Pressure and density dependence of the boson peak in polymers

The nature of the low-frequency vibrations, the so-called boson peak, in spectra of glass-forming systems remains a subject of active discussions. It appears that densification of glasses leads to significant change of the boson peak vibrations and opens additional possibility to verify different model predictions. We present light (Raman and Brillouin) scattering studies of the influence of pressure (up to 1.5 GPa) on the boson peak vibrations and elastic properties of five different polymers. We demonstrate that the pressure-induced shift of the boson peak frequency in all cases is significantly stronger than change of sound velocities. This result clearly shows the failure of the homogeneous elastic continuum approximation. The boson peak amplitude decreases strongly with pressure. However, the major part of these variations (but not all) can be related to the change of the Debye level. We emphasize a correlation between pressure-induced variations of the boson peak frequency and intensity. Surprisingly, the spectral shape of the boson peak remains the same at all pressures indicating that the frequency distribution of the vibrational modes remains essentially unaltered even when the boson peak frequency doubles. The results are compared to predictions of different models and results of recent computer simulations.