RESUMEN
In previous papers, we found that the mean square displacement [u(2)] in polystyrene thin films decreased with film thickness and discussed two possible causes for the decrease. One is the hard interface layer between the polymer thin film and the substrate, and the other is the spatial confinement effect of polymer chains. In order to disclose the cause of the decrease of [u(2)] with film thickness, we studied the molecular weight M(w) dependence of [u(2)] in polystyrene thin films. We found that [u(2)] was independent of M(w) within experimental error, concluding that the interface effect is a main reason for the decrease of [u(2)] with film thickness.
Asunto(s)
Biofisica/métodos , Neutrones , Polímeros/química , Membranas Artificiales , Modelos Estadísticos , Peso Molecular , Poliestirenos/química , Dispersión de Radiación , Propiedades de Superficie , TemperaturaRESUMEN
We studied the low-energy excitations in the meV region as well as the picosecond fast process in polystyrene thin supported films using inelastic and quasielastic neutron scattering in a temperature range from 11 to 430 K , covering the bulk glass transition temperature Tg. It was found that the mean square displacement u2 decreased with the film thickness below and above the glass transition temperature Tg, suggesting that the hardening occurs with decreasing the film thickness. Corresponding to the decrease in u2, it was also found that the density of phonon states G(omega) decreased with film thickness. This decrease occurs mainly in the Debye mode while the contribution of the boson peak mode also decreases with the film thickness without changing the boson peak energy. As for the fast process, which appeared above at about 150 K as a quasielastic scattering, the fraction A(fast)(Q) decreased with the film thickness in a similar manner with the boson peak without changing the relaxation rate, suggesting a common origin for the boson peak and the fast process. These observations showing the hardening were well explained by assuming a hard dense layer at the interface, where the numbers of the boson peak mode as well as the fast process are less than the bulk state.
RESUMEN
We report inelastic neutron scattering measurements on polystyrene thin films in a glassy state in the meV region. We found in elastic scattering that the mean square displacement