How Polyelectrolyte Adsorption Depends on History: A Combined Fourier Transform Infrared Spectroscopy in Attenuated Total Reflection and Surface Forces Study. Sukhishvili, Svetlana A.; Dhinojwala, Ali; Granick, Steve. Department of Materials Science and Engineering, University of Illinois, Urbana, IL, USA. Langmuir (1999), 15(24), 8474-8482. CODEN: LANGD5 ISSN: 0743-7463. Journal written in English. CAN 131:337626 AN 1999:619832 CAPLUS

We present a systematic study of how adsorption history affects the thickness, surface forces, and interfacial rheol. of a model cationic polymer. The polymer was quaternized poly(4-vinylpyridine), QPVP (wt.-av. d.p. nw = 325 and 98% quaternized with Et bromide). The main comparisons concerned one-step adsorption from soln. at a variable salt concn. up to 0.5 M NaCl, vs. two-step adsorption (initial adsorption from buffer soln. without added salt, then NaCl added later). The aq. solns. were buffered at pH = 9.2 such that the surfaces (mica in the case of surfaces forces (SFA) expts., oxidized silicon in the case of in situ IR (FTIR-ATR) expts.) in each case carried a large neg. charge. The SFA and FTIR-ATR expts. gave consistent ests. of the amt. of polymer adsorbed, confirming the expectation that adsorption should be driven by electrostatic attraction to the surface of large opposite charge. The adsorbed amt. showed little dependence on path, validating the common assumption of equilibration in this respect. However the layer thickness measured by surface forces, the shear nanorheol. response at a given surface force, and the dichroism of pendant side groups of the polymer all showed a pronounced dependence on the path to reach the adsorbed state. We interpret the measurements to suggest that two-step adsorption produces an inhomogeneous layer comprised of a dense layer of segments closest to the solid surface and a sparse outer layer. In particular, two-step adsorption produced thicker layers and a greater tendency to decouple shear forces from those that resist compression in the normal direction, thereby lessening the shear forces at a given level of normal force.