Internal Structure of Layer-by-Layer Adsorbed Polyelectrolyte Films: A Neutron and X-Ray Reflectivity Study

Abstract

The internal structure of ultrathin polymer films physisorbed to surface-modified Si wafers by electrostatic deposition of polyelectrolytes from aqueous solutions has been investigated by measuring the X-ray and neutron reflectivity from partially deuterated samples. For the first time it has been demonstrated that the preparation procedure, which involves repeated dipping of the substrate into solutions of polycations and polyanions in an alternating sequence, leads to the deposition of continuous molecular layers that form a polymer fii with a well-defined supramolecular structure. Thus, the structure of a polymer film deposited from solutions with high ionic strength (2 M NaCl) and comprising 48 molecular strata, which were organized in a superlattice of 5 perprotonated layers and 1 perdeuterated layer in 8 repeat units, has been resolved in full detail: the overall layer thickness was found to be 1205 (\pm) 20 Å, the layers deposited close to the substrate had a thickness significantly smaller than their equilibrium thickness far from the substrate, and the equilibrium thickness of an individual polycation layer was (\sim)20 Å and that of the polyanion layer (\sim)35 Å. The surface roughnesses at the substratelfilm and the film/air interface were significantly different, (\sim)4 and (\sim)13 Å, respectively, but even the larger number for the film/air interface shows that continuous and molecularly smooth layers are formed. The interdigitation between neighboring polymer layers was estimated to be (\sim)12 Å; the water content of the film has been determined, and the concentration of counterions deposited with the polyelectrolyte has been estimated.