Low-frequency vibrations in monomers, dimers and polymers of propylene glycol

Abstract

We have studied the low-frequency dynamics of oligomers (n = 1 and n = 2) and polymers (n = 45) of propylene glycol using molecular dynamics (MD) simulations. The polymer structure was built from a reverse Monte Carlo (RMC) simulation of the static structure factors S(Q) obtained for ordinary and deuterated poly(prolylene oxide) from neutron diffraction. The fraction of the different stereo-isomers obtained in this way was checked by performing ¹³C nuclear magnetic resonance studies on the polymers. The RMC polymer structure was used as a starting structure for MD simulations of the polymer, whereas the shorter-chain oligomers were simulated using random starting structures. The vibrational density of states was calculated from the Fourier transform of the velocity autocorrelation function. Our results indicate that the low-frequency peak below 100 cm⁻¹, generally referred to as the boson peak, is to a large extent due to intermolecular degrees of freedom, the peak position and shape being rather insensitive to changes in the intrachain dynamics.