Polyelectrolyte Multilayer Films from Mixtures of Polyanions: Different Compositions in Films and Deposition Solutions

Abstract

Polyelectrolyte multilayers are usually prepared from polydisperse polyelectrolytes. It is desirable to measure and control multilayer composition when it deviates from the deposition solution. With neutron reflectivity, multilayers prepared from PDADMA and binary mixtures of long deuterated PSSd long (80.8 kDa) and short protonated PSS short (10.6 kDa) were investigated. A small amount of PSS long in the deposition solution led to a disproportionate increase of PSS long in the film, and completely suppressed the exponential growth regime. Adsorption kinetics were studied with in situ ellipsometry: (i) During adsorption of a PSS layer, the PSS long fraction increased with adsorption time; PSS short could desorb, while PSS long adsorbed irreversibly. (ii) During adsorption of a PDADMA layer, a fast thickness increase was followed by a slow thickness decrease. This was attributed to the formation of PDADMA/PSS short complexes, which eventually desorbed. The desorbed thickness depended on the number of layers deposited. The mechanism of layer formation is attributed to the asymmetric growth of PDADMA/PSS multilayers, as supported by the adsorption kinetics of multilayers prepared from one kind of PSS.

Self-Patterning Polyelectrolyte Multilayer Films: Influence of Deposition Steps and Drying in a Vacuum

“…The same surface roughness (σ AFM = 1.5 nm) was obtained as that with X-ray and neutron reflectometry. 27,44 In contrast, the film consisting of PSS short shows domains with an average distance exceeding 100 nm. The surface roughness was almost an order of magnitude larger (σ AFM = 21 nm).…”

“…Therefore, while the stresses are probably similar, domain formation is made possible by the increased mobility of PDADMA/PSS short complexes. 27 To monitor the formation of domains during film growth, the surface topology of different LbL films was imaged in dependence on the number of deposited PDADMA/PSS short bl. The obtained AFM images are shown in Figure 2.…”

“…These thin filaments are attributed to PDADMA/ PSS short complexes which shuttled between the domains during PDADMA adsorption and adjusted the domain separation. 27 The very different lateral structure of PDADMA and PSS short terminated films is attributed to the different configuration of PSS and PDADMA on PDADMA/PSSfilms: PSS with its large linear charge density adsorbs flatly, it compensates positive charges at the film surface. One PSS chain can adsorb on adjacent domains.…”

“…38 In situ monitoring of multilayer build-up indicated that fast adsorption of short PSS is followed by slow desorption of the PDADMA/PSS complexes. 27 In contrast to PSS, in all studies with low molecular weight PDADMA, the polycation was immobile within the film, even though the film thickness increase in the linear growth region was significantly decreased due to the low molecular weight of PDADMA. 37 The picture is simpler for PSS long with M w (PSS long ) > M w threshold (PSS) ≈ 25 kDa.…”

“…As the average distance between domains increases when additional layers are deposited, not only mobile PEs in the bulk, but also PEs with an adsorption/desorption equilibrium at the surface may be involved. 27,30 Additional stresses are applied to the LbL film postpreparation when the film is exposed to vacuum. Then, not only the weakly bound water leaves the film, but also the tightly bound water.…”