62 Sara Llamas, Laura Fernández-Peña, Ana Mateos-Maroto et al.
Figure 11. Schematic representation of the two classical growth mechanisms found in
polyelectrolyte multilayers.
The linear growth is characterized by a linear dependence of the thickness
of the multilayer on N, appearing in multilayers such as (PAH + PSS)n [30,
207] or (PDADMAC + PSS)n when the assembly is performed under low ionic
strenght conditions [26]. For non-linear growth, the thickness dependence on
N is faster than linear (it is called frequently exponential growth). This type of
growth is found in (PDADMAC + PSS)n multilayers when the charge density
of the polyelectrolyte is low (i.e., high ionic strength solutions) [26, 31]. This
second growth mechanism also appears for multilayer containing biopolymers
[214-216]. Figure 11 represents the two different dependences of the adsorbed
amount on N found in LbL polyelectrolyte multilayers.
The explanation of the appearance of different growth mechanisms have
been source of strong controversy for long time and several explanations have
been developed. The first explanation is based on differences of the multilayer
roughness as the film growth [26, 31, 217]. An alternative explanation
considers the non-linear growth a consequence of the appearance of
interdiffusion phenomena of at least one of the polyelectrolytes along the
multilayer architecture during the assembly of the successive layers [218,
219].
The explanation of the change from a linear growth to a non-linear growth
due to modifications on the multilayer roughness are based on the fact that the
film roughness remains almost constant independently of the number of
adsorbed layers for linear growth multilayers, whereas a strong increase of the
roughness is observed as N increases for non-linear growth multilayers. This