429 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
Bound states in a nonlinear Kronig-Penney model
We study the bound states of a Kronig Penney potential for a nonlinear
one-dimensional Schroedinger equation. This potential consists of a large, but
not necessarily infinite, number of equidistant delta-function wells. We show
that the ground state can be highly degenerate. Under certain conditions
furthermore, even the bound state that would normally be the highest can have
almost the same energy as the ground state. This holds for simple periodic
potentials as well.Comment: TeX file, figures available as postscript files upon reques
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
Exposure to Perchlorate Induces the Formation of Macrophage Aggregates in the Trunk Kidney of Zebrafish and Mosquitofish
Environmental contamination of ground and surface waters by perchlorate, derived from ammonium perchlorate (AP) and other perchlorate salts, is of increasing concern. Exposure to perchlorate can impair the thyroid endocrine system, which is thought to modulate renal and immune function in vertebrates. This study with zebrafish Danio rerio and eastern mosquitofish Gambusia holbrooki examined the histological effects of perchlorate on the trunk kidney, which in teleosts serves excretory and hemopoietic functions and therefore may be a target of perchlorate effects. Adult zebrafish of both sexes were exposed in the laboratory to waterborne, AP-derived perchlorate at measured concentrations of 18 mg/L for 8 weeks. Adult male mosquitofish were exposed to waterborne sodium perchlorate at measured perchlorate concentrations of 1–92 mg/L for 8 weeks. Control fish were kept in untreated water. The region of the body cavity containing the trunk kidney was processed from each fish for histological analysis. Macrophage aggregates (MAs), possible markers of contaminant exposure or immunotoxic effect, were present in the hemopoietic region of the kidney in both species exposed to perchlorate. The estimated percent area of kidney sections occupied by MAs was greater in zebrafish exposed to perchlorate at 18 mg/L (P , 0.05) than in controls. In male mosquitofish, the incidence of renal MAs increased proportionally with sodium perchlorate concentration and was significantly different from that of controls at 92 mg/L (P , 0.05). These observations confirm that in fish the kidney is affected by exposure to perchlorate. The concentrations of perchlorate at which the effects were noted are relatively high but within the range reported in some contaminated habitats
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
A Study of the Soft X-Ray Emission Lines in NGC 4151. I. Kinematic Properties of the Plasma Wind
We present our analysis of the narrow emission lines produced in the plasma regions within the bright active galactic nucleus of NGC 4151, from an ORBYTS research-with-schools public engagement project. Our goal was to test whether the properties of these plasma regions changed between XMM-Newton observations spanning 15 yr from 2000 to 2015, by measuring the outflow velocities and distances. From this study, we found that NGC 4151 has at least two to three plasma regions. There is no evidence of the outflowing wind properties changing as the velocities and distances are consistent throughout the observations
- …