Polyelectrolyte Multilayers for Fibre Engineering - The Nanoscience and Technology of Renewable Biomaterials - page 129

4.5

PAH pH 7.5/PAA pH 7.5

3.5

PAH PAA PAH

2.5

Rinse

1.5

0.5

− 0.5

0 1000 2000 3000 4000 5000 6000

Time (s)

(c)

Figure5.3 (continued).

adsorbed layers which changes the polarisability of the adsorbed layer. As can be seen
in Figure 5.3b, this decrease cannot be detected in the frequency spectra, indicating that
no desorption is taking place during the adsorption of the PAA in the 2nd, 4th layer
etc. On the other hand, the data in Figure 5.3c show that there is a large decrease in
the dissipation as the PAA is adsorbed. The data in Figures 5.3a-c thus indicate that the
structure of the PEM is highly dependent on the type of polyelectrolyte adsorbed in the
outermost layer of the PEM. If the PAH is in the outermost layer, the PEM has a lower
viscosity and a lower shear modulus than if the PAA is adsorbed in the outermost layer.
As will be shown later, this has a large influence on the effect of the PEM. The exact
reason to this change in properties of the PEM depending of which polyelectrolyte that
is adsorbed in the outermost layer is yet not known.

5.3 Formation of PEM with Different Polyelectrolytes and the Properties

of the Layers Formed

As mentioned in the earlier section, the adsorption on silicon oxide using Stagnation Point
Adsorption Reflectometry (SPAR) has been utilised in order to establish that polyelec-
trolyte multilayers are formed from PDADMAC/PSS, PAH/PAA and PAH/PEDOT:PSS
and to predict that PEMs are also formed from these polyelectrolytes on wood fibres.
Figure 5.4 shows the formation of PEMs from the strong polyelectrolyte pair
PDADMAC/PSS on silicon oxide, where the adsorption was conducted at different
concentrations of NaCl (29). Figure 5.4a shows the stepwise increase in adsorption as
polylelectrolytes are consecutively added and a polyelectrolyte multilayer is formed,
and figure 5.4b shows the corresponding saturation signals for these experiments. From
this it was obvious that a larger amount was adsorbed when the NaCl concentration