705 research outputs found
Star-Like Micelles with Star-Like Interactions: A quantitative Evaluation of Structure Factor and Phase Diagram
PEP-PEO block copolymer micelles offer the possibility to investigate phase
behaviour and interactions of star polymers (ultra-soft colloids). A star-like
architecture is achieved by an extremely asymmetric block ratio (1:20).
Micellar functionality f can be smoothly varied by changing solvent composition
(interfacial tension). Structure factors obtained by SANS can be quantitatively
described in terms of an effective potential developed for star polymers. The
experimental phase diagram reproduces to a high level of accuracy the predicted
liquid/solid transition. Whereas for intermediate f a bcc phase is observed,
for high f the formation of a fcc phase is preempted by glass formation.Comment: 5 pages, 4 figures, PRL in pres
Influence of long-range correlated quenched disorder on the adsorption of long flexible polymer chains on a wall
The process of adsorption on a planar wall of long-flexible polymer chains in
the medium with quenched long-range correlated disorder is investigated. We
focus on the case of correlations between defects or impurities that decay
according to the power-low for large distances , where . Field theoretical approach in and directly in
dimensions up to one-loop order for the semi-infinite m-vector
model (in the limit ) with a planar boundary is used. The whole set of
surface critical exponents at the adsorption threshold , which separates
the nonadsorbed region from the adsorbed one is obtained. Moreover, we
calculate the crossover critical exponent and the set of exponents
associated with them. We perform calculations in a double and
expansion and also for a fixed dimension , up to one-loop
order for different values of the correlation parameter .
The obtained results indicate that for the systems with long-range correlated
quenched disorder the new set of surface critical exponents arises. All the
surface critical exponents depend on . Hence, the presence of long-range
correlated disorder influences the process of adsorption of long-flexible
polymer chains on a wall in a significant way.Comment: 4 figures, 2 table
Muon spin rotation and neutron scattering study of the non-centrosymmetric tetragonal compound CeAuAl3
We have investigated the non-centrosymmetric tetragonal heavy-fermion
compound CeAuAl3 using muon spin rotation (muSR), neutron diffraction (ND) and
inelastic neutron scattering (INS) measurements. We have also revisited the
magnetic, transport and thermal properties. The magnetic susceptibility reveals
an antiferromagnetic transition at 1.1 K with a possibility of another magnetic
transition near 0.18 K. The heat capacity shows a sharp lambda-type anomaly at
1.1 K in zero-filed, which broadens and moves to higher temperature in applied
magnetic field. Our zero-field muSR and ND measurements confirm the existence
of a long-range magnetic ground state below 1.2 K. Further the ND study reveals
an incommensurate magnetic ordering with a magnetic propagation vector k = (0,
0, 0.52) and a spiral structure of Ce moments coupled ferromagnetically within
the ab-plane. Our INS study reveals the presence of two well-defined crystal
electric field (CEF) excitations at 5.1 meV and 24.6 meV in the paramagnetic
phase of CeAuAl3 which can be explained on the basis of the CEF theory.
Furthermore, low energy quasi-elastic excitations show a Gaussian line shape
below 30 K compared to a Lorentzian line shape above 30 K, indicating a
slowdown of spin fluctuation below 30 K. We have estimated a Kondo temperature
of TK=3.5 K from the quasi-elastic linewidth, which is in good agreement with
that estimated from the heat capacity. This study also indicates the absence of
any CEF-phonon coupling unlike that observed in isostructural CeCuAl3. The CEF
parameters, energy level scheme and their wave functions obtained from the
analysis of INS data explain satisfactorily the single crystal susceptibility
in the presence of two-ion anisotropic exchange interaction in CeAuAl3.Comment: 28 pages and 17 figure
Micellization of Sliding Polymer Surfactants
Following up a recent paper on grafted sliding polymer layers (Macromolecules
2005, 38, 1434-1441), we investigated the influence of the sliding degree of
freedom on the self-assembly of sliding polymeric surfactants that can be
obtained by complexation of polymers with cyclodextrins. In contrast to the
micelles of quenched block copolymer surfactants, the free energy of micelles
of sliding surfactants can have two minima: the first corresponding to small
micelles with symmetric arm lengths, and the second corresponding to large
micelles with asymmetric arm lengths. The relative sizes and concentrations of
small and large micelles in the solution depend on the molecular parameters of
the system. The appearance of small micelles drastically reduces the kinetic
barrier signifying the fast formation of equilibrium micelles.Comment: Submitted to Macromolecule
The sustainable worldwide offshore wind energy potential: A systematic review
The offshore wind industry is expanding rapidly around the world due to several factors enabling this source of renewable energy. Stronger wind resources in offshore areas, lack of social and geographical constraints related to onshore wind power, the evolution of technology, and increasing demand for electricity in coastal regions as a result of a massive increase in population are some of the factors favoring the use of wind energy. The assessment of the potential global capacity that considers the different economic, environmental, and social factors and the dynamics of market, policy, and technology are vital for estimating the competitiveness of offshore wind energy in the future energy profile. There are several studies and technical reports that evaluate the potential of offshore wind energy in different countries or regions. They used a different source of data, metrics, and quantitative approaches in appraising the potential offshore wind power capacity and its cost efficiency. The critical factors that have been considered are geographical, technical, economic, environmental, and social and market elements. This paper provides a systematic review for analyzing the studies that address the potential offshore wind energy around the world and published during the 2000–2016 period. This study highlights the key criteria for assessing the potential for offshore wind energy deployment and the related tools and methods
