LOW-FREQUENCY VIBRATIONS IN MONOMERS, DIMERS AND POLYMERS OF PROPYLENE-GLYCOL

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 Mont e Carlo (RMC) simulation of the static structure factors S(Q) obtained for ordinary and deuterated poly(prolylene oxide) from neutron diffra ction. The fraction of the different stereo-isomers obtained in this w ay was checked by performing C-13 nuclear magnetic resonance studies o n the polymers. The RMC polymer structure was used as a starting struc ture for MD simulations of the polymer, whereas the shorter-chain olig omers were simulated using random starting structures. The vibrational density of states was calculated from the Fourier transform of the ve locity autocorrelation function. Our results indicate that the low-fre quency peak below 100 cm(-1), generally referred to as the boson peak, is to a large extent due to intermolecular degrees of freedom, the pe ak position and shape being rather insensitive to changes in the intra chain dynamics.