PDADMAC/PSS multilayers

FIG. 2 -potential as a function of layer number for PDADMAC/PSS multilayers on sulfate-stabilized polystyrene (PS) latices. The multilayers were assembled onto the negatively charged PS latices ( -potential of ca. -65 mV, layer number = 0) by the consecutive deposition of PDADMAC (odd layers) and PSS (even layers). Positive values are observed for PDADMAC deposition, and negative values for PSS adsorption. The alternating values are characteristic of stepwise growth of multilayer films on colloids. 

FIG. 3 Normalized SPLS intensity distributions of (from left to right) neat PS latices and PS lat-ices coated with one, five, and nine PDADMAC/PSS multilayers. (From Ref. 50.)... 

FIG. 4 Thickness of PDADMAC/PSS multilayers assembled on PS latices as a function of layer number. The layers were assembled by the consecutive adsorption of PDADMAC and PSS. The thicknesses were determined from SPLS data. 

Multilayer assemblies were furthermore investigated in dependence of the number of layers, , where an interesting influence of the outer layer potential on the multilayer system was found (see Fig. 26). Here, is displayed, which is a measure for the total IH immobilisation in the multilayer arrangement. In PDADMAC/PSS multilayers (squares) a monotonous increase of R2sp with layer number was observed and attributed to the additional water immobilised in each adsorbed layer. 

Fig. 1 Charge inversion for polyelectrolyte multilayers represented as the surface potential dependence on the number of adsorbed layer, N. a Charge inversion as it was obtained by the Kelvin probe for (PDADMAC + PSS) multilayers adsorbed from a NaCl solution of concentration 50 mM onto a flat surface. Adapted from Ref. [80] with permission from The Royal Society of Chemistry, b Zeta potential of (PSS + PAH)n multilayers deposited onto colloidal microparticles of methylformamide with a positive bare charge. Adapted with permission from Ref. [52]. Copyright (1998) American Chemical Society, c Zeta potential changes evaluated using Streaming potential for multilayers built with PSS and PAH, the results are represented as zeta potential against number of measurement. The number of measurements is related to the number of times that each single layer was measured and it shows as accumulative number with the increase of the layer number. Adapted with permission from Ref. [83]. Copyright (2000) American Chemical Society...

Fig. 3 Surface density of monomer, pmonomer, for polyanion and polycation layers in (PDADMAC + PSS) multilayers with different ionic strength. Notice that the different amount of adsorbed PDADMAC and PSS is a strong evidence of the existence of extrinsic compensation. Reproduced from Ref. [80] with permission from The Royal Society of Chemistry...

Fig. 5 Changes on the UV-visible spectrum of (PDADMAC + PSS) multilayers adsorbed from pure water solutions with the number of bilayers. Note that only PSS layers show absorbance in this region. The inserted figure represents the dependence of the absorbance on the number of adsorbed bilayers. This latter is a representation of the adsorbed amount...

It is worth mentioning that currently most of the works in the literature are devoted to the study of multilayers of synthetic poly electrolytes. Some iconic examples of these systems are multilayers of type (PDADMAC + PSS)n (PDADMAC poly(diallyl-dimethyl-ammonium chloride), and (PSS poly(4-sty-rene sulfonate of sodium)) where the subindex n indicates the number of bilayers in the multilayer [80-82] or (PAH - - PSS) (PAH poly(aIlylamine hydrochloride)) [83]. However, there is a growing interest in the last years on the fabrication of biocompatible systems, e.g. biomacromolecules [84, 85]. Some examples are multilayers of type (CHI - - HEP)n (being CHI Chitosan and HEP Heparin) [86] (PLL + HA)n (PLL is poly(L-lysine) and HA is hyaluronic acid) or (PLL + PGA)n (PGA is poly(glutamic acid)) [87, 88]. 

