Polyelectrolyte Multilayers for Fibre Engineering - The Nanoscience and Technology of Renewable Biomaterials - page 139

layers and/or a high degree of interpenetration of polymer chains between the different
layers. The results for the PEMs formed from low molecular PAH/PAA at pH 7.5/3.5
and pH 5/5 indicate that the layer was thicker and better defined when the PEMs were
formed at pH 7.5/7.5.
A difference in contact angle was also detected for high molecular mass PDADMAC/
PSS, PAH/PEDOT:PSS and PAH/CMC. In comparison, PAH/CMC showed the largest
difference on the wettability depending on the polyelectrolyte in the external layer. When
5-9 layers were adsorbed, the advancing contact angle was calculated to be 110-115 ◦
when PAH was in the outermost layer, and about 40 ◦ when the PEM was capped by CMC.
The reasons behind the difference in wettability for different polyelectrolytes is still
under debate, but it has recently been shown that it is not the cationic or anionic nature
of the polyelectrolyte that determines the wettability of a polyelectrolyte, but rather the
intrinsic hydrophobicity (47) of the polymer chain.

5.5 Influence of PEM on Properties of Fibre Networks

Sheets made from cellulose fibres carrying a PEM have, for different polymer systems,
shown a great increase in paper strength compared to sheets made from nontreated
fibres (Figures 5.15 and 5.16) (27, 35). In the case of sheets made from fibres treated
with low molecular mass PAH/PAA (27) a tensile index of about 55 kNm/kg was found,
compared with a value of about 20 kNm/kg for sheets made from nontreated fibres. With
this polymer system, the increase was larger for the low molecular mass combination.
In the case of sheets made from fibres treated with highly charged PDADMAC/PSS
(29), the increase was lower, and in contrast to PAH/PAA, the improvement was most
significant for the high molecular mass combination of polyelectrolytes; about 80% when

Strain at break, PAH/PAA, 15 k/5 k
Strain at break, PAH/PAA, 70 k/240 k

6

55

50

45

4

40

35

30

25

2

Tensile index, PAH/PAA, 15 k/5 k
Tensile index, PAH/PAA, 70 k/240 k

20

15

0123456789

Number of layers

Figure5.15 Tensileindexandstrainatbreakof sheetsmade fromfibres treatedwithPEMs
formedfromPAH70kandPAA240k(35)andfromPAH15kandPAA5k(27),presented
asfunctionsof thenumberof layersinthePEM.