Determination of the whole set of elastic constants of a polymeric Langmuir-Blodgett film by Brillouin spectroscopy

Abstract

We report Brillouin light scattering experiments on polymeric uniaxial Langmuir-Blodgett films, which have their preferred axis, the c axis, in the plane of the film. Both acoustic surface and bulk modes were observed. In addition to the usual Rayleigh and Sezawa film modes, a longitudinal guided acoustic phonon was identified. Also observed, and verified by computer simulations, was a resonant enhancement of the scattering cross section for some Sezawa modes due to an additional bulk coupling mechanism of the scattered light to the film modes. The complete set of elastic constants as well as the refractive indices were determined from the spectra. For the samples that we had measured, with more than ten molecular monolayers, the elastic constants do not depend on the film thickness. The ratio between the transverse and the longitudinal sound velocities is found to be much smaller than for ordinary solids.