Polarized vibrational density of states of conducting polymers from neutron scattering.

Summary form only given. We establish the feasibility of using incoherent inelastic neutron scattering from stretch-oriented conjugated polymers films to obtain both polarization and energy information about the low-frequency modes: interchain phonons and librons, intrachain torsional motions, ring-librations. The polarized densities of states of low-frequency modes of polyacetylene [cis- and trans-(CH)/sub x/,, and Na-doped (CH)/sub x/] have been obtained. To assign the different features of the density of states and the changes upon doping, the neutron investigation is combined with molecular dynamics simulations. For instance, in polyacetylene, the out-of-phase translation of the two chains of the primitive cell (longitudinal optical phonon mode) has been identified at 12 cm/sup -1/ in cis-(CH)/sub x/, and 32 cm/sup -1/ in trans (CH)/sub x/, and the dorninent features of the density of states, at 120 cm/sup -1/ in cis-(CH)/sub x/ and 170 cm/sup -1/l in trans-(CH)/sub x/, are assigned to intramolecular torsional motions (transverse modes). Upon doping, marked changes in the density of states are observed, a broad minimum appearing at 130 cm/sup -1/, in the spectral region dominated by intramolecular torsional displacements. Incoherent neutron scattering experiments in polyanifine and poly(p-phenylenevinylene) provide, for the first time, important insight into the dynamics of phenyl-ring librations. These modes are identified and their temperature dependencies are studied.