Molecular dynamics simulation for investigation of Boson peak in a polymer system

We performed a molecular dynamics simulation to investigate Boson peak (BP) in a united atom polyethylene system. We computed vibrational density of states (DOS), dynamic structure factor (DSF) and wave-vector-dependent dynamic susceptibility (WDS) in a wide range of the angular frequency ω and the wave vector q. The BP was successfully observed in the profiles of DOS and DSF at the temperatures near and below the glass transition point. We determined the peak position of the BP (ω_BP/2π=0.25 THz) from the fitting results of DOS; the peak position does not change with changing temperature. The BP was also recognized as an isolated peak on the 3D graph of DSF; the peak position along the q axis (q_BP) on the line of ω=ω_BP shifts toward high q with decreasing temperature T, where q_BP~T^-0.5. Using a simple harmonic oscillation model, we estimated the number of monomers n_BP giving rise to the BP; n_BP=3.2. In the 3D graph of the real part of WDS, 2 distinct steps intersect on the line of ω=ω_BP; and in that of the imaginary part, a ridge laying diagonally from high q and ω region to low q and ω region terminates on the same line. From these results, we conclude that the BP in this simulation originates from the localized oscillation of the monomer groups consisting of 3.2 monomers.