Polyelectrolyte Multilayers for Fibre Engineering - The Nanoscience and Technology of Renewable Biomaterials - page 136

10

8

6

4

2

0 layer 4

layer 3

− 2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Fibre distance (mm)

Figure5.10 Forcetracesfromthewashingstepofafibretreatedwith3and4layersofhigh
molecularmassPDADMAC/PSStoadepthof0.8mmandthenwashedtoadepthof1.2mm.
The lower curves show the force trace when the fibre was immersed (advancing) and the
uppercurvesshowtheforcetracewhenthefibrewaswithdrawn(receding). Theadsorption
was carried out with a background electrolyte concentration of 0.1 M NaCl without any
furtherpHadjustment (pH5.5-6).Datafrom(29).

force depending on the polymer adsorbed in the outermost layer, with a lower force, i.e.
a lower wettability, when PDADMAC was adsorbed than when PSS was adsorbed in
the outermost layer. A similar trend can be seen in Figure 5.11 which shows the force
curve for a similar analysis of an individual fibre treated with 8 and 9 layers of PAH and
PAH (41), adsorbed at pH 5. This fibre was treated to a depth of 1.0 mm and washed
to a depth of 1.7 mm.
In order to obtain at least a semi-quantitative estimate of the influence of the polymer
in the outer layer on the wettability of the PEM, the advancing contact angles of the
films were calculated from the Cahn balance measurements. Assuming the receding
contact angle to be 0 ◦ , the perimeter of each fibre was calculated over certain regions of
the treated part of the fibre. The validity of this assumption was supported by previous
Dynamic contact angle (DCA) measurements of single, nontreated, wood fibres (42-45)
showing that the receding contact angle is close to 0 ◦ , indicating complete wetting of the
fibres. These perimeter-values were then used when the average values of the advancing
contact angle were calculated.
Contact angles for individual fibres treated by high molecular mass (70 k/240 k)
(35) and low molecular mass (15 k/8 k) PAH/PAA (41) using different pH strate-
gies are presented in Figure 5.12. Figure 5.13 shows corresponding results for high
molecular mass PDADMAC/PSS (29) and low molecular mass PDADMAC/PSS (46).
Figure 5.14 shows the advancing contact angle data for PEO/PAA (41), PAH/CMC (46)
and PAH/PEDOT:PSS (36). All, except PEO/PAA, demonstrated a difference in contact
angle depending on the polymer adsorbed in the outermost layer, the contact angle being
higher when the individual fibre was capped with the cationic polymer. In the case of
PAH/PAA, there was also a difference depending on the pH strategy. Comparing the
data for the LM w combination of PAH/PAA at pH 7.5/7.5 and at pH 7.5/3.5, it was found