Dispersity As a Design Parameter for Tuning Conformation and Response of Polymer Brushes

Polyelectrolyte brushes attached to surfaces change their conformation when exposed to solutions of varying pH. As responsive materials, polyelectrolyte brushes are widely employed to control surface wettability, adhesion, friction, and biofouling. Many of these applications require reversible pH-cycling of the surface properties, and therefore it is essential to understand brush response to variations in pH. Despite widespread use of brushes across a broad range of applications, the role of polyelectrolyte molar mass dispersity and brush length in the observed pH-response is not well understood. Here, we synthesized poly(acrylic acid) (PAA) brushes with controlled brush length and dispersity, grafted to both planar and spherical surfaces at constant grafting density. We investigated the role of dispersity in hysteretic memory behavior in flat PAA brushes, in which the response was dependent on the direction of pH change. Disperse brushes exhibited hysteretic memory behavior, and their water contact angle measured at low pH increased with increasing dispersity. Furthermore, we examined the effect of brush parameters, thickness and molar mass dispersity, on the attachment and detachment of bacteria to PAA brushes. Bacterial adhesion was minimized within an optimal range of brush thicknesses, and was unaffected by brush dispersity. By contrast, increases in both brush length and dispersity mediated the detachment of bacteria as the pH was increased from 4 (at which PAA is collapsed) to 9 (at which PAA is extended). In spherical brushes of low weight-average degree of polymerization (N_w), brush length of low-dispersity brushes was weakly dependent on pH, whereas that of high-dispersity brushes was greater, and drastically increased with pH. At a higher N_w, however, brush length of PAA brushes showed similar pH-response regardless of dispersity. We hypothesize that the drastic differences in brush length are related to differences in conformation among low and high dispersity brushes.