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Coupling pH-responsive polymer brushes to electricity: switching thickness and creating waves of swelling or collapse.

Electrolysis of water is proposed as a method to couple the pH-responsive behavior of polymer brushes to an electrical stimulus. It is shown that an electrode in close proximity to a pH-responsive polymer brush can change the local solution pH, inducing either swelling or collapse of the polymer brush. By alternating the bias of the voltage applied to the electrode, either acidic or alkaline conditions can be generated, and reproducible cycles of polymer brush swelling and collapse can be achieved. It was found that the length of time which the electrical stimulus is applied to the electrodes can be as short as 10 s and that, once "switched", polymer brushes remain in the switched state for many minutes after the electrical stimulus is turned off. In other experiments, two electrodes were positioned 10 cm apart with a pH-responsive brush in between. Under these conditions waves of either acidic or alkaline solution pH could be generated which caused a coincident wave of polymer brush swelling or collapse. These waves originate from one electrode and travel across the brush surface toward the opposite electrode with a velocity of ~40 μm s(-1).