Apparent hydrodynamic thickness of densely grafted polymer layers in a theta solvent
✍ Scribed by Yoon-Kyoung Cho; Ali Dhinojwala; Steve Granick
- Publisher
- John Wiley and Sons
- Year
- 1997
- Tongue
- English
- Weight
- 157 KB
- Volume
- 35
- Category
- Article
- ISSN
- 0887-6266
No coin nor oath required. For personal study only.
✦ Synopsis
We study the drainage of a near-theta solvent through densely grafted polymer layers and compare to recent notions that these layers display little permeability 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 24ЊC). 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 copolymer. The experimental probe was a surface forces apparatus modified to apply smallamplitude 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 ( R H ) was less than the coil thickness ( L o ) 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 separation was reduced from 3 L o to 0.3 L o , the apparent hydrodynamic thickness ( R* H ) decreased monotonically to values R* H Ӷ R H . 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 stretching of densely end-grafted polymers in a theta solvent.