Polyelectrolyte Multilayers for Fibre Engineering - The Nanoscience and Technology of Renewable Biomaterials - page 123

5

Polyelectrolyte Multilayers
for Fibre Engineering

Rikard Lingstr om, Erik Johansson and Lars Wagberg

5.1 Background

The method of forming Polyelectrolyte Multilayers (PEM) was introduced as a general
method in the early 1990s by Decher (1), but the principle was discussed already in
the 1960s by Iler (2). A multilayer is formed by adsorbing an oppositely charged
polyelectrolyte to a charged substrate. The substrate is recharged and in a second step,
an oppositely charged polyelectrolyte can be adsorbed. By repeating this process, layers
consisting of a large number of individual layers can be formed (3).
In early publications, the recharging of the surface was considered to be a necessary
condition for the formation of PEM (4). Electrokinetic measurements conducted for
PEMs formed from different combinations of polylelectrolytes showed that there was a
change in the sign of the surface potential depending on the polyelectrolyte adsorbed in
the outermost layer (5, 6). Recent investigations have, however, indicated that recharging
is not a prerequisite for the PEM formation (7). This indicates that the entropic gain due
to the release of counter ions and immobilized water molecules is the most important
factor for the PEM formation to take place (8).
Each adsorption step follows the fundamentals of polyelectrolyte adsorption, but
the mechanisms behind the recharging of the substrate are still not theoretically fully
understood. Different theories have been proposed, based on both equilibrium and
nonequilibrium models, but there is still no general theory available to describe the
PEM formation. Dobrynin et al . (9) have proposed a mechanism based on PEM as a
thermodynamically favourable state, i.e. the PEM-structure that is formed minimises the