Isochronal superposition and density scaling of the $\alpha$-relaxation from pico- to millisecond

TL;DR

This study investigates the alpha relaxation dynamics in molecular liquids across a wide timescale, demonstrating that density scaling and isochronal superposition apply broadly but break down for intra-molecular motions separated from alpha relaxation.

Contribution

It extends the understanding of density scaling and superposition to hydrogen-bonding liquids and explores their limitations for intra-molecular dynamics.

Findings

Isochronal superposition applies to both van der Waals and hydrogen-bonding liquids.

Density scaling exponent is smaller for hydrogen-bonding liquids.

Breakdown of scaling occurs for intra-molecular dynamics separated from alpha relaxation.

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.