Forces between Two Layers of Adsorbed Polystyrene Immersed in Cyclohexane below and above the θ Temperature

Forces between polystyrene layers absorbed on mica and immersed in cyclohexane have been measured. The measurements were made on two different molecular weights (6 X 10⁵ and 9 X 10⁵) in two different laboratories and therefore support the quantitative reliability of the results. We have reproduced previous results of this type of measurement below the θ temperature and extended the force measurement to (a) adsorbed layers at lower surface coverage and (b) temperatures above the bulk polystyrene-cyclohexane θ temperature. At a coverage of about 1.1 mg/m² of polystyrene, which is on the order of 20-30% of saturation, we found strongly attractive forces below T_θ, detectable at separations of about 300 Å between the bare mica surfaces. The forces can be measured very accurately and precisely in this situation. The force reaches a minimum at 46 ± 2 Å and becomes strongly repulsive closer in. The long-range attractive portion of the force curve is very nearly exactly exponential, with a decay length of 45 ± 2 Å. For saturated surfaces with about 4.5 ± 1 mg/m² the force is detectably attractive at larger distances (600-1200 Å) both below (23 and 26 °C) and above (37 ± 2 °C) the temperature (34.5 °C). For each molecular weight the positions of the minimum in the F(D) curve and of the short-range repulsive barrier are at smaller separations at T > T_θ. The magnitudes of the minima are smaller at T > T_θ as well. Both of these new results suggest strongly that the forces, especially the attractive components, between the polymer surfaces are influenced by effects in addition to the usual segment-segment interactions which determine bulk thermodynamic properties.