66 Sara Llamas, Laura Fernández-Peña, Ana Mateos-Maroto et al.
The adsorption times are strongly dependent on the adsorption conditions
used for the assembly, and especially on the nature of the material used for the
multilayer building [29, 30].
5.5. Water Content, Mechanical Properties,
Permeability and Porosity
The role of water in multilayer architectures is critical becauses it is
correlated to the dynamics and structure of the formed films due to their
importance for controlling the molecular interactions, especially those related
to the complexation between the polyelectrolytes in adjacent layers [23, 24,
35, 221-225]. In most of the multilayers, the water content is in the range 20-
80% of the total weight of the multilayer [226] and can be easily controlled
tuning the conditions used for the assembly of the layers and/or the charge
density of the polymers [224, 225]. Figure 15 shows the water content of
(PDADMAC + PSS)n multilayers with different number of bilayers adsorbed
from solutions with different ionic strengths. This high water content plays a
central role in the fabrication of cargo systems for the encapsulation of active
matter [227]. Most of the systems show a decrease of the water content with N
(see Figure 15) [224, 225, 228].
In addition to the hydration water, it is also necessary to consider the
swelling water [229]. In most of the cases, the dependence of the hydration
and swelling water on N is the opposite [31]. It is worth mentioning that the
degree of swelling of the multilayers play a key role on many applications of
the multilayers such as controlled release or separation of materials [127, 230].
The control of this parameter can be carried out by modifications on the
environmental humidity or on the ionic strength of the solutions. This allows
modifying the ionic pairing between adjacent layers, and consequently the
ionic cross-linking and the mechanical properties of the films [230]. These
latter plays are essential for the development of new applications of the
supramolecular architectures [24, 35].
The mechanical properties of polyelectrolite multilayers is strongly
dependent on the specific interactions occurring between the polyelectrolytes
and on the water content [231], behaving in most of the cases as polymer gel
due to the high water content [212, 232, 233]. The modification of the ionic
pairing due to the change of the assembly conditions also plays a key role for
modifying the the mechanical properties of the multilayers [234].