a) b - École Polytechnique de Montréal

2.4.1.1.4 Various Mechanisms of Film Growth
Understanding how the multilayers are structured and what governs their assembly is of great
importance for tuning their physico-chemical properties.
Two main modes of growth of polyelectrolyte multilayer films, namely linear and non-linear
(supralinear or sublinear), have been recognized. The extent of deposited polyelectrolyte per
adsorption step for consecutive steps distinguishes the two modes. Generally, strong
polyelectrolytes with high charge densities and low ionic strength result in a linear growth. In
this case, polyelectrolytes from the solution interact mostly with constituents of the film surface
and less with more deeply buried molecules. In linear growth, combination of different
techniques such as ellipsometry, SPM, X-ray and neutron reflectivity led researchers to establish
a three zone model of the multilayer films(Decher, 1997; Ladam et al., 2000). The first zone
consists of the first few polyelectrolyte layers being deposited close to the substrate surface.
Typically, the amount of adsorbed polymer per deposition cycle increases during the initial 1 to 5
deposition cycles, and then approaches a constant value(Gero Decher, Lvov, & Schmitt, 1994;
Lvov & Decher, 1994). It was observed that for poorly charged surfaces, this transition regime
can even stretch over much more than 5 deposition cycles(Kotov et al., 1997; Lutt, Fitzsimmons,
& DeQuan, 1998). The thickness per layer in this zone is smaller than in the second zone. The
second zone is the bulk film, which contains interpenetrating layers of polyelectrolytes. Due to
the proximity of the outer part of the multilayer, the local properties vary because of the solution
environment and the increase in surface roughness(Decher, 1997). The third zone is comprised
of one or a few polyelectrolyte layers close to the multilayer surface, including a diffuse double
layer of counterions to maintain overall electroneutrality(Ladam, et al., 2000).
On the contrary, the mechanism of exponential buildup processes is not clearly understood. The
first suggestion for the phenomenon was an increase in film roughness with the number of
deposited layers. Later experiments(Picart et al., 2002) suggested that non-linear (exponential)
growth results from diffusion of polyelectrolyte molecules inward and outward from a film,
while the film thickness is proportional to the amount of migrating polyion. The inward
polyelectrolyte diffusion step causes supralinear growth and is favored by a non-homogeneous
multilayer (Lavalle et al., 2002). Deposition of poly(L-lysine) (PLL) in the presence of a
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