Influence of Solvent Quality on Surface Forces between Polystyrene Layers Adsorbed onto Mica

Force-distance profiles were strikingly sensitive to small differences in temperature and solvent composition. Surfaces forces were measured at 18 and 28°C between mica surfaces bearing polystyrene (M_w = 490,000 g·mol^-1) adsorbed from dilute cyclopentane solution (θ temperature T_θ = 19.5°C). Forces at long range were monotonically repulsive at 28°C and mildly attractive at 18°C provided that a cyclopentane containing 5% linear pentanes were used. This contrasts with experiments in 99% pure cyclopentane, in which the range of detectable forces was (2-3)R_G (unperturbed radius of gyration) at 18°C and (5-6)R_G at 28°C, and the forces at long range were strongly attractive at 18°C and weakly attractive even at 28°C. A change of sign of the long-range interactions between adsorbed homopolymers, over a small range of temperature, does not appear to have been reported previously.