Detailed structure of molecularly thin polyelectrolyte multilayer films onsolid substrates as revealed by neutron reflectometry

Using neutron reflectometry we have resolved-to high resolution-the interna l structure of self-assembled polyelectrolyte multilayer films and have dev eloped a detailed molecular picture of such systems by analyzing the data w ith a composition-space refinement technique. We show that such surface fil ms consist of stratified structures in which polyanions and polycations of individual layers interdigitate one another intimately. Nevertheless, the d eposition technique leads to results that are predictable, if well-defined and constant environmental conditions are maintained during the preparation . For alternating layers of poly(styrenesulfonate) (PSS) and poly(allylamin e hydrochloride) (PAH), adsorbed onto atomically flat surfaces, a roughenin g of successively deposited layers leads to a progressively larger number o f adsorption sites for consecutive generations of adsorbed polymer, and thu s to an increase in layer thicknesses with an increasing number of deposite d layers. Because of the interpenetration of adjacent polyelectrolyte speci es, however, this increase settles quickly into an equilibrium thickness. I n fully hydrated films (100% relative humidity), water occupies greater tha n or equal to 40% of the volume within the films. About twice as much water (by volume) is associated with PSS as with PAH. Incorporated inorganic sal t plays a minor role only, if any. The equilibrium thickness of the deposit ed layer structure may be fine-tuned via the ionic strength, I, of the solu tions used for the preparation. We show that the dependence of the thicknes s d(1p) per layer pair on I is linear, with a sensitivity, Delta d(1p)/Delt a I = 16 Angstrom x L/mol. Concurrently with the layer thickness the interf ace roughness sigma between adjacent layers increases: sigma similar to 0.4 x d(1p). In contrast to the ionic strength of the deposition solutions, th e degree of polymerization of the polyanions used in the preparation plays a minor role only in determining the overall structure of the deposited fil ms. The results reported here are quantitatively consistent with those of a recent study (Tarabia et al. J. Appl. Phys. 1998, 83, 725-732), if one ass umes that the hydration of the polyelectrolyte molecules in the sample film s investigated in the two studies is similar.