| Abstract
| - Alternated deposition of polyanions and polycations on a charged solid substrate leads to the buildup of polyelectrolytemultilayer (PEM) films. Two types of PEM films were reported in the literature: films whose thickness increaseslinearly and films whose thickness increases exponentially with the number of deposition steps. However, it wasrecently found that, for exponentially growing films, the exponential increase of the film thickness takes place onlyduring the initially deposited pairs of layers and is then followed by a linear increase. In this study, we investigatethe growth process of hyaluronic acid/poly(l-lysine) (HA/PLL) and poly(l-glutamic acid)/poly(allylamine) (PGA/PAH) films, two films whose growth is initially exponential, when the growth process enters the linear regime. Wefocus, in particular, on the influence of the molecular weight (Mw) of the polyelectrolytes. For both systems, we findthat the film thickness increment per polyanion/polycation deposition step in the linear growth regime is fairly independentof the molecular weights of the polyelectrolytes. We also find that when the (HA/PLL)n films are constructed withlow molecular weight PLL, these chains can diffuse into the entire film during each buildup cycle, even for very thickfilms, whereas the PLL diffusion of high molecular weight chains is restricted to the upper part of the film. Our resultslead to refinement of the buildup mechanism model, introduced previously for the exponentially growing films, whichis based on the existence of three zones over the entire film thickness. The mechanism no longer needs all the “in”and “out” diffusing polyanions or polycations to be involved in the buildup process to explain the linear growth regimebut merely relies on the interaction between the polyelectrolytes with an upper zone of the film. This zone is constitutedof polyanion/polycation complexes which are “loosely bound” and rich in the polyelectrolyte deposited during theformer deposition step.
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