One of the most critical parameters in the control of the compensation mechanism is the ionic strength of the solutions. This is because the changes on the ionic equilibrium in the multilayers are associated to the modification of the charge density by ion condensation mechanism [111]. This fact has been pointed out for one of the classical model of PEMs, (PDADMAC PSS)n, by Schlenofl and... 

In general, two well separated kinetic processes are observed and were identified by Bertrand et al. [172] as a first fast nucleation of domains followed by a second slower reorganization of the polymer chains in the multilayer. A more detailed picture allows one to describe the first step as a diffusive process coupled to the adsorption through an electrostatic or steric barrier [166, 173], whereas the second step implies aU the reorganization steps that lead to the multilayer till the stationary state [167]. This latter step includes both in plane reorganization of the polymeric chains and the interdififusion of the polyelectrolyte chains along the whole multilayer structure [99]. Figure 13 shows the adsorption kinetics of a PDADMAC layer onto a (PDADMAC - - PSS)n film. 

Fig. 15 Kinetic times for (PDADMAC + PSS)n and (PAH + PSS) multilayers obtained using Eq. (3). a, b Times for the adsorption of PDADMAC open circle, filled square) and PAH layers, c, d Times for the adsorption of PSS in (PDADMAC + PSS) open triangle) and (PAH + PSS) (filled diamond) multilayers. Reprinted fiom Ref. [97], Copyright (2011), with permission from Elsevier...

A last parameter involved in the control of the solvent quality is the temperature of the solutions, and its influence can be even stronger than that of I and pH. Despite its importance, low attention has been paid to this parameter in the literature. Salomaki et al. [102] pointed out that the modification of the temperature allows the modification of the growth trend of both (PDADMAC + PSS)n and (PAH + PSS)n multilayers. Special interest present the transition between linear and non-linear growth for (PAH + PSS)n multilayers mediated by the temperature since this system presents a linear growth trend in almost all the I and pH assembling conditions studied in the literature [83, 151]. 

The effect of the supporting electrolyte is mainly governed by the well-known Hofmeister serie [186]. In general, the effect of the cations in the growth of polyelectrolyte multilayers is quite limited [187], at least in aqueous systems [183]. The increase of the ions hydrophobicity increases the thickness of the multilayer. Salomaki et al. [188, 189] used different sodiiun salts, and pointed out that for (PDADMAC - - PSS)n, the multilayer thickness decrease according to Br > N03 > C103 > Cr > BrOs > HCOO" > F" (see Fig. 19). More... 

Fig. 19 Effect of the counter-anion in the thickness of (PDADMAC -I- PSS) multilayers. Reprinted with permission from Ref. [189].Copyright (2004) American Chemical Society...

PDADMAC + PSS)n multilayers. They found that the last layer play a key role in the hydration of the multilayers. PD ADM AC-capped films present a higher amount of associated water than those terminated in PSS due to the different ability to associate water of the different polymers. 

Electrophoresis measurements provide a qualitative indication of the assembly of polymer multilayers on colloids [49,50], The -potential as a function of polyelectrolyte layer number for negatively charged polystyrene (PS) particles coated with poly(diallyldimethylam-monium chloride) (PDADMAC) and poly(styrenesulfonate) (PSS) are displayed in Figure... 

Polyelectrolyte multilayers on silica colloids are investigated here using a combination of fast MAS and DQ solid-state NMR techniques. 2D DQ H NMR spectra of the bulk complex and the multilayer films are found to be similar, revealing complexation between the alternating layers of poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC). 

Layer-by-layer deposition from aqueous solutions of two types of oppositely charged polymers, usually positively charged polydiallyldimethylammonium chloride (PDADMAC, polycation) and negatively charged sodium polystyrene sulfonate (PSS, polyanion) produces a multilayer film via electrostatic interaction between oppositely charged macromolecules. The thickness of such films can be controlled precisely by a number of deposited layers from approximately 1.2 nm for one layer to 33.9 nm for 21 layers [5]. Finally, oppositely charged (relatively to PDADMAC) QDs can be deposited on the surface of the polyelectrolyte spacer as a submonolayer film. 

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