STATISTICAL THERMODYNAMICS OF BLOCK COPOLYMER ADSORPTION .3. INTERACTION BETWEEN ADSORBED LAYERS OF BLOCK COPOLYMERS

Recently, we presented a self-consistent-field theory for the adsorption of block copolymers from a multicomponent mixture. In this paper the theory is applied to the interaction between two layers of adsorbed block copolymers. The free energy of interaction is derived for two types of equilibrium: (i) all molecules are free to diffuse out of the gap (full equilibrium), and (ii) the amount between the surfaces of some molecules is constant (restricted equilibrium). For diblock copolymers with one strongly adsorbing and one weakly adsorbing or nonadsorbing block we find repulsive interaction curves at full equilibrium because bridging is unfavorable. This is in contrast with homopolymers, which always give attraction in full equilibrium. If the total amount of diblock copolymer is kept constant, the interaction in a good solvent is always repulsive. With increasing amounts the repulsion becomes stronger. The onset of interaction is found at a surface separation that is approximately twice the hydrodynamic layer thickness. This separation depends highly on the length of both blocks. We show how the interaction curves at different block lengths can be scaled onto one master curve. In a poor solvent attraction is found at large separations due to osmotic forces. For ABA triblock copolymers with adsorbing A segments and nonadsorbing B segments in a solvent that is good for B, we find attraction at large separations due to bridging.