Polyelectrolyte Multilayers (under construction)
UNDERSTANDING THE STRUCTURE, STABILITY AND DYNAMICS OF POLYELECTROLYTE MULTILAYERS
- Self-assembly processes of charged polymers (polyelectrolytes) involving electrostatic interactions can be used to build-up multilayered materials with unique properties. In the early 90's Decher et al  demonstrated the feasibility of the self-assembly of polyelectrolyte multilayers (PEMs) using the so-called Layer-by-Layer (LbL) technique. The versatility of the LbL process has allowed the fabrication of thin multilayer films made of synthetic polyelectrolytes (PE), DNA, lipids and proteins, which has resulted in a boost of novel applications in recent years. For instance, PEMs are used as matrix materials for enzymes and proteins in sensor applications , and also as a matrix for active components in solar cells. PEMs are used as a coating for protecting and control the healing process of damaged arteries . In addition, PEM's can be used as permeable membranes for nanofiltration , gas separation, and fuel cells. Furthermore, PEM's are also used in the fabrication of non-linear optical materials , coloured electrochromic electrodes (future display devices), and to tailor the properties of photonic crystalls . Other uses of PEM's include analyte separation processes (chromatography) , and the fabrication of thin-walled hollow micro- and nanocapsules (see , and ref. therein). These capsules have a great potential for drug carrier and nanoreactors.
- The present knowledge about PEMs has been summarized in a few reviews [9-13]. 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 . 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 . 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 . 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 . 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 : 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 . 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 ). 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 ). 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 ( 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. Thus, the complex nature of PEMs possesses a challenge when one tries to choose a PEM system for a particular application. Therefore, one must first try to learn more about the fundamental properties of PEMs before it is possible to understand how to use these films for specific applications without a large and exhausting process of trial and error.
- There have been very few attempts to describe theoretically the electrostatic self-assembly, and all of them are build-up on severe assumptions hard to test experimentally; specifically they assume that PEM are equilibrium structures. Netz and Joanny  considered the formation of multilayers in a system of semiflexible polyelectrolytes (PE), assuming that the deposited layer structure was fixed, providing a solid charged substrate for the next layer. Mayes et al  applied a similar idea to flexible PE. Both these models neglect interpenetration (interdigitation) and chain complexation between the layers, which are usual features found in experimental works about PEMs. The oppositely limit of strong intermixing of PE between neighbouring layers was considered by Castlenovo and Joanny  by incorporating the complex formation between oppositely PEs into self-consistent field equations. Unfortunately this equations are limited to solutions of high ionic strength, were electrostatic interactions can effectively be treated as short-range interactions. Despite these huge efforts, the strong correlations existing between oppositely charged polyions, provides a formidable challenge for their theoretical description. In this respect, numerical simulations could be of great help.
- There exist few computational studies about electrostatic self-assembly of PEM. In a seminal paper on layer formation on a spherical substrate  it has been demonstrated that additional non-electrostatic forces are needed to produce nice PEMs. This suggests that multilayering is possibly not an equilibrium phenomenon evidence of which has been also observed experimentally . Similar conclusions have been drawn for other geometries using either Molecular Dynamics (MD) or Monte Carlo (MC) techniques [24, 25]. Another outcome is, that these studies have confirmed the fuzzy nature of PEMs (molecules in one layer interpenetrate other layers) , although almost perfect periodic oscillations of the density differences between monomers belonging to positively and negatively charged PE have been found. MC studies (see refs. in ) have reported stability as well as the microstructure of the PE layers to be especially sensitive to the strength of the non-electrostatic short range attraction between the PE and the charged substrate. In addition, the thickness of the adsorbed layer is observed to decrease when increasing PE concentration or surface charge density, although the total adsorbed amount displays a non-monotonic dependence on polymer concentration. It was also demonstrated that the formation of multilayers as well as the extent of layer intermixing depends on the molecular weight of the PE chains and the fraction of charge on its backbone. The presence of ionic pairs between oppositely charged PE that form the layers have been claimed as a possible important factor in stabilizing the multilayer films. In the last year, MD studies for the assembly of flexible PE onto flat charged surfaces  have suggested the formation of thermodynamically stable structures from bulk mixture solutions. This is a controversial, very basic issue that opposes to some experimental reports, and we will try to resolve this issue.
- In short, despite the amount of work done during the last 15 years, the understanding of the multilayer formation process and the knowledge about how slight differences during the growth process are able to strongly modify the properties of the multilayer materials is still in its infancy. Doubtless, the understanding of such issues is of paramount importance to improve current building-up methods and devices, tune finely the properties of such materials for specific purposes, and in turn devise new potential applications for such materials. Such knowledge will not be only of benefit for the Scientific Community but also for industry as well as society due to the huge potentiality of such materials for new devices and applications.
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Aim of the Research
- The present project is intended to shed light on some still not clearly understood aspects governing multilayer formation and the control of their properties. At this stage, numerical simulations that use the state-of-the-art algorithms to deal with charged soft matter offer a very valuable and useful tool in order to elucidate the mechanisms governing multilayering assembly and the properties of PEMs. These numerical simulations can build a bridge between the detailed experimental results and the relatively coarse grained analytical models, helping us to understand several non-well understood issues.