Effect of ionic strength and type of ions on the structure of water swollen polyelectrolyte multilayers

Abstract

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...

Polyelectrolyte Multilayers: Towards Single Cell Studies

“…In contrast, these LbL films swell more strongly in water than the ones built up in presence of small ions. Due to the lower density of complexation sites the mesh sizes are larger and can take up more water [52]. The same phenomenon was observed for increasing salt concentration during film formation.…”

“…This in turn leads to thicker films with a more mobile polymer matrix [47] related to an exponential growth with endothermic formation of complexes [51]. In case of pronounced extrinsic charge compensation the LbL films are denser with fewer voids in the dry state due to stronger screening of the polyelectrolyte charges [52]. This gives a higher flexibility of the polymer chains and an easier adjustment of the chains to the environment.…”

Layer-by-Layer Formation of Oligoelectrolyte Multilayers: A Combined Experimental and Computational Study

“…Therefore, this calculation of the water mass fraction per layer from the simulation data should be considered just a rough estimation to support a qualitative estimation of layer thickness from the experimental data. Consequently, the values of water contents per layer used for the experimental data were selected as follows: for the first layer, a reference value of water content of 40% was taken from data reported in literature, 40 which is similar to our own swelling measurements carried out on high molecular weight (PSS/PDADMAC) PEMs (data not reported here). From this reference value, the variations determined from the simulation data were applied to the next layers.…”

Temperature responsive behavior of polymer brush/polyelectrolyte multilayer composites

“…Detailed descriptions can be found in previous studies. 36,37 Heavy water, D 2 O, was injected into the trough, and the samples were left equilibrating for at least 1 h prior to measurement. The Motofit package 38 running in the Igor Pro environment (Wavemetrics) was used to fit the measured reflectivity curves, using a model to gain the corresponding SLD profiles.…”