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 saturation 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 shortrange repulsion was substantially less at 28 "C and higher temperature than at 23 "C and lower temperature. 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 longrange 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 simple pattern. The range of attraction at temperatures above T, passed through a maximum with increasing 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 limitations is included.

Shear rheology in a confined geometry: polysiloxane melts

“…There is uncertainty regarding absolute calibration of the micamica thickness, and consequently of the film thickness, as we have discussed elsewhere. 25 The uncertainty refers to the effect of adsorbed gases during the experiment. If these dissolve into the liquid under study during the course of the experiment, then the convention used in some experiments13 is appropriate to define zero as the mica-mica thickness measured at the end of the experiment after water has been added.…”

Non-equilibrium in adsorbed polymer layers

“…These systems have been modeled using equilibrium concepts [146][147][148][149]. Significant hysteresis effects are, however, common: for PS adsorption onto mica in near-theta solvents, Hu and Granick [150] and Ruths et al [151] have observed that following the first compression run, the force as a function of surface separation during decompression is different and corresponds to layers of smaller thicknesses. Ruths et al observed that force profiles upon recompression did not relax back to the original profile for days.…”

Surface phase transitions in polymer systems

“…Their results for specific polymer systems may be compared directly with experiments such as surface force experiments on polymer brushes (e.g. , Hu and Granick, 1990;Taunton et al , 1990).…”