697 research outputs found
Shear stress relaxation and ensemble transformation of shear stress autocorrelation functions revisited
We revisit the relation between the shear stress relaxation modulus ,
computed at finite shear strain , and the shear stress
autocorrelation functions and computed,
respectively, at imposed strain and mean stress . Focusing on
permanent isotropic spring networks it is shown theoretically and
computationally that in general
for with being the static equilibrium shear modulus.
and thus must become different for solids and it is impossible
to obtain alone from as often assumed. We comment
briefly on self-assembled transient networks where must vanish for
a finite scission-recombination frequency . We argue that should reveal an intermediate plateau set by the
shear modulus of the quenched network.Comment: 8 pages, 4 figure
Molecular-Dynamics Simulation of a Glassy Polymer Melt: Incoherent Scattering Function
We present simulation results for a model polymer melt, consisting of short,
nonentangled chains, in the supercooled state. The analysis focuses on the
monomer dynamics, which is monitored by the incoherent intermediate scattering
function. The scattering function is recorded over six decades in time and for
many different wave-vectors. The lowest temperatures studied are slightly above
the critical temperature of mode-coupling theory (MCT), which was determined
from a quantitative analysis of the beta- and alpha-relaxations. We find
evidence for the space-time factorization theorem in the beta-relaxation
regime, and for the time-temperature superposition principle in the
alpha-regime, if the temperature is not too close to the critical temperature.
The wave-vector dependence of the nonergodicity parameter, of the critical
amplitude, and the alpha-relaxation time are in qualitative agreement with
calculations for hard spheres. For wave-vectors larger than the maximum of the
structure factor the alpha-relaxation time already agrees fairly well with the
asymptotic MCT-prediction. The behavior of the relaxation time at small
wave-vectors can be rationalized by the validity of the Gaussian approximation
and the value of the Kohlrausch stretching exponent.Comment: 23 pages of REVTeX, 13 PostScript figures, submitted to Phys. Rev.
Monte Carlo Simulation of Long Chain Polymer Melts: Crossover from Rouse to Reptation Dynamics
We present data of Monte Carlo simulations for monodisperse linear polymer
chains in dense melts with degrees of polymerization between N=16 and N=512.
The aim of this study is to investigate the crossover from Rouse-like dynamics
for short chains to reptation-like dynamics for long chains. To address this
problem we calculate a variety of different quantities: standard mean-square
displacements of inner monomers and of the chain's center of mass, the recently
proposed cubic invariant, persistence of bond-vector orientation with time, and
the auto-correlation functions of the bond vector, the end-to-end vector and
the Rouse modes. This analysis reveals that the crossover from non- to
entangled dynamics is very protracted. Only the largest chain length N=512,
which is about 13 times larger than the entanglement length, shows evidence for
reptation.Comment: 38 pages of REVTeX, 14 PostScript figure
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