Investigations in the initial build-up stages of polyelectrolyte multilayers by laser reflectometry and atomic force microscopy

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

Self-assembled film thickness determination by focused ion beam

“…This quasi plateau is attributed to the polymer film concentration. After the formation of coacervates between PSS and PAH on the substrate in the first layer, there are densification of the aggregates by diffusion of the PSS into the aggregates [11]. Similar results were found by Buron et al [21] for MAQUAT/PAA.…”

“…However, this structure can be obtained by the conformation of the polymer during the adsorption. Indeed, in a previous paper [11], we have shown that the polyelectrolytes form PAH/PSS aggregates on the silica surface and that PAH can adsorb on the top of the aggregates and so coalesce to aggregates by the summits. In this process, cavities form between two aggregates and can be observed by FIB.…”

“…We are entitled to wonder if these defects do not result from sample drying and\or from the vaccum of the FIB chamber. A previous study has shown that the drying step do not change the morphology of the deposit beyond 5 bilayers [11]. However, this structure can be obtained by the conformation of the polymer during the adsorption.…”

“…The thickness increment is usually much smaller in the linear regime than in the exponential one (typically a few nanometers versus hundreds of nanometers). For the couple of polymers studied here, the two types of growth process are found: linear [1] or exponential [11], depending on the pH of construction condition.…”

“…The same calculation can be done for the other films. For the thin polymer film, the thickness can be determined by laser reflectometry [11], and the Fig. 3 compares the thickness found by the two methods.…”