This study addresses the effect of ionic strength and type of ions on the structure and water content of polyelectrolyte multilayers. Polyelectrolyte multilayers of poly(sodium-4-styrene sulfonate) (PSS) and poly(diallyl dimethyl ammonium chloride) (PDADMAC) prepared at different NaF, NaCl and NaBr concentrations have been investigated by neutron reflectometry against vacuum, H 2 O and D 2 O. Both thickness and water content of the multilayers increase with increasing ionic strength and increasing ion size. Two types of water were identified, ''void water'' which fills the voids of the multilayers and does not contribute to swelling but to a change in scattering length density and ''swelling water'' which directly contributes to swelling of the multilayers. The amount of void water decreases with increasing salt concentration and anion radius while the amount of swelling water increases with salt concentration and anion radius. This is interpreted as a denser structure in the dry state and larger ability to swell in water (sponge) for multilayers prepared from high ionic strengths and/or salt solution of large anions. No exchange of hydration water or replacement of H by D was detected even after eight hours incubation time in water of opposing isotopic composition. IntroductionThin polymer films are widely used for modification and functionalization of surfaces. The adsorption from solution is a particularly refined technology for modifying surfaces for advanced applications. It is well documented that the physisorption or chemisorption of polyelectrolytes or reactive polymers onto surface-functionalized substrates can lead to the deposition of molecularly thin surface films. [1][2][3][4][5][6] Reflectivity techniques, especially neutron and X-ray reflectometry, are well suited for the characterization of multilayer films, as they allow the determination of the concentration gradient along the layer normal. X-Ray reflectometry has only exhibited Kiessig fringes that arise from the interference of X-ray beams reflected at the substrate-film and film-air interfaces.7-9 Neutron reflectivity measurements of polymer films of a superlattice structure showed that the polyelectrolytes are deposited as layers with an interdigitation smaller or equal to a single layer thickness and indicate that there is no distinct layer-by-layer separation between polyelectrolytes of opposite charges. [10][11][12] The main driving force of film assembly is entropy due to release of counterions. Measurements of the surface potential resulted in a change of (surface) potential after each adsorption step, i.e. after each additional single deposited polyelectrolyte layer. [13][14][15][16] For alternating layers of poly(styrene sulfonate), PSS, and poly(diallyl dimethyl ammonium chloride), PDADMAC, adsorbed onto smooth surfaces, a roughening of successively deposited layers leads to a progressively larger number of adsorption sites for consecutive generations of adsorbed polymers, and thus to an increase in layer thicknes with increasing nu...

Polymer coatings are frequently utilized to control and modify substrate properties. The performance of the coatings is often determined by the first polymer layers between the substrate and the bulk polymer material, which are termed interphase. Standard methods have failed to completely characterize this interphase, because its properties change significantly over a few nanometers. Here we determine the spatially resolved adhesion properties of the interphase in polyelectrolyte multilayers (PEMs) by desorbing a single polymer covalently bound to an atomic force microscope cantilever tip from PEMs with varying thickness. We show that the adhesion properties of the first few layers (up to three double layers) is dominated by the surface potential of the substrate, while thicker PEMs are controlled by cohesion in between the PEM polymers. For cohesion, the local film conformation is the crucial parameter. This finding is generalized by utilizing oligoelectrolyte multilayer (OEM) as coatings and both hydrophilic and hydrophobic polymers as polymeric force sensors.

The swelling behavior of polyelectrolyte multilayers (PEMs) of poly(sodium-4 styrene sulfonate) (PSS) and poly(diallyl dimethyl ammonium chloride) (PDADMAC) prepared from aqueous solution of 0.1 M and 0.5 M NaCl are investigated by ellipsometry and Atomic Force Microscopy (AFM). From 1 double-layer up to 4 double-layers of 0.1 M NaCl, the amount of swelling water in the PEMs decreases with increasing number of adsorbed double layers due to an increase in polyelectrolyte density as a result of the attraction between the positively charged outermost PDADMAC layer and the Si substrate. From 6 double layers to 30 double layers, the attraction is reduced due to a much larger distance between substrate and outermost layer leading to a much lower polyelectrolyte density and higher swelling water. In PEMs prepared from aqueous solution of 0.5 M NaCl, the amount of water constantly increases which is related to a monotonically decreasing polyelectrolyte density with increasing number of polyelectrolyte layers. Studies of the surface topology also indicate a transition from a more substrate affected interphase behavior to a continuum properties of the polyelectrolyte multilayers. The threshold for the transition from interphase to continuum behavior depends on the preparation conditions of the PEM.

a Successful layer-by-layer (LbL) growth of short chain (B30 repeat units per chain) poly(sodium styrene sulfonate) (PSS)-poly(diallyl dimethylammonium chloride) (PDADMAC) multilayers is presented for the first time and compared with the growth of equivalent long chain polyelectrolyte multilayers (PEMs). A detailed study performed by quartz crystal microbalance with dissipation (QCM-D) is carried out and three main processes are identified: (i) initial mass uptake, (ii) adsorption-desorption during layer equilibration and (iii) desorption during rinsing. In contrast to the high stability and strong layer increment of high molecular weight (HMW) PEMs, layer degradation characterizes low molecular weight (LMW) multilayers. In particular, two different instability phenomena are observed: a constant decrease of sensed mass during equilibration after PDADMAC adsorption, and a strong mass loss by salt-free rinsing after PSS adsorption. Yet, an increase of salt concentration leads to much stronger layer growth.First, when the rinsing medium is changed from pure water to 0.1 M NaCl, the mass loss during rinsing is reduced, irrespective of molecular weight. Second, an increase in salt concentration in the LMW PE solutions causes a larger increment during the initial adsorption step, with no effect on the rinsing.Finally, the mechanical properties of the two systems are extracted from the measured frequency and dissipation shifts, as they offer a deeper insight into the multilayer structures depending on chain length and outermost layer. The paper enriches the field of PE assembly by presenting the use of very short PE chains to form multilayers and elucidates the role of preparation conditions to overcome the limitation of layer stability.

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