Robust Pickering emulsions stabilized with pH-responsive self-assembled amphiphilic tadpole bottlebrush copolymers:...

The newly synthesized amphiphilic tadpole bottlebrush copolymer can exist in solution as unimers (disassembled polymer chains) or micelles depending on pH. Owing to this specific structure, these assemblies are at thermodynamic equilibrium, allowing for dynamic transitions between unimer and aggregated states. We hypothesize that the same copolymer can stabilize emulsions with distinct properties at varying pH, whether in its unimer or micellar state (Pickering emulsions).

We characterized the copolymer behavior in aqueous solutions across pH range using dynamic light scattering (DLS), contact angle measurements, and dynamic tensiometry. Dodecane-in-water emulsions were prepared using the copolymer at various pH values. Emulsion characteristics were studied using optical microscopy and laser granulometry, complemented by visual observations to assess stability over time. The adsorption of polymer micelles at the emulsion droplet surface was investigated using transmission electron microscopy (TEM) of freeze-fractured samples.

Above pH 5-6, the copolymer acts as a macromolecular surfactant, resulting in emulsions with short-term stability. At lower pH (pH <5-6), when the copolymer self-assembles into micelles, very stable emulsions are obtained, exhibiting long-term stability (> 2 years) even at low copolymer concentrations (as low as 0.001 wt% with respect to total sample weight). Drop size is tunable with the copolymer concentration. TEM analysis of freeze-fractured emulsions reveals micelle adsorption at the droplet surface at low pH, highlighting their efficiency as Pickering emulsion stabilizers. Despite the copolymer reversible assembly in solution, no emulsion breakup occurs when pH increases to reach the unimer state domain. This unexpected behavior suggests that adsorbed copolymer micelles lose pH-sensitivity at the water-dodecane interface, demonstrating a unique system where interfacial behavior differs from solution behavior.