Pentaphenyl trimethyl trisiloxane is an isotropic liquid at room temperature with a dynamic glass transition at 224 K. Using x-ray reflectivity, we see surface density oscillations (layers) develop below 285 K, similar to those seen in other metallic and dielectric liquids and in computer simulations. The layering threshold is $\sim 0.23$ times the liquid-gas critical temperature. Upon cooling further, there is a sharp increase at 224 K in the persistence of the surface layers into the bulk material, i.e., an apparently discontinuous change in static structure at the glass transition.

• Received 14 August 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.175701