1998-08-12
10:15 at CAB D 28

SURFACE EFFECTS ON THE CONFORMATION AND THE RHEOLOGY OF POLYMER SOLUTIONS

Antony N. Beris

Dept of Chemical Engineering, University of Delaware

A conformation tensor, self-consistent mean-field theory is presented for the study of polymer flows above a solid surface, either neutral or adsorbing. The analysis involves consideration of the polymer chain conformations at the microscopic level and their interactions with an imposed flow at the continuum level. The projection of information from the microscopic to the macroscopic level is accomplished through a thermodynamic approach which provides an extension of the Hamiltonian description of conservative flow phenomena to account for dissipation, using the principles of irreversible thermodynamics. The present contribution extends previous efforts restricted to a flow of a polymer solution past a neutral surface to the case of a polymer solution above an adsorbing surface. Under static conditions (zero flow), the governing equations reduce to a minimization problem for the free energy, which results into a set of equations that are shown to be the continuum analog of those corresponding to the lattice model of Scheutjens and Fleer. To extend the study under flow conditions, the assumption has been made (based on the extent that different parts of the adsorbed polymer layer venture into the bulk of the fluid) that the imposed flow field affects only the conformations of the adsorbed tails. To describe the tail deformation, a modified Smoluchowski equation has been formulated and numerically solved. It is found that although the flow field substantially affects the conformations of the adsorbed tails, forcing them to orient parallel to the surface, the density distribution of segments in the tails remains unaltered. In contrast, the effect of the adsorbed layer on the velocity profile is very strong: in agreement with previous studies, the development of a boundary layer near the surface is predicted, of the size of the polymer radius of gyration, where the solvent flow remains almost stagnant. This results into an effective hydrodynamic layer thickness which is found to remain unaffected by the strength of the imposed bulk flow.