Coupling of Caged Molecule Dynamics to JG β-Relaxation: I

Abstract

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.

Terahertz response of organic amorphous systems: experimental concerns and perspectives

“…Terahertz spectroscopy is by far not the only technique that can be used to measure the VDOS. Similar measurements can be obtained by methods such as neutron scattering, light scattering, Brillouin scattering, low-frequency Raman scattering and optical heterodyne detected optical Kerr effect techniques [2]. The advantage of THz-TDS is its simplicity and robustness of use, with minimal practical constraints on performing the experiments in a laboratory setting, straightforward and accurate data extraction and the availability of commercial terahertz spectroscopy systems.…”

“…A further examination of these results and other high-frequency data on amorphous systems clearly revealed that the vitrification of JG-beta relaxation is a universal process observed across a wide size and chemistry of glass-forming systems, and that the sensitivity of the high-frequency techniques to this effect is linked to the associated changes in the caged dynamics [2,7,8].…”

“…When cooling a liquid, two thermal events can be resolved by the response of the absorption at terahertz frequencies to the change in temperature: at T α ≈ T g and T β < T g [5]. These events strongly coincide with the vitrification temperature of α and the JG β-relaxation, respectively [2]. Generally, T α and T β can be determined from terahertz data by a simple linear fit of α THz (T ) or THz (T ) [5].…”

“…It has been recently discussed that it is not likely for the α and JG β-relaxation to directly contribute to terahertz dynamics close to T g and T β , respectively. Instead, the vitrification of the dielectric relaxations appears to affect the terahertz response via the cage dynamics, or nearly constant losses (NCL) [2,16,17]. This explanation still does not explain the linearity of THz (T ) nor does it explain why the NCL amplitude is proportional to the changes in the amplitude or relaxation time of the α and JG β-relaxations, as a simple addition is certainly not a suitable explanation given the difference in the thermal behaviour that is observed for τ α (T ) and τ β (T ).…”

“…A number of experimental techniques have been used to investigate such materials and their complex properties. Commonly used, wellestablished, experimental methods include thermal analysis, dielectric spectroscopy, as well as a broad range of scattering methods such as X-ray, neutron, Brillouin, light and Raman scattering amongst others [2].…”