Interfacial Phase Separation of Polymers and Polymer-Grafted Nanoparticles: "Lily Pads" at Liquid Interfaces

Controlled assembly of polymers and polymer-grafted nanoparticles (NPs) at liquid interfaces could open the door to numerous new thin film technologies. Visualization of liquid interfacial assembly processes has been vastly expanded by variable-pressure scanning electron microscopy (VPSEM), which extends real-time, in situ SEM imaging to non-conductive, finite volatility systems. For this study, ligands of varied polymer chemistry [i.e., poly(HEMA), poly(N-VP), and poly(tBuMA)] were grafted to ~150-nm silica NPs, which were imaged during their subsequent adsorption and assembly at air/glycerol or air/ionic liquid interfaces. Unexpected two-dimensional phase separation was observed for most systems (all that were not crystallizable), generating circular “lily pads”, i.e., one particle-thick islands of several micron-diameter. The free (non-grafted) polymer counterparts of these NPs provided similar lily pad morphologies, with thickness now on the order of polymer radius of gyration, thereby increasing the aspect ratio to more than 1000. Novel transfer and staining procedures were developed to characterize lily pad shape and thickness by TEM. Time-dependent imaging by VPSEM and fluorescence microscopy gave insights into growth mechanisms, which span minutes to hours.