Explaining the Absence of High-Frequency ViscoelasticRelaxation Modes of Polymers in Dilute Solutions.

Using multiscale modeling, includingmolecular dynamics simulations, with both united-atom and coarse-grainedforce fields, as well as Brownian dynamics simulations with stillhigher levels of coarse-graining, we explain the long-mysterious absenceof high frequency modes in the viscoelastic spectrum of isolated polymerchains in good solvents, reported years ago by Schrag and co-workers.The relaxation spectrum we obtain for a chain of 30 monomers at atomisticresolution is, remarkably, a single exponential, while that of a coarse-grainedchain of 100 monomers is well fit by only two modes. These resultsare very surprising in view of the many degrees of freedom possessedby these chains, and in view of the many relaxation modes presentin melts of such chains. However, the result agrees perfectly withexperimental observations of Schrag and co-workers. We also performed Browniandynamics (BD) simulations in which the explicit solvent moleculesare replaced by a viscous continuum, using chain models of varyingdegrees of resolution, both in the presence and absence of hydrodynamicinteractions (HI). Although the local dynamics is suppressed by theaddition of bending, torsion, side groups and excluded volume interactions(as suggested in Jain and Larson(15)), none of the BD simulations predicta single exponential relaxation for a short polymer chain. The comparisonof the relaxation of the bond vectors from different models indicatesan additional slow-down in the presence of explicit solvent molecules,which is critical to obtain a single exponential relaxation for shortchains. Our results indicate that the chain dynamics at small lengthscales (down to a few Kuhn steps) are significantly different fromthe predictions of models based on a continuum solvent, and finallyhelp explain the experimental results of Schrag and co-workers. [ABSTRACT FROM AUTHOR]