Polyelectrolyte Multilayers for Fibre Engineering - The Nanoscience and Technology of Renewable Biomaterials - page 134

14

PDADMAC, without addition
PDADMAC, 0.01 M NaCl
PDADMAC, 0.05 M NaCl
PDADMAC, 0.1 M NaCl
PSS, 0.1 M NaCl

12

10

8

6

4

2

0123456789 0 1

Number of layers

Figure 5.8 Amount of PDADMAC and PSS adsorbed per gram of fibres, determined by
nitrogen and sulphur analysis, respectively. The adsorptionmeasurementswere conducted
withoutanyextraadditionofNaClandwiththeadditionof0.01,0.05,and0.1MNaCl.The
fibresanalysedwithrespecttoadsorbedPSSweretreatedwith0.1MNaCl.Datafrom(29).

overcharging of the fibre surface due to the formation of large tails and loops of the
polyelectrolyte on the fibre surface. This increased adsorption in the initial layer then
propagates throughout the consecutive layers of the PEM.
Polyelectrolyte titration has also been used in order to study the formation of PEMs
from high and low molecular mass PDADMAC/PSS, and the results were similar to
those obtained using elementare analysis. In the case of one layer of PDADMAC, it
has also been shown that an increase in adsorption time from 10 to 30 minutes did not
result in any increase in the adsorbed amount.
Having adsorption data both for fibres and for SiO 2 surfaces with the same polyelec-
trolyte systems, it is possible to compare the adsorption on wood fibres to the adsorption
on silicon oxide as the substrate. To be able to make this comparison, the S/S 0 sig-
nal from SPAR was converted to the amount of adsorbed polymer, the adsorption in
equivalents/m 2 onto fibres being calculated assuming a specific surface area of the fibres
of 1.37 m 2 /g (39). This value is probably low since it corresponds to the external
part of the fibre and the PDADMAC molecules probably have access to a somewhat
larger surface area. Figure 5.9 shows the adsorbed amount of charges of high molecular
PDADMAC/PSS on both SiO 2 and wood fibres. In contrast to what was found when
the amount adsorbed on wood fibres was determined, the amount adsorbed on SiO 2
was not higher in the first layer than in the subsequent layers. This difference may be
explained by the macroscopic structure of the wood fibre, and by the different charges
of the respective surfaces. However, the trends observed for the two surfaces are very
similar and, considering the low specific surface area assumed for the fibres, there is
also a fairly good agreement between the absolute values achieved with the different
methods.
As has also been shown, the amount of polyelectrolytes adsorbed onto wood fibres did
not show any deviation from linearity with the number of layers as the salt concentration
was increased. The reason for the difference between wood fibres and SiO 2 surfaces in