Interfacial assembly of star block copolymers at the water-oil interface

We synthesized a series of star block copolymers (s-BCPs) polystyrene-b-poly(2-vinylpyrindine) (PS-b-P2VP), where the PS blocks form the core and P2VP blocks comprise the corona. These s-BCPs were fully dissolved in toluene and the interfacial behavior of droplets of these s-BCPs solutions against water was investigated by pendant drop tensiometry. The P2VP becomes hydrophilic when it come into contact with the aqueous phase and is protonated. This converts the P2VP block into a strongly hydrophilic block, forcing a configurational change of the s-BCPs at the interface. By varying the pH, the degree of protonation can be changed.  The lowest interfacial tension was found near a pH of 3.10, while a spontaneous wrinkling of the interfacial assembly was observed for the pH of 0.65, indicating an oversaturation of the interface with highly protonated s-BCPs. s-BCPs can also be introduced to the interface by dispersing the s-BCPs in the aqueous phase. Behavior like that seen with functionalized nanoparticles was observed. The arm number, molecule weight and volume ratio were varied to investigate the dependence of evolution of the interfacial tension on these parameters and elucidate different kinetic pathways for the assembly.