MEASUREMENT OF FORCES IN SYMMETRICAL AND ASYMMETRIC INTERACTIONS BETWEEN DIBLOCK COPOLYMER LAYERS ADSORBED ON MICA

Measurements have been made of the forces of interaction between pairs of layers of diblock copolymers adsorbed on mica from toluene in a surface forces apparatus. The copolymers studied were poly(2-vinylpyridine)-polyisoprene (PVP-PI) and poly(2-vinylpyridine)-polystyrene (PVP-PS), over a range of molecular weights of each of the blocks. Previous studies of adsorption of similar polymers under similar conditions have shown that the PVP blocks (''anchors'') preferentially adsorb and anchor the PS and PI blocks (''buoys'') effectively by their ends at a density high enough to cause substantial overlap among the buoys. The ranges of the forces measured in this study are consistent with previous observations that this overlap causes the PS or PI chains to be tethered to the anchor layer in a configuration that is extended in the direction normal to the surface relative to the dimensions of a free chain of equal molecular weight in toluene solution, thereby creating a polymer ''brush''. All of the force versus distance data on six pairs of symmetrical interactions between these layers collapse with reasonable accuracy in a reduced plot, the variables of which are suggested by models of the interactions between grafted layers developed by Patel et al. and by Milner et al. The shape of this universal profile is predicted well by both models, with a small advantage to the latter. There are no free parameters in this comparison; the necessary data on solvent quality, segment size, and number density of chains on the surface were determined independently. Both models underpredict the force and its range by a factor of about 2 when the osmotic pressure data for homopolymer solutions are used in the calculation, while good agreements are found when the osmotic pressure is determined from the surface force data at high compression. Measurements have also been made of three distinct situations of asymmetric interaction between dissimilar layers. These asymmetries are chemical, between a PS layer and a PI layer at equal surface density and equal brush height, molecular weight, between two different PS brushes that have equal surface number density of chains but different chain lengths, and structural, between two different PI layers that have different molecular weights and surface number densities but about the same weight per unit area of polymer brush chain bound to the surface. The first two cases give results that are readily anticipated from the knowledge of the behavior of symmetrical interactions between brushes which shows that there is little interpenetration between two equally dense brushes. Structural asymmetry appears to give rise to more substantial interpenetration, or rearrangement, producing forces of a magnitude that are somewhat smaller than those anticipated from the noninterpenetration idea. Pull-off forces were also measured between thoroughly dried layers of the PS- and PI-containing copolymers. These data are converted into adhesive energies and compared with literature values for surface energies of bulk PS and Pl.