Isochronal superposition and density scaling of the α-relaxation from pico- to millisecond

Abstract

The relaxation dynamics in two van der Waals bonded and one hydrogen-bonding molecular liquids is studied as a function of pressure and temperature by incoherent neutron scattering using simultaneous dielectric spectroscopy. The dynamics is studied in a range of alpha relaxation times from nano-to milliseconds, primarily in the equilibrium liquid state. In this range we find that isochronal superposition and density scaling work not only for the two van der Waals liquids, but also for the hydrogen-bonding liquid, though the density scaling exponent is much smaller for the latter. Density scaling and isochronal superposition are seen to break down for intra-molecular dynamics when it is separated in time from the alpha relaxation in close agreement with previous observations from molecular dynamics simulations.

Testing density scaling in nanopore-confinement for hydrogen-bonded liquid dipropylene glycol

“…25 Recently, Hansen and co-workers have demonstrated that the density scaling idea applies also for DPG, on the condition that the time scale of the a-relaxation refers to the equilibrium liquid far from the glass transition. 26 Based on the experimental results it was suggested that the TV g scaling works for most of the glass-forming systems only when considering a relatively small density changes. 13,27 On the other hand, in the literature we can also nd some evidence that it can be valid even up to 4.55 GPa (for cumene).…”

Dynamic Properties of Supercooled Chlorinated Biphenyls

“…An invariance of the shape of the α-dispersion to temperature and pressure at a fixed value of the relaxation time, τ (defined from the inverse of the peak frequency), is known as isochronal superpositioning. , In Figure , the α-peaks of the five Aroclors, measured at different temperatures for which their peak frequencies were approximately equal, are seen to essentially superpose (after vertical shifts to account for differences in the respective dielectric strengths). There is an almost negligible change in the peak shape with chlorine content.…”

Challenges of interpreting dielectric dilatometry for the study of pressure densification

“…Due to its relatively high glass transition temperature (T g ) at atmospheric pressure, 245 K, 5PPE suits the technical specifications of our high-pressure dielectric setup. Furthermore, 5PPE fulfils several prerequisites to be considered an R-simple liquid [9,[32][33][34], just like tetramethyl-tetraphenyl-siloxane studied in Ref. [16].…”

“…[16]. In 5PPE we thus expect the existence of isomorphs, that is, lines in the phase diagram along which several structural, dynamic and thermodynamic properties are invariant [9,23,[32][33][34]. Isomorphs are also isochrones, which are curves in the phase diagram with constant relaxation time.…”