of PS. We demonstrate that the generic blob-based model
can be parameterized to reproduce the mesoscale properties of a specific
polymer -- here PS. The moderately CG model captures stereochemistry. To
perform backmapping we improve and adjust several fine-graining techniques. We
prove equilibration of backmapped PS melts by comparing their structural and
conformational properties with reference data from smaller systems,
equilibrated with less efficient methods.Comment: 18 page
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Comparing equilibration schemes of high-molecular-weight polymer melts with topological indicators.
Recent theoretical studies have demonstrated that the behaviour of molecular knots is a sensitive indicator of polymer structure. Here, we use knots to verify the ability of two state-of-the-art algorithms-configuration assembly and hierarchical backmapping-to equilibrate high-molecular-weight (MW) polymer melts. Specifically, we consider melts with MWs equivalent to several tens of entanglement lengths and various chain flexibilities, generated with both strategies. We compare their unknotting probability, unknotting length, knot spectra, and knot length distributions. The excellent agreement between the two independent methods with respect to knotting properties provides an additional strong validation of their ability to equilibrate dense high-MW polymeric liquids. By demonstrating this consistency of knotting behaviour, our study opens the way for studying topological properties of polymer melts beyond time and length scales accessible to brute-force molecular dynamics simulations