Self-aggregation of triblock copolymers at the solid silica-water interface

We report on the adsorption and surface aggregation behavior of triblock copoly(ethylene oxide-tetrahydrofuran-ethylene oxide) copolymers, EOn/2THFmEOn/2, at the silica-water interface. In this work, we have used both time-resolved fluorescence spectroscopy and atomic force microscopy to study the morphology of the adsorbed layers. We confirm previous suggestions that the copolymers self-assemble at the silica surface to form discrete micelle-like structures. The surface aggregation, however, starts at a much lower copolymer concentration than the corresponding micellization process in solution. The time-resolved fluorescence measurements reveal that the copolymer aggregates are smaller at the surface than those in bulk solution. The AFM measurements that were performed clearly confirm the existence of discrete micelle-like copolymer aggregates at the silica surface. An interesting result seen in the AFM surface topography images is that, for the high molecular weight copolymer, we observe no surface aggregates at some parts of surface. We believe this is related to the polydispersity of the copolymer. Single high molecular weight copolymers may effectively compete for surface area with surface aggregates formed by a copolymer fraction of lower molecular weight.