211 research outputs found
Polymers as compressible soft spheres
We consider a coarse-grained model in which polymers under good-solvent
conditions are represented by soft spheres whose radii, which should be
identified with the polymer radii of gyrations, are allowed to fluctuate. The
corresponding pair potential depends on the sphere radii. This model is a
single-sphere version of the one proposed in Vettorel et al., Soft Matter 6,
2282 (2010), and it is sufficiently simple to allow us to determine all
potentials accurately from full-monomer simulations of two isolated polymers
(zero-density potentials). We find that in the dilute regime (which is the
expected validity range of single-sphere coarse-grained models based on
zero-density potentials) this model correctly reproduces the density dependence
of the radius of gyration. However, for the thermodynamics and the
intermolecular structure, the model is largely equivalent to the simpler one in
which the sphere radii are fixed to the average value of the radius of gyration
and radiiindependent potentials are used: for the thermodynamics there is no
advantage in considering a fluctuating sphere size.Comment: 21 pages, 7 figure
Elastic Lattice Polymers
We study a model of "elastic" lattice polymer in which a fixed number of
monomers is hosted by a self-avoiding walk with fluctuating length . We
show that the stored length density scales asymptotically
for large as , where is the
polymer entropic exponent, so that can be determined from the analysis
of . We perform simulations for elastic lattice polymer loops with
various sizes and knots, in which we measure . The resulting estimates
support the hypothesis that the exponent is determined only by the
number of prime knots and not by their type. However, if knots are present, we
observe strong corrections to scaling, which help to understand how an entropic
competition between knots is affected by the finite length of the chain.Comment: 10 page
Individual Entanglements in a Simulated Polymer Melt
We examine entanglements using monomer contacts between pairs of chains in a
Brownian-dynamics simulation of a polymer melt. A map of contact positions with
respect to the contacting monomer numbers (i,j) shows clustering in small
regions of (i,j) which persists in time, as expected for entanglements. Using
the ``space''-time correlation function of the aforementioned contacts, we show
that a pair of entangled chains exhibits a qualitatively different behavior
than a pair of distant chains when brought together. Quantitatively, about 50%
of the contacts between entangled chains are persistent contacts not present in
independently moving chains. In addition, we account for several observed
scaling properties of the contact correlation function.Comment: latex, 12 pages, 7 figures, postscript file available at
http://arnold.uchicago.edu/~ebn
Identification of a polymer growth process with an equilibrium multi-critical collapse phase transition: the meeting point of swollen, collapsed and crystalline polymers
We have investigated a polymer growth process on the triangular lattice where
the configurations produced are self-avoiding trails. We show that the scaling
behaviour of this process is similar to the analogous process on the square
lattice. However, while the square lattice process maps to the collapse
transition of the canonical interacting self-avoiding trail model (ISAT) on
that lattice, the process on the triangular lattice model does not map to the
canonical equilibrium model. On the other hand, we show that the collapse
transition of the canonical ISAT model on the triangular lattice behaves in a
way reminiscent of the -point of the interacting self-avoiding walk
model (ISAW), which is the standard model of polymer collapse. This implies an
unusual lattice dependency of the ISAT collapse transition in two dimensions.
By studying an extended ISAT model, we demonstrate that the growth process
maps to a multi-critical point in a larger parameter space. In this extended
parameter space the collapse phase transition may be either -point-like
(second-order) or first-order, and these two are separated by a multi-critical
point. It is this multi-critical point to which the growth process maps.
