Using multiscale modeling, including
molecular dynamics simulations, with both united-atom and coarse-grained
force fields, as well as Brownian dynamics simulations with still
higher levels of coarse-graining, we explain the long-mysterious absence
of high frequency modes in the viscoelastic spectrum of isolated polymer
chains in good solvents, reported years ago by Schrag and co-workers.
The relaxation spectrum we obtain for a chain of 30 monomers at atomistic
resolution is, remarkably, a single exponential, while that of a coarse-grained
chain of 100 monomers is well fit by only two modes. These results
are very surprising in view of the many degrees of freedom possessed
by these chains, and in view of the many relaxation modes present
in melts of such chains. However, the result agrees perfectly with
experimental observations of Schrag and co-workers. We also performed Brownian
dynamics (BD) simulations in which the explicit solvent molecules
are replaced by a viscous continuum, using chain models of varying
degrees of resolution, both in the presence and absence of hydrodynamic
interactions (HI). Although the local dynamics is suppressed by the
addition of bending, torsion, side groups and excluded volume interactions
(as suggested in Jain and Larson), none of the BD simulations predict
a single exponential relaxation for a short polymer chain. The comparison
of the relaxation of the bond vectors from different models indicates
an additional slow-down in the presence of explicit solvent molecules,
which is critical to obtain a single exponential relaxation for short
chains. Our results indicate that the chain dynamics at small length
scales (down to a few Kuhn steps) are significantly different from
the predictions of models based on a continuum solvent, and finally
help explain the experimental results of Schrag and co-workers.