Polymer Brushes
Surface architectures with distinct properties
Schematic representation of a polymer brush.
The term "polymer brush" was coined by deGennes to describe an architecture in which polymer chains are terminally tethered to a surface at a very high denisty (anchoring density is much smaller than coil dimensions). Due to this overcrowding the chains stretch perpendicular to the surface until the loss of possible conformations (loss in entropy) balances the enthalpic gain of segment dilution. The situation is illustrated in the image to the right in a somewhat simplified way as the chains are drawn invidually as if there would be no overlap.
The best way to realize such an architecture is to grow the chains directly at the surface of the substrate. This approach is called grafting from polymerization or surface-initiated polymerization. In a first step, the surface is decorated with an monolayer of an initiator which can then be used to "grow a lawn" of polymer molecules. Using this approach we are able to create monolayers of a variety of polymers with a thickness between a few nanometers and a few micrometers. The properties of the layers can be readily adjusted by choosing an appropriate monomer.
Example projects:
Polyelectrolyte BrushesPolyelectrolyte Brushes are surface architectures with rather special properties. Their swelling behavior is unique and shows strong and often times unespected dependencies on pH and ionic strength of the medium. |
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Mixed Polymer BrushesBlockcopolymer brushes or mixed polymer brushes combine two immiscible polymers in very close proximity. Phase seggregation is limited to nano scales and can be switched using selective or non-selective solvents. |
Brush DegradationPolymer brushes are intrinsically meta-stable architectures. In contact with solvent they experience a strong osmotic pressure forcing them to swell strongly. Most uncharged brushes easily withstand this force, yet for many charged polymer brushes the swelling leads to a drag on the chains that is stronger than chemical bonds. Catastrophic degrafting is often times the result. |
Gradient SurfacesCombining surface-initiated polymerization with interference lithography provides means to generate topological gradients on the nanoscale. |
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