Universal and system-specific features of surface forces between adsorbed polystyrene in a near-Θ solvent

Abstract

Surface forces were measured between mica surfaces bearing polystyrene adsorbed to satu- ration from dilute cyclopentane solution (e temperature, T, = 19.5 C). The temperatures were 18, 23,28, and 34 C. Four conclusions emerge. First, for M, = 515 000, the adhesive minimum grew with the time (up to 14.5 h) that the adsorbed layers were kept together. This suggests the existence of extremely slow chain relaxations. Second, for M, = 490 000 and 515 000, the layer thickness at the point of steep short- range repulsion was substantially less at 28 C and higher temperature than at 23 C and lower tempera- ture. This shows that the mass adsorbed was remarkably sensitive to small changes in solvent quality. Third, for M, = 115 000,490 000, 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,, 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,) 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 sim- ple pattern. The range of attraction at temperatures above T, passed through a maximum with increas- ing molecular weight and increasing temperature. The relative influence of kinetics and thermodynamics on the force-distance profiles is discussed. The initially distinct polymer layers appeared to interdiffuse to only a limited extent over the time scale of the experiment, which implies that the present systems were in metastable states far from thermodynamic equilibrium. A discussion of error and of experimental lim- itations is included.