Abstract
This work aims to investigate the initial build-up stages of polyelectrolyte multilayers formed by the successive deposition of the weakly charged Poly(Allyl amine, HCl), PAH and the strong acid Poly(Styrene Sulfonate), PSS, on silica at pH = 9. Two complementary tools were used: laser reflectometry to determine the step-by-step deposited weight of each polymer and Atomic Force Microscopy (AFM) to characterize the topography and measure the thickness of the film. The experiments show that the deposited weight of PAH is the same at each step (around 0.5 mg.m − 2), whereas that of PSS increases more or less linearly along with the step number. Starting at the second polymer deposit (PSS), the structure of the film is very heterogeneous, with a thin polymer layer and bumps that are attributed to the presence of PSS/PAH hydrophobic complexes. The amount of bumps and their height were determined at each of the ten first polymer deposits. The film thickness between the bumps was evaluated by scratching small areas with an AFM tip. Our results support a former description of film growth, based on the electrostatic interactions between the two polymers and the substrate. They also reveal different the roles of the two polymers in the growth of the films: PAH is responsible for the progressive bi-dimensional coverage of the surface area and PSS for the increase in the thickness of the film.
Highlights
The build-up of polyelectrolyte multilayers (PEM) on plane or particle surfaces has been widely studied in the last two decades, due both to the fundamental interest of scientists and to the large potential of applications [1]
We suggested that Poly(Styrene Sulfonate) (PSS) could penetrate the polycation–polyanion aggregates formed in former deposition layers; the driving force for this phenomenon was that the high charge density of the polymer enabled it capable to increase the ionization of PAH and diffuse within the aggregates to unionized amine groups
Calculation of the charge balance after the successive deposition steps showed that the polyelectrolytes did not mutually neutralize their own charge, but that most of the PSS charge density was neutralized by co-adsorbed Na+ counterions
Summary
The build-up of polyelectrolyte multilayers (PEM) on plane or particle surfaces has been widely studied in the last two decades, due both to the fundamental interest of scientists and to the large potential of applications [1]. PEM consists in the successive and alternate deposition of two oppositely charged polyelectrolytes. Difficult this technique may be, due to its numerous variables, its outcome is, very promising. In order to improve control in the assembly of macromolecules, the main experimental factors (polymer charge densities, pH, ionic strength, type of electrolytes) in film construction need to be better understood and chemical data, linked with morphological and structural characteristics. We addressed the influence of pH and salt concentration on the progressive weight uptake of alternating layers of a strong cationic polyelectrolyte, poly (dimethylaminoethyl methacrylate chloride), MADQUAT, and a weak polyacid, Poly(Acrylic Acid), PAA [4]. It was seen that much information could be drawn from a simple charge balance and that this reflects the contribution of electrolyte (counter) ions in the neutralization of the layer
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