Apparent Hydrodynamic Thickness of Densely Grafted Polymer Layers in a Theta Solvent
We study the drainage of a near-theta solvent through densely grafted polymer layers and compare to recent notions that these layers display little permeabil- ity to solvent flow at surface separations less than a ''hydrodynamic thickness.'' The solvent is trans-decalin (a near-theta solvent at the experimental temperature of 247C). The polymer is polystyrene (PS) end-attached to two opposed mica surfaces via the selective adsorption of the polyvinylpyridine (PVP) block of a PS-PVP diblock copoly- mer. The experimental probe was a surface forces apparatus modified to apply small- amplitude oscillatory displacements in the normal direction. Out-of-phase responses reflected viscous flow of solvent alone—the PS chains did not appear to contribute to dissipation over the oscillation frequencies studied. The value of the hydrodynamic thickness (RH) was less than the coil thickness (Lo) measured independently from the onset of surface-surface interactions in the force-distance profile, implying significant penetration of the velocity field into the polymer layer. As the surface-surface separa- tion was reduced from 3Lo to 0.3Lo, the apparent hydrodynamic thickness (R* H) de- creased monotonically to values R* H ! RH. Physically, this indicates that the ''slip plane'' moved progressively closer to the solid surfaces with decreasing surface-surface separation. This was accompanied by augmentation of the effective viscosity by a factor of up to approximately 5, indicating somewhat diminished permeability of solvent through the overlapping polymer layers. Similar results hold for the flow through surface-anchored polymers in a good solvent. It is interesting to note the strong stretch- ing of densely end-grafted polymers in a theta solvent.