  Synchrotron X-ray reflectivity (XRR) confirms the formation of a
quasi-immobilized layer in thin films of polydimethylsiloxane (PDMS) melts near
silica surfaces. This layer (40-60A) has a lower density than the bulk value,
and its thickness varies slightly with PDMS molecular weight. Formation of this
layer is very rapid for PDMS melts with low molecular weights (below
entanglement limit for these molecules) but takes 5-10 hours for higher
molecular weights (close to and above their entanglement value).
