Using Brownian dynamics simulation, we determine the chain orientation
and stretching and their connection to stress overshoot in an entangled
polymer melt undergoing startup shear at rates lower than the reciprocal
of the Rouse time yet higher than the reciprocal of the reptation
time. In this rate regime, the prevailing tube theory attributes the
stress overshoot to alignment of the primitive chain. In contrast,
our results reveal that there is substantial chain stretching that
persists well beyond the Rouse time, and it contributes significantly
to the initial stress growth. In particular, stress overshoot is found
to be primarily due to chain retraction after considerable stretching
rather than chain orientation.
We investigate the flow-induced linear and ring polymer translocation through nanopores using a hybrid simulation method that combines a lattice-Boltzmann approach for the fluid with a molecular dynamics model for the polymer chain. Our results illustrate that the critical velocity flux for the linear and ring chain exhibits characteristic differences: for the linear chain, the critical velocity flux is independent of the chain length, which is in agreement with previous work; whereas, for the ring chain, the critical velocity flux decreases slowly with the increase of the chain length and gets closer to the critical velocity flux of the linear chain. In addition, we find that the ring chain shows much faster translocation compared to the linear chain with the same chain length. Moreover, the ring chain and the folded linear chain display the similar configurational deformation during the translocation. These results above indicate that with the increase of the chain length, the separation between the linear and ring polymers in the flow-induced translocation through nanopores becomes increasingly difficult.
We
present a general theory for the intrinsic viscosity of flexible
polymers of arbitrary architecture. The theory is based on a partially
permeable sphere model for which we introduce two phenomenological
functions, the drag function ξ and the drainage function κ,
that are determined by the density profile of the polymer. At the
mean-field level, these functions capture the long-range, multibody,
accumulative hydrodynamic interactions, that are responsible for the
frictional dissipation in and around a polymer. The density profiles
for a diversity of chain architectures are obtained by Monte Carlo
simulation. Predictions from our theory are in good agreement with
experimental data on all the polymer structures examined, ranging
from linear, ring, and stars to hyperbranched and dendrimers. The
concepts and methods we introduce in this work should be useful for
studying other dilute solution frictional properties, such as the
self-diffusivity, and provide a convenient framework for understanding
the relationship between the molecular architecture and their dilute
solution properties.
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