Project B2: Polymer multilayers at solid substrates

Principal Investigators: J. Rühe (Freiburg) / G. Decher (Strasbourg)
Collaborators: P. Schaaf (Strasbourg)
PhD Students: Marcel Rothfelder / Christophe Higy / Christoph Scheibelein

Current state of the research

Electrostatic layer-by-layer (LBL) assembly leads to polyelectrolyte (PE) multilayers which are interesting surface architectures that provide good structural control and permit arrangement of different materials in an organized manner along the layer normal [1]. This enables the tuning of position-dependent properties among the different components, which has led to advances in many fields of research. The LBL method is simple, applicable to readily available materials, and allows for efficient positioning of inorganic and bioactive species in a polymer matrix. Applications of such films range from thin film devices (e.g. magnetic or optolelectronic properties) to the control of cell transfection by controlling the density and conformation of plasmid DNA at interfaces. Polymer brushes are end-attached polymeric monolayers in which the chain conformation is controlled by the lateral density. The Freiburg group has published some first experiments on fabricating PE multilayers on top of PE brushes [2], which induces very rapid linear growth in such films. In doing so, one provides a potentially strongly swollen starting layer and allows for the study of systems that originate from surface-attached PE complexes. Such systems are characterized by a wealth of possible polymer conformations ranging from largely collapsed first layers to strongly swollen complex layers. Despite of the broad interest in this area, there are still a many open questions on the equilibrium electrostatics of such systems [3], especially for the case of polymeric counterions [4] with very little experimental work dedicated to brushes [e.g. 5]. Especially interesting is how the conformational orientation of the interfacial polymer brush can be transferred to subsequent PE layers (“brush-templating” effect).
 

Contributions of the participating groups

The Freiburg group has a long standing experience in the generation of surface-attached polymer layers and ultrathin polymer networks. In the past years, considerable effort has been placed on the synthesis of and the understanding of the properties of PE brushes [6]. PE brushes consisting of weak and strong, positively and negatively charged chains were synthesized with varying molecular weight and graft density. The influence of counter- and co-ions, pH value, graft density and other parameters on the swelling of the surface-attached PE chains was evaluated. Moreover, a significant effort has been placed on the generation of multilayer constructs based on deposition of polymers by techniques, such as dip coating, spray coating, doctor blading or spin coating followed by crosslinking. The Strasbourg group has pioneered the development of PE multilayers (Fig. 1) and possesses extensive competence in preparing and characterizing such systems,  also with respect to biological applications [7]. The group has longstanding experience in the characterization of these systems by neutron scattering and reflectometry techniques, which led to the concept of the fuzzy nanolayer assembly [1].

Research project and collaborations

In previous experiments, it was shown that PE brushes could serve as a good example for a soft, water-swollen, charged interface. Thus, PE brushes are almost idealmodel systems; as the polymer chains are covalently bound to the surface, desorption is impossible and the number of surface charges remains the same, irrespective of conditions.

The interaction of this layer with other (charged) molecules has already been studied to some extent [2]. For PE complex formation (Fig.2), the adsorbed amount depends largely on the interplay between the number of charges in the film (for weak PEs, it is a function of the conditions of the adsorption process) and the solubility/swellability of the forming PE complex in the contacting aqueous solution. Details of the mechanism and the exact dependence on various parameters during the adsorption remain to be worked out to get a clearer understanding of the overall process. Such knowledge is crucial if additional layers are to be adsorbed and multilayer assemblies are to be built.

As a second approach to polymer multilayers we will study systems in which polymers with benzophenone groups will be deposited by dip coating onto solid substrates. After crosslinking through UV-irradiation, this procedure will be continued and pre-selected polymers will be deposited layer-by-layer. The structure of these multilayers will be studied together with the Strasbourg group who will carry out fluorescence microscopy investigations of such multilayers. Particularly interesting is the case in which polymer chains or networks are generated on an elastomeric substrate (collaboration with P. Schaaf). In this case, the grafting density of the surface-attached chains (or sub-chains in the network case) depends strongly on the degree of stretching of the elastomer, which will eventually permit us to study different parameters in parallel. Such studies will be carried out in collaboration with project C2.

References
[1]Decher, G. (1997). Science 277, 1232.
[2]Zhang, H. N.; Rühe, J. (2005). Macromolecules 38, 10743.
[3]Wang, K., R. A. Zangmeister, et al. (2009). J. Am. Chem. Soc. 131, 318.
[4]Biesalski, M., D. Johannsmann and Rühe, J. (2004). J. Chem. Phys. 120, 8807.
[5]Tran, Y. and P. Auroy (2001). Eur. Phys. J. E 5, 65.
[6]Rühe J. et al. In “Advances in Polymer Science” Vol. 165 (2004).
[7]N. Jessel et al. In Macromolecular Engineering Vol. 2, (Y. Gnanou, L. Leibler and K. Matyiaszewski, Eds.), Wiley-VCH: Weinheim (2007) 1249.