Interactions between mica surfaces in a polystyrene?cyclopentane solution near the ?-temperature
The forces F(D) between smooth mica surfaces immersed in cyclopentane have been measured as a function of their separation D both in the absence and in the presence of polystyrene (of two molar masses, 6 × 105 and 2 × 106) adsorbed onto the mica from the solvent, at a temperature (23 + I°C) slightly above the 0-temperature for this system (19.6°C). The interactions between the bare mica surfaces in the cyclopentane were short-ranged (~<10 nm) and attractive. After addition of polymer to the solvent the surfaces were incubated in the solution at small surface separation to limit the rate of polymer adsorption, and interactions were measured following progressively longer incubation periods. The following trends were noted for both molar masses: (i) Following initial adsorption of polymer, a long-ranged attraction between the surfaces was followed by an ultimate repulsion as the surfaces were compressed close together. (ii) At longer incubation times in the solution, the magnitude of the attraction decreased while the range of interaction and the ultimate repulsive regime moved to larger D values. (iii) After overnight incubation at large surface separation the interaction became monotonically repulsive, starting at a surface separation D - 1.7Rg (unperturbed radius of gyration) for both molar masses used. This behavior did not change on further incubation. (iv) When the solution was replaced by pure solvent after overnight incubation, the monotonic repulsion changed to an initial weak attraction followed by an ultimate repulsion, very similar to the behavior following partial adsorption. Overall, the results indicate that attraction between the polymer covered mica surfaces is strongly correlated with extent of polymer adsorption, and disappears following full adsorption. The origin of the attraction is probably due to "bridging" effects, while the ultimate repulsion most likely occurs because of the dominance of osmotic interactions. © 1985 Academic Press, Inc.