Nanostructured Polyelectrolyte Brushes
Final Report Abstract
Polymer chains which are tethered with one end to a surface at a high graft density are strongly stretched away from the surface. In the project performed by two groups at Cornell University and the University of Freiburg such brushes were nanopatterned by following deep top-down and bottom-up UV lithography approaches and the behavior of such patterned films was studied. A special focus was placed on brushes which carry charged groups on the main chain, so-called polyelectrolyte brushes. The extent of chain stretching can reach up to 70% of the contour length of the polymer molecules, leading to a very strong loss of entropy. It is found, that which morphology is observed when patterned brushes with pattern sizes ranging from a few hundred nanometers to several micrometers are obtained, is quite complex and depends strongly on the sample history and the finer details of sample generation. When such brushes are exposed to an aqueous environment, the strong stretching of the polymer chains (entropic spring) can lead to the breaking of covalent chemical bonds and the loss of chains from the surface. This so-called degrafting behavior of strong (nanopatterned) polyelectrolyte PMeVP brushes is due to a complex interaction of kinetic and thermodynamic effects. From a thermodynamic point of view the stability of polymer brushes depends strongly on intrinsic polymer brush properties as the degree of charging, molecular weight, grafting density and pattern size. Additionally, also extrinsic properties, i.e. parameters due to the properties of its environment, like solvent quality and temperature influence the stability strongly. All of these parameters work in concert to increase or decrease the extension of the polymer chains and respectively tension or release the ‘entropic springs’. However, also kinetic effects such as how the brushes are immersed into the solvent (‘osmotic shock’) and entanglements can play a big role. These kinetic parameters influence the way how the stress is imparted into the chains and how is concentrated to certain chain segments or distributed among them. This can lead to the counterintuitive behavior observed in nanobrushes, where tethered chains in such small structures, which are prone to less stress from a thermodynamic point of view, degraft even faster than their peers consisting of microbrushes. The broad knowledge achieved in this work on the control of the nanomorphology of patterned polymer brushes and the understanding of the complexity of the combination of kinetic and thermodynamic effects of the degrafting behavior of nanostructured polymer brushes might help in the future to improve the design of more sophisticated brush-based materials. However, it also broadens the range of possibilities for brush applications as it will help finding the optimal brush properties and experimental conditions for improved stability of the obtained layers.
Publications
- Morphology of Nanostructured Polymer Brushes Dependent on Production and Treatment. Macromolecules, 2017, 50, 4715-4724
Wei-Liang Chen, Matthias Menzel, Oswald Prucker, Endian Wang,Christopher K. Ober, and Jürgen Rühe
(See online at https://doi.org/10.1021/acs.macromol.7b00714) - Reduced Lateral Confinement and Its Effect on Stability in Patterned Strong Polyelectrolyte Brushes. Langmuir, 2017, 33, 3296-3303
Wei-Liang Chen, Matthias Menzel, Tsukasa Watanabe, Oswald Prucker, Jürgen Rühe, and Christopher K. Ober
(See online at https://doi.org/10.1021/acs.langmuir.7b00165)
