19,570 research outputs found
A self consistent field approach to surfaces of compressible polymer blends
A self consistent field theory for compressible polymer mixtures is developed
by introducing elements of classical density functional theory into the
framework of the Helfand theory. It is then applied to study free surfaces of
binary (A,B) polymer blends. Density profiles in the one- and two-phase region
are calculated as well as chain end distributions and chain orientations of the
minority and the majority component. In the ideally symmetric mixture, in which
the individual properties of polymers A and B are the same and both have the
same surface energy, polymers of the minority component segregate to the
surface, where they are exposed to less polymers of the majority component.
This effect can only be captured correctly, if one accounts for the fact that
the monomer-monomer interaction has finite range. As a consequence, the
Flory-Huggins-parameter varies in space and depends on the concentration
profiles and their derivatives. The surface segregation calculated with such an
ansatz, without any fit parameter, is in reasonable quantitative agreement with
data from recent Monte Carlo simulations.Comment: 45 pages, latex, 12 figures, accepted by the Journal of Chemical
physic
Reply to Comment on: "Are stress-free membranes really 'tensionless'?"
This is a reply to a comment on the paper arXiv:1204.2075 "Are stress-free
membranes really tensionless ?" (EPL 95,28008 (2011))
Phase behavior of grafted chain molecules: Influence of head size and chain length
Constant pressure Monte Carlo simulations of a coarse grained off-lattice
model for monolayers of amphiphilic molecules at the air/water interface are
presented. Our study focusses on phase transitions within a monolayer rather
than on self aggregation. We thus model the molecules as stiff chains of
Lennard-Jones spheres with one slightly larger repulsive end bead (head)
grafted to a planar surface. Depending on the size of the head, the temperature
and the pressure, we find a variety of phases, which differ in tilt order
(including tilt direction), and in positional order. In particular, we observe
a modulated phase with a striped superstructure. The modulation results from
the competition between two length scales, the head size and the tail diameter.
As this mechanism is fairly general, it may conceivably also be relevant in
experimental monolayers. We argue that the superstructure would be very
difficult to detect in a scattering experiment, which perhaps accounts for the
fact that it has not been reported so far. Finally the effect of varying the
chain length on the phase diagram is discussed. Except at high pressures and
temperatures, the phase boundaries in systems with longer chains are shifted to
higher temperatures.Comment: To appear in J. Chem. Phy
Monte Carlo simulations of interfaces in polymer blends
We review recent simulation studies of interfaces between immiscible
homopolymer phases. Special emphasis is given to the presentation of efficient
simulation techniques and powerful methods of data analysis, such as the
analysis of capillary wave spectra. Possible reasons for polymer
incompatibility and ways to relate model dependent interaction parameters to an
effective Flory Huggins parameter are discussed. Various interfaces are then
considered and characterised with respect to their microscopic structure and
thermodynamic properties. In particular, interfaces between homopolymers of
equal or disparate stiffness are studied, interfaces containing diblock
copolymers, and interfaces confined in thin films. The results are related to
the phase behaviour of ternary homopolymer/copolymer systems, and to wetting
transitions in thin films.Comment: To appear in Annual Reviews of Computational Physics, edt. D.
Stauffe
Errors in Monte Carlo simulations using shift register random number generators
We report large systematic errors in Monte Carlo simulations of the
tricritical Blume-Capel model using single spin Metropolis updating. The error,
manifest as a asymmetry in the magnetisation distribution, is traced to
the interplay between strong triplet correlations in the shift register random
number generator and the large tricritical clusters. The effect of these
correlations is visible only when the system volume is a multiple of the random
number generator lag parameter. No such effects are observed in related models.Comment: 7 pages Revtex, 4 ps figures (uuencoded). Paper also available from:
http://moses.physik.uni-mainz.de/~wilding/home_wilding.htm
Grafted Rods: A Tilting Phase Transition
A tilting phase transition is predicted for systems comprising rod like
molecules which are irreversibly grafted to a flat surface, so that the non
interacting rods are perpendicularly oriented. The transition is controlled by
the grafting density . It occurs as increases as a result of the
interplay between two energies. Tilt is favoured by the van-der-Waals
attraction between the rods. It is opposed by the bending elasticity of the
grafting functionality. The role of temperature is discussed, and the tilting
mechanism is compared to other tilting transitions reported in the literature.Comment: 21 pages, 2 figures, to appear in Journal de Physique I
Silicon ingot casting: Heat exchanger method. Multi-wire slicing: Fixed abrasive slicing technique, phase 3
In the area of ingot casting the proof of concept of heat exchanger method (HEM) was established. It was also established that HEM cast silicon yielded solar cell performance comparable to Czochralski grown material. Solar cells with conversion efficiencies of up to 15% were fabricated. It was shown that square cross-section ingots can be cast. In the area of crystal slicing, it was established that silicon can be sliced efficiently with the fixed abrasive slicing technique approach. This concept was carried forward to 10 cm diameter workpiece
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