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ToF-SIMS study of alternate polyelectrolyte thin films: Chemical surface characterization and molecular secondary ions sampling depth

Multilayered assemblies of alternate polyelectrolytes have been synthesized by dipping charged silicon wafers successively into solutions of polyelectrolytes of opposite charge. In this study, three types of assemblies and several thicknesses are investigated by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), in combination with other characterization techniques (X-Ray Photoelectron Spectroscopy (XPS), X-Ray Reflectivity (XRR) and Atomic Force Microscopy (AFM)). The sensitivity of ToF-SIMS to the extreme surface provides a powerful tool to verify the chemical structure, as well as the spatial homogeneity of the topmost layers. Monolayers of complex polyelectrolytes differing only by the end of the pendant group or by the monomer chain length can be distinguished easily, notwithstanding the interference with the information coming from the underlying layers. The chemical imaging capability of ToF-SIMS allows the identification of the defects and contaminants in the surface layer, as well as the verification of the thickness uniformity at a local scale (similar to 1 mu m). In addition, the proof of a regular build-up is given by the disappearance of the substrate signal (Sif) when the number of layers increases. On the other hand, the question of the information depth in ToF-SIMS, which constitutes an important issue for the characterization of very thin films, is addressed. The attenuation depth in the organic film is determined for atomic and molecular secondary ions (Si+, SiOH+, SiO3H-), mainly by the correlation with XPS and XRR data. The decay of the mean emission depth when the ion size increases makes the largest molecular ions the most surface sensitive.

Structural studies on thin organic coatings built by repeated adsorption of polyelectrolytes

Polyelectrolyte multilayers are obtained by repeated physisorption of oppositely charged polyions. We report on the effect of the linear charge density of a series of aromatic ionenes on the growth of stable ionene/poly(vinyl sulfate) multilayers. Stable multilayers could not be grown in water below a critical linear charge density, corresponding to a charge parameter of about 0.7 (in water at 20 degrees C). We then present two potential ways to overcome the critical charge density limitation. The first one relies on the existence of a specific matching between a polycation and a polyanion, partly based on non-electrostatic intramolecular interactions, giving rise to a surface-constrained complexation leading to multilayers of extremely high supramolecular order. The growth mechanism of the multilayers is different from what is the usual case for stronger polyelectrolytes, since it occurs in a loose unstructured boundary layer at the sample surface, and involves organization processes similar to those occurring upon formation of insoluble structured polyelectrolyte complexes. The relationship between thickness and number of deposition cycles (d vs, n) could be described by an n(1.36) scaling law. The second way is to use a chemical reactivation of the outermost polyelectrolyte layer after its deposition by electrostatic adsorption, in order to regenerate a net charge at the interface allowing further adsorption. This technique is larger in scope, since it may lead to interesting properties such as non-centrosymmetry of the multilayers

Characterization of polyacrylonitrile films grafted onto nickel by ellipsometry, atomic force microscopy and X-ray reflectivity

The thickness and roughness of polyacrylonitrile films electrografted on a nickel surface have been measured by ellipsometry atomic force microscopy and X-ray reflectivity. From combined ellipsometry and X-ray reflectivity measurements, accurate values for the refractive indices of polyacrylonitrile and nickel have been derived at a 6328-Angstrom wavelength. Dependence of the film thickness on the monomer concentration has been quantified for the first time. Furthermore, the thickness of the polyacrylonitrile (PAN) film is related to the nature of the solvent, depending on whether it is a good solvent for PAN (dimethylformamide; DMF) or not (acetonitrile; ACN)

Electrodeposition of mixed adherent thin films of poly(ethyl acrylate) and polyacrylonitrile onto nickel

Abstract Adherent thin polymer films have been prepared by sequential electrodeposition of ethyl acrylate (EA) and acrylonitrile (AN) onto nickel. Their composition has been studied by IR spectroscopy and time of flight - secondary ion mass spectrometry. Morphology and thickness have been analyzed by atomic force microscopy and ellipsometry, respectively, and compared to single component films of PEA and PAN. No microphase separation was detected in the mixed PEA/ PAN films. These show a granular morphology comparable to that of PAN films. The grains contain the two constitutive polymers, as confirmed by the selective thermal degradation of PEA.

Ultrathin multilayers made by alternate deposition of ionenes and polyvinylsulfate: from unstable to stable growth

We report on the influence of ionic strength of polyelectrolyte solutions on the growth of multilayers made of polyvinylsulfate and a series of aromatic ionenes of decreasing charge density. Stable multilayers growth was only possible for polyelectrolytes of sufficiently high charge density, confirming the existence of a critical charge density for the formation of multilayers. We also show that, for systems near the critical charge density, the addition of salt to the solution may result in a transition from unstable to stable multilayer growth