180 research outputs found
Polymer chain stiffness versus excluded volume: A Monte Carlo study of the crossover towards the wormlike chain model
When the local intrinsic stiffness of a polymer chain varies over a wide
range, one can observe both a crossover from rigid-rod-like behavior to
(almost) Gaussian random coils and a further crossover towards self-avoiding
walks in good solvents. Using the pruned-enriched Rosenbluth method (PERM) to
study self-avoiding walks of up to steps and variable flexibility,
the applicability of the Kratky-Porod model is tested. Evidence for
non-exponential decay of the bond-orientational correlations for large distances along the chain contour is presented, irrespective
of chain stiffness. For bottle-brush polymers on the other hand, where
experimentally stiffness is varied via the length of side-chains, it is shown
that these cylindrical brushes (with flexible backbones) are not described by
the Kratky-Porod wormlike chain model, since their persistence length is
(roughly) proportional to their cross-sectional radius, for all conditions of
practical interest.Comment: 6 pages, 5 figures, to be published in Europhys. Lett. (2010
Why polymer chains in a melt are not random walks
A cornerstone of modern polymer physics is the `Flory ideality hypothesis'
which states that a chain in a polymer melt adopts `ideal' random-walk-like
conformations. Here we revisit theoretically and numerically this pivotal
assumption and demonstrate that there are noticeable deviations from ideality.
The deviations come from the interplay of chain connectivity and the
incompressibility of the melt, leading to an effective repulsion between chain
segments of all sizes . The amplitude of this repulsion increases with
decreasing where chain segments become more and more swollen. We illustrate
this swelling by an analysis of the form factor , i.e. the scattered
intensity at wavevector resulting from intramolecular interferences of a
chain. A `Kratky plot' of {\em vs.} does not exhibit the plateau
for intermediate wavevectors characteristic of ideal chains. One rather finds a
conspicuous depression of the plateau, ,
which increases with and only depends on the monomer density .Comment: 4 pages, 4 figures, EPL, accepted January 200
Freezing of Spinodal Decompostion by Irreversible Chemical Growth Reaction
We present a description of the freezing of spinodal decomposition in
systems, which contain simultaneous irreversible chemical reactions, in the
hydrodynamic limit approximation. From own results we conclude, that the
chemical reaction leads to an onset of spinodal decomposition also in the case
of an initial system which is completely miscible and can lead to an extreme
retardation of the dynamics of the spinodal decomposition, with the probability
of a general freezing of this process, which can be experimetally observed in
simultaneous IPN formation.Comment: 10 page
Orthorhombic Phase of Crystalline Polyethylene: A Constant Pressure Path Integral Monte Carlo Study
In this paper we present a Path Integral Monte Carlo (PIMC) simulation of the
orthorhombic phase of crystalline polyethylene, using an explicit atom force
field with unconstrained bond lengths and angles. This work represents a
quantum extension of our recent classical simulation (J. Chem. Phys. 106, 8918
(1997)). It is aimed both at exploring the applicability of the PIMC method on
such polymer crystal systems, as well as on a detailed assessment of the
importance of quantum effects on different quantities. We used the
ensemble and simulated the system at zero pressure in the temperature range 25
- 300 K, using Trotter numbers between 12 and 144. In order to investigate
finite-size effects, we used chains of two different lengths, C_12 and C_24,
corresponding to the total number of atoms in the super-cell being 432 and 864,
respectively. We show here the results for structural parameters, like the
orthorhombic lattice constants a,b,c, and also fluctuations of internal
parameters of the chains, such as bond lengths and bond and torsional angles.
We have also determined the internal energy and diagonal elastic constants
c_11, c_22 and c_33. We discuss the temperature dependence of the measured
quantities and compare to that obtained from the classical simulation. For some
quantities, we discuss the way they are related to the torsional angle
fluctuation. In case of the lattice parameters we compare our results to those
obtained from other theoretical approaches as well as to some available
experimental data. In order to study isotope effects, we simulated also a
deuterated polyethylene crystal at a low temperature. We also suggest possible
ways of extending this study and present some general considerations concerning
modeling of polymer crystals.Comment: 18 pages, RevTex, 18 figures, 3 tables, submitted to Phys. Rev.
Intrinsic profiles and capillary waves at homopolymer interfaces: a Monte Carlo study
A popular concept which describes the structure of polymer interfaces by
``intrinsic profiles'' centered around a two dimensional surface, the ``local
interface position'', is tested by extensive Monte Carlo simulations of
interfaces between demixed homopolymer phases in symmetric binary (AB)
homopolymer blends, using the bond fluctuation model. The simulations are done
in an LxLxD geometry. The interface is forced to run parallel to the LxL planes
by imposing periodic boundary conditions in these directions and fixed boundary
conditions in the D direction, with one side favoring A and the other side
favoring B. Intrinsic profiles are calculated as a function of the ``coarse
graining length'' B by splitting the system into columns of size BxBxD and
averaging in each column over profiles relative to the local interface
position. The results are compared to predictions of the self-consistent field
theory. It is shown that the coarse graining length can be chosen such that the
interfacial width matches that of the self-consistent field profiles, and that
for this choice of B the ``intrinsic'' profiles compare well with the
theoretical predictions.Comment: to appear in Phys. Rev.
Portfolio Vol. I N 2
Wiley, Thomas R. In the Cathedral, Mexico City . Picture. 2.
Whitehead, Richard Jr. Izzy was a Lady, After All . Prose. 3.
Beckham, Adela. Rain on a March morning . Poem. 6.
Beckham, Adela. Heaven . Poem. 6.
Deane, Dorothy. Temptation . Poem. 6.
Kellogg, Elizabeth. Gruess Dich Gott . Prose. 7.
Nadel, Norman. The Duchess . Poem. 8.
Dick, Pewilla. The Sligo Fisherman . Prose. 9.
Deane, Dorothy. Against the Winter . Poem 12.
Flory, Doris Jean. A problem . Poem 12.
Travis, Paul Bough. My First View of the Congo Forest . Picture. 13.
Bellows, George. Stag at Sharkey\u27s . Picture. 13.
B.C.W. Aspiration . Poem. 14.
Stewart, John. On Record . Prose 14.
Sweitzer, Harry J. Playing Around . Prose. 15.
Ellsberg, Edward. Book Parade: Hell on Ice . Prose. 15.
B.C.W. End of Winter . Poem. 16.
Wiley, Thomas R. End of Winter . Picture. 16.
Deeter, Robert. Television, How, Where, and When . Prose. 17.
Brush, Jane. Love A La Mode . Poem. 20.
Brush, Jane. Radio! . Poem. 20.
Brush, Jane. Backward Glance . Poem. 20.
Brush, Jane. Homo Paradoxus . Poem. 20.
Brush, Jane. The Sardonic Slant . Poem. 20.
Brush, Jane. Baths . Prose. 20.
Wilson, Gordon. Brushword . Cartoon. 20
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The effect of elevated carbon dioxide on a Sierra-Nevadan dominant species: Pinus ponderosa
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