We studied the swelling of polyelectrolyte (PE) multilayers (PEM) in water (H2O) vapors. The PEM were made from polyanion poly(styrene sulfonate) (PSS) and polycation poly(diallyldimethylammonium chloride)-N-methyl-N-vinylacetamide (pDADMAC-NMVA). While PSS is a fully charged polyanion, pDADMAC-NMVA is a random copolymer made of charged pDADMAC and uncharged NMVA monomer units. Variation of the relative amount of these two units allows for controlling the charge density of pDADMAC-NMVA. The degree of swelling was studied as a function of the relative humidity in the experimental chamber (respectively water concentration in the gas phase) for PEM prepared from PSS and pDADMAC-NMVA with their different charge densities--100%, 89% and 75%. The films were prepared by means of spraying technique and consisted of six PE couples-PSS/pDADMAC-NMVA. Neutron reflectometry was applied as main tool to observe the swelling process. The technique allows to obtain in a single experiment information about film thickness and amount of water in the film. The experiments were complemented with AFM measurements to obtain the thickness of the films. It was found that the film thickness increases when the charge density of the polycation decreases. The swelling of the PEM increases with the relative humidity and it depends on the charge density of pDADMAC-NMVA. The swelling behavior is 2-fold, splitting up in a charge dependent mode with relatively little volume increase, and a second mode with high volume expansion, which is independent from charge density of PEM. The "swelling transition" occurs for all samples at a relative humidity about 60% and a volume increase of ca. 20%. The results were interpreted according to the Flory-Huggins theory which assumes a phase separation in PEM network at higher water contents.

The layer-by-layer (LbL) deposition technique of anionic and cationic polyelectrolytes allows formation of thin and ultrathin nanostructured films with tunable properties and, therefore, functionality. Here, we report on the synthesis and characterization of new polyelectrolyte multilayer system consisting of anionic copolymer poly(sodium 3-(4-vinylbenzylsulfanyl)-propane-1-sulfonate)-co-poly(2-hydroxyethyl-vinylbenzyl sulfoxide) with different charge densities (21, 36, 65, 81, and 100%) and cross-linkable cationic poly(4-methyl-1-(4-vinylbenzyl)-pyridinium chloride. We study the impact of salt concentration on film thickness and roughness for the different charge densities and discuss the observed trends with regard to charge effects. Furthermore we covalently cross-link the films by an aldol reaction, which preserves the number of charged groups while being easily followed via UV-vis spectroscopy because of the characteristic absorbance band of the coupling product. Cross-linking increases film stability markedly. Thus the new multilayer system allows tuning of both ionic and covalent cross-linking in a quantitative fashion

Polyelectrolyte multilayers were prepared by the layer-by-layer (LbL) technique from polyanions bearing aldehyde and polycations with 4-methylpyridinium moieties. The aldol reaction of these complementary reactive groups can be followed by the formation of fluorescent merocyanine dyes, resulting in cross-linked, ultrathin polymer films. The efficient stabilization of the polymer films allows for their intact removal from high surface energy supports, such as glass or surface oxidized silicon wafers, by simple treatment with salt solutions, yielding free-standing membranes. Increasing separation of the reactive polycation and polyanion layers with layers of inert polycation and polyanion analogues only gradually prevents the coupling reaction. From this dependence, polyions assembled in consecutive adsorption layers seem to be able to penetrate into as far as three neighboring layers.

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