Multiscale Modeling of Polymer Dynamics under Equilibrium and Non-Equilbrium Conditions.

In this dissertation, we have explored various aspects of polymer dynamics in dilute solutions, at equilibrium and non-equilibrium conditions, with computational models ranging over multiple length and time scales, using both molecular dynamics and Brownian dynamics (BD) simulations. Our investigations on equilibrium polymer dynamics helps resolve the puzzle of the absence of high frequency modes in the relaxation spectrum of the chain, reported decades ago by Schrag and coworkers. For this, we systematically introduce the various mechanisms acting on the atoms of a short polystyrene chain. Our simulations clearly show that, in terms of a re-scaled time that is based on the chain size and diffusivity of the center of mass, each additional detail causes a slow-down in the relaxation dynamics of the chain backbone bonds. Finally, when all mechanisms are present, including the explicit solvent molecules, the bond relaxation dynamics shows a single exponential decay for a short polystyrene chain with a time scale that is temporally indistinguishable from that of the end-to-end relaxation dynamics, in excellent agreement with the earlier experiments. Next, we investigate the conformational and dynamic behavior of polymer chains in shear flows using BD simulations with chain models encompassing widely varying resolutions. Our simulations reveal multiple regimes for chain deformation as the flow rates range from weak to ultra-high. We show that, in the absence of any excluded volume (EV), the chain compression obtained at high shear rates in several earlier simulations is an artifact of insufficient chain discretization. In the presence of EV, an orientational "locking" at the fully extended state is observed at extremely high shear rates. We show that the chain tumbling at strong shear rates occurs by the formation of loops whose length is limited by the time required to stretch them, and derive scaling laws from a balance of convection and diffusion of monomers in those loops. Our model and results presented here corrects the previously reported scaling laws obtained by using coarse-grained bead-spring models, which fails to capture the correct physics at strong shear rates that can excite a single spring away from equilibrium.PHDChemical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/100065/1/indrasd_1.pd