368 research outputs found
Exploring Disordered Morphologies of Blends and Block Copolymers for Light-Emitting Diodes with Mesoscopic Simulations
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
Coarse-Grained Models of Biological Membranes within the Single Chain Mean Field Theory
The Single Chain Mean Field theory is used to simulate the equilibrium
structure of phospholipid membranes at the molecular level. Three levels of
coarse-graining of DMPC phospholipid surfactants are present: the detailed
44-beads double tails model, the 10-beads double tails model and the minimal
3-beads model. We show that all three models are able to reproduce the
essential equilibrium properties of the phospholipid bilayer, while the
simplest 3-beads model is the fastest model which can describe adequately the
thickness of the layer, the area per lipid and the rigidity of the membrane.
The accuracy of the method in description of equilibrium structures of
membranes compete with Monte Carlo simulations while the speed of computation
and the mean field nature of the approach allows for straightforward
applications to systems with great complexity.Comment: Accepted for publication in Soft Matte
The biology and ecology of Valencia letourneuxi Sauvage 1880 (Valenciidae) - Prospects for conservation
Data are provided on the distribution, abundance, early development and biology of the endangered Greek endemic species Valencia letourneuxi Sauvage 1880, along with a record of its occurrence at new localities. V. letourneuxi is a small-bodied and short-lived insectivorous species, exhibiting cryptic colouration and sexual dimorphism. It matures in the first year of life, reproduces serially in late spring and summer, and deposits spherical eggs, around 2 mm, on aquatic plants. Most morphometric characters show size-specific trends, which complicate comparisons among populations or with other species. Of specific systematic importance is the relative position of the anal and dorsal fins, which remains almost unaltered throughout development, and allows safe distinction from A. fasciatus. The species was found mostly in deep areas with clean and slow running water, usually associated with freshwater springs. Rich submerged vegetation is the prominent ecological feature of all sites in which the species was found. Using as criteria of rarity the limited geographic distribution, the confinement of the species in few localities of each aquatic system and the low local densities, V. letourneuxi can be characterised as a "restricted and locally rare species". The restricted distribution, coupled with the narrow ecological requirements, makes the species vulnerable to extinction. Its disappearance from at least four aquatic systems and the serious population decline in a number of other systems seems to be connected with habitat loss or degradation caused by human activities. The prospects of conservation are discussed
Mesoscopic Modeling of a Highly-Ordered Sanidic Polymer Mesophase and Comparison With Experimental Data
[Image: see text] Board-shaped polymers form sanidic mesophases: assemblies of parallel lamellae of stacked polymer backbones separated by disordered side chains. Sanidics vary significantly with respect to polymer order inside their lamellae, making them “stepping stones” toward the crystalline state. Therefore, they are potentially interesting for studying crystallization and technological applications. Building on earlier mesoscopic models of the most disordered sanidics Σ(d), we focus on the other extreme, near-crystalline order, and develop a generic model that captures a highly ordered Σ(r) mesophase. Polymers are described by generic hindered-rotation chains. Anisotropic nonbonded potentials, with strengths comparable to the thermal energy, mimic board-like monomer shapes. Lamellae equilibrated with Monte Carlo simulations, for a broad range of model parameters, have intralamellar order typical for Σ(r) mesophases: periodically stacked polymers that are mutually registered along their backbones. Our mesophase shows registration on both monomer and chain levels. We calculate scattering patterns and compare with data published for highly ordered sanidic mesophases of two different polymers: polyesters and polypeptoids. Most of the generic structural features that were identified in these experiments are present in our model. However, our mesophase has correlations between chains located in different lamellae and is therefore closer to the crystalline state than the experimental samples
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