Evolution of the dynamic susceptibility in molecular glass formers: Results from light scattering, dielectric spectroscopy, and NMR

Abstract

Although broadly studied, molecular glass formers are not well investigated above their melting point. Correlation times down to 10(-12) s are easily accessible when studying low-T(g) systems by depolarized light scattering, employing a tandem-Fabry-Perot interferometer and a double monochromator. When combining these techniques with state-of-the-art photon correlation spectroscopy (PCS), broad band susceptibility spectra become accessible which can compete with those of dielectric spectroscopy (DS). Comparing the results with those from DS, optical Kerr effect, and NMR, we describe the evolution of the susceptibilities starting from the boiling point T(b) down to T(g), i.e., from simple liquid to glassy dynamics. Special attention is given to the emergence of the excess wing contribution which is also probed by PCS and which signals a crossover of the spectral evolution. The process is attributed to a small-angle precursor process of the α-relaxation, and the apparent probe dependent stretching of the α-process is explained by a probe dependent contribution of the excess wing. Upon cooling, its emergence is linked to a strong decrease of the strength of the fast dynamics which is taken as reorientational analog of the anomaly of the Debye-Waller factor. Many glass formers show in addition a slow β-process which manifests itself rather universally in NMR, in DS, however, with different amplitudes, but not at all in PCS experiments. Finally, a three-parameter function is discussed interpolating τ(α)(T) from T(b) to T(g) by connecting high- and low-temperature dynamics.

Elastically cooperative activated barrier hopping theory of relaxation in viscous fluids. II. Thermal liquids

“…Rossler et.al. have convincingly shown this picture can empirically fit relaxation data on many molecular liquids over 14 orders of magnitude based on [22,40]:…”

“…This temperature is quite low, and corresponds to   of order 10 -2 s. As discussed further below, we believe this temperature is related to a reference temperature extracted by Rossler et al [22,40], T R , where the uncooperative barrier equals its collective analog. Finally, T g is identified with    100s .…”

“…Tarjus and Kivelson [19,20,39], Rossler and coworkers [22,40] and others [72] have suggested phenomenological 2-barrier models where the high temperature process is Arrhenius. Rossler et.al.…”

“…At high temperatures and fast relaxation times, an apparent Arrhenius law is often found to fit experiments [4,19,22,39,40,64]:…”

“…Section VI presents calculations for the effect of pressure on the alpha relaxation, and also analyzes an "activation volume" that grows with cooling and is strongly correlated with the number of cooperatively moving molecules and other measures of dynamic heterogeneity [13,14]. Connections of our approach with diverse alternative theories and models, including Arrhenius, mode coupling theory (MCT) [15], entropy crisis [16,17], dynamic facilitation [18], and phenomenological two-barrier approaches [19][20][21][22] is the subject of section VII. Our theoretical results are treated as "data" and we explore how well these models can fit our calculations.…”