Capillary wave dynamics of thin polymer films over submerged nanostructures.

The surface dynamics of thin molten polystyrene films supported by nanoscale periodic silicon line-space gratings were investigated with x-ray photon correlation spectroscopy. Surface dynamics over these nanostructures exhibit high directional anisotropy above certain length scales, as compared to surface dynamics over flat substrates. A cutoff length scale in the dynamics perpendicular to the grooves is observed. This marks a transition from standard over-damped capillary wave behavior to suppressed dynamics due to substrate interactions.

Elastic constants and dimensions of imprinted polymeric nanolines determined from Brillouin light scattering.

Elastic constants and cross-sectional dimensions of imprinted nanolines of poly(methyl methacrylate) (PMMA) on silicon substrates are determined nondestructively from finite-element inversion analysis of dispersion curves of hypersonic acoustic modes of these nanolines measured with Brillouin light scattering. The results for the cross-sectional dimensions, under the simplifying assumption of vertical sides and a semicircular top, are found to be consistent with dimensions determined from critical-dimension small-angle x-ray scattering measurements. The elastic constants C(11) and C(44) are found to be, respectively, 11.6% and 3.1% lower than their corresponding values for bulk PMMA. This result is consistent with the dimensional dependence of the quasi-static Young's modulus determined from buckling measurements on PMMA films with lower molecular weights. This study provides the first evidence of size-dependent effects on hypersonic elastic properties of polymers.

Incoherent neutron scattering and the dynamics of confined polycarbonate films.

Incoherent elastic neutron scattering measurements are performed on thin (75 to 1015 A) polycarbonate films supported on Si wafers. We find that the mean-square atomic displacement is diminished by thin film confinement. For film thicknesses comparable to the unperturbed dimensions of the macromolecule, we observe two characteristic crossover temperatures in as a function of temperature T, one above and the other below the bulk T(g). Furthermore, the harmonic force constant kappa, defined by the low temperature dependence of (i.e., kappa approximately k(B)T/), increases as the film thickness decreases. These observations suggest that the atoms are more strongly localized in the thin supported films.

Correlation of positron annihilation and other dynamic properties in small molecule glass-forming substances.

Positron annihilation spectroscopy is used to characterize the ortho-positronium lifetime, tau(3), in a broad range of small molecule organic glasses over a wide temperature range. The magnitude and thermal variations of tau(3) reflect changes in the dielectric alpha-relaxation time tau(alpha) and its non-Arrhenius or "fragile" characteristics. tau(3) also displays striking similarities with the fast relaxations reflected in the hydrogen-weighted mean square atomic displacements . The parallel temperature variations of , tau(3)(T/T(g)), and tau(alpha)(T/T(g)), and their similar patterns of change between the different glasses, are discussed in terms of anharmonicity in the intermolecular caging potentials and the subnanometer density heterogeneities.