Polyelectrolyte Multilayers for Fibre Engineering - The Nanoscience and Technology of Renewable Biomaterials - page 142

for both systems was also higher when the cationic polymer was adsorbed outermost,
indicating a difference in the PEM structure depending on the polymer adsorbed in the
outermost layer. This is in agreement with the larger strength found in sheets made
from PEM-covered fibres when the PEM was capped by the cationic polyelectrolyte.
To test this hypothesis further the adhesion between PEMs was directly measured in
model experiments with the AFM colloidal probe technique. In these experiments, the
pull-off force was directly measured for PEMs formed from low and high molecular
mass PAH/PAA (35, 51) (Figure 5.18), both adsorbed at pH 7.5. The adhesion results
show that there is an increase in adhesion between the layers with increasing number of
layers. It can also be concluded that the adhesion was higher when PAH was adsorbed
in the outermost layer and that the effect was more significant for the low molecular
mass than for the high molecular mass combination.
Figure 5.18 shows that there was a significant increase in the adhesion when the
contact time at maximum load was increased from 0 to 5 s. This indicates that, if the
chains are given a longer time to diffuse across the interface, a stronger adhesion is
developed between the PEM covered-surfaces.
Since it has been shown that the number of chain ends rather than the number of
loops is important to achieve a strong adhesion between surfaces (50), and since the
number of free chain ends decreases when the molecular mass of the interacting chains is
increased, higher molecular mass polymers would in general tend to give a less significant
contribution to the adhesion at least at short contact times, when the high molecular mass
polyelectrolytes will not have sufficient time to diffuse the required distance across the
PEM/PEM interface. Low molecular mass polymers are assumed to possess a higher
mobility and a higher rate of interpenetration than the high molecular fractions and it
can also be assumed that they contribute to a more significant improvement in adhesion.
For the very thin and rigid layers of PDADMAC/PSS, it is however reasonable that
the low molecular mass PDADMAC/PSS gives fewer and shorter interacting chains due
to the flat conformation of these polyelectrolytes, which explains the comparably small
improvement in paper strength compared to that obtained with the high molecular mass
combination. This process of chain mixing between two opposite surfaces carrying
PEMs is schematically shown in Figure 5.19.
For individual fibre-fibre joints (52) of fibres treated with PAH/PAA adsorbed at pH
7.5/3.5, a study using light microscopy and specific staining of nonbonded areas showed
that the molecular degree of contact was increased from about 18% to 32% for fibres
treated with five layers compared to nontreated fibres. The data for layers 3-5 also
indicated that the degree of contact, in the fibre/fibre contact, was increased when PAH
was adsorbed in the outermost layer. This shows that a high contact area between the
fibres, in addition to a strong interaction due to entanglement between the layers, is very
important for the development of strong fibre-fibre joints. The results also clearly shows
the need for clear-cut model experiments in order to elucidate the strength-enhancing
mechanism of different additives.
It has recently been shown that fibres having the lowest wettability, both for
PDADMAC/PSS and PAH/PAA, when the cationic polylelectrolyte is in the outermost
layer, also show the strongest adhesion. This seems contradictory to a recently
published hypothesis where it is suggested that a more hydrophilic (53) agent will
more efficiently improve the strength of papers made of treated fibres. However, the