Jin X, Guo Y, Feng S, Capaccioli S, Ngai KL, Wang LM. 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.
J Phys Chem B 2020;
124:6690-6697. [PMID:
32633964 DOI:
10.1021/acs.jpcb.0c04444]
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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 fα(T,P) constant. This isochronal superposition property of the α-relaxation holds for different choices of fα(T,P). The invariant frequency dispersions for the same fα(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 fα(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.
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