249 research outputs found
Structure of bottle-brush brushes under good solvent conditions. A molecular dynamics study
We report a simulation study for bottle-brush polymers grafted on a rigid
backbone. Using a standard coarse-grained bead-spring model extensive molecular
dynamics simulations for such macromolecules under good solvent conditions are
performed. We consider a broad range of parameters and present numerical
results for the monomer density profile, density of the untethered ends of the
grafted flexible backbones and the correlation function describing the range
that neighboring grafted bottle-brushes are affected by the presence of the
others due to the excluded volume interactions. The end beads of the flexible
backbones of the grafted bottle-brushes do not access the region close to the
rigid backbone due to the presence of the side chains of the grafted
bottle-brush polymers, which stretch further the chains in the radial
directions. Although a number of different correlation lengths exist as a
result of the complex structure of these macromolecules, their properties can
be tuned with high accuracy in good solvents. Moreover, qualitative differences
with "typical" bottle-brushes are discussed. Our results provide a first
approach to characterizing such complex macromolecules with a standard bead
spring model.Comment: To appear in Journal of Physics Condensed Matter (2011
Phase behavior of symmetric linear multiblock copolymers
Molecular dynamics simulations are used to study the phase behavior of a
single linear multiblock copolymer with blocks of A- and B-type monomers under
poor solvent conditions, varying the block length , number of blocks ,
and the solvent quality (by variation of the temperature ). The fraction
of A-type monomers is kept constant and equal to 0.5, and always the lengths of
A and B blocks were equal (), as well as the number of blocks
(). We identify the three following regimes where: (i) full
microphase separation between blocks of different type occurs (all blocks of
A-type monomers form a single cluster, while all blocks of B-type monomers form
another), (ii) full microphase separation is observed with a certain
probability, and (iii) full microphase separation can not take place. For very
high number of blocks and very high (not accessible to our simulations)
further investigation is needed.Comment: 5 pages, 4 figures, to be published in Europhys. Let
Universality from disorder in the random-bond Blume-Capel model
Using high-precision Monte Carlo simulations and finite-size scaling we study the effect of quenched disorder in the exchange couplings on the Blume-Capel model on the square lattice. The first-order transition for large crystal-field coupling is softened to become continuous, with a divergent correlation length. An analysis of the scaling of the correlation length as well as the susceptibility and specific heat reveals that it belongs to the universality class of the Ising model with additional logarithmic corrections observed for the Ising model itself if coupled to weak disorder. While the leading scaling behavior in the disordered system is therefore identical between the second-order and first-order segments of the phase diagram of the pure model, the finite-size scaling in the ex-first-order regime is affected by strong transient effects with a crossover length scale for the chosen parameters
Mesophase formation in two-component cylindrical bottle-brush polymers
When two types of side chains (A,B) are densely grafted to a (stiff) backbone
and the resulting bottle-brush polymer is in a solution under poor solvent
conditions, an incompatibility between A and B leads to microphase separation
in the resulting cylindrical brush. The possible types of ordering are
reminiscent of the ordering of block copolymers in cylindrical confinement.
Starting from this analogy, Leibler's theory of microphase separation in block
copolymer melts is generalized to derive a description of the system in the
weak segregation limit. Also molecular dynamics simulation results of a
corresponding coarse-grained bead-spring model are presented. Using side chain
lengths up to N = 50 effective monomers, the ratio of the Lennard-Jones energy
parameter between unlike monomers and monomers of the same
kind is varied. Various correlation
functions are analyzed to study the conditions when (local) Janus cylinder-type
ordering and when (local) microphase separation in the direction along the
cylinder axis occurs. Both the analytical theory and the simulations give
evidence for short range order due to a tendency towards microphase separation
in the axial direction, with a wavelength proportional to the side chain
gyration radius, irrespective of temperature and grafting density, for a wide
range of these parameters.Comment: 26 pages, 19 figure
Interfacial adsorption in two-dimensional pure and random-bond Potts models
We study using Monte Carlo simulations the finite-size scaling behavior of
the interfacial adsorption of the two-dimensional square-lattice -states
Potts model. We consider the pure and random-bond versions of the Potts model
for and , thus probing the interfacial properties at the
originally continuous, weak, and strong first-order phase transitions. For the
pure systems our results support the early scaling predictions for the size
dependence of the interfacial adsorption at both first- and second-order phase
transitions. For the disordered systems, the interfacial adsorption at the
(disordered induced) continuous transitions is discussed, applying standard
scaling arguments and invoking findings for bulk critical properties. The
self-averaging properties of the interfacial adsorption are also analyzed by
studying the infinite limit-size extrapolation of properly defined
signal-to-noise ratios.Comment: 7 pages, 5 figures, final version to be published in Phys. Rev. E.
