1,092 research outputs found
Lifshitz points in blends of AB and BC diblock copolymers
We consider micro- and macro-phase separation in blends of AB and BC flexible diblock copolymers. We show that, depending on architecture, a number of phase diagram topologies are possible. Microphase separation or macrophase separation can occur, and there are a variety of possible Lifshitz points. Because of the rich parameter space, Lifshitz points of multiple order are possible. We demonstrate Lifshitz points of first and second order, and argue that, in principle, up to 5th-order Lifshitz points are possible
Anomalous structural and mechanical properties of solids confined in quasi one dimensional strips
We show using computer simulations and mean field theory that a system of
particles in two dimensions, when confined laterally by a pair of parallel hard
walls within a quasi one dimensional channel, possesses several anomalous
structural and mechanical properties not observed in the bulk. Depending on the
density and the distance between the walls , the system shows
structural characteristics analogous to a weakly modulated liquid, a strongly
modulated smectic, a triangular solid or a buckled phase. At fixed , a
change in leads to many reentrant discontinuous transitions involving
changes in the number of layers parallel to the confining walls depending
crucially on the commensurability of inter-layer spacing with . The solid
shows resistance to elongation but not to shear. When strained beyond the
elastic limit it fails undergoing plastic deformation but surprisingly, as the
strain is reversed, the material recovers completely and returns to its
original undeformed state. We obtain the phase diagram from mean field theory
and finite size simulations and discuss the effect of fluctuations.Comment: 14 pages, 13 figures; revised version, accepted in J. Chem. Phy
Micro- vs. macro-phase separation in binary blends of poly(styrene)-poly(isoprene) and poly(isoprene)-poly(ethylene oxide) diblock copolymers
In this paper we present an experimentally determined phase diagram of binary blends of the diblock copolymers poly(styrene)-poly(isoprene) and poly(isoprene)-poly(ethylene oxide). At high temperatures, the blends form an isotropic mixture. Upon lowering the temperature, the blend macro-phase separates before micro-phase separation occurs. The observed phase diagram is compared to theoretical predictions based on experimental parameters. In the low-temperature phase the crystallisation of the poly(ethylene oxide) block influences the spacing of the ordered phase
Multi-scale coarse-graining of diblock copolymer self-assembly: from monomers to ordered micelles
Starting from a microscopic lattice model, we investigate clustering,
micellization and micelle ordering in semi-dilute solutions of AB diblock
copolymers in a selective solvent. To bridge the gap in length scales, from
monomers to ordered micellar structures, we implement a two-step coarse
graining strategy, whereby the AB copolymers are mapped onto ``ultrasoft''
dumbells with monomer-averaged effective interactions between the centres of
mass of the blocks. Monte Carlo simulations of this coarse-grained model yield
clear-cut evidence for self-assembly into micelles with a mean aggregation
number n of roughly 100 beyond a critical concentration. At a slightly higher
concentration the micelles spontaneously undergo a disorder-order transition to
a cubic phase. We determine the effective potential between these micelles from
first principles.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett
Monte Carlo Study of the Axial Next-Nearest-Neighbor Ising Model
The equilibrium phase behavior of microphase-forming substances and models is
notoriously difficult to obtain because of the extended metastability of the
modulated phases. We develop a simulation method based on thermodynamic
integration that avoids this problem and with which we obtain the phase diagram
of the canonical three-dimensional axial next-nearest-neighbor Ising model. The
equilibrium devil's staircase, magnetization, and susceptibility are obtained.
