Chain length and grafting density impact on the molecular structure of PMMA polymer brushes

Marcus Hildebrandt, University of Duisburg-Essen, Essen, Germany
Jochen Gutmann, University of Duisburg-Essen, Essen, Germany

Polymer brushes are thin films consisting of polymer chains, which are tethered with one end to a solid surface. At high grafting densities (polymer chains per surface area), repulsive forces between neighbouring chains lead to chain stretching and a brush conformation is obtained. These thin films arouse interest from a theoretical, fundamental and applied science point of view, such as roughness correlation. Polymer films with an overall thickness down to a few nanometres can show a large relative deviation in local film thickness due to interfacial roughness between polymer layer and substrate. In polymer electronics, like organic emitting diodes however, a constant quality across the entire diode is required. Polymer brushes could transfer the roughness profile from one interface to an adjacent one and ensure a locally defined film thickness across a large area, that is conformal brushes (roughness correlation). By varying molecular weight (brush length) and grafting density of the brushes, the coherence between these parameters and correlated roughness can be studied. In this work, roughness correlation of poly(methyl methacrylate) (PMMA) brush films, synthesized via ‘grafting-from’ method, was investigated by realizing grafting density and molecular weight gradients on a silicon substrate. Therefore both parameters were varied in orthogonal directions and hence a two-dimensional system is obtained. For characterization of those systems, scattering methods are required like Grazing Incidence Small Angle X-Ray Scattering (GISAXS). In preliminary GISAXS experiments at the GALAXI beamline we were able to prove the existence of roughness correlation and the dependence on grafting density and molecular weight in PMMA-brushes on a silicon wafer.

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