Interpreting the nonlinear dielectric response of glass-formers in terms of the coupling model

Uncovering the bridging role of slow atoms in unusual caged dynamics and β-relaxation of binary metallic glasses

The origin of β-relaxation in metallic glasses is still not fully understood, and the guidance of slow atoms for caged dynamics and β-relaxation is rarely mentioned. Using molecular dynamics simulations, we reveal the bridging role of slow atoms on unusual caged dynamics and β-relaxation. In the stage of unusual caged dynamics, slow atoms are bounded by neighboring atoms. It is difficult for the slow atoms to break the cage, producing more high-frequency vibration, which causes more atoms to jump out of the cage randomly in the next stage. Precisely, the movement of the slow atoms changes from individual atoms vibrating inside the cage and gradually breaking out of the cage into a string-like pattern. The string-like collective atomic jumps cause decay of the cages, inducing β-relaxation. This situation generally exists in binary systems with the large atomic mass difference. This work offers valuable insights for understanding the role of slow atoms in unusual caged dynamics and β-relaxation, complementing studies on the origin of β-relaxation in metallic glasses and their glass-forming liquids.

Studies on dynamics and isomerism in supercooled photochromic compound Aberchrome 670 with the use of different experimental techniques

Differential Scanning Calorimetry (DSC), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) and Broadband Dielectric (BD) spectroscopies were applied to investigate the thermal, structural, photochemical and dynamical properties of a fulgide-type photochromic compound, Aberchrome 670 (Ab670). In the original crystals, characterized by a pale yellow color, molecules take the E conformation. However, upon UV irradiation of either the crystalline or glassy compound, it isomerizes to the closed (C) form, characterized by the intense red tone. Although, we have found that such conversion is not complete (far below 100%). It was shown that due to UV irradiation as well as heating of the studied fulgide to high temperature (above the melting point), the Z isomer is formed. Further FTIR measurements performed on the UV irradiated and molten compound indicated that upon annealing of the sample in the vicinity of the glass transition temperature the Z isomer reverts back to the original E form. The final confirmation of this supposition has come from BDS studies, where the strong shift of the structural relaxation process during time-dependent isothermal measurements was noticed. One can add that a similar pattern of behavior has been observed previously by some of us in the case of tautomerism or mutarotation [Z. Wojnarowska et al., J. Chem. Phys., 2010, 133, 094507; W. Kossack et al., J. Chem. Phys., 2014, 140, 215101; P. Wlodarczyk et al., J. Phys. Chem. B, 2009, 113, 4379-4383; P. Wlodarczyk et al., J. Non-Cryst. Solids, 2010, 356, 738-742]. From the analysis of the time variation of the structural relaxation times, the activation barrier, E_A = 18 kJ mol^-1, for Z to E isomerization in Ab670 was calculated. Interestingly, it agrees well with the one determined for a similar kind of transformation in stilbenes. Therefore, we found that dielectric spectroscopy can be a very useful technique to track Z to E interconversion in the highly viscous supercooled state. Consequently, a unique opportunity to follow this kind of isomerism at high pressures, high electric fields and under nanometric spatial confinement in pure supercooled compounds appeared.

Nonlinear dielectric effects in liquids: a guided tour

Dielectric relaxation measurements probe how the polarization of a material responds to the application of an external electric field, providing information on structure and dynamics of the sample. In the limit of small fields and thus linear response, such experiments reveal the properties of the material in the same thermodynamic state it would have in the absence of the external field. At sufficiently high fields, reversible changes in enthalpy and entropy of the system occur even at constant temperature, and these will in turn alter the polarization responses. The resulting nonlinear dielectric effects feature field induced suppressions (saturation) and enhancements (chemical effect) of the amplitudes, as well as time constant shifts towards faster (energy absorption) and slower (entropy reduction) dynamics. This review focuses on the effects of high electric fields that are reversible and observed at constant temperature for single component glass-forming liquids. The experimental challenges involved in nonlinear dielectric experiments, the approaches to separating and identifying the different sources of nonlinear behavior, and the current understanding of how high electric fields affect dielectric materials will be discussed. Covering studies from Debye's initial approach to the present state-of-the-art, it will be emphasized what insight can be gained from the nonlinear responses that are not available from dielectric relaxation results obtained in the linear regime.

Dynamics of hydrated proteins and bio-protectants: Caged dynamics, β-relaxation, and α-relaxation

BACKGROUND

The properties of the three dynamic processes, α-relaxation, ν-relaxation, and caged dynamics in aqueous mixtures and hydrated proteins are analogous to corresponding processes found in van der Waals and polymeric glass-formers apart from minor differences.

