Ordered polyelectrolyte "multilayers". 1. Mechanisms of growth and structure formation: A comparison with classical fuzzy "multilayers"

The growth and structuring of polyelectrolyte self-assemblies (so-called "multilayers") made from a lyotropic ionene and a strong polyelectrolyte are examined in depth using X-ray reflectometry among other techniques. We show that highly ordered polyelectrolyte films may be obtained, consisting of a regular lamellar nanostructure extending over considerable distances in the films, with preferential orientation of chain fragments occurring in the films. This is in marked contrast with classical, "fuzzy" multilayers, for which no internal structure was reported so far. From our set of results, including a comparison of the structures of "multilayers" and bulk complexes, we propose that three mechanisms govern film growth and structuring: adsorption of the polyelectrolyte (governed by electrostatic balance), diffusion of the polyelectrolyte into the previously adsorbed film (which is the blurring step), and surface-constrained complexation between the polyanion and the polycation resulting from the mixing due to diffusion. Depending on whether the polyelectrolytes are capable of forming structured complexes or not, the self-assembled film will present different levels of internal organization. These findings have important implications for the general understanding of electrostatic self-assembly and for possible applications therefrom

An attempt to separate roughness from interdiffusion in the interfacial broadening between two immiscible polymers

A detailed study of the interface formation between polystyrene (PS) and chlorinated polybutadiene (CPB) at 150 degrees C is carried out on bilayer samples by X-ray reflectometry(XRR) and X-ray diffuse scattering (XDS). An equilibrium melt interfacial width sigma(m)(150 degrees C) = 18.8 +/- 0.3 Angstrom is determined by XRR. Lateral fluctuations being averaged over the coherence area in this method, a, must be considered as originating from both homopolymer interdiffusion (sigma(CPB/PS)) and interfacial roughness (sigma(r)): sigma(m)(2) = sigma(CPB/PS)(2) + sigma(r)(2). Interfacial roughness is generally described in the literature to be due to either a residual roughness from sample preparation (hypothesis 1) or capillary fluctuations building up at the interface (hypothesis 2). The nature and importance of the interfacial roughness are assessed by XDS scans, which include the detection of the diffuse (off-specular) scattering arising from lateral correlations in the roughness profile. While hypothesis 1 can be unambiguously rejected from the analysis of our XDS data, at least two equally valuable fits of theory to data are found under the hypothesis that the interfacial roughness changes during annealing. In the first one, interdiffusion is restricted, and the interfacial width is dominated by roughness (sigma(r) > sigma(CPB/PS)). In the second one, the reverse is true: sigma(CPB/PS) much greater than sigma(r). The former picture is closer to predictions from existing molecular theories of capillarity and gives for the Flory-Huggins interaction parameter computed through mean-field arguments a better agreement with the interaction parameter obtained from solubility parameters of CPB and PS

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. (C) 1998 Elsevier Science S.A. All rights reserved

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). (C) 1997 Elsevier Science S.A

Polyelectrolytes bearing azobenzenes for the functionalization of multilayers

Thin polymeric films are assembled by the alternating adsorption of oppositely charged polyelectrolytes. The polyions are functionalized by azobenzenes, typically carrying donor-acceptor substituents. The azobenzene chromophores are exploited as versatile analytical tools, to study the assembling process, and to control the film quality. A high concentration of ionic groups does not seem to be advantageous per se for good film growth, but rather the matching of the charge densities of the polyelectrolyte pair used seems to be important. Also, the influence of the strongly interacting, form-anisotropic character of the azobenzenes on the internal film structure was investigated. Although even high concentrations of azobenzenes and of other mesogens do not induce particular ordering, a few polymer pairs allowed the construction of real multilayer films, exhibiting e.g. Bragg peaks

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