Incoherent neutron scattering and the dynamics of confined polycarbonate films

Incoherent elastic neutron scattering measurements are performed on thin (75 to 1015 Angstrom) polycarbonate films supported on Si wafers. We find that the mean-square atomic displacement [u(2)] is diminished by thin film confinement. For film thicknesses comparable to the unperturbed dimensions of the macromolecule, we observe two characteristic crossover temperatures in (u(2)) as a function of temperature T, one above and the other below the bulk T-g. Furthermore, the harmonic force constant K, defined by the low temperature dependence of (u(2)) (i.e., kappa approximate to k(B)T/[u(2)]), increases as the film thickness decreases. These observations suggest that the atoms are more strongly localized in the thin supported films.