PEM experimental studies

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Experiments have established several important features for PEM's: the thickness of each adsorbed layer shows almost a linear dependence on the salt concentration. Flexible polyelectrolytes in two component multilayers are known to intermix over several adjacent layers. This layer intermixing can be suppressed by using more rigid blocks for the assembly [13]. Intrinsic charge compensation by polyions accompanied by overcharging and the kinetically irreversible nature of deposition has also been reported.

In the case of weak polyelectrolytes, very interesting variations of the stability, the thickness, the stiffness, the permeability and the porosity of PEMs have been observed by adjusting the pH of the dipping solutions; this change not only modifies the new layers, but also has an effect on previously adsorbed layers [14]. The measurements of the Young's modulus of PEMs show a strong correlation with ionomers, and therefore it could be possible that the structure of PEMs is close to that of ionomers. The role that the charge density of the PE plays on PEMs growth is somewhat unclear. Despite the existence of considerable data in the literature concerning the influence of the charge density on PEM formation, there is little consensus, and conflicting results have often been reported [15]. As an example of such controversy, some works have reported the existence of a critical minimum charge density below which is not possible the formation of PEMs whereas other works have observed multilayer buildup with PE of very low charge density [15].

From the extensive research over the past few years on PEMs, it is clear that unlike single layer films or polymer gels there is no general description of the physicochemical properties of different PEMs systems, especially for those assembled from weak polyelectrolytes. Another clear example of the previous claim is found in the buildup mechanism of PEMS [16]: whereas strong polyelectrolytes usually observe a linear growth for the PEM's thickness, weak polyelectrolytes do not always follow this pattern and can exhibit an exponential growth. The assembly conditions, even in the case of using the same PE, can modify the buildup regime. Thus, for instance, it has been shown very recently that an increase in temperature has a profound influence on the rate of the layer-by-layer buildup [17]. Furthermore, an elevated temperature is shown to swap the buildup from a linear to an exponential regime. The exact mechanism behind the phenomenon of the nonlinear growth is unknown at present. There are studies that suggest the surface roughening as responsible of the exponential growth (see ref. in [18]). Instead, other models are concerned with a certain active volume of the PEM, or the diffusion of polyions in and out of the film (see ref. in [17]). Another example of the complexity of the behaviour of weak PE is the shift in the charge density of weak PE depending on if they are adsorbed to the PEM or remain in solution. Furthermore, recent experimental reports ([18] and ref. therein) suggest that non-electrostatic short-range interactions, like for instance the hydrophobic interaction, also play an important role in the multilayering process. Moreover, it has been reported that the structures formed usually are metastable at some film buildup stages and little knowledge exist about the film relaxation during these stages.