College Park, Maryland June 6 - 10 , 2004
T3-B2 (4:15 PM): Dynamics in Thin Polymer Films
C. L. Soles, J. F. Douglas, W.-L. Wu (NIST Polymer Division)
Incoherent neutron scattering is presented as a powerful tool for interpreting changes in the molecular dynamics as a function of film thickness for a range of polymers. Motions on a time scale of approximately a nanosecond and faster are quantified in terms of a mean-square atomic displacement (MSD) from the Debye-Waller factor. We find that thin film confinement generally leads to a reduction of the MSD in comparison to the bulk material, this effect becoming especially pronounced when the film thickness approaches the unperturbed dimensions of the macromolecule. Generally we always observe a suppression (never an enhancement) of the MSD at temperatures T above the bulk calorimetric glass transition temperature, Tg. Below Tg, the reduction in the magnitude of the MSD depends upon the polymer and the length scales being probed. Polymers with extensive segmental or local mobility in the glass are particularly susceptible to reductions of the MSD with confinement, especially at the Q vectors probing longer length scales, while materials lacking these sub-Tg motions are relatively less sensitive. We also demonstrate that a reduced MSD correlates with a reduced mobility at long time and spatial scales, as measured by diffusion in the thin polymer films. Finally, we find that this reduced thin film mobility is not reliably predicted by thermodynamics assessments of an apparent Tg, as measured by discontinuities or kinks in the T dependence of the thermal expansion, specific volume, index of refraction, specific heat, etc. This is illustrated through series polycarbonate films simultaneously characterized with specular x-ray reflectivity and beam positron annihilation lifetime spectroscopy. In total, these measurements illustrate that the MSD is a powerful and predictive tool for understanding dynamic changes in thin polymer films.
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