Absorption and velocity dispersion of acoustic waves in polybutylacrylate films Brillouin scattering

Relaxation processes in polybutylacrylate (PBA) films were studied by means of Brillouin light scattering and ultra-acoustic techniques over a range of temperatures from 170 K to 460 K. Brillouin scattering (BS) measurements were taken from PBA films spun on silicon substrates in order to detect thermally activated acoustic phonons in the range of frequencies between 1 and 20 GHz. Both longitudinal bulk phonons and the longitudinal guided mode are revealed. Ultrasonic measurements are done at 18.1 MHz on shear horizontal waves propagating in PBA films deposited on a ST-cut quartz plate. BS and ultra-acoustic data give evidence for a considerable velocity dispersion associated with a maximum of acoustic attenuation in the temperature range analyzed. These results are interpreted in terms of an α-relaxation process characterized by a Kohlraush-Williams-Watt relaxation function and by a non-Arrhenius temperature dependence of the relaxation time