Glass transition is still an unsolved problem in condensed matter physics and chemistry. In this paper, we critically reexamine experimental data and theoretical interpretations of dynamic properties of various processes seen over a wide time range from picoseconds to laboratory time scales. In order of increasing time, the ubiquitous processes considered include (i) the dynamics of caged molecular units with motion confined within the anharmonic intermolecular potential and where no genuine relaxation has yet taken place; (ii) the onset of the Johari-Goldstein secondary relaxation involving rotation or translation of the entire molecular unit and causing the decay of the cages, to be followed by the cooperative and dynamically heterogeneous motions participated by increasing number of molecules or length scale; and (iii) the terminal primary alpha-relaxation with the maximum cooperative length-scale allowed by the intermolecular interaction and constraints of the glass former. Some general and important properties found in each of these processes are shown to be interrelated, indicating that the processes are connected, with one being the precursor of the other following it. Thus, a theory of glass transition is neither complete nor fundamental unless all of these processes and their inter-relations have been accounted. In addition to published data, new experimental data are reported here to provide a limited collection of critical experimental facts having an impact on current issues of glass transition research and servingas a guide for the construction of a complete and successful theory in the future.

The paper (Sibik, J.; Elliott, S. R.; Zeitler, J. A. J. Phys. Chem. Lett. 2014, 5, 1968-1972) used terahertz time-domain spectroscopy (THz-TDS) to study the dynamics of the polyalcohols, glycerol, threitol, xylitol, and sorbitol, at temperatures from below to above the glass transition temperature Tg. On heating the glasses, they observed the dielectric losses, ε″(ν) at ν = 1 THz, increase monotonically with temperature and change dependence at two temperatures, first deep in the glassy state at TTHz = 0.65Tg and second at Tg. The effects at both temperatures are most prominent in sorbitol but become progressively weaker in the order of xylitol and threitol, and the sub-Tg change was not observed in glycerol. They suggested this feature originates from the high-frequency tail of the Johari-Goldstein (JG) β-relaxation, and the temperature region near 0.65Tg is the universal region for the secondary glass transition due to the JG β-relaxation. In this paper, we first use isothermal dielectric relaxation data at frequencies below 10(6) Hz to locate the "second glass transition" temperature Tβ at which the JG β-relaxation time τJG reaches 100 s. The value of Tβ is close to TTHz = 0.65Tg for sorbitol (0.63Tg) and xylitol (0.65Tg), but Tβ is 0.74Tg for threitol and 0.83Tg for glycerol. Notwithstanding, the larger values of Tβ of glycerol are consistent with the THz-TDS data. Next, we identify the dynamic process probed by THz-TDS as the caged molecule dynamics, showing up in susceptibility spectra as nearly constant loss (NCL). The caged molecule dynamics regime is terminated by the onset of the primitive relaxation of the coupling model, which is the precursor of the JG β-relaxation. From this relation, established is the connection of the magnitude and temperature dependence of the NCL and those of τJG. This connection explains the monotonic increase of NCL with temperature and change to a stronger dependence after crossing Tβ giving rise to the sub-Tg behavior of ε″(ν) observed in experiment. Beyond the polyalcohols, we present new dielectric relaxation measurements of flufenamic acid and recall dielectric, NMR, and calorimetric data of indomethacin. The data of these two pharmaceuticals enables us to determine the value of Tβ = 0.67Tg for flufenamic acid and Tβ = 0.58Tg or Tβ = 0.62Tg for indomethacin, which can be compared with experimental values of TTHz from THz-TDS measurements when they become available. We point out that the sub-Tg change of NCL at Tβ found by THz-TDS can be observed by other high frequency spectroscopy including neutron scattering, light scattering, Brillouin scattering, and inelastic X-ray scattering. An example from neutron scattering is cited. All the findings demonstrate the connection of all processes in the evolution of dynamics ending at the structural α-relaxation.

