Universal and System-Specific Features of Surface Forces Between Adsorbed Polystyrene in a Near-θ Solvent

Surface forces were measured between mica surfaces bearing polystyrene adsorbed to saturation from dilute cyclopentane solution (Θ temperature, T_Θ = 19.5°C). The temperature were 18, 23, 28, and 34°C. Four conclusions emerge. First, for M_w = 515000, the adhesive minimum grew with the time (up to 14.5 h) that the adsorbed layers were kept together. Second, for M_w = 490000 and 515000, the layer thickness at the point of steep shortrange repulsion was substantially less at 28°C and higher temperature than at 23°C and lower temperature. Third, for M_w = 115000, 490000, and 1 080 000 at fixed temperatures, the layer thickness at the point of steep short-range repulsion scaled approximately with the radius of gyration, R_G, just as is expected for the layer thickness at a single surface under poor solvent conditions. Fourth, in comparisons of the long-range forces (>R_G) as a function of molecular weight and temperature after forces had equilibrated for up to 2-5 min, the force-distance profiles were reversible, but their sign, strength, and range did not fit a simple pattern.