ABSORPTION AND VELOCITY DISPERSION OF ACOUSTIC WAVES IN POLYBUTYLACRYLATE FILMS

Relaxation processes in polybutylacrilate (PBA) films have been studied by means of Brillouin light scattering and ultracoustic techniques over a range of temperatures covering from 170K to 460K. Brillouin scattering (BS) measurements have been taken from PBA films spinned on silicon substrates 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, characteristic of supported films, have been revealed. Ultrasonic measurements were done at 18.1 MHz on shear horizontal waves propagating in PBA films deposited on a ST-cut Quartz plate. BS and ultracoustic data give evidence for a considerable velocity dispersion associated with a maximum of acoustic attenuation in the temperature range analyzed. These results have been interpreted in terms of an a-relaxation process characterized by a Kohlraush-Williams-Watt relaxation function having a constant stretching exponential &=0.45, and by a non- Arrhenius temperature dependence of the relaxation time.