Molecular ordering in thin liquid films of polydimethylsiloxanes

X-ray reflectivity has been used for investigations of molecular ordering i n thin liquid films of polydimethylsiloxanes (PDMS) of low molecular weight s deposited on a Relished silicon wafer. The liquid films we studied were s imilar to 40-90 Angstrom thick. Evidence of molecular layering induced by g eometrical confinement by a hard wall is obtained for thin films of lowest molecular weight PDMS. The positions of the secondary maxima in the Patters on functions, P(z), for these samples reveal a periodicity of about 10 Angs trom, consistent with the size of PDMS molecules. Further increasing the mo lecular weight leads to suppression of P(z) oscillations, causing the elect ron density profile to become more uniform. For higher molecular weight PDM S, a flatlike conformation of molecules absorbed on a solid surface is obse rved.