Polyelectrolyte Multilayers for Fibre Engineering - The Nanoscience and Technology of Renewable Biomaterials - page 124

free energy of the system via lateral correlation of adsorbed polyelectrolytes leading to a
recharging of the system. An alternative to this model has been proposed by Cohen Stu-
art (10) arguing that the recharging is due to the kinetic locking of a structure consisting
of loops and tails.
By using different combinations of polyelectrolytes and nano-particles (4), PEMs
showing very different properties can be formed and the PEM-properties can also be
influenced by varying parameters such as the salt concentration (11), type of salt, tem-
perature (12), molecular mass of the polyelectrolytes (13), type of counter-ions of the
polyelectrolytes (8, 14), and naturally the charge of the polyelectrolytes (4, 15, 16).
Highly charged polyelectrolytes are known to form thin layers when they are adsorbed
onto a solid substrate. Schlenoff et al . (17) have shown that the thickness of a single
layer of poly(diallyldiamethylammonium chloride) (PDADMAC), adsorbed in the pres-
ence of 0.5 M NaCl, and determined in the dry state by ellipsometry, is less than 1 nm.
PEMs formed from PSS as the anionic polyelectrolyte and a copolymer of PDADMAC
and N-methyl-N-vinylformamide as the cationic polymer were studied by Glinel et al .
(18) for different charge densities, and they reported a decrease in thickness when the
charge density was increased. In another study, Shiratori and Rubner (16) have shown
that polyallylamine (PAH) and poly acrylicacid (PAA), which are both weak polyelec-
trolytes (i.e. sensitive to pH changes) form very thin PEMs (less than 10 A in bilayer
thickness(dry)) and very thick PEMs ( > 120 A(dry)) depending on the pH strategies used
the during formation of the PEM. Later investigations using Quartz Crystal Microbalance
(QCM) have indicated that PEMs formed from PAH/PAA at pH 7.5/3.5 are thicker than
layers formed at pH 5/5 and pH 7.5/7.5 (19).
During the last decade, the LbL method has been developed as a noncomplicated
and general protocol to improve the properties of any solid substrate. PEM treatment
is already in use in several applications such as sensor technology (20) and contact
lens coating (20). Highly efficient membranes (21) can also be formed using the PEM
technique as well as hollow capsules for the controlled release of active chemicals
(22, 23).
Since PEM treatment influences the properties of the substrate, it can also be used
as a way of improving the adhesion between surfaces. Investigations during the last
decade have shown the potential of the PEM technique as a way of improving the
adhesion between wood fibres and thereby improving paper strength (24). PEMs were
first used on wood fibres in 1998 (25, 26) and improvements were observed in the
tensile strength of papers made from PEM-treated fibres, quantitatively comparable to
those achieved by mechanical beating. Sheets made from fibres carrying PEMs formed
from polyallylaminehydrochloride (PAH) and polyacrylic acid (PAA) (27) showed an
increase in tensile index from about 20 kNm/kg for sheets made from nontreated fibres
to 55 kNm/kg for sheets made from fibres having PEMs formed from 8 individual layers.
PEMs from cationic and anionic starch have also shown very promising results, and a
tensile index higher than 60 kNm/kg has been achieved when fully bleached chemical
softwood fibres were pre-treated with three layers of starch (28).
One interesting feature is that the tensile strength seems to be dependent on the
polymer adsorbed in the outermost layer (24, 27). Different explanations of this have
been proposed (19), and in this chapter the phenomenon is discussed in terms of the
PEM properties and the wettability of the PEM-treated fibres.