Charge Ordering and Ferroelectricity in Half-doped Manganites
By means of density-functional simulations for half-doped manganites, such as
pseudocubic Pr0.5Ca0.5MnO3 and bilayer PrCa2Mn2O7, we discuss the occurrence of
ferroelectricity and we explore its crucial relation to the crystal structure
and to peculiar charge/spin/orbital ordering effects. In pseudocubic
Pr0.5Ca0.5MnO3, ferroelectricity is induced in the Zener polaron type
structure, where Mn ions are dimerized. In marked contrast, in bilayer
PrCa2Mn2O7, it is the displacements of apical oxygens bonded to either Mn3+ or
Mn4+ ions that play a key role in the rising of ferroelectricity. Importantly,
local dipoles due to apical oxygens are also intimately linked to charge and
orbital ordering patterns in MnO2 planes, which in turn contribute to
polarization. Finally, an important outcome of our work consists in proposing
Born effective charges as a valid mean to quantify charge disproportionation
effects, in terms of anisotropy and size of electronic clouds around Mn ions.Comment: 5 pages, 2 figures, submitted for publicatio
Critical behaviour of the Rouse model for gelling polymers
It is shown that the traditionally accepted "Rouse values" for the critical
exponents at the gelation transition do not arise from the Rouse model for
gelling polymers. The true critical behaviour of the Rouse model for gelling
polymers is obtained from spectral properties of the connectivity matrix of the
fractal clusters that are formed by the molecules. The required spectral
properties are related to the return probability of a "blind ant"-random walk
on the critical percolating cluster. The resulting scaling relations express
the critical exponents of the shear-stress-relaxation function, and hence those
of the shear viscosity and of the first normal stress coefficient, in terms of
the spectral dimension of the critical percolating cluster and the
exponents and of the cluster-size distribution.Comment: 9 pages, slightly extended version, to appear in J. Phys.
Dragging a polymer chain into a nanotube and subsequent release
We present a scaling theory and Monte Carlo (MC) simulation results for a
flexible polymer chain slowly dragged by one end into a nanotube. We also
describe the situation when the completely confined chain is released and
gradually leaves the tube. MC simulations were performed for a self-avoiding
lattice model with a biased chain growth algorithm, the pruned-enriched
Rosenbluth method. The nanotube is a long channel opened at one end and its
diameter is much smaller than the size of the polymer coil in solution. We
analyze the following characteristics as functions of the chain end position
inside the tube: the free energy of confinement, the average end-to-end
distance, the average number of imprisoned monomers, and the average stretching
of the confined part of the chain for various values of and for the number
of monomers in the chain, . We show that when the chain end is dragged by a
certain critical distance into the tube, the polymer undergoes a
first-order phase transition whereby the remaining free tail is abruptly sucked
into the tube. This is accompanied by jumps in the average size, the number of
imprisoned segments, and in the average stretching parameter. The critical
distance scales as . The transition takes place when
approximately 3/4 of the chain units are dragged into the tube. The theory
presented is based on constructing the Landau free energy as a function of an
order parameter that provides a complete description of equilibrium and
metastable states. We argue that if the trapped chain is released with all
monomers allowed to fluctuate, the reverse process in which the chain leaves
the confinement occurs smoothly without any jumps. Finally, we apply the theory
to estimate the lifetime of confined DNA in metastable states in nanotubes.Comment: 13pages, 14figure
Interactions between proteins bound to biomembranes
We study a physical model for the interaction between general inclusions
bound to fluid membranes that possess finite tension, as well as the usual
bending rigidity. We are motivated by an interest in proteins bound to cell
membranes that apply forces to these membranes, due to either entropic or
direct chemical interactions. We find an exact analytic solution for the
repulsive interaction between two similar circularly symmetric inclusions. This
repulsion extends over length scales of order tens of nanometers, and contrasts
with the membrane-mediated contact attraction for similar inclusions on
tensionless membranes. For non circularly symmetric inclusions we study the
small, algebraically long-ranged, attractive contribution to the force that
arises. We discuss the relevance of our results to biological phenomena, such
as the budding of caveolae from cell membranes and the striations that are
observed on their coats.Comment: 22 pages, 2 figure
Theta-point universality of polyampholytes with screened interactions
By an efficient algorithm we evaluate exactly the disorder-averaged
statistics of globally neutral self-avoiding chains with quenched random charge
in monomer i and nearest neighbor interactions on
square (22 monomers) and cubic (16 monomers) lattices. At the theta transition
in 2D, radius of gyration, entropic and crossover exponents are well compatible
with the universality class of the corresponding transition of homopolymers.
Further strong indication of such class comes from direct comparison with the
corresponding annealed problem. In 3D classical exponents are recovered. The
percentage of charge sequences leading to folding in a unique ground state
approaches zero exponentially with the chain length.Comment: 15 REVTEX pages. 4 eps-figures . 1 tabl
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