Isochronal Superposition of the Structural α-Relaxation and Invariance of Its Relation to the β-Relaxation to Changes of Thermodynamic Conditions in Methyl m-Toluate

Abstract

The dielectric spectra of methyl m-toluate (MMT) in supercooled liquid and glassy states were measured over wide ranges of temperature T at ambient and elevated pressures P. We found that the frequency dispersion of the loss peak contributed by the structural α-relaxation is invariant to changes of P and T, while keeping the loss peak frequency f α(T,P) constant. This isochronal superposition property of the α-relaxation holds for different choices of f α(T,P). The invariant frequency dispersions for the same f α(T,P) are also indicated by the fractional exponent βKWW in the Fourier transform of the Kohlrausch–Williams–Watts (KWW) function. Similarly, the fragility m index of MMT keeps approximately constant on varying pressure, largely different from H-bonded glass formers. The secondary β-relaxation at a frequency higher than f α(T,P) is found to shift to lower frequencies by elevating pressure in concert with the α-relaxation. The ratio τα(T,P)/τβ(T,P) is approximately unchanged to variations of T and P while keeping τα(T,P) constant. These properties observed in MMT offer experimental evidence of the dynamic correlation between α- and β-relaxations in pure small-molecule glass-formers.

Highly tunable β-relaxation enables the tailoring of crystallization in phase-change materials

“…Such properties are not observed in the non-PCM Ge 15 Te 85 despite their similarity in chemical compositions. At last, we note that modification of β-relaxation strength can be achieved by other methods such as varying processing conditions 37 , composition 58,59 , temperature, pressure 60,61 , and strain. A study of Pd-based metallic glasses showed that when the ultrasonic vibrations are stochastically resonant with the atomic motion associated with β-relaxations, crystallization can be accelerated by nearly one order of magnitude 62,63 .…”

Influence of the preparation method (thermal quench, ball-milling, dehydration and freeze-drying) on the dynamical properties of glassy trehalose

“…This result is consistent with the fact that quenched liquid glass is obtained from a higher density state than other glasses obtained from the crystalline state. The influence of density on intramolecular processes has been demonstrated by measurements of dielectric relaxation under pressure [26,29,30].…”

“…The origin of the two β-relaxation processes must then be necessarily be attributed, either to intramolecular degrees of freedom, or to intermolecular secondary relaxation process characteristic of the glassy state (also called Johari-Goldstein relaxation). A clear difference between these two relaxation processes concerns their sensitivity to the pressure parameter and to ageing which is high in the case of intermolecular relaxation and nearly inexistent in the case of intramolecular relaxation [28][29][30][31]. Broadband dielectric measurements on Leucrose [32], a dissacharide similar to trehalose, have shown the existence of two secondary relaxation modes below T g , the slower process being very sensitive to the pressure while the faster is not.…”