Lattice dynamics of pristine and doped phases of conjugated polymers

Peter Papanek, University of Pennsylvania

Abstract

Conjugated polymers are a new class of electronic materials that show a significant increase of electronic conductivity upon doping with various electron-donating or electron-accepting species. Vibrational spectroscopic techniques such as infrared and Raman spectroscopy have been widely employed in the characterization of these systems, yet little was known about the lattice and molecular vibrations in the low-frequency region not accessible to the optical methods. Inelastic neutron scattering is well suited for the study of these vibrational modes. In this work, we present the results of neutron scattering experiments on pristine cis- and trans-polyacetylene (CH)$\sb{x},$ sodium- and potassium-doped polyacetylene, poly(p-phenylene vinylene) (PPV) and polyaniline. By using stretch-oriented samples, we were able to identify the polarization of observed vibrational modes with respect to polymer axes. The experiments were performed using the time-of-flight and the filter-analyzer spectrometers. Pronounced differences were found between the vibrational spectra of cis and trans-polyacetylene due to different molecular geometries of both isomers and the tighter packing of chains in the trans-(CH)$\sb{x}$ crystal. This explanation is supported by single-chain vibrational analyses based on force constants calculated by the semiempirical AM1 method and by molecular-dynamics simulations. The latter also confirmed the anisotropic expansion of lattice parameters of the trans-(CH)$\sb{x}$ crystal with increasing temperature. The observed changes in the density of modes of (CH)$\sb{x}$ induced by alkali-metal doping suggest increasingly Coulombic character of interchain interactions, and the formation of a charged soliton-antisoliton lattice. The persistence of the soliton lattice into the metallic phase contradicts the simple-metal theories that predict undimerized chains. The spectra of PPV demonstrate the complex dynamics of the phenyl rings. The temperature dependent features were explained by the anharmonic character of the ring-torsional potential. Similar effects could not be identified in polyaniline due to low instrumental resolution and poor sample quality.

Subject Area

Materials science|Condensation|Chemistry

Recommended Citation

Papanek, Peter, "Lattice dynamics of pristine and doped phases of conjugated polymers" (1996). Dissertations available from ProQuest. AAI9627981.
https://repository.upenn.edu/dissertations/AAI9627981

Share

COinS