a) b - École Polytechnique de Montréal

Microstructural information collected by SEM (Figure 5-6c) and AFM (Figure 5-6d) depicts a
relatively thick polyelectrolyte multilayer as wide as 5.5μm for 38 layers of PSS and PANI on
the surface of the filled sample (B). Picart et al.(Picart, Lavalle, et al., 2001) also reported a very
thick multilayer film of 1μm for 16 layers. They proposed that this thickness increase can be
attributed to a diffusion of polyelectrolyte inwards and outwards in the multilayer during the LbL
process. They also reported an exponential growth of layer thickness similar to the mechanism of
mass growth in this work.
5.4.5 Mass Deposition Dynamics of PSS/PANI
Although the structural information for the construction of ultra-thin multi-layers is extremely
difficult to obtain, gravimetric measurements can interpret the molecular topology of the films
that play the most important role in the formation of multi-layers. Figure 5-7 shows the
quantification of mass deposition of each layer as measured by gravimetric techniques for
various substrates in both the absence and presence of salt. It shows an unusual oscillating
behavior of thickness growth. After dipping the substrate in the PSS solution, the deposition
mass increases, followed by a decrease in deposition mass in a PANI solution due to the partial
removal of a previously adsorbed layer. Some previous works(Dubas & Schlenoff, 2001c;
Schoeler, et al., 2002; Tjipto, Quinn, & Caruso, 2007) have also reported on the oscillating
behavior of PSS polyelectrolytes due to the partial removal of previously adsorbed layers in
subsequent operations. Schoeler et al.(Schoeler, et al., 2002) related the extent of removed
material to the critical charge density of the polyelectrolyte. Charge density for a polyelectrolyte
is defined as the fraction of monomers in the polymer which are charged. If this fraction is much
smaller than 1, the polyelectrolyte is weakly charged. Normally, weak polyelectrolytes tend to
demonstrate an oscillating deposition. Dubas et al.(Dubas & Schlenoff, 2001c) concluded that
the drawing out of water molecules unassociated with specific ion pairs due to increasing
external osmotic pressure is the main reason for the decrease in thickness at low salt
concentrations. Tjipto et al.(Tjipto, et al., 2007) related the oscillating behavior of a multilayer
formed of poly(4-styrenesulfonic acid-co-maleic acid) and PDADMAC to the heterogeneity and
roughness of the samples. Later in this paper it will be shown that all of these explanations are
likely responsible for the oscillating deposition observed in this work. The addition of 1 molar
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