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

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• 作者：S. Capaccioli ; K. L. Ngai ; M. Shahin Thayyil ; D. Prevosto
• 刊名：Journal of Physical Chemistry B
• 出版年：2015
• 出版时间：July 16, 2015
• 年：2015
• 卷：119
• 期：28
• 页码：8800-8808
• 全文大小：402K
• ISSN：1520-5207

文摘

The paper (Sibik, J.; Elliott, S. R.; Zeitler, J. A. J. Phys. Chem. Lett. 2014, 5, 1968鈥?972) 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 T_g. On heating the glasses, they observed the dielectric losses, 蔚鈥?谓) at 谓 = 1 THz, increase monotonically with temperature and change dependence at two temperatures, first deep in the glassy state at T_THz = 0.65T_g and second at T_g. The effects at both temperatures are most prominent in sorbitol but become progressively weaker in the order of xylitol and threitol, and the sub-T_g change was not observed in glycerol. They suggested this feature originates from the high-frequency tail of the Johari鈥揋oldstein (JG) 尾-relaxation, and the temperature region near 0.65T_g is the universal region for the secondary glass transition due to the JG 尾-relaxation. In this paper, we first use isothermal dielectric relaxation data at frequencies below 10⁶ Hz to locate the 鈥渟econd glass transition鈥?temperature T_尾 at which the JG 尾-relaxation time 蟿_JG reaches 100 s. The value of T_尾 is close to T_THz = 0.65T_g for sorbitol (0.63T_g) and xylitol (0.65T_g), but T_尾 is 0.74T_g for threitol and 0.83T_g for glycerol. Notwithstanding, the larger values of T_尾 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 尾-relaxation. From this relation, established is the connection of the magnitude and temperature dependence of the NCL and those of 蟿_JG. This connection explains the monotonic increase of NCL with temperature and change to a stronger dependence after crossing T_尾 giving rise to the sub-T_g behavior of 蔚鈥?谓) 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 T_尾 = 0.67T_g for flufenamic acid and T_尾 = 0.58T_g or T_尾 = 0.62T_g for indomethacin, which can be compared with experimental values of T_THz from THz-TDS measurements when they become available. We point out that the sub-T_g change of NCL at T_尾 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 伪-relaxation.