The critical exponents confirm the XY nature of the disorder-modulated phase
transition beyond the Lifshitz point. The results identify the limitations of
various approximation schemes used to analyze this basic microphase-forming
model.Comment: 4 pages, 3 figure
Photoassociation spectroscopy of a Spin-1 Bose-Einstein condensate
We report on the high resolution photoassociation spectroscopy of a Rb
spin-1 Bose-Einstein condensate to the excited
molecular states. We demonstrate the use of spin dependent photoassociation to
experimentally identify the molecular states and their corresponding initial
scattering channel. These identifications are in excellent agreement with the
eigenvalues of a hyperfine-rotational Hamiltonian. Using the observed spectra
we estimate the change in scattering length and identify photoassociation laser
light frequency ranges that maximize the change in the spin-dependent
mean-field interaction energy.Comment: 5 pages, 4 figure
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Influence of a non-ionic amphiphilic copolymer on the self-assembly of a peptide amphiphile that forms nanotapes
The influence of a non-ionic polymeric surfactant on the self-assembly of a peptide amphiphile (PA)
that forms nanotapes is investigated using a combination of microscopic, scattering and spectroscopic
techniques. Mixtures of Pluronic copolymer P123 with the PA C16-KTTKS in aqueous solution were
studied at a fixed concentration of the PA at which it is known to self-assemble into extended
nanotapes, but varying P123 concentration. We find that P123 can disrupt the formation of C16-
KTTKS nanotapes, leading instead to cylindrical nanofibril structures. The spherical micelles formed
by P123 at room temperature are disrupted in the presence of the PA. There is a loss of cloudiness in the
solutions as the large nanotape aggregates formed by C16-KTTKS are broken up, by P123
solubilization. At least locally, b-sheet structure is retained, as confirmed by XRD and FTIR
spectroscopy, even for solutions containing 20 wt% P123. This indicates, unexpectedly, that peptide
secondary structure can be retained in solutions with high concentration of non-ionic surfactant. Selfassembly
in this system exhibits slow kinetics towards equilibrium, the initial self-assembly being
dependent on the order of mixing. Heating above the lipid chain melting temperature assists in
disrupting trapped non-equilibrium states
Moxidectin: an oral treatment for human onchocerciasis
Moxidectin is a milbemycin endectocide recently approved for the treatment of human onchocerciasis. Onchocerciasis, earmarked for elimination of transmission, is a filarial infection endemic in Africa, Yemen, and the Amazonian focus straddling Venezuela and Brazil. Concerns over whether the predominant treatment strategy (yearly mass drug administration (MDA) of ivermectin) is sufficient to achieve elimination in all endemic foci have refocussed attention upon alternative treatments. Moxidectin’s stronger and longer microfilarial suppression compared to ivermectin in both phase II and III clinical trials indicates its potential as a novel powerful drug for onchocerciasis elimination
Viscoelastic Properties of Dynamically Asymmetric Binary Fluids Under Shear Flow
We study theoretically the viscoelastic properties of sheared binary fluids
that have strong dynamical asymmetry between the two components. The dynamical
asymmetry arises due to asymmetry between the viscoelastic stresses,
particularly the bulk stress. Our calculations are based on the two-fluid model
that incorporates the asymmetric stress distribution. We simulate the phase
separation process under an externally imposed shear and compare the asymmetric
case with the usual phase separation under a shear flow without viscoelastic
effects. We also simulate the behavior of phase separated stable morphologies
under applied shear and compute the stress relaxation.Comment: 10 pages text, 9 figure
A numerical renormalization group study of laser induced freezing
We study the phenomenon of laser induced freezing, within a numerical
renormalization scheme which allows explicit comparison with a recent defect
mediated melting theory. Precise values for the `bare' dislocation fugacities
and elastic moduli of the 2-d hard disk system are obtained from a constrained
Monte Carlo simulation sampling only configurations {\em without} dislocations.
These are used as inputs to appropriate renormalization flow equations to
obtain the equilibrium phase diagram which shows excellent agreement with
earlier simulation results. We show that the flow equations need to be correct
at least up to third order in defect fugacity to reproduce meaningful results.Comment: Minor Corrections; Combined version of Europhys. Lett. 67 (2004) p.
814 and Europhys. Lett. 68 (2004) p. 16
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