Imaging adsorption, assembly, and morphology of homopolymers, nanoparticles, and their mixtures at liquid interfaces

Adsorption, assembly, and dynamics of nanoparticles (NPs) at liquid interfaces will be key to many new technologies. Scanning electron microscopy (SEM) has enough resolution to image and track individual NPs at air/liquid interfaces in real-time, and a new SEM variation, variable pressure SEM, extends these capabilities to low volatility liquids. This approach can address many fundamental questions about interfacial NP behaviors. NPs typically are decorated with polymeric ligands of comparable size, and ligands have profound impacts on stability, structure, and dynamics. In this study, ligands of different polymer chemistry were attached to silica NPs and subsequently adsorbed and/or spread on glycerol. Unexpected 2D phase separation behaviors were observed, including formation of lily pads, one-particle thick circular islands of micron-sized diameter. The same polymers, introduced without NPs, also form lily pads, now with thicknesses approaching the polymer radius of gyration. NP-polymer lily pad composites can equally be constructed. Numerous parameters that affect the lily pad morphology have been probed, including polymer-liquid solubility, polymer Tg, cosolvent type and evaporation rate, liquid viscosity, and post-assembly surface modification.