Surface Forces in the Tapping Mode: Solvent Permeability and Hydrodynamic Thickness of Adsorbed Polymer Brushes. Dhinojwala, Ali; Granick, Steve. Department of Materials Science and Engineering, Urbana, IL, USA. Macromolecules (1997), 30(4), 1079-1085. CODEN: MAMOBX ISSN: 0024-9297. Journal written in English. CAN 126:104688 AN 1997:113061 CAPLUS

A surface forces app. modified to apply small-amplitude oscillatory displacements in the normal direction was used to measure the drainage of tetradecane (a good solvent) past polybutadiene (PB) brushes end-attached to 2 opposed mica surfaces. The PB was attached by selective adsorption of the poly(vinylpyridine) (PVP) block of a PB-PVP diblock copolymer. In-phase motion in the normal direction reflected elastic forces; these were found to be equiv. to the static force-distance profile measured directly. Out-of-phase motions reflected viscous flow of solvent since the PB chains did not contribute to dissipation over the oscillation frequencies studied. No frequency dependence was obsd. from 1 to 100 Hz. The hydrodynamic forces at a given plate sepn. (D) implied an effective plate sepn. less than D by a const. hydrodynamic thickness (RH) but otherwise the flow of a Newtonian liq. with viscosity same as in the bulk. The value of the hydrodynamic thickness was less than the value (L0) measured in the equil. force-distance profile, implying significant penetration of the velocity field into the brush layer. The value of RH diminished monotonically as the plate sepn. was reduced from 4L0 to 0.2L0. In other words, the slip plane changed monotonically with decreasing film thickness. The magnitude of hydrodynamic forces grew in proportion to D-1.2. This would be expected from scaling arguments for a Q solvent but deviates decidedly from the predicted D-0.5 from scaling arguments for semidil. good solvent conditions. This could reflect inapplicability of the Brinkman equation or could reflect different scaling behavior of the static and hydrodynamic screening lengths.