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a) b - École Polytechnique de Montréal

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polyanion such as hyaluronan (HA)(Picart, Ladam, et al., 2001) or poly(L-glutamic acid)<br />

(PGA)(Lavalle, et al., 2002) revealed that PLL chains diffuse in and out of the film during<br />

buildup. At the subsequent rinsing step, some of these free chains entrapped in the multilayer<br />

diffuse outward from the film but lots of chains still remain insi<strong>de</strong>. In the next step, these<br />

remaining free chains were brought into contact with polyanions (HA or PGA) in solution. Due<br />

to electrostatic interactions between them, free PLL chains diffuse out of the film, resulting in<br />

the formation of PLL/polyanion complexes at the outer layer of the film. The amount of PLL<br />

chains that diffuse out of the multilayer in presence of the polyanion in solution <strong>de</strong>termines the<br />

thickness of the new forming layer on top of the multilayer. The ability of the polyelectrolytes to<br />

diffuse inward and outward of the films (mostly seen for weak polyelectrolytes) can lead to an<br />

exponential growth system rather than a linear one. Film roughness is also increased with layer<br />

number due to charge mismatch between the polyanions and polycations, <strong>de</strong>monstrating more<br />

non-linear behavior(Bertrand, 2000). Hence, weak polyelectrolytes can display linear or<br />

exponential growth, but strong ones always show linear behavior.<br />

2.4.1.1.5 Effect of Polyelectrolyte Concentration on Multilayers<br />

The available amount of polymer to be adsorbed is greater in solutions with higher polymer<br />

concentration than in solutions with lower polymer concentration, which leads to greater<br />

adsorption(Ferreira & Rubner, 1995). The main reason for greater adsorption in higher<br />

concentration solutions is the presence of more anchor sites, including loops and tails, on the<br />

substrate surface. This forces molecules to adopt a more compact orientation because of the<br />

severe competition of the polymeric chains. In lower polymer concentration solutions, there is<br />

less competition, and thus polymer chains straighten out, resulting in thinner layers and smaller<br />

adsorption. It was shown(Cheung, et al., 1997) that the bilayer thickness of polyaniline/PSS has<br />

an average value between 12 Å and 36 Å (Table 1), which increases by a factor of about three<br />

when the polyanion (PSS) solution concentration increases from 10 -4 to 10 -2 M.<br />

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