Colloids and Surfaces A: Physicochemical and Engineering Aspects
Responsive polyelectrolyte multilayers
Introduction
Among the methods developed to prepare thin films, the layer-by-layer (LbL) deposition of polyelectrolytes has aroused considerable attention due to its simplicity and versatility [1], [2]. In addition this technique has the advantage of enabling nanoscale control of the thickness, the structure and the composition of the films. Beside the preparation of polyelectrolyte multilayers (PEMs) on flat substrates, the LbL technology has been extended to produce well-defined hollow capsules [3]. These microcontainers made by layer-by-layer deposition of oppositely charged polyelectrolytes onto colloid templates and subsequent removal of the core, are particularly attractive due to their promises for microreactors, microsensors, and drug-delivering systems [4]. Recent developments and potential applications of polyelectrolyte films and hollow microcapsules are described in details in various past reviews [5], [6], [7], [8], [9], [10].
One of the significant research interests emerging recently from the PEM area is the development of environmental responsive devices due to their potential applications in medicine, cosmetics, pharmaceutics and textile industry [11]. Here, the challenge is the preparation of thin films or shells showing a predictable response of their functional properties such as wettability, adhesivity or porosity when an external condition is varied. A common approach to produce such systems consists in using the propensity of some polyelectrolytes to change their macromolecular characteristics following a variation of external stimuli such as pH, temperature, ionic strength, magnetic field or light. The present review focuses on our recent work on the preparation and characterization of PEM films and microcapsules showing a variation of their characteristics in response to external stimuli, with emphasis placed on the influence of the structure and nature of the polyelectrolytes on the their responsiveness.
The first part of this paper is dedicated to the preparation of thermoresponsive multilayers. The most developed approach to produce such systems is to use derivatives of poly(N-isopropylacrylamide) (PNIPAM). PNIPAM is a polymer that at room temperature is water soluble through strong hydrogen bonding with water [12]. However, above 32 °C the hydrogen bonding is disrupted and water no longer acts as a suitable solvent. As a consequence, above this temperature called lower critical solution temperature (LCST), PNIPAM chains undergo a coil to globule transition where they preferentially make hydrophobic bonds with neighboring polymer chains [12], [13], [14]. Due to its LCST close to physiological temperature, PNIPAM is a very attractive candidate for biomedical applications [15]. The first approach to introduce PNIPAM into multilayers is to prepare hydrogen-bonded self-assemblies. Caruso et al. found that multilayers of hydrogen-bonded PNIPAM and poly(acrylic acid) could be reversibly loaded with a dye and subsequently emptied by varying the temperature [16]. However, such systems disintegrate when the pH exceeds a certain value. For this reason, other approaches using charged derivatives of PNIPAM were also tested. Different charged PNIPAM derivatives differing by their structure and their composition, were successfully synthesized and incorporated within polyelectrolyte assemblies [17], [18], [19], [20], [21], [22]. However, the responsiveness of the resulting assemblies was shown to strongly depend on the microstructure of the polyelectrolyte used. This aspect will be discussed here by considering the thermosensitive behavior of different systems based on mono- or multilayers and incorporating charged copolymers of PNIPAM of various structures (Scheme 1). Moreover, the preparation of microcapsules filled with a high molar mass PNIPAM is outlined as an alternative approach to prepare thermoresponsive devices.
The second part of this review deals with the pH-responsive PEMs. Due to their pH sensitivity, weak polyelectrolytes were extensively exploited to prepare responsive films and shells. Indeed, the variation of the pH induces a change in the charge density of the chains which affects the interactions between the polyelectrolytes and results in a change of the morphology or of the permeability of the films. Reversible swelling and porosity transitions in multilayers, tuned by pH changes, have been described in the case where poly(allylamine hydrochloride) (PAH) was associated to the strong polyanion poly(sodium styrene sulfonate) (PSS) [23]. The reversible formation of pores is crucial for the control of loading and release of substances. Others pH-sensitive membranes have been described, containing always at least one weak polyelectrolyte such as poly(acrylic acid) (PAA) [24], poly(methacrylic acid) (PMA) [25] or others weak co-polymers [26]. The thickness and the stability of the films were always determined by the pH of the dipping solutions. Stable films have been obtained after crosslinking the multilayers, due to the formation of strong covalent bonds [27]. A particular interest is in the preparation of responsive capsules whose permeability can be tuned with pH for drug delivery applications [28], [29], [30]. The preparation of such devices will be discussed here. In addition, a second approach which consists in embedding a polypeptide showing a change of its conformation with pH will also be described.
Section snippets
Responsive behavior of PNIPAM copolymers adsorbed in monolayers
An interesting question is how the phase transition properties of thermoreversible polymers would change in adsorbed layers on solid surfaces, and how electrostatic interactions would affect them. The phase transition of uncharged PNIPAM adsorption layers was studied by temperature-dependent dynamic light scattering (DLS), where a transition with a pronounced hysteresis occurred. The occurrence of the hysteresis was interpreted as a kinetically unstable ‘extended brushlike conformation’ [31].
Systems based on weak polyelectrolytes
Because the linear charge density along weak polyelectrolyte backbones is a function of pH, the electrostatic interaction within PEMs incorporating such polyelectrolytes can be easily tuned. This feature was widely exploited to trigger the properties of PEM films such as permeability, morphology or wettability. A particular interest is in the design of the capsules whose the shell permeability can be controlled for encapsulation and release procedures. Since the layer-by-layer (LbL) adsorption
Conclusion
This brief review of selected multilayers based on electrostatically interacting polymers illustrates the potential interests and pitfalls of LbL assembly for the preparation of responsive systems. Clearly, the knowledge accumulated on these systems is important, but we are still lacking a complete view of the dynamics of these systems, where the principles governing the rearrangement and motion of specific segments would be fully understood. This will be required before applications emerge,
Acknowledgments
The major part of the work reviewed in this paper was performed within the German-French Collaborative Research Group ‘Complex Fluids: from three to two Dimensions’. This program is jointly funded by the Deutsche Forschungsgemeinschaft (DFG, Germany) (Scho 636/3-1 to -2 and Fi 235/14-1 to -4) and the Centre National de la Recherche Scientifique (CNRS, France).
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Present address: Institut de Chimie Séparative de Marcoule, F-30207 Bagnols sur Cèze, France.