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In several current important problems in different areas of soft matter physics, controversy persists in interpreting the molecular dynamics observed by various spectroscopies including dielectric relaxation, light scattering, nuclear magnetic resonance, and neutron scattering. Outstanding examples include: (1) relaxation of water in aqueous mixtures, in molecular sieves and silica-gel nanopores, and in hydration shell of proteins; and (2) dynamics of each component in binary miscible polymer blends, in mixtures of an amorphous polymer with a small molecular glassformer, and in binary mixtures of two small molecular glassformers. We show the applications of calorimetry to these problems have enhanced our understanding of the dynamics and eliminated the controversies

Although by now the glass transition temperature of uncrystallized bulk water is generally accepted to manifest at temperature Tg near 136 K, not much known are the spectral dispersion of the structural α-relaxation and the temperature dependence of its relaxation time τ α,bulk (T). Whether bulk water has the supposedly ubiquitous Johari-Goldstein (JG) β-relaxation is a question that has not been answered. By studying the structural α-relaxation over a wide range of temperatures in several aqueous mixtures without crystallization and with glass transition temperatures Tg close to 136 K, we deduce the properties of the α-relaxation and the temperature dependence of τ α,bulk (T) of bulk water. The frequency dispersion of the α-relaxation is narrow, indicating that it is weakly cooperative. A single Vogel-Fulcher-Tammann (VFT) temperature dependence can describe the data of τ α,bulk (T) at low temperatures as well as at high temperatures from neutron scattering and GHz-THz dielectric relaxation, and hence, there is no fragile to strong transition. The Tg-scaled VFT temperature dependence of τ α,bulk (T) has a small fragility index m less than 44, indicating that water is a "strong" glass-former. The existence of the JG β-relaxation in bulk water is supported by its equivalent relaxation observed in water confined in spaces with lengths of nanometer scale and having Arrhenius T-dependence of its relaxation times τ conf (T). The equivalence is justified by the drastic reduction of cooperativity of the α-relaxation in nanoconfinement and rendering it to become the JG β-relaxation. Thus, the τ conf (T) from experiments can be taken as τ β,bulk (T), the JG β-relaxation time of bulk water. The ratio τ α,bulk (Tg)/τ β,bulk (Tg) is smaller than most glass-formers, and it corresponds to the Kohlrausch α-correlation function, exp[−(t/τ α,bulk ) 1−n ], having (1−n) = 0.90. The dielectric data of many aqueous mixtures and hydrated biomolecules with Tg higher than that of water show the presence of a secondary ν-relaxation from the water component. The ν-relaxation is strongly connected to the α-relaxation in properties, and hence, it belongs to the special class of secondary relaxations in glass-forming systems. Typically, its relaxation time τν(T) is longer than τ β,bulk (T), but τν(T) becomes about the same as τ β,bulk (T) at sufficiently high water content. However, τν(T) does not become shorter than τ β,bulk (T). Thus, τ β,bulk (T) is the lower bound of τν(T) for all aqueous mixtures and hydrated biomolecules. Moreover, it is τ β,bulk (T) but not τα(T) that is responsible for the dynamic transition of hydrated globular proteins.

Fenofibrate is mainly used to reduce cholesterol level in patients at risk of cardiovascular disease. Thermal transition study with the help of differential scanning calorimetry (DSC) shows that the aforesaid active pharmaceutical ingredient (API) is a good glass former. Based on our DSC study, the molecular dynamics of this API has been carried out by broadband dielectric spectroscopy (BDS) covering wide temperature and frequency ranges. Dielectric measurements of amorphous fenofibrate were performed after its vitrification by fast cooling from a few degrees above the melting point (Tm=354.11 K) to deep glassy state. The sample does not show any crystallization tendency during cooling and reaches the glassy state. The temperature dependence of the structural relaxation has been fitted by single Vogel–Fulcher–Tamman (VFT) equation. From VFT fit, glass transition temperature (Tg) was estimated as 250.56 K and fragility (m) was determined as 94.02. This drug is classified as a fragile glass former. Deviations of experimental data from Kohlrausch–Williams–Watts (KWW) fits on high-frequency flank of α-peak indicate the presence of an excess wing in fenofibrate. Based on Ngai׳s coupling model, we identified the excess wing as true Johari–Goldstein (JG) process. Below the glass transition temperature one can clearly see a secondary relaxation (γ) with an activation energy of 32.67 kJ/mol.

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