Longitudinal acoustic mode in polymers: Influence of defects

Normal mode calculations have been done on planar zigzag chains Cn (n=22 to 82), terminated or connected by various fold structures and subjected to force and mass perturbations, to determine how the longitudinal acoustic mode (LAM) is influenced by such defects. We find that in general LAM-like displacements are distributed over a frequency range (depending on the stem length), with a total intensity smaller than that for the unperturbed case. Only when the fold comprises ∼3% or less of the mass of the stem does it behave like a point mass perturbation. These results show that the perturbed elastic rod model has limited validity and that caution is necessary in attributing the shape of a LAM band entirely to a distribution of stem lengths.