Anomalous Confinement Slows Surface Fluctuations of Star Polymer Melt Films.

The unusually large film thickness at which confinement effects manifest themselves in surface fluctuations of unentangled four-arm star polymers has been defined using film thicknesses from 10 to 107. For 15k four-arm star polystyrene (SPS), confinement appears at a thickness between 112 nm (40) and 72 nm (26), which is remarkably larger than the thicknesses at which confinement appears for unentangled 6k linear (<15 nm, <7) and 6k and 14k cyclic (24 and 22 nm, respectively) polystyrenes. Data for 15k star films can be rationalized using a two-layer model with a 17 nm (6) thick highly viscous layer at the substrate, which is significantly thicker than the 1 thick "irreversibly adsorbed" layer. For a 29 nm (10) thick film, more striking confinement occurs due to the combined influence of both interfaces. These results underscore the extraordinary role long-chain branching plays in dictating surface fluctuations of thin films.