Raman scattering from systems of linear chain molecules (poly(propylene oxide)) of different lengths: a comparison with molecular dynamics simulations

摘要

We present low-frequency Raman scattering results on CH3-terminated poly(propylene oxide) of different molecular weights, corresponding to n = 1, 2, 7 and 70, n being the number of repeat units of the chains. The results are compared with the density of states obtained from molecular dynamics simulations of a similar set of systems (n = 1, 2 and 45). The measurements were performed over a temperature range (10-300K) covering the glass transition temperature. The low-frequency features typical of such systems, namely the boson peak (BP) and the disordered longitudinal acoustic mode (DLAM), were detected. We show that the shape and frequency position of the DLAM change dramatically with change in chain length, in accordance with theoretical models. In contrast, the shape and frequency position of the BP do not exhibit any strong n dependence, while its intensity increases on decreasing the chain length. The former observation suggests that BP reflects the collective dynamics governed by intermolecular interactions within relatively large regions, such that the presence of additional correlations along the chains appears of little consequence. The behaviour of the intensity of the BP can qualitatively be explained by changes in the number of intermolecular degrees of freedom and by changes in the density of states. [References: 16]