Direct visualization of the real time swelling and collapse of a poly(methacrylic acid) brush using atomic force microscopy

The reversible height change dynamics of a poly(methacrylic acid) brush, a weak polyelectrolyte in response to pH changes, were observed using atomic force microscopy. The brush thickness could be repeatably and reversibly switched between 40 nm at pH 3 and 120 nm at pH 10.5. The swelling and collapse transitions took 6 s in the AFM measurements, but high resolution force spectroscopy measurements showed that the collapse can be made to happen in less than a second. We conclude that the response time of these switches is limited in practise by the physical time taken to exchange solvents and by the intrinsic slow dynamics in the brush.