Tuning the pH sensitivity of poly(methacrylic acid) brushes

The pH-induced swelling and collapse of surface-tethered, weak polyelectrolyte brushes is of interest for the development of actuators or to allow pH controlled transport or adsorption. This contribution discusses results of an extensive series of quartz crystal microbalance (QCM) experiments that aimed at (i) further understanding the influence of brush thickness and density on the pH responsiveness of poly(methacrylic acid) (PMAA) brushes and (ii) developing strategies that allow one to engineer the pH responsiveness and dynamic response range of PMAA based brushes. It was observed that, due to their high grafting density, the apparent pK(a) of surface-tethered PMAA differs from that of the corresponding free polymer in solution and also covers a broader pH range. The pK(a) of the PMAA brushes was found to depend on both brush thickness and density; thicker brushes showed a higher pK(a) value, and brushes of higher density started to swell at higher pH. The second part of the paper demonstrates the feasibility of the N-hydroxysuccinimide-mediated post-polymerization modification to engineer the pH responsiveness of the PMAA brushes. By using appropriate amine functionalized acids, it was possible to tune both the pH of maximum response as well as the dynamic response range of these PMAA based polyelectrolyte brushes.