ADSORPTION OF IONIC BLOCK-COPOLYMERS - SELF-CONSISTENT-FIELD ANALYSIS AND SCALING PREDICTIONS

The subject of this paper is the adsorption of ionic diblock copolymers on an uncharged surface. One block, the ''anchor'', consists of N(A) uncharged, adsorbing A segments, whereas the ''buoy'' block has N(B) segments which carry a fixed charge and are nonadsorbing. Upon adsorption these molecules form a layer that resembles a brush of B segments. In contrast with a usual brush, however, the molecules in the adsorbed layer are in equilibrium with those in solution. Consequently, the chain density of the brush is not fixed; its value is determined by the dynamic equilibrium between polymer adsorption and desorption. We interpret the results in terms of the limiting behavior in four regimes, indicated as HU, HC, LU, and LC. The system is classified as either at high (H) or at low (L) anchor density and, irrespective of the anchor density, as either in the charged (C) or in the uncharged (U) regime. We find scaling relations for the adsorbed amount and layer thickness as a function of the block lengths N(A) and N(B), the charge alpha(B)e on the B segments, and the salt concentration phi(s)b in each of the four regimes. The scaling relations are checked using a self-consistent-field (SCF) lattice theory. The existence of two regimes for uncharged molecules has been reported previously. We argue that those HU and LU regimes are closely related to the two regimes HC and LC we find for charged molecules. Scaling relations can be translated from the uncharged to the corresponding charged regimes by replacing the excluded volume parameter v(B) of the buoy segments by an effective electrostatic excluded volume parameter v(e) = alpha(B)2/phi(s)b. In the LC regime the chain density sigma scales as sigma is-proportional-to (N(A)/N(B))3/2v(e)-1 and the layer thickness L as L is-proportional-to (N(A)N(B))1/2. The latter scaling is independent of v(e). Using the SCF model, these relations are found to be valid for an adsorbed amount of A segments below 10% of monolayer coverage. In the HC regime the adsorption is dominated by the anchoring block and the scaling relation for the chain density sigma is-proportional-to 1/N(A) is identical to that for uncharged molecules. The SCF calculations show that this regime will not be reached in practical situations.