Adsorption of Polyelectrolytes onto Charged Surfaces 53polyelectrolytes, and the retained water due to the inhomogeneous structure of
the adsorbed layers in which the polyelectrolyte chains are adsorbed as
isolated pancakes [41]. Therefore, low charged polyelectrolytes lead to a
higher water content in the adsorbed layers due to their fuzzy structure [26].
The hydration degree of the layers is responsible of the mechanical properties
of the films due to the plastifier effect associated with water. Thus, fuzzy
layers, with high water content, presents rubber-like behaviour being softer
than those layers formed by polyelectrolytes adsorbed in a flat conformation.
The hydration degree of polyelectrolyte layers is generally high with values
around the 20 and 80% of the total mass of the film. This high water content
leads to film with gel like behaviour in the hydrated state. The drying of
polyelectrolytes layers provokes an important rigidification due to the water
loss [126-128].
5. POLYELECTROLYTE MULTILAYERS:
LAYER-BY-LAYER METHOD
An interesting approach to take advantage of the characteristics and
properties of polyelectrolyte layers is the fabrication of materials by sequential
stacking of opossitely charged polyelectrolyte, leading to the so-called
polyelectrolyte multilayers (PEMs) assembled by the Layer-by-Layer self-
assembly method (LbL) [129-131]. This method was introduced by Decher et
al. [132-134] almost 30 years ago and it undergone a fast development,
becoming a powerful methodology for the fabrication of self-assembled
nanomaterials [131, 135] due to its simplicity, low cost, and versatility
[103, 104]. The LbL approach provides the bases for the fabrication
of polyelectrolyte multilayer architectures with well-defined thickness,
composition and including multiple chemical functionalities [104]. The LbL
method consists on the alternate deposition of polyelectrolytes bearing
oppositely charges onto a solid surface that is a template for the assembly
[136, 137]. The main driving forces governing the deposition are similar to the
above discussed for single polyelectrolyte layers with the direct electrostatic
interaction and the entropy gain associated with the counterions release
playing a central role.
The list of components that can be used for the multilayer assembly is not
limited to polyelectrolytes, including different types of materials both charged
and non-charged, which expands the LbL assembly beyond the electrostatic