Controlling the Molecular Weight Distribution of Polymer Brushes to Tune Stimulus-Response and Bacterial Adhesion

Bacterial biofilms ubiquitously foul surfaces in technological settings – damaging oil pipelines,1-3 medical implants,4-6 and marine engineering equipment.7-8 Stimuli-responsive polymer brushes provide one route to control adhesion of bacteria to surfaces by tuning bacterial-surface interactions, and prevent long-term bacterial attachment and fouling through brush conformational changes triggered by external stimuli (pH, temperature). Both thickness and molecular-weight distribution in brushes can change the conformation of a stimulus-responsive polymer brush,9-10 suggesting that these parameters may play distinct roles in detachment of bacteria. Surprisingly, how molecular weight distribution (quantified as dispersity) in stimuli-responsive brushes affects the response and fouling-release properties is largely unexplored. First, we developed a method to systematically increase the dispersity of polymers synthesized through a widely-applicable polymerization method, atom transfer radical polymerization (ATRP), by identifying a reagent (phenylhydrazine) capable of reacting with propagating polymer chains. Polymerizations conducted with phenylhydrazine exhibited chain termination and increased dispersity, controlled through the phenylhydrazine concentration. Reaction kinetics in the presence of phenylhydrazine deviated from that observed in typical ATRP syntheses, and a theoretical model was developed that showed excellent agreement with experimental data. Second, we investigated the effects of brush dispersity on the pH-response of poly(acrylic acid) (PAA) brushes. Increase in brush dispersity lead to greater pH-response at low pH, and additionally, variation in contact angle measurements at intermediate pH (hysteretic memory behavior) was observed when the pH was decreased from 10 to 3 and increased to 10 thereafter. We posited that dispersity-driven conformational changes at low pH lead to observed hysteretic behavior. Finally, we studied the effect of PAA brush thickness and dispersity on bacterial attachment at pH 4 and detachment at pH 9. Increasing either thickness or dispersity led to greater bacterial detachment. Bacterial attachment depended non-monotonically on brush thickness, with brushes of thickness between 13 – 18 nm showing lowest attachment, and attachment was independent of brush dispersity. Together, these results identify brush dispersity as a design parameter to tune pH-response with applications in sensors, controlled drug-release, and separation processes. Separate control over bacterial attachment and detachment via the molecular-weight distribution, demonstrated here, opens new options for smart antifouling surfaces.Chemical and Biomolecular Engineering, Department o