6,189 research outputs found
Topological Constraints at the Theta Point: Closed Loops at Two Loops
We map the problem of self-avoiding random walks in a Theta solvent with a
chemical potential for writhe to the three-dimensional symmetric
U(N)-Chern-Simons theory as N goes to 0. We find a new scaling regime of
topologically constrained polymers, with critical exponents that depend on the
chemical potential for writhe, which gives way to a fluctuation-induced
first-order transition.Comment: 5 pages, RevTeX, typo
Pseudo-Casimir force in confined nematic polymers
We investigate the pseudo-Casimir force in a slab of material composed of
nematically ordered long polymers. We write the total mesoscopic energy
together with the constraint connecting the local density and director
fluctuations and evaluate the corresponding fluctuation free energy by standard
methods. It leads to a pseudo-Casimir force of a different type than in the
case of standard, short molecule nematic. We investigate its separation
dependence and its magnitude and explicitly derive the relevant limiting cases.Comment: 7 pages, 2 figure
Solution of a model of SAW's with multiple monomers per site on the Husimi lattice
We solve a model of self-avoiding walks which allows for a site to be visited
up to two times by the walk on the Husimi lattice. This model is inspired in
the Domb-Joyce model and was proposed to describe the collapse transition of
polymers with one-site interactions only. We consider the version in which
immediate self-reversals of the walk are forbidden (RF model). The phase
diagram we obtain for the grand-canonical version of the model is similar to
the one found in the solution of the Bethe lattice, with two distinct
polymerized phases, a tricritical point and a critical endpoint.Comment: 16 pages, including 6 figure
Thermodynamic approach to the dewetting instability in ultrathin films
The fluid dynamics of the classical dewetting instability in ultrathin films
is a non-linear process. However, the physical manifestation of the instability
in terms of characteristic length and time scales can be described by a
linearized form of the initial conditions of the films's dynamics. Alternately,
the thermodynamic approach based on equating the rate of free energy decrease
to the viscous dissipation [de Gennes, C. R. Acad. Paris.v298, 1984] can give
similar information. Here we have evaluated dewetting in the presence of
thermocapillary forces arising from a film-thickness (h) dependent temperature.
Such a situation can be found during pulsed laser melting of ultrathin metal
films where nanoscale effects lead to a local h-dependent temperature. The
thermodynamic approach provides an analytical description of this
thermocapillary dewetting. The results of this approach agree with those from
linear theory and experimental observations provided the minimum value of
viscous dissipation is equated to the rate of free energy decrease. The flow
boundary condition that produces this minimum viscous dissipation is when the
film-substrate tangential stress is zero. The physical implication of this
finding is that the spontaneous dewetting instability follows the path of
minimum rate of energy loss.Comment: 8 pages, 3 figures. Under revie
Long-range Casimir interactions between impurities in nematic liquid crystals and the collapse of polymer chains in such solvents
The elastic interactions between objects embedded in a nematic liquid crystal
are usually caused by the average distorsion-rather than by the fluctuations-of
the nematic orientational field. We argue that for sufficiently small
particles, the nematic-mediated interaction originates purely from the
fluctuations of the nematic director. This Casimir interaction decays as
d^(-6), d being the distance between the particles, and it dominates van der
Waals interactions close to the isotropic-to-nematic transition. Considering
the nematic as a polymer solvent, we show that the onset of this Casimir
interaction at the isotropic-to-nematic transition can discontinuously induce
the collapse of a flexible polymer chain from the swollen state to the globular
state, without crossing the Theta-point.Comment: 6 pages, 1 figur
Non-Equilibrium in Adsorbed Polymer Layers
High molecular weight polymer solutions have a powerful tendency to deposit
adsorbed layers when exposed to even mildly attractive surfaces. The
equilibrium properties of these dense interfacial layers have been extensively
studied theoretically. A large body of experimental evidence, however,
indicates that non-equilibrium effects are dominant whenever monomer-surface
sticking energies are somewhat larger than kT, a common case. Polymer
relaxation kinetics within the layer are then severely retarded, leading to
non-equilibrium layers whose structure and dynamics depend on adsorption
kinetics and layer ageing. Here we review experimental and theoretical work
exploring these non-equilibrium effects, with emphasis on recent developments.
The discussion addresses the structure and dynamics in non-equilibrium polymer
layers adsorbed from dilute polymer solutions and from polymer melts and more
concentrated solutions. Two distinct classes of behaviour arise, depending on
whether physisorption or chemisorption is involved. A given adsorbed chain
belonging to the layer has a certain fraction of its monomers bound to the
surface, f, and the remainder belonging to loops making bulk excursions. A
natural classification scheme for layers adsorbed from solution is the
distribution of single chain f values, P(f), which may hold the key to
quantifying the degree of irreversibility in adsorbed polymer layers. Here we
calculate P(f) for equilibrium layers; we find its form is very different to
the theoretical P(f) for non-equilibrium layers which are predicted to have
infinitely many statistical classes of chain. Experimental measurements of P(f)
are compared to these theoretical predictions.Comment: 29 pages, Submitted to J. Phys.: Condens. Matte
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
Collision of Polymers in a Vacuum
In a number of experimental situations, single polymer molecules can be
suspended in a vacuum. Here collisions between such molecules are considered.
The limit of high collision velocity is investigated numerically for a variety
of conditions. The distribution of contact times, scattering angles, and final
velocities are analyzed. In this limit, self avoiding chains are found to
become highly stretched as they collide with each other, and have a
distribution of scattering times that depends on the scattering angle. The
velocity of the molecules after the collisions is similar to predictions of a
model assuming thermal equilibration of molecules during the collision. The
most important difference is a significant subset of molecules that
inelastically scatter but do not substantially change direction.Comment: 7 pages, 6 figure
Continuum theory of tilted chiral smectic phases
We demonstrate that the sequence of distorted commensurate phases observed in
tilted chiral smectics is explained by the gain in electrostatic energy due to
the lock-in of the unit cell to a number of layers which is the integer closest
to the ratio pitch over thickness of the subjacent Sm-C phase. We
also explain the sign change of the helicity in the middle of the sequence by a
balance between two twist sources one intrinsic and another due to the
distortion of the Sm-C
Density profiles and surface tensions of polymers near colloidal surfaces
The surface tension of interacting polymers in a good solvent is calculated
theoretically and by computer simulations for a planar wall geometry and for
the insertion of a single colloidal hard-sphere. This is achieved for the
planar wall and for the larger spheres by an adsorption method, and for smaller
spheres by a direct insertion technique. Results for the dilute and semi-dilute
regimes are compared to results for ideal polymers, the Asakura-Oosawa
penetrable-sphere model, and to integral equations, scaling and renormalization
group theories. The largest relative changes with density are found in the
dilute regime, so that theories based on non-interacting polymers rapidly break
down. A recently developed ``soft colloid'' approach to polymer-colloid
mixtures is shown to correctly describe the one-body insertion free-energy and
the related surface tension
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