FORCES BETWEEN LAYERS OF ADSORBED BLOCK COPOLYMERS.

The principal technique that we have used to study the adsorbed layers is the surface forces balance designed by J. N. Israelachvili. This method, applied to our system, brings two adsorbed layers into direct contact with one another across a solvent gap. The layers are adsorbed on atomically smooth basal planes of Muscovite mica crystals; the technique yields the profile of force as a function of separation between the mica substrates. The range and profile of forces gives information on the polymer density profile in the gap; naturally, too, the forces themselves are the quantities of interest in understanding the physics of colloid stability and adhesion. The results on a series of di-blocks of roughly equal PVP molecular weight and varying PS chain length are given. Two features of these data are particularly interesting. While the smooth monotonic repulsion resulting from the overlap of the PS layers in the good solvent toluene is anticipated, the very long range of the forces is noteworthy. The radii of the PS portions of these chains would be in free toluene solution approximately, in ascending order, 70, 90 and 100A, respectively.