Brillouin light scattering studies of the mechanical properties of thin freely standing polystyrene films

We have used Brillouin light scattering (BLS) to measure the room-temperature, high-frequency mechanical properties of thin freely standing polystyrene (PS) films. We have investigated the effects of chain confinement and the free surface on the mechanical properties by measuring the velocity of film-guided acoustic phonons in films with thicknesses ranging from less than to greater than the average end-to-end distance of the unperturbed molecules $R_{\mathrm{EE}}.$ We find that the measured, room-temperature sound velocities are the same, to within $\pm 1\%,$ for all films that have glass transition temperatures that differ by as much as 65 K. Our results, which differ markedly from those of recent picosecond acoustic measurements, are discussed in terms of models proposed to explain anomalous glass transition behavior in thin polymer films. A careful analysis of the BLS data reveals that, at room temperature, the mechanical stiffness, mass density, and thermal expansion of thin, freely standing PS films in the glassy state are consistent with bulk values for all film thicknesses.