Correlations of the boson peak with positron annihilation in series of polycarbonate copolymers

The low-frequency vibrations of the Boson peak are observed with inelastic neutron and Raman scattering in a homologous series of polycarbonate copoly mers. In accordance with the notion that the Boson peak is predominantly an acoustic phenomenon, the Boson peak energy is found to correlate with the velocity of sound. However, a striking inverse correlation is observed betw een the Boson peak energy and the average size of the unoccupied volume ele ments (approximately 5-6 Angstrom in diameter) evidenced by positron annihi lation lifetime spectroscopy (PALS). The apparent correlation between the B oson peak energy and PALS data is discussed in terms of a relationship betw een the elastic modulus and the electron density heterogeneities.