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

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