Phase Behavior and Mechanics of Bridging Gels with pH-Tunable Attractions

We study the phase behavior and mechanics of a colloid-polymer model system with bridging attractions, in which the strength of the polymer adsorption can be tuned through the pH of the system. We induced bridging interactions between trifluoromethyl methacrylate-co-tert-butyl methacrylate (TtMA) particles by adding poly(acrylic acid) (PAA). A bridging attraction at low pH is likely driven by hydrogen bonding between PAA and the stabilizers on the surface of the particles; these bonds weaken as the pH is increased. We find that at both low (φ = 0.15) and high (φ = 0.40) particle volume fractions, suspensions of TtMA particles and PAA polymers undergo a transition from fluid to gel as pH is decreased or polymer concentration is increased, and show preliminary data on suspension structure, dynamics, and mechanics. We expect that this model system may be useful for investigating mechanisms of gelation that are not driven by frustrated phase separation, and for understanding flocculation processes in practical settings such as wastewater treatment.