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

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 ina wide dynamical window, corresponding to 10(-3)-10(14) 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 spec trometer techniques were used. We observe four distinguishable kinds of dynamics; (i) slow normal-mode dynamics, (ii) the main (alpha) relaxation which is related to the viscosity, (iii) a faster (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 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. [S0163-1829(97)02042-0].