PDMS melts on mica studied by confocal Raman scattering

We report surprising surface-induced torsional alignment of polydimethylsiloxane (PDMS) chains in contact with the muscovite (001) mica surface with and without confinement. The alignment was measured by polarized confocal Raman spectroscopy over diffraction-limit circular spots with approximately 0.3 microm diameter. Our discussion here focuses on the intense symmetric methyl-group vibration centered at 2907 cm(-1), whose Raman scattering intensity is found to depend on whether incident light is polarized in the x or y direction of the surface, the x direction being parallel to one of the mica optical axes. Furthermore, the Raman peak broadens significantly relative to that of bulk PDMS while remaining Lorentzian in shape, implying slower but homogeneous vibrational dephasing. However, the preferred orientation differs, apparently stochastically, from spot to spot on the surface. Possible origins of this heterogeneous surface-induced structure are discussed.