BLOCK COPOLYMER ADSORPTION IN A SELECTIVE SOLVENT - A KINETIC-STUDY

We study the adsorption kinetics of diblock copolymers onto a wall in a highly selective solvent good for the nonadsorbing block B and poor for the adsorbing block A. We consider only the adsorption from a micellar phase. The potential barrier for direct adsorption of chains belonging to micelles is so high that only free chains adsorb on the wall. The adsorption of free chains creates a large region in the vicinity of the wall where the free chains are depleted and where the equilibrium between free chains and micelles is broken. The equilibrium is restored by micelle relaxation through the expulsion of chains one by one. Taking the micelle relaxation into account, we write a diffusion-relaxation equation for the adsorption kinetics and distinguish four different time regimes. In the very early stages the adsorption is dominated by the diffusion of free chains toward the wall; at larger times the limiting step is the expulsion of chains by the micelles; the surface coverage increases then linearly with time and the copolymer builds up a grafted B brush on the wall. When the brush is dense enough, the potential barrier for adsorption of new chains is high and the passage through the barrier becomes the limiting step; the surface coverage increases then very slowly (logarithmically) with time. Finally the coverage relaxes to its equilibrium value but over times that are so long that it is not clear whether the equilibrium can be reached experimentally.