Asymmetric Growth in Polyelectrolyte Multilayers

Abstract

Radioactive counterions were used to track the ratio of positive to negative polymer repeat units within a polyelectrolyte multilayer made from poly(diallyldimethylammonium chloride), PDADMAC, and poly(styrene sulfonate), PSS. For this widely employed pair of "linearly" assembled polyelectrolytes it was found that the accepted model of charge overcompensation for each layer is incorrect. In fact, overcompensation at the surface occurs only on the addition of the polycation, whereas PSS merely compensates the PDADMAC. After the assembly of about a dozen layers, excess positive sites begin to accrue in the multilayer. Treating the surface as a reaction-diffusion region for pairing of polymer charges, a model profile was constructed. It is shown that different reaction-diffusion ranges of positive and negative polyelectrolyte charge lead to a blanket of glassy, stoichiometric complex growing on top of a layer of rubbery, PDADMAC-rich complex. Though overcompensation and growth was highly asymmetric with respect to the layer number, entirely conventional "linear" assembly of the multilayer was observed. The impact of asymmetric growth on various properties of multilayers is discussed.

Short versus long chain polyelectrolyte multilayers: a direct comparison of self-assembly and structural properties

“…Accordingly, an excess of positive charges inside the PEM after PDADMAC deposition has been reported, in contrast to the strong intrinsic charge compensation after PSS adsorption. 63 In the case of LMW PEs, neither PSS nor PDADMAC shows a secondary uptake during layer equilibration. On the contrary, a loss of mass is observed, in particular during the equilibration which follows PDADMAC adsorption.…”

“…5a it can be seen that there is always a gain of mass during equilibration after HMW PSS adsorption, irrespective of the rinsing medium. Based on the diffusion model reported for PEMs 61,62 and on the distribution of extrinsic charge compensation described by Schlenoff and coworkers for PDADMAC-PSS multilayers, 63 this secondary mass adsorption can be explained by the presence of a reservoir of mobile PDADMAC chains in the multilayer, which diffuse back to the interface during PSS deposition and enhance the uptake. In the case of HMW PDADMAC, a small loss is measured during layer equilibration, which could be due to some water release upon complexation between the polyions.…”

Polyelectrolyte Multilayers on Silica Surfaces: Effect of Ionic Strength and Sodium Salt Type

“…Similar behaviour has been already observed elsewhere in NaCl solution. [10,28] The influence of various counteranions on the properties of PDADMAC-PSS multilayers (but for thin dry PEMs) was studied using ellipsometry and AFM by Salomäki et al [29] who found out that the thickness follows the Hofmeister series reasonably well.…”

Development of a Biocompatible Layer-by-Layer Film System Using Aptamer Technology for Smart Material Applications

“…A more recent study by Ghostine et al with PSS/PDDA films addresses a lot of the current issues in this field. Among other things, they found after the deposition of 12-14 bilayers, PSS is no longer able to compensate for all positive charges of PDDA [51]. As a result, counter-ions from the surrounding solution must permeate the film to neutralize these fixed charges.…”