Mechanical Properties of Freely-Standing Polymer Films Manuscript

BRILLOUIN LIGHT SCATTERING STUDIES OF THE MECHANICAL PROPERTIES OF THIN FREELY-STANDING POLYSTYRENE FILMS

James Forrest, Kari Dalnoki-Veress, and John Dutcher,
Phys. Rev. E 58, 6109-6114 (1998).

ABSTRACT

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 that are both less than and greater than the average end-to-end distance of the unperturbed molecules R_EE. We find that the measured, room-temperature sound velocities are the same, to within +/- 1%, for all film thicknesses which have glass transition temperatures wihch 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 the thermal expansion of thin, freely-standing PS films in the glassy state are consistent with bulk values for all film thicknesses.