Dynamics around the liquid-glass transition in poly(propylene-glycol) investigated by wide-frequency-range light-scattering techniques

Abstract

The structural dynamics of poly(propylene-glycol) of molecular weight ${M}_{W}=4000$ have been investigated over a large temperature range 10--375 K and in a wide dynamical window, corresponding to ${10}^{\ensuremath{-}3}--{10}^{14}\mathrm{Hz},$ using various light-scattering techniques. The slow dynamics were investigated using a wide time-range photon-correlation spectroscopy; for the faster dynamics a combination of interferometric and grating spectrometer techniques were used. We observe four distinguishable kinds of dynamics; (i) slow normal-mode dynamics, (ii) the main $(\ensuremath{\alpha})$ relaxation which is related to the viscosity, (iii) a faster (\ensuremath{\beta}) relaxation, and (iv) a low-frequency vibrational peak. The data are discussed in relation to the mode-coupling theory (MCT) for the liquid-glass transition. Surprisingly, the slow dynamics observed using the PCS technique close to the glass transition temperature ${T}_{g}$ are found to be in good agreement with predictions of MCT and a ${T}_{c}=236\mathrm{K}$ can be extracted. In contrast, the high-frequency data taken above ${T}_{c}$ are not consistent with MCT. In this range a strong vibrational peak, the so-called boson peak, seriously affects the relaxational spectrum and simple MCT analysis cannot be applied. This finding is in agreement with recent light- and neutron-scattering investigations of other hydrogen-bonded intermediate glass formers and also strong covalently bonded systems.