arXiv admin note: text overlap with arXiv:1504.0742
Bose-Einstein condensates in standing waves: The cubic nonlinear Schroedinger equation with a periodic potential
We present a new family of stationary solutions to the cubic nonlinear
Schroedinger equation with a Jacobian elliptic function potential. In the limit
of a sinusoidal potential our solutions model a dilute gas Bose-Einstein
condensate trapped in a standing light wave. Provided the ratio of the height
of the variations of the condensate to its DC offset is small enough, both
trivial phase and nontrivial phase solutions are shown to be stable. Numerical
simulations suggest such stationary states are experimentally observable.Comment: 4 pages, 4 figure
Enhanced erythrocyte antioxidant status following an 8-week aerobic exercise training program in heavy drinkers
© 2017 Elsevier Inc. Alcohol-induced oxidative stress is involved in the development and progression of various pathological conditions and diseases. On the other hand, exercise training has been shown to improve redox status, thus attenuating oxidative stress-associated disease processes. The purpose of the present study was to evaluate the effect of an exercise training program that has been previously reported to decrease alcohol consumption on blood redox status in heavy drinkers. In a non-randomized within-subject design, 11 sedentary, heavily drinking men (age: 30.3 ± 3.5 years; BMI: 28.4 ± 0.86 kg/m2) participated first in a control condition for 4 weeks, and then in an intervention where they completed an 8-week supervised aerobic training program of moderate intensity (50–60% of the heart rate reserve). Blood samples were collected in the control condition (pre-, post-control) as well as before, during (week 4 of the training program), and after intervention (week 8 of the training program). Samples were analyzed for total antioxidant capacity (TAC), thiobarbituric acid reactive substances (TBARS), protein carbonyls (PC), uric acid (UA), bilirubin, reduced glutathione (GSH), and catalase activity. No significant change in indices of redox status in the pre- and post-control was observed. Catalase activity increased (p < 0.05) after 8 weeks of intervention compared to week 4. GSH increased (p < 0.05) after 8 weeks of intervention compared to the control condition and to week 4 of intervention. TAC, UA, bilirubin, TBARS, and PC did not significantly change at any time point. Moreover, concentrations of GSH, TBARS, and catalase activity negatively correlated with alcohol consumption. In conclusion, an 8-week aerobic training program enhanced erythrocyte antioxidant status in heavy drinkers, indicating that aerobic training may attenuate pathological processes caused by alcohol-induced oxidative stress.This study was co-financed by the European Union [European Social Fund (ESF)] and Greek national funds through the Operational Program ‘Education and Lifelong Learning’ of the National Strategic Reference Framework-Research Funding Program: THALES. Investing in knowledge society through the ESF.Published versio
Superspreading: Mechanisms and Molecular Design
The
intriguing ability of certain surfactant molecules to drive
the superspreading of liquids to complete wetting on hydrophobic substrates
is central to numerous applications that range from coating flow technology
to enhanced oil recovery. Despite significant experimental efforts,
the precise mechanisms underlying superspreading remain unknown to
date. Here, we isolate these mechanisms by analyzing coarse-grained
molecular dynamics simulations of surfactant molecules of varying
molecular architecture and substrate affinity. We observe that for
superspreading to occur, two key conditions must be simultaneously
satisfied: the adsorption of surfactants from the liquid–vapor
surface onto the three-phase contact line augmented by local bilayer
formation. Crucially, this must be coordinated with the rapid replenishment
of liquid–vapor and solid–liquid interfaces with surfactants
from the interior of the droplet. This article also highlights and
explores the differences between superspreading and conventional surfactants,
paving the way for the design of molecular architectures tailored
specifically for applications that rely on the control of wetting
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