5,237 research outputs found
Single chain elasticity and thermoelasticity of polyethylene
Single-chain elasticity of polyethylene at point up to 90% of
stretching with respect to its contour length is computed by Monte-Carlo
simulation of an atomistic model in continuous space. The elasticity law
together with the free-energy and the internal energy variations with
stretching are found to be very well represented by the wormlike chain model up
to 65% of the chain elongation, provided the persistence length is treated as a
temperature dependent parameter. Beyond this value of elongation simple ideal
chain models are not able to describe the Monte Carlo data in a thermodynamic
consistent way. This study reinforces the use of the wormlike chain model to
interpret experimental data on the elasticity of synthetic polymers in the
finite extensibility regime, provided the chain is not yet in its fully
stretched regime. Specific solvent effects on the elasticity law and the
partition between energetic and entropic contributions to single chain
elasticity are investigated.Comment: 32 pages with 5 figures included. Accepted as a regular paper on The
Journal of Chemical Physics, August 2002. This article may be downloaded for
personal use only. Any other use requires prior permission of the author and
the American Institute of Physic
On the numerical integration of motion for rigid polyatomics: The modified quaternion approach
A revised version of the quaternion approach for numerical integration of the
equations of motion for rigid polyatomic molecules is proposed. The modified
approach is based on a formulation of the quaternion dynamics with constraints.
This allows to resolve the rigidity problem rigorously using constraint forces.
It is shown that the procedure for preservation of molecular rigidity can be
realized particularly simply within the Verlet algorithm in velocity form. We
demonstrate that the presented method leads to an improved numerical stability
with respect to the usual quaternion rescaling scheme and it is roughly as good
as the cumbersome atomic-constraint technique.Comment: 14 pages, 2 figure
On the force-velocity relationship of a bundle of rigid bio-filaments
In various cellular processes, bio-filaments like F-actin and F-tubulin are able to exploit chemical energy associated with polymerization to perform mechanical work against an obstacle loaded with an external force. The force-velocity relationship quantitatively summarizes the nature of this process. By a stochastic dynamical model, we give, together with the evolution of a staggered bundle of Nfrigid living filaments facing a loaded wall, the corresponding force-velocity relationship. We compute the evolution of the model in the infinite wall diffusion limit and in supercritical conditions (monomer density reduced by critical density ρ^1>1), and we show that this solution remains valid for moderate non-zero values of the ratio between the wall diffusion and the chemical time scales. We consider two classical protocols: the bundle is opposed either to a constant load or to an optical trap setup, characterized by a harmonic restoring force. The constant load case leads, for each F value, to a stationary velocity Vstat(F;Nf,ρ^1) after a relaxation with characteristic time τmicro(F). When the bundle (initially taken as an assembly of filament seeds) is subjected to a harmonic restoring force (optical trap load), the bundle elongates and the load increases up to stalling over a characteristic time τOT. Extracted from this single experiment, the force-velocity VOT(F;Nf,ρ^1) curve is found to coincide with Vstat(F;Nf,ρ^1), except at low loads. We show that this result follows from the adiabatic separation between τmicroand τOT, i.e., τmicro≈ τOT
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