Biomedical Engineering Reference
In-Depth Information
90
30
80
25
70
60
20
50
15
40
30
10
Fibre
20
Silicon oxide
5
10
0
0
0
2
4
6
8
10
Number of layers
Figure5.9
Amountofadsorbedpolyelectrolytechargespersquaremeterofsubstrateduring
PEM formation from high molecular mass PDADMAC/PSS on wood fibres and on SiO
2
.
The background electrolyte concentration was 0.1 MNaCl and a specific surface area of
1.37 m
2
/g, for the fibres, was used to recalculate the data to equivalents/m
2
(39). The
measurementswereconductedwithout anyadjustment of thepHwhichwasdetermined to
be5.5-6.Datafrom(29).
this respect is not obvious, and the conclusion is that the data for multilayer formation
on flat surfaces must be translated to porous wood fibres with care, especially at high
salt concentration (40) when the porous structure of the fibres has a larger importance
due to the extensive coiling of polyelectrolytes. The contributions from nonelectrostatic
interactions are different for cellulose and SiO
2
, and these also have an influence on the
adsorbed amount.
Despite these differences (Figure 5.9), it must be concluded that fully bleached chem-
ical softwood fibres and SiO
2
show very similar trends in terms of PEM formation, and
that SiO
2
can be used as a convenient model surface for screening measurements to
predict PEM formation on this type of wood fibres.
The formation of the PEM on fibres was also followed by single fibre measurements
using the Cahn balance set-up shown in Figure 5.1. With this technique it is possible to
determine the wetting force created by the interaction between the fibres and the liquid
and this value can then be used, if the fibre perimeter is known to calculate a contact
angle between the liquid and the fibre. Typical results from this type of measurement are
shown in Figure 5.10, recently published by Lingstrom
et al
. (29), where the force-traces
for an individual fibre coated with three and four individual layers of high molecular
mass PDADMAC/PSS, treated to a depth of 0.8 mm and washed to a depth of 1.2 mm,
are shown. It can be seen that the fibre showed a certain difference in advancing wetting
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