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 using the grafting-from approach at a constant grafting density. We developed a modified atom transfer radical polymerization method for dispersity control in brush synthesis. We then investigated the role of dispersity in hysteretic memory behavior in 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).

Dispersity enhanced responsive properties in polyelectrolyte brushes

ACS Fall 2021

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