EFFECTS OF TIME AND COMPRESSION ON THE INTERACTIONS OF ADSORBED POLYSTYRENE LAYERS IN A NEAR-THETA SOLVENT

We have investigated the effects of molecular weight, adsorption time, force-measuring rate, and repeated compression-decompression cycles ( previous history) on the interactions between high molecular weight po lystyrenes adsorbed on mica from dilute cyclopentane solutions under n ear-Theta solvent conditions. On the first slow approach, a long-range bridging attraction, which needs several minutes to fully develop for each incremental movement of the surfaces, is found at separations be low 6R(g). The interaction becomes a steep hard-wall repulsion at sepa rations below 0.5R(g). With increasing molecular weight the thickness of the adsorbed layers increases, and the attractive bridging force on approach and adhesion on separation decreases. Self-consistent field theory underestimates the onset location and depth of the attractive m inima but accurately predicts the location of the hard-wall repulsion. For shorter (nonequilibrium) adsorption times, the bridging attractio n and adhesion are stronger and occur at smaller separations, consiste nt with incomplete coverage. If the first approach and compression are done slowly, the layers become ''irreversibly'' compressed and no lon g-range bridging attraction is observed on subsequent approaches, even after allowing the polymer layers to relax for several days.