Nanostructured Polyelectrolyte Brushes

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This proposal describes an international collaborative program focused on the study of polyelectrolyte brushes that involves the research groups of Christopher Ober (Cornell University) and Jürgen Rühe (University of Freiburg). Poly(electrolyte) (PEL) brushes are of particular interest because the presence of charges and the geometric constraints arising both from the thin film format as well as from the surface attachment of each chain, lead to a rich physical chemistry of such systems. The loss of entropy induced by chain stretching generates a very strong pull on the bonds through which the chains are linked to the surface. It has recently been reported, that the pull can be especially in osmotic brushes so very strong that breaking of chemical bonds of the polymer chain occurs (entropic death of the chains). We will explore this rather unusual case of mechanochemistry, where the origin of the mechanical forces leading to rupture of chemical bonds has no external, but an internal origin. We plan to elucidate this by generating patterned PEL brushes of precisely tailored composition with structure sizes from infinite (i.e. homogenous thin film) down to feature sizes whose footprints are comparable to the swollen layer thickness (i.e. < 100 nm). In the former case the brushes represent essentially two 2D- structures as the layer can only swell by stretching away from the surface while the latter allows a more 3D-swelling. We plan to address the following key questions: How does the swelling of the PEL brushes scale with size of the nanostructure and how does confinement of polyelectrolyte brushes in nanostructures influence chain stability and the interaction with ions in the surrounding medium as a function of environment, segment/charge distribution and charge state? To study this, we plan to grow polyelectrolyte brushes from poly(styrene sulfonate), quaternized poly(vinyl pyridine) and poly(2-methacryloxyethyl trimethylammonium chloride) and then create nanostructured brushes using benign orthogonal patterning. We will characterize the brushes using synchrotron-based methods such as X-ray reflectometry, grazing incidence small angle X-ray scattering (GISAXS) studies and near edge X-ray absorption fine structure (NEXAFS). In addition both XPS and multiple angle ellipsometry will provide complementary information, as will more routine methods such as ATR-FTIR and SFM. As a result we hope to obtain a deeper understanding of the complex interplay between confinement, swelling and stability of PEL brushes and obtain new insights into the complex physicochemistry of such systems.