Lattice dynamics of polyaniline and poly(p-pyridyl vinyline) : first-principles determination.

Abstract

First-principles density functional studies of the dynamical properties of the conjugated polymers polyaniline and poly(p-pyridyl vinyline) are presented in this work. We have employed linear response within density functional perturbation theory, as implemented in the CASTEP code, to investigate the Born effective charges, polarizabilities, and vibrational properties. With regard to the last, we have calculated the vibrational frequencies and made assignments of the modes for the two polymers. Most of the phonon modes have been classified and we have shown that the higher frequency modes are associated with C - H and C = N stretching modes. We also present the results of calculations of the polarizability and permittivity of the materials which are in reasonable agreement with the typical values of conjugated polymers. Dynamical Born effective charges have been calculated and compared with the Mulliken population atomic charges. It is found that notable differences exist between the Born effective charges for the nitrogen atoms in the conducting polymers, and we conclude that effective charges are more appropriate for use in the study of the dynamics of the systems. Differences are found in the ir absorption spectra obtained for the two polymers, which can be attributed to the structural differences of the two materials. It is found that the presence of the nitrogen atom plays an important role in determining their lattice dynamics.