613 research outputs found
Correlated disorder in random block-copolymers
We study the effect of a random Flory-Huggins parameter in a symmetric
diblock copolymer melt which is expected to occur in a copolymer where one
block is near its structural glass transition. In the clean limit the
microphase segregation between the two blocks causes a weak, fluctuation
induced first order transition to a lamellar state. Using a renormalization
group approach combined with the replica trick to treat the quenched disorder,
we show that beyond a critical disorder strength, that depends on the length of
the polymer chain, the character of the transition is changed. The system
becomes dominated by strong randomness and a glassy rather than an ordered
lamellar state occurs. A renormalization of the effective disorder distribution
leads to nonlocal disorder correlations that reflect strong compositional
fluctuation on the scale of the radius of gyration of the polymer chains. The
reason for this behavior is shown to be the chain length dependent role of
critical fluctuations, which are less important for shorter chains and become
increasingly more relevant as the polymer length increases and the clean first
order transition becomes weaker.Comment: 11 pages, 5 figures, submitted to PR
Deviations from the mean field predictions for the phase behaviour of random copolymers melts
We investigate the phase behaviour of random copolymers melts via large scale
Monte Carlo simulations. We observe macrophase separation into A and B--rich
phases as predicted by mean field theory only for systems with a very large
correlation lambda of blocks along the polymer chains, far away from the
Lifshitz point. For smaller values of lambda, we find that a locally
segregated, disordered microemulsion--like structure gradually forms as the
temperature decreases. As we increase the number of blocks in the polymers, the
region of macrophase separation further shrinks. The results of our Monte Carlo
simulation are in agreement with a Ginzburg criterium, which suggests that mean
field theory becomes worse as the number of blocks in polymers increases.Comment: 6 pages, 4 figures, Late
Increase in Arctic coastal erosion and its sensitivity to warming in the twenty-first century
Arctic coastal erosion damages infrastructure, threatens coastal communities and releases organic carbon from permafrost. However, the magnitude, timing and sensitivity of coastal erosion increase to global warming remain unknown. Here we project the Arctic-mean erosion rate to increase and very likely exceed its historical range of variability before the end of the century in a wide range of emission scenarios. The sensitivity of erosion to warming roughly doubles, reaching 0.4–0.8 m yr−1 °C−1 and 2.3–4.2 TgC yr−1 °C−1 by the end of the century. We develop a simplified semi-empirical model to produce twenty-first-century pan-Arctic coastal erosion rate projections. Our results will inform policymakers on coastal conservation and socioeconomic planning, and organic carbon flux projections lay out the path for future work to investigate the impact of Arctic coastal erosion on the changing Arctic Ocean, its role as a global carbon sink, and the permafrost–carbon feedback. © 2022, The Author(s)
Separation of VUV/UV photons and reactive particles in the effluent of a He/O2 atmospheric pressure plasma jet
Cold atmospheric pressure plasmas can be used for treatment of living tissues
or for inactivation of bacteria or biological macromolecules. The treatment is
usually characterized by a combined effect of UV and VUV radiation, reactive
species, and ions. This combination is usually beneficial for the effectiveness
of the treatment but it makes the study of fundamental interaction mechanisms
very difficult. Here we report on an effective separation of VUV/UV photons and
heavy reactive species in the effluent of a micro scale atmospheric pressure
plasma jet (-APPJ). The separation is realized by an additional flow of
helium gas under well-defined flow conditions, which deflects heavy particles
in the effluent without affecting the VUV and UV photons. Both components of
the effluent, the photons and the reactive species, can be used separately or
in combination for sample treatment. The results of treatment of a model plasma
polymer film and vegetative Bacillus subtilis and Escherichia coli cells are
shown and discussed. A simple model of the He gas flow and reaction kinetics of
oxygen atoms in the gas phase and at the surface is used to provide a better
understanding of the processes in the plasma effluent. The new jet
modification, called X-Jet for its appearance, will simplify the investigation
of interaction mechanisms of atmospheric pressure plasmas with biological
samples.Comment: 10 pages, 7 figures, submitted to Journal of Physics D: Applied
Physic
A Model Ground State of Polyampholytes
The ground state of randomly charged polyampholytes is conjectured to have a
structure similar to a necklace, made of weakly charged parts of the chain,
compacting into globules, connected by highly charged stretched `strings'. We
suggest a specific structure, within the necklace model, where all the neutral
parts of the chain compact into globules: The longest neutral segment compacts
into a globule; in the remaining part of the chain, the longest neutral segment
(the 2nd longest neutral segment) compacts into a globule, then the 3rd, and so
on. We investigate the size distributions of the longest neutral segments in
random charge sequences, using analytical and Monte Carlo methods. We show that
the length of the n-th longest neutral segment in a sequence of N monomers is
proportional to N/(n^2), while the mean number of neutral segments increases as
sqrt(N). The polyampholyte in the ground state within our model is found to
have an average linear size proportional to sqrt(N), and an average surface
area proportional to N^(2/3).Comment: 8 two-column pages. 5 eps figures. RevTex. Submitted to Phys. Rev.