Furthermore, we provide evidence that in addition to the high-temperature
gas-like swollen polymer phase (coil) and the low-temperature liquid drop-like
collapse phase (globule) there is also a maximally dense crystal-like phase
(crystal) at low temperatures dependent on the parameter values. The
multi-critical point is the meeting point of these three phases. Our
hypothesised phase diagram resolves the mystery of the seemingly differing
behaviours of the ISAW and ISAT models in two dimensions as well as the
behaviour of the trail growth process
Metal-Insulator transition in the Generalized Hubbard model
We present the exact ground-state wave function and energy of the generalized
Hubbard model, subjected to the condition that the number of double occupied
sites is conserved, for a wide, physically relevant range of parameters. For
one hole and one double occupied site the existence of the ferromagnetic
ground-state is proved which allow one to determine the critical value of the
on-site repulsion corresponding to the point of metal-insulator transition. For
the one dimensional model the exact solution for special values of the
parameters is obtained.Comment: 20 pages, LaTex. Mod.Phys.Lett.B 7 (1993) 1397; Journal of Physics:
Condensed Matter (to appear
A multi-blob representation of semi-dilute polymer solutions
A coarse-grained multi-blob description of polymer solutions is presented,
based on soft, transferable effective interactions between bonded and
non-bonded blobs. The number of blobs is chosen such that the blob density does
not exceed their overlap threshold, allowing polymer concentrations to be
explored deep into the semi-dilute regime. This quantitative multi-blob
description is shown to preserve known scaling laws of polymer solutions and
provides accurate estimates of amplitudes, while leading to orders of magnitude
increase of simulation efficiency and allowing analytic calculations of
structural and thermodynamic properties.Comment: 4 pages, 4 figure
Renormalization of the one-loop theory of fluctuations in polymer blends and diblock copolymer melts
Attempts to use coarse-grained molecular theories to calculate corrections to
the random-phase approximation (RPA) for correlations in polymer mixtures have
been plagued by an unwanted sensitivity to the value of an arbitrary cutoff
length, {\it i.e.}, by an ultraviolet (UV) divergence. We analyze the UV
divergence of the inverse structure factor predicted by a
`one-loop' approximation similar to that used in several previous studies. We
consider both miscible homopolymer blends and disordered diblock copolymer
melts. We show, in both cases, that all UV divergent contributions can be
absorbed into a renormalization of the values of the phenomenological
parameters of a generalized self-consistent field theory (SCFT). This
observation allows the construction of a UV convergent theory of corrections to
SCFT phenomenology. The UV-divergent one-loop contribution to are
shown to be the sum of: (i) a -independent contribution that arises from a
renormalization of the effective parameter, (ii) a -dependent
contribution that arises from a renormalization of monomer statistical segment
lengths, (iii) a contribution proportional to that arises from a
square-gradient contribution to the one-loop fluctuation free energy, and (iv)
a -dependent contribution that is inversely proportional to the degree of
polymerization, which arises from local perturbations in fluid structure near
chain ends and near junctions between blocks in block copolymers.Comment: 35 pages, 2 figure
Phase diagram of mixtures of colloids and polymers in the thermal crossover from good to solvent
We determine the phase diagram of mixtures of spherical colloids and neutral
nonadsorbing polymers in the thermal crossover region between the
point and the good-solvent regime. We use the generalized free-volume theory
(GFVT), which turns out to be quite accurate as long as
( is the radius of gyration of the polymer and is the colloid
radius). Close to the point the phase diagram is not very sensitive to
solvent quality, while, close to the good-solvent region, changes of the
solvent quality modify significantly the position of the critical point and of
the binodals. We also analyze the phase behavior of aqueous solutions of
charged colloids and polymers, using the extension of GFVT proposed by Fortini
et al., J. Chem. Phys. 128, 024904 (2008)
Renormalized one-loop theory of correlations in polymer blends
The renormalized one-loop theory is a coarse-grained theory of corrections to
the self-consistent field theory (SCFT) of polymer liquids, and to the random
phase approximation (RPA) theory of composition fluctuations. We present
predictions of corrections to the RPA for the structure function and to
the random walk model of single-chain statics in binary homopolymer blends. We
consider an apparent interaction parameter that is defined by
applying the RPA to the small limit of . The predicted deviation of
from its long chain limit is proportional to , where
is chain length. This deviation is positive (i.e., destabilizing) for weakly
non-ideal mixtures, with \chi_{a} N \alt 1, but negative (stabilizing) near
the critical point. The positive correction to for low values of
is a result of the fact that monomers in mixtures of shorter
chains are slightly less strongly shielded from intermolecular contacts. The
depression in near the critical point is a result of long-wavelength
composition fluctuations. The one-loop theory predicts a shift in the critical
temperature of , which is much greater than the predicted
width of the Ginzburg region. Chain dimensions deviate
slightly from those of a random walk even in a one-component melt, and contract
slightly with increasing . Predictions for and single-chain
properties are compared to published lattice Monte Carlo simulations.Comment: submitted to J. Chem. Phy
Surface segregation of conformationally asymmetric polymer blends
We have generalized the Edwards' method of collective description of dense
polymer systems in terms of effective potentials to polymer blends in the
presence of a surface. With this method we have studied conformationally
asymmetric athermic polymer blends in the presence of a hard wall to the first
order in effective potentials. For polymers with the same gyration radius
but different statistical segment lengths and the excess
concentration of stiffer polymers at the surface is derived as % \delta \rho
_{A}(z=0)\sim (l_{B}^{-2}-l_{A}^{-2}){\ln (}R_{g}^{2}/l_{c}^{2}{)%}, where
is a local length below of which the incompressibility of the polymer
blend is violated. For polymer blends differing only in degrees of
polymerization the shorter polymer enriches the wall.Comment: 11 pages, 7 figures, revtex
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