AFM study of cationically charged polymer brushes: switching between soft and hard matter

AFM studies on cationic polymer brushes in water (a good solvent) show that brushes in an extended conformation can be easily deformed or indented by an AFM tip. Experiments on polymer brushes in a more collapsed conformation, using methanol-water as a `poor solvent' environment, show similar properties. Conversely, this `soft' behaviour is dramatically different in the presence of electrolytes containing anions that are strongly coordinated to cationic groups of the polymer brush. Our initial studies in electrolyte solutions at different concentrations show that these brushes become so rigid that they cannot be indented or deformed by the AFM tip, even at high loads.