METHODS

Collection of various experimental data enables us to characterize the structural α-relaxation of the protein coupled to hydration water (HW), the secondary or ν-relaxation of HW, and the caged HW process.

RESULTS

From the T-dependence of the ν-relaxation time of hydrated myoglobin, lysozyme, and bovine serum albumin, we obtain Ton at which it enters the experimental time windows of Mössbauer and neutron scattering spectroscopies, coinciding with protein dynamical transition (PDT) temperature Td. However, for all systems considered, the α-relaxation time at Ton or Td is many orders of magnitude longer. The other step change of the mean-square-displacement (MSD) at Tg_alpha originates from the coupling of the nearly constant loss (NCL) of caged HW to density. The coupling of the NCL to density is further demonstrated by another step change at the secondary glass temperature Tg_beta in two bio-protectants, trehalose and sucrose.

CONCLUSIONS

The structural α-relaxation plays no role in PDT. Since PDT is simply due to the ν-relaxation of HW, the term PDT is a misnomer. NCL of caged dynamics is coupled to density and show transitions at lower temperature, Tg_beta and Tg_alpha.

GENERAL SIGNIFICANCE

The so-called protein dynamical transition (PDT) of hydrated proteins is not caused by the structural α-relaxation of the protein but by the secondary ν-relaxation of hydration water. "This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo".

Observation of the nearly constant loss in super rigid saccharides: in search of a hidden crossover in dynamics deep in the glassy state

The molecular dynamics of three saccharides: D-glucose, 1,6-anhydro-D-glucose (levoglucosan) and 1,6:2,3-dianhydro-β-D-mannopyranose of various degrees of freedom, number of hydroxyl groups and internal structures was investigated over a wide range of temperatures and frequencies by means of Broadband Dielectric Spectroscopy (BDS). Despite the pronounced variety in the physicochemical properties of the carbohydrates, no change in the shape of the structural relaxation process was observed in the vicinity of the glass transition temperature (β(KWW) = 0.5). On the other hand further studies of the Debye-Stokes-Einstein relationship between dc conductivity and structural dynamics revealed some significant changes connected with the ability to form strong H-bonded structures. Moreover the presence of nearly constant loss (NCL) at moderate frequencies and just below the T(g) in the glassy state of levoglucosan and 1,6:2,3-dianhydro-β-D-mannopyranose was noticeable. We followed the temperature evolution of ε'' located at frequencies f = 0.1 kHz and f = 1 kHz, where the NCL is detected. Interestingly, a clear change in the dynamics far below the glass transition was observed in both compounds. This crossover (T(c)), found in different materials, and studied by various experimental techniques, is usually interpreted as being caused by the freezing of the Johari-Goldstein (JG) relaxation process. Alternatively it can also be due to the increasing anharmonicity in the density of vibrational states. Interestingly, it was shown that the slope of ε''(T) measured above the T(c) slightly changes while below the T(c) stays constant after physical aging. This is related to the densification of the sample that might result in steric hindrance and suppression of some kind of motion occurring in the glassy state, involving the larger parts of the molecules.

Coupling of Caged Molecule Dynamics to JG β-Relaxation III: van der Waals Glasses

In the first two papers separately on the polyalcohols and amorphous polymers of this series, we demonstrated that the fast dynamics observed in the glassy state at high frequencies above circa 1 GHz is the caged dynamics. We showed generally the intensity of the fast caged dynamics changes temperature dependence at a temperature THF nearly coincident with the secondary glass transition temperature Tgβ lower than the nominal glass transition temperature Tgα. The phenomenon is remarkable, since THF is determined from measurements of fast caged dynamics at short time scales typically in the ns to ps range, while Tgβ characterizes the secondary glass transition at which the Johari-Goldstein (JG) β-relaxation time τJG reaches a long time of ∼10(3) s, determined directly either by positronium annihilation lifetime spectroscopy, calorimetry, or low frequency dielectric and mechanical relaxation spectroscopy. The existence of the secondary glass transition originates from the dependence of τJG on density, previously proven by experiments performed at elevated pressure. The fact that THF ≈ Tgβ reflects the density dependence of the caged dynamics and coupling to the JG β-relaxation. The generality of the phenomenon and its theoretical rationalization implies the same should be observable in other classes of glass-formers. In this paper, III, we consider two archetypal small molecular van der Waals glass-formers, ortho-terphenyl and toluene. The experimental data show the same phenomenon. The present paper extends the generality of the phenomenon and explanation from the polyalcohols, a pharmaceutical, and many polymers to the small molecular van der Waals glass-formers.