From Exponential to Linear Growth in Polyelectrolyte Multilayers

Abstract

There exist two types of polyelectrolyte multilayers: those whose thickness increases linearly with the number of deposition steps, which are nicely structured, and those whose thickness increases exponentially, which resembles hydrogels. This simple picture has recently slightly evolved with the finding that some exponentially growing films enter into a linear growth phase after a certain number of deposition steps. In this study, we investigate the buildup process of hyaluronic acid/poly(L-lysine) (HA/PLL) multilayers that constitute one of the best known exponentially growing systems. The films are built by using two deposition methods: the well-known dipping method and the more recent spraying method where the polyelectrolyte solutions are sprayed alternately onto a vertical substrate. The goal of this study is twofold. First, we investigate the influence of the main parameters (i.e., spraying rate and spraying time) of the spraying method on the film growth process. We find that, as for the dipping method, the film thickness first evolves exponentially with the number of deposition steps, and after a given number of deposition steps, it follows a linear evolution. We find that similar behavior is observed with the dipping method. Second, because the spraying method allows the very fine variation of the different parameters of the buildup, we use this method to investigate the exponential-to-linear transition. We find that this transition always takes place after about 12 deposition steps whatever the values of the parameters controlling the deposition process. We discuss our results in light of a model proposed by Hübsch et al. (Hübsch, E.; Ball, V.; Senger, B.; Decher, G.; Voegel, J. C.; Schaaf, P. Langmuir 2004, 20, 1980-1985) and later by Salomäki et al. (Salomäki, M.; Vinokurov, I. A.; Kankare, J. Langmuir 2005, 21, 11232-11240) in which it is assumed that the exponential-to-linear transition is due to a film restructuring that progressively forbids the diffusion of one of the polyelectrolytes constituting the film over part of the film. This "forbidden" zone then grows with the number of deposition steps so that the outer zone of the film that is still concerned with diffusion keeps a constant thickness and moves upward as the total film thickness increases.

Biomimetic assembly of polyelectrolyte multilayers containing phosvitin monitored with reflectometric interference spectroscopy

“…Furthermore, the film growth follows an exponential regime which is described in more detail elsewhere. 18,24,25 To transform the measured optical thickness ͑n ‫ء‬ d͒ into a film thickness ͑d͒, we assumed a refractive index for the PEM film of 1.42 according to data published by Richert et al 26 The resulting data for the ͑PLL-PGA͒ i film construction are in good accordance with the data reported in the literature in terms of adsorption kinetics and actual layer thickness. 23 After monitoring the PEM film growth in situ ͓dark gray bar framed in white, Fig.…”

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

“…Several studies have shown that exponentially growing films will exhibit linear growth after the deposition of ten or more bilayers [21,25,[48][49][50]. It was theorized that exponentially growing films become too thick and evolve three distinct domains within their architecture: Domain I which is in contact with the substrate, Domain III which is at the solution interface and remains permeable to diffusing PE, and eventually Domain II (restructuring zone) which lies between Domains I and III and is constantly in a state of re-organization to a denser film structure making it impermeable to diffusing PE [21,25,48,49]. As a diffusing PE can only permeate so far in a finite amount of time, the film begins to grow linearly [47].…”

“…Depending on the speed of restructuring, the film may not have time (in solution) to re-establish the preferred hydrated morphology. This has been a concern of more than one author studying the exponential to linear transition; however it was ruled out by Porcel et al who saw no changes in growth rate, mechanism, and film thickness [49]. Despite this revelation, the films used were constructed by a spray method rather than dipping and could be the result of the deposition procedure.…”

Temperature-induced molecular transport through polymer multilayers coated with PNIPAM microgels

“…11,[25][26][27][28][29] The so-called exponential-like growth of the multilayers is caused by polymer diffusion in and out of the multilayers during the polymer assembly process. [30][31][32][33] The exponential-like growth allows the production of micrometer thick films with high loading capacities for various biomolecules. 10,30 The multilayers can release biomolecules reaching the cells thereby inducing cellular response such as adhesion, migration or differentiation.…”