Propagating to nonpropagating vibrational modes in amorphous polycarbonate

Comparisons are made between the low-frequency Raman and inelastic neutron-scattering spectra obtained from amorphous bisphenol A polycarbonate at low temperature. The vibrational density of states and light-vibration coupling coefficient are determined. The frequency dependences of these parameters are explained by propagating vibration modes up to an energy of about 1 meV, and fractonlike modes in more cohesive domains at higher energies. The vibrational dynamics are strongly dependent on disorder in the glass: this disorder comes from fluctuations in bond strengths or elasticity constants, and not from changes in density.