Inelastic-neutron-scattering studies of poly(p-phenylene vinylene)

Mode polarization and energy spectra for the lattice modes of poly(p-phenylene vinylene) are obtained by inelastic-incoherent-neutron-scattering methods. Experiments were performed on stretch-oriented, highly crystalline films with time-of-flight and filter-analyzer spectrometers. By measuring all-hydrogen and vinylene-deuterated samples, we show that almost all of the observed modes can be attributed to phenylene motions. The low-energy features in the vibrational density of states consist of a longitudinal 2.5-meV mode assigned to a damped translational motion along the chain axis, a 7-meV phenyl ring in-phase librational mode (transverse with respect to the chain axis), and strong bands at 15 and 25 meV showing mixed polarization, the latter also displaying negative frequency shift with increasing temperature. Higher-frequency modes are in excellent agreement with published infrared and Raman studies, as well as with the results of our vibrational analysis based on the semiempirical AM1 method.