412 research outputs found
Strain-dependent localization, microscopic deformations, and macroscopic normal tensions in model polymer networks
We use molecular dynamics simulations to investigate the microscopic and
macroscopic response of model polymer networks to uniaxial elongations. By
studying networks with strands lengths ranging from to 200 we cover
the full crossover from cross-link to entanglement dominated behavior. Our
results support a recent version of the tube model which accounts for the
different strain dependence of chain localization due to chemical cross-links
and entanglements
The Electrostatic Persistence Length of Polymers beyond the OSF Limit
We use large scale Monte Carlo simulations to test scaling theories for the
electrostatic persistence length of isolated, uniformly charged polymers
with \DH intrachain interactions in the limit where the screening length
exceeds the intrinsic persistence length of the chains. Our
simulations cover a significantly larger part of the parameter space than
previous studies. We observe no significant deviations from the prediction
by Khokhlov and Khachaturian which is based on applying
the Odijk-Skolnick-Fixman theory to the stretched de
Gennes-Pincus-Velasco-Brochard polyelectrolyte blob chain. A linear or
sublinear dependence of the persistence length on the screening length can be
ruled out. We argue that previous numerical results pointing into this
direction are probably due to a combination of excluded volume and finite chain
length effects. The paper emphasizes the role of scaling arguments in the
development of useful representations for experimental and simulation data.Comment: 11 pages, 7 figure
Self-similar chain conformations in polymer gels
We use molecular dynamics simulations to study the swelling of randomly
end-cross-linked polymer networks in good solvent conditions. We find that the
equilibrium degree of swelling saturates at Q_eq = N_e**(3/5) for mean strand
lengths N_s exceeding the melt entanglement length N_e. The internal structure
of the network strands in the swollen state is characterized by a new exponent
nu=0.72. Our findings are in contradiction to de Gennes' c*-theorem, which
predicts Q_eq proportional N_s**(4/5) and nu=0.588. We present a simple Flory
argument for a self-similar structure of mutually interpenetrating network
strands, which yields nu=7/10 and otherwise recovers the classical Flory-Rehner
theory. In particular, Q_eq = N_e**(3/5), if N_e is used as effective strand
length.Comment: 4 pages, RevTex, 3 Figure
Simulating Van der Waals-interactions in water/hydrocarbon-based complex fluids
In systems composed of water and hydrocarbons Van der Waals-interactions are
dominated by the non-retarded, classical (Keesom) part of the
Lifshitz-interaction; the interaction is screened by salt and extends over
mesoscopic distances of the order of the size of the (micellar) constituents of
complex fluids. We show that these interactions are included intrinsically in a
recently introduced local Monte Carlo algorithm for simulating electrostatic
interactions between charges in the presence of non-homogeneous dielectric
media
Stress Relaxation of Entangled Polymer Networks
The non-linear stress-strain relation for crosslinked polymer networks is
studied using molecular dynamics simulations. Previously we demonstrated the
importance of trapped entanglements in determining the elastic and relaxational
properties of networks. Here we present new results for the stress versus
strain for both dry and swollen networks. Models which limit the fluctuations
of the network strands like the tube model are shown to describe the stress for
both elongation and compression. For swollen networks, the total modulus is
found to decrease like (V_0/V)^{2/3} and goes to the phantom model result only
for short strand networks.Comment: 9 pages, 3 figures, RevTe
DNA nano-mechanics: how proteins deform the double helix
It is a standard exercise in mechanical engineering to infer the external
forces and torques on a body from its static shape and known elastic
properties. Here we apply this kind of analysis to distorted double-helical DNA
in complexes with proteins. We extract the local mean forces and torques acting
on each base-pair of bound DNA from high-resolution complex structures. Our
method relies on known elastic potentials and a careful choice of coordinates
of the well-established rigid base-pair model of DNA. The results are robust
with respect to parameter and conformation uncertainty. They reveal the complex
nano-mechanical patterns of interaction between proteins and DNA. Being
non-trivially and non-locally related to observed DNA conformations, base-pair
forces and torques provide a new view on DNA-protein binding that complements
structural analysis.Comment: accepted for publication in JCP; some minor changes in response to
review 18 pages, 5 figure + supplement: 4 pages, 3 figure
Multi-scale modeling of diffusion-controlled reactions in polymers: Renormalisation of reactivity parameters
The quantitative description of polymeric systems requires hierarchical modeling schemes, which bridge the gap between the atomic scale, relevant to chemical or biomolecular reactions, and the macromolecular scale, where the longest relaxation modes occur. Here, we use the formalism for diffusion-controlled reactions in polymers developed by Wilemski, Fixman, and Doi to discuss the renormalisation of the reactivity parameters in polymer models with varying spatial resolution. In particular, we show that the adjustments are independent of chain length. As a consequence, it is possible to match reactions times between descriptions with different resolution for relatively short reference chains and to use the coarse-grained model to make quantitative predictions for longer chains. We illustrate our results by a detailed discussion of the classical problem of chain cyclization in the Rouse model, which offers the simplest example of a multi-scale descriptions, if we consider differently discretized Rouse models for the same physical system. Moreover, we are able to explore different combinations of compact and non-compact diffusion in the local and large-scale dynamics by varying the embedding dimension. Z9
Topological versus rheological entanglement length in primitive path analysis protocols
Primitive path analysis algorithms are now routinely employed to analyze
entanglements in computer simulations of polymeric systems, but different
analysis protocols result in different estimates of the entanglement length,
N_e. Here we argue that standard PPA measures the rheological entanglement
length, typically employed by tube models and relevant to quantitative
comparisons with experiment, while codes like Z or CReTA also determine the
topological entanglement length. For loosely entangled systems, a simple
analogy between between phantom networks and the mesh of entangled primitive
paths suggests a factor of two between the two numbers. This result is in
excellent agreement with reported values for poly-ethylene, poly-butadiene and
bead-spring polymer melts.Comment: 3 pages, no figure
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