Xuesong Hu

Anomalies in the Optical Index of Refraction of Spun Cast Polystyrene Thin Films

We used x-ray reflectivity in combination with optical ellipsometry to measure the optical index of refraction, n, in thin spun cast polystyrene films. We have found that n is independent of the molecular weight, but is a sharp function of the film thickness for films less than 100 nm. In all cases the deviation from the bulk, Δn, is negative and varies linearly with wavelength in the visible region. The magnitude of Δn, was found to be as large as 0.25 for films 7 nm thick. The bulk index of refraction was recovered in all films after annealing for2habove T g at 160 °C. X-ray reflectivity measurements of the scattering critical angle show minimal density deviations from the bulk (less than 0.5%) between the annealed and unannealed films. Consequently the large molecular-weight-independent value of Δn is interpreted as being due to a radially symmetric segmental orientation induced by the spinning process.

Synchrotron x-ray scattering studies of the surface structure and dynamics of liquids and liquid films.

The dynamics of surface fluctuations in thin supported polystyrene films have been investigated using x-ray photon correlation spectroscopy (XPCS) in reflection geometry. The results from the films thicker than four times of the radius of gyration (Rg) of polystyrene show the behavior of the capillary waves expected in viscous liquid. However, thinner films show a deviation indicating the need to account for viscoelasticity. Theoretical considerations with viscoelastic liquid model has been performed by introducing frequency dependent viscosity and compared with Fredricksons brush model (Macromolecules, 25, 2882 (1992)). The theory has been extended to the surface and interfacial modes in a bilayer film system. The results will be discussed in terms of surface tension, viscosity, and shear modulus.

Dynamics of Buried Polymer -polymer Interfaces in Thin Films

Submitted for the MAR07 Meeting of The American Physical Society Dynamics from Buried Polymer-Polymer Interfaces in Thin Films JYOTSANA LAL, XUESONG HU, Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL-60439, USA., ZHANG JIANG, SUNIL K. SINHA, Department of Physics, University of California, San Diego, La Jolla, CA-92093, USA., SURESH NARAYANAN, ALEC R. SANDY, Advanced Photon Source, Argonne National Laboratory, Argonne, IL-60439, USA., XUESONG JIAO, LAURENCE B. LURIO, Department of Physics, Northern Illinois University, DeKalb, IL-60115, USA. — We report a further development of X-ray photon correlation spectrocopy (XPCS) in order to probe capillary wave dynamics at a buried polymer interface of a bilayer. The bi-layer was chosen so that the critical angle for total external reflection for the top layer is smaller than that for the bottom layer. When x-rays are incident below the critical angle of the top layer only the structure and dynamics of the top layer are probed. When x-rays are incident above the critical angle of the top layer but below that of the bottom layer, a standing wave is set up. The phase of this standing wave can be adjusted to have a maxima at the polymer-polymer interface and simultaneously a node at the polymer-air interface. Consequently, one can isolate the static scattering and XPCS from the buried layer. Results on a system consisting of a 100 nm polystyrene (PS) film on top of an 100 nm polybromostyrene (PBrS) film, supported on a Si substrate will be reported. The dynamics are consistent with a low-viscosity mixed layer between the PS and PBrS. Jyotsana Lal Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, IL-60439, USA. Date submitted: 25 Sep 2006 Electronic form version 1.4