Conformational transitions of heteropolymers in dilute solutions
In this paper we extend the Gaussian self-consistent method to permit study
of the equilibrium and kinetics of conformational transitions for
heteropolymers with any given primary sequence. The kinetic equations earlier
derived by us are transformed to a form containing only the mean squared
distances between pairs of monomers. These equations are further expressed in
terms of instantaneous gradients of the variational free energy. The method
allowed us to study exhaustively the stability and conformational structure of
some periodic and random aperiodic sequences. A typical phase diagram of a
fairly long amphiphilic heteropolymer chain is found to contain phases of the
extended coil, the homogeneous globule, the micro-phase separated globule, and
a large number of frustrated states, which result in conformational phases of
the random coil and the frozen globule. We have also found that for a certain
class of sequences the frustrated phases are suppressed. The kinetics of
folding from the extended coil to the globule proceeds through non-equilibrium
states possessing locally compacted, but partially misfolded and frustrated,
structure. This results in a rather complicated multistep kinetic process
typical of glassy systems.Comment: 15 pages, RevTeX, 20 ps figures, accepted for publication in Phys.
Rev.
Polyelectrolyte multilayer formation: electrostatics and short-range interactions
We investigate the phenomenon of multilayer formation via layer-by-layer
deposition of alternating charge polyelectrolytes. Using mean-field theory, we
find that a strong short-range attraction between the two types of polymer
chains is essential for the formation of multilayers. The dependence of the
required short-range attraction on the polymer charge fraction and salt
concentration is calculated. For weak short-range attraction between any two
adjacent layers, the adsorbed amount (per added layer) decays as the distance
from the surface increases, until the chains stop adsorbing altogether. For
strong short-range attraction, the adsorbed amount per layer increases after an
initial decrease, and finally it stabilizes in the form of a polyelectrolyte
multilayer that can be repeated many times.Comment: 8 pages, 7 figure
First neutral dinuclear cobalt complex formed by bridging [μ-O2P(H)R]- ligands: Synthesis, X-ray crystal structure and quantum-chemical study
© 2015 Mendeleev Communications. Published by Elsevier B.V. The reaction of cobalt dibromide hexahydrate with 2,2′-bipyridine (bpy) and 9-anthrylphosphinic acid AntP(O)(OH)H (Ant = 9-anthryl) leads to the first example of a neutral dinuclear cobalt(ii) complex {Co2Br2[μ-O2P(H)Ant]2(bpy)2} formed by two bridging [μ-O2P(H)Ant]- ligands. The complex has been characterized by X-ray diffraction analysis and quantum-chemical calculations
Synthesis, X-ray crystal structure and quantum-chemical study of new dinuclear cobalt complex {Co2[mmm-O2P(H)Mes] 2(bpy)4}Br2
The reaction of cobalt dibromide hexahydrate with 2,2'-bipyridine (bpy) and mesitylphosphinic acid MesP(O)(OH)H (Mes = = 2,4,6-trimethylphenyl) under solvothermal synthesis conditions leads to a new dinuclear cobalt(ii) complex {Co2[μ-O2P(H)Mes]2- (bpy) 4}Br2 formed by two bridging {μ-O2P(H)Mes}- ligands. X-ray crystal structure analysis of the complex displayed that cobalt ions have distorted octahedral coordination and are doubly bridged by two mesitylphosphinato ligands. © 2013 Mendeleev Communications. All rights reserved
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