Structure - mechanical property relations of polyelectrolyte multilayer and free-standing membranes
Final Report Abstract
The aim of this project was the establishment of correlations between molecular scale architecture of polyelectrolytes and structural / mechanical properties of multilayers assembled from these molecules by electrostatic layer-by-layer deposition. In the long term, the potential of freestanding membranes based on these layers for applications should be evaluated. For this purpose, we combined synthesis of well-defined model polyelectrolytes with in depth physico chemical characterization of multilayer structure and mechanics. New strong polyelectrolytes with well-defined variable charge densities were developed: Ideal statisitical copolymerization of strong electrolyte monomers with non-ionic ones proved to be an effective synthetic strategy. However, a minimum content of hydrophobic fragments in the polyelectrolytes turned out to be necessary, to avoid the formation of soluble stoichiometric polyelectrolyte complexes, and to allow successful build up of polyelectrolyte multilayers (PEMs). Spray coating was shown to be an efficient and more rapid alternative to the classical dip coating process for assembling especially thicker films (> 50 nm). Polymeric sulfonates and pyridinium salts derived from vinylbenzylchloride were found to be particularly suited for making PEMs, forming very stable multilayers, and allowing for extensive charge dilution. The use of 4-methylpyridinium derivatives enabled the cross-linking of the PEMs under mild conditions (ambient temperature), either by dialdehydes, or by using aldehyde-functionalized polyanions, without changing the content of ionic groups in the system. This coupling reaction produces a strong chromophore (and fluorophore), so that success, extent and kinetics of the cross-linking can be easily monitored. The cross-linked PEMs are stable against powerful solvent-salt mixtures, as well as drying in air. Charge density was corroborated to be a key parameter to control multilayer thickness and roughness. In fact, both values increase with decreasing ion content in the polyelectrolytes. Also, the swelling ability increases. The dependency of multilayer swelling on charge density of the compounds could for the first time be determined using Neutron reflectometry measurements. From the water uptake as function of relative vapor pressure, the Flory- Huggins parameter of the polyelectrolyte systems as a key-quantitiy for thermodynamic considerations could be estimated. Tools for measuring the mechanical properties of these films were developed and adapted, notably the SIEBIMM method (strain-induced elastic buckling instability for mechanical measurements). It was shown that the E-modulus is controlled by the charge density, too, decreasing with decreasing ion content. Another key factor is the water content (directly controlled by the ambient humidity), as water is an efficient plasticizer for PEMs. The use of the newly developed polyelectrolytes with a partially hydrophobic character resulted in very stable PEMs with high moduli (> 1 GPa). Methods to prepare free-standing PEM membranes were developed, either based on the use of sacrifial, pH-sensitive coatings, or by exploiting the high solvent stability of the covalently cross-linked films under neutral conditions. The pressure dependent bending of the former PEMs allowed to study their deformation characteristics. The latter PEMs were so stable that they could be separated even from high energy surfaces without the need of sacrifical layers, and transferred onto various supports. These free-standing films can not only be prepared in rather large dimensions (at least mm range, aspect ratio > 1000:1), but are also stable after drying. Thus, first steps towards applications in sensing could be established.
Publications
- Freestanding Polyelectrolyte Multilayers as Functional and Construction Elements. IEE Proc. Nanobiotechnology (2006) 153 (4): 112-120
M. Nolte and A. Fery
- Polyelectrolyte Freestanding Films as Sensors for Osmotic pressures. Chem Phys Chem (2006) 7 (9): 1985-1989
M. Nolte, I. Dönch, and A. Fery
- Neutron Reflectometry Study of Swelling of Polyelectrolyte Multilayers in Water Vapours: Influence of Charge Density of the Polycation. Langmuir (2009) 25, 11576-11585
R. Köhler, I. Dönch, P. Ott, A. Laschewsky, A. Fery, R. Krastev
(See online at https://doi.org/10.1021/la901508w) - Cross-linkable polyelectrolyte multilayer films of tailored charge density. Chem. Mater. (2010) 22, 3223-3231
P. Ott, Patrick; J. Gensel, S. Roesler, K. Trenkenschuh, D. Andreeva, A. Laschewsky, A. Fery
(See online at https://doi.org/10.1021/cm903384e)
