The structural dynamics of poly(propylene-glycol) of molecular weight MW=4000 have been investigated over a large temperature range 10--375 K and in a wide dynamical window, corresponding to 10^-3--10¹⁴Hz, 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 (α) relaxation which is related to the viscosity, (iii) a faster ({β}) 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 Tg are found to be in good agreement with predictions of MCT and a Tc=236K can be extracted. In contrast, the high-frequency data taken above Tc 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.
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Bergman et al. (1997) studied this question.
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