(203f) Internal Structure and Charge Compensation of Polyelectrolyte Multilayers

Using a continuum self-consistent field theory, we have modeled the sequential process of layer-by-layer assembly of flexible polyelectrolytes on flat surfaces as a series of kinetically trapped states. Up to 60 depositions of oppositely charged polyelectrolytes are performed, each followed by a washing step. The multilayer has a three-zone structure. An exponential growth is found for the first several layers, followed by a linear growth for subsequent layers evolving towards a steady state. While adjacent layers are highly interpenetrating, stratification can be seen for every four or more layers. Our results also reveal that each layer inverts the total charges of the multilayer film (including the bare substrate charges). At the steady state, however, the charges carried by the newly adsorbed polymers are not related to the total charges of the existing film, but are just the opposite to the charges carried by the previous layer; this appears to be a general rule of charge compensation in the multilayer. We have further examined the effects of surface charge density, degree of ionization of polyelectrolytes, bulk salt concentration, bulk solution pH and solvent quality on the thickness and internal structure of the multilayer. Our results agree with most experimental findings on polyelectrolyte layer-by-layer assembly.