(40f) Stimuli-Responsive Polyelectrolyte Brushs At The Solid/water Interface: Reversible Activation By Changes In Ionic Strength
Polyelectrolytes anchored at surfaces are important in various applications and also form a challenging topic for fundamental studies. In this work, a monolayer of the diblock copolymer poly(ethyl ethylene)-b-poly(styrene sulfonic acid) (PEE114-PSS83) was transferred from the air/water interface to a silicon substrate coated with a deuterated polystyrene layer (dPS/Si surface), to evaluate its behaviour as a tunable polyelectrolyte brush. The grafting density and thickness of the block copolymer film was controlled by applying different lateral pressures during the deposition. X-ray reflectivity and AFM measurements showed that a homogeneous layer of the block copolymer was formed, whose thickness increased up to about 8 nm with increasing grafting density. The structure and responsive behaviour of this polyelectrolyte brush at the interface against an aqueous solution in D2O was then studied by neutron reflectivity. It was established that the block copolymer film has a well-defined structure, its hydrophobic PEE block being anchored to the dPS/Si surface and the polystyrene sulfonate (PSS) block forming a carpet layer. The effect of salt concentration on the brush nanostructure was investigated in aqueous solutions containing up to 1M NaCl. It was found that the brush thickness decreases at salt concentrations above 0.1M, but that the original thickness of the brush was regained when the electrolyte was replaced by pure D2O. These results confirm the reversible activation of the brush at the solid/liquid interface by changing the ionic strength of the subphase. This block copolymer is suitable as a model system for stimuli-responsive polymer layers, both in fundamental studies and biological applications. However, no such responsive behaviour was found in analogous experiments with a diblock copolymer having a longer polyelectrolyte block (PEE144-PSS136). Tentatively, this is attributed to sterical effects, probably due to the entanglement of the longer PSS chains.