High pressure specific heat spectroscopy reveals simple relaxation behavior of glass forming molecular liquid

TL;DR

This study uses high-pressure specific heat spectroscopy to show that certain molecular liquids exhibit a unified relaxation behavior governed by a single underlying process, supporting the isomorph theory for van der Waals liquids.

Contribution

It demonstrates that the characteristic timescales from specific heat and dielectric spectroscopy are proportional across temperature and pressure, indicating simple relaxation dynamics in these liquids.

Findings

The ratio of specific heat and dielectric relaxation times is pressure and temperature independent.

The results support the isomorph theory for van der Waals liquids.

Molecular liquids can exhibit unified relaxation behavior governed by a single inner clock.

Abstract

The frequency dependent specific heat has been measured under pressure for the molecular glass forming liquid 5-polyphenyl-4-ether in the viscous regime close to the glass transition. The temperature and pressure dependence of the characteristic timescale associated with the specific heat is compared to the equivalent timescale from dielectric spectroscopy performed under identical conditions. It is shown that the ratio between the two timescales is independent of both temperature and pressure. This observation is non-trivial and demonstrates the existence of specially simple molecular liquids in which different physical relaxation processes are both as function of temperature and pressure/density governed by the same underlying "inner clock". Furthermore, the results are discussed in terms of the recent conjecture that van der Waals liquids, like the measured liquid, comply to the isomorph theory.