ADSORPTION OF IONIC BLOCK-COPOLYMERS - SELF-CONSISTENT-FIELD ANALYSISAND SCALING PREDICTIONS

The subject of this paper is the adsorption of ionic diblock copolymer s 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 adsorpt ion these molecules form a layer that resembles a brush of B segments. In contrast with a usual brush, however, the molecules in the adsorbe d layer are in equilibrium with those in solution. Consequently, the c hain density of the brush is not fixed; its value is determined by the dynamic equilibrium between polymer adsorption and desorption. We int erpret 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-consiste nt-field (SCF) lattice theory. The existence of two regimes for unchar ged 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 f or charged molecules. Scaling relations can be translated from the unc harged to the corresponding charged regimes by replacing the excluded volume parameter v(B) of the buoy segments by an effective electrostat ic excluded volume parameter v(e) = alpha(B)2/phi(s)b. In the LC regim e 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 seg ments 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.