Interfacial Assembly and Jamming Behavior of Polymeric Janus Particles at Liquid Interfaces

The self-assembly and interfacial jamming of spherical Janus nanoparticles (JNPs) at the water/oil interface were investigated. Polymeric JNPs, made by cross-linking polystyrene-block-polybutadiene-block-poly(methyl methacrylate) (PS-PB-PMMA), with a high interfacial activity assemble at the water/oil interface. During the self-assembly at the interface, the interfacial energy was reduced and a dynamic interlayer was observed that is responsive to the pH of the aqueous phase. Unlike hard particles, the JNPs are composed of polymer chains that can spread at the liquid-liquid interface to maximize coverage at relatively low areal densities. In a pendant drop geometry, the interfacial area of a water droplet in oil was significantly decreased and the JNPs were forced to pack more closely. Entangling of the polymer chains causes the JNPs to form a solid-like interfacial assembly, resulting in the formation of wrinkles when the interfacial area is decreased. The wrinkling behavior, the retention of the wrinkles, or the slow relaxation of the liquid drop back to its original equilibrium shape was found to depend upon the pH.