Shear moduli of polystyrene thin films determined with quartz crystal resonators in the sandwich configuration

We report on the determination of the shear compliance of spin-cast polysty rene films with quartz crystal resonators. The films with a thickness betwe en 10 and 93 nm were sandwiched between the quartz blank and an overlayer o f aluminum, which had been evaporated on top of the polymer film in order t o enhance the shear stress. The shear compliance was inferred from the freq uency shifts on the various overtones. For ease of analysis, the film was p laced directly onto the quartz blank with no electrodes underneath, the osc illation being excited externally across an air gap. The derived shear modu li are consistent with the literature values for the bulk material determin ed from ultrasonic spectroscopy. However, they depend on thickness. For fil ms thicker than 10 nm the apparent compliance decreases with film thickness , which presumably is a consequence of surface roughness. The 10 nm specime n had a much increased compliance, which we attribute to a less dense packi ng of the polymer chains caused by geometric confinement. (C) 2000 American Institute of Physics. [S0021-8979(00)01509-7].