A detailed study of the role that solution pH plays in the layer-by-layer processing of the weak polyelectrolytes poly(acrylic acid) and poly(allylamine hydrochoride) was carried out. It was found that dramatically different polymer adsorption behavior is observed as one systematically increases (or decreases) the charge density of a weak polyelectrolyte including transitions from very thick adsorbed layers (ca. 80 Å) to very thin adsorbed layers (ca. 4 Å) over a very narrow pH range. By controlling pH, it is possible to vary the thickness of an adsorbed polycation or polyanion layer from 5 to 80 Å. In addition, control over the bulk and surface composition of the resultant multilayer thin films is readily achieved via simple pH adjustments. These studies have provided new insights into the polyelectrolyte sequential adsorption process and have already opened up some interesting technological applications.

Variations in the linear charge density of a weak polyacid brought about by controlling solution pH in a layer-by-layer sequential adsorption process were used to systematically control the layer thickness, level of layer interpenetration, and surface wettability of sequentially adsorbed layers of poly(acrylic acid) (PAA) and poly(allylamine) (PAH). The thickness contributed by an individual polyion layer was found to depend primarily on the pH of the polymer's dipping solution and, within the pH range examined, was not influenced by the thickness or level of interpenetration of the previously adsorbed layer. Contact angle and methylene blue adsorption measurements revealed that the deposited layers are typically highly interpenetrated and that the deposition process is a surface charge dominated adsorption process. Using this simple molecular-level blending approach, it is possible to create surfaces with advancing water contact angles that vary from essentially zero (completely wettable surfaces) to as high as 50°, all using the same simple polycation/polyanion combination.

The effects of charge density on weak polyelectrolyte multilayer growth were investigated by assembling several strong polyelectrolytes with either poly(allylamine hydrochloride) (PAH) or poly(acrylic acid) (PAA) to keep one polymer fully charged while varying the degree of ionization of the other. The degree of ionization of the weak polyelectrolytes in solution and in a multilayer film was controlled by pH adjustment and quantified by Fourier transform infrared spectroscopy analyses. In all cases, the bilayer thickness exhibited a dramatic change from molecularly thin bilayers (case of two fully charged polymers) to much thicker bilayers when the degree of ionization of the weak polyelectrolyte decreased below 70−90% charged units. We also confirmed and quantified earlier observations that both the charge density and the effective pK a of a weak polyelectrolyte can change substantially from solution-state values when the polymer is incorporated into a multilayer film. In addition, it was found that the degree of ionization of PAA in a multilayer film is also influenced strongly by the type of polycation used in the assembly process.