Polymer dynamics at interfaces

Abstract

Using classical molecular dynamics (MD) simulations we study the role of interface on structural and dynamical properties of a single polymer chain adsorbed at solid-liquid interface. This study was motivated by the recent experiments of linear flexible polymer chain adsorbed on a solid substrate at dilute surface coverage, which exhibit a scaling dependence of chain diffusivity, D, as a function of the degree of polymerization, N, as D ~ N-1.5. By contrast, a scaling dependence of D ~ N-1 was observed for DNA molecules bound to fluid cationic lipid bilayers.

We show that the mechanism for motion of adsorbed polymer chain is determined by the type of substrate. A polymer chain confined in two dimensions in presence of randomly placed impenetrable obstacles, satisfies the scaling, D ~ N-1.5, when end-to-end distance of the polymer chain is greater than the average separation between the obstacles. This complies with the experimental results of polymer adsorbed on hard surfaces. In three dimensions and absence of obstacles, the surface diffusion of a polymer chain adsorbed on analytically smooth surface is controlled by hydrodynamics leading to a scaling exponent of -0.75. the mechanism of polymer motion agrees with Zimm behavior. For atomistically corrugated surfaces, a decoupling takes place between the solvent hydrodynamics and the polymer chain motion. This changed the scaling exponent to -1.0 which matches with the experimental results for DNA adsorbed on lipid bilayers. Thus we may conclude that screening of hydrodynamic interactions takes place on solid as well as fluid substrate.

In this thesis the adsorption-desorption transition of polymer chain in explicit solvent and effect of wetting characteristics of the solvent on chain conformations as it transforms from an ellipsoidal to pancake conformation was extensively studied. Finite size effects due to long range hydrodynamic interactions posed a serious challenge in our simulations with explicit solvent and anisotropic box dimensions. We developed correction procedures to overcome